Article

Deglacial changes in dust flux in the eastern equatorial Pacific

May 2007
Earth and Planetary Science Letters 257(1):215-230

May 2007
257(1):215-230

DOI:10.1016/j.epsl.2007.02.033

Authors:

David McGee

Massachusetts Institute of Technology

Franco Marcantonio

Texas A&M University

Atmospheric dust levels may play important roles in feedbacks linking continental source areas, tropical convection, marine productivity, and global climate. These feedbacks appear to be particularly significant in the tropical Pacific, where variations in local convection and productivity have been demonstrated to have impacts on climate at higher latitudes. Modeling of past dust levels and related feedbacks has been limited, however, by a paucity of observational data. In this study we present a temporal and spatial survey of dust fluxes to the eastern equatorial Pacific over the past 30 kyr. Glacial and Holocene fluxes of 232Th, a proxy for continental material, were calculated by normalization to 230Th from a north–south transect of cores along 110°W between 3°S and 7°N (ODP sites 848–853). Fluxes were 30–100% higher during the last glacial, suggesting increased dustiness in both hemispheres during the glacial period. In both time periods, dust fluxes decrease towards the south, reflecting scavenging of Northern Hemisphere dust by precipitation at the ITCZ. The Holocene meridional dust flux gradient between 7°N and 3°S is characterized by a steep drop in dust levels at the southern edge of the modern range of the ITCZ, while the gradient is shallower and more nearly linear during the last glacial. This change may indicate that the glacial ITCZ in this region was a less effective barrier to inter-hemispheric dust transport, most likely due to a decrease in convective intensity and precipitation during the last glacial; alternatively, the change in gradient may be explained by increased variability in the location of the glacial ITCZ. Our data do not appear to require a mean southerly displacement of the glacial ITCZ, as suggested by the results of other studies.

Dust flux in the eastern equatorial Pacific, supplement to: McGee, David; Marcantonio, Franco; Lynch-Stieglitz, Jean (2007): Deglacial changes in dust flux in the eastern equatorial Pacific. Earth and Planetary Science Letters, 257(1-2), 215-230

Data

January 2007

David McGee · Franco Marcantonio · Jean Lynch-Stieglitz

Response of the Central Pacific Intertropical Convergence Zone to Northern Hemisphere Cooling During the Last Glacial Maximum and Heinrich Stadial 1

Article

Full-text available

Feb 2024
GEOPHYS RES LETT

The latitudinal position of the Intertropical Convergence Zone (ITCZ) reflects the energy imbalance between the hemispheres. Southward displacements of the ITCZ during the Last Glacial Maximum (LGM; 19–26.5 ka) and Heinrich Stadial 1 (HS1; 14.6–17.5 ka), are widely accepted, but their magnitude is controversial. Geochemistry of detrital fractions in down‐core sediments collected from 6°N to equator along the 131.5°W transect reveal a distinct shift in εNd, La/Yb, and La–Sc–Th composition from predominantly northern hemisphere‐sourced to mixed northern and southern hemisphere‐sourced signal at 3°N–4°N during the LGM and 3°N–6°N during HS1. These contrasting provenance signals point to the past ITCZ functioning as a dust barrier. Given that a comparable geochemical demarcation currently occurs at 6°N–7°N, our data suggest that the ITCZ migrated southward by ∼3° during the LGM and ∼1°–3° during HS1 relative to its modern position in the central Pacific.

The Spatial Distribution of Aeolian Dust and Terrigenous Fluxes in the Tropical Atlantic Ocean Since the Last Glacial Maximum

Article

Full-text available

Feb 2021

The flux of terrestrial material from the continents to the oceans links the lithosphere, hydrosphere, and biosphere through physical and biogeochemical processes, with important implications for Earth's climate. Quantitative estimates of terrigenous fluxes from sources such as rivers, aeolian dust, and resuspended shelf sediments are required to understand how the processes delivering terrigenous material respond to and are influenced by climate. We compile thorium‐230 normalized ²³²Th flux records in the tropical Atlantic to provide an improved understanding of aeolian fluxes since the Last Glacial Maximum (LGM). By identifying and isolating sites dominated by aeolian terrigenous inputs, we show that there was a persistent meridional gradient in dust fluxes in the eastern equatorial Atlantic at the LGM, arguing against a large southward shift of the intertropical convergence zone during LGM boreal winter. The ratio of LGM to late‐Holocene ²³²Th fluxes highlights a meridional difference in the magnitude of variations in dust deposition, with sites <10°N showing larger changes over time. This supports an interpretation of increased trade wind strength at the LGM, potentially combined with differential changes in soil moisture and reductions in higher altitude summer winds. Our results also highlight the persistent importance of continental margins as sources of high terrigenous flux to the open ocean. This is especially evident in the western tropical Atlantic, where study locations reveal the primary influence of the South American continent up to >700 km away, characterized by ²³²Th fluxes approximately twice as large as aeolian‐dominated sites in the east.

Productivity patterns in the Equatorial Pacific over the last 30,000 years

Article

Full-text available

May 2017

The Equatorial Pacific traverses a number of productivity regimes, from the highly productive coastal upwelling along Peru to the near gyre-like productivity lows along the international dateline, making it an ideal target for investigating how biogeochemical systems respond to changing oceanographic conditions over time. However, conflicting reconstructions of productivity during periods of rapid climate change, like the last deglaciation, render the spatio-temporal response of Equatorial Pacific productivity ambiguous. In this study, surface productivity since the last glacial period (30,000 years ago) is reconstructed from seven cores near the Line Islands, Central Equatorial Pacific, and integrated with productivity records from across the Equatorial Pacific. Three coherent deglacial patterns in productivity are identified: 1) a monotonic glacial-Holocene increase in productivity, primarily along the Equator, associated with increasing nutrient concentrations over time, 2) a deglacial peak in productivity ~15,000 years ago due to transient entrainment of nutrient rich southern-sourced deep waters and 3) possible precessional cycles in productivity in the Eastern Equatorial Pacific that may be related to ITCZ migration and potential interactions with ENSO dynamics. These findings suggest that productivity was generally lower during the glacial period, a trend observed zonally across the Equatorial Pacific, while deglacial peaks in productivity may be prominent only in the East.

Helium and thorium isotope constraints on African dust transport to the Bahamas over recent millennia

Article

Nov 2016
EARTH PLANET SC LETT

Despite its potential linkages with North Atlantic climate, the variability in Saharan dust transport to the western North Atlantic over the past two millennia has not been well-characterized. A factor of 4 increase in dust production in sub-Saharan Africa has been attributed to the onset of Sahelian agriculture 200 yr ago. The regional extent of this anthropogenic dust increase, however, remains uncertain. Additionally, while millennial-scale cold periods of the last deglaciation have been associated with strong increases in North African dust emissions, few adequate records exist to observe dustiness during the Little Ice Age, a century-scale cooling of the North Atlantic (AD 1400–1800). In this study, we develop a new technique for the paired use of ²³⁰Th-normalized ²³²Th fluxes and ³He-normalized ⁴He fluxes in Bahamian tidal flat sediments. After justifying the fact that ²³⁰Th and ³He have had relatively constant sources to tidal flat and banktop waters, and accounting for the smoothing effect of bioturbation, a factor of 4 change in far-field dust transport to the western North Atlantic between the pre-industrial and modern era is not supported by our dust proxies over the past 2000 yr. Furthermore, we speculate why the response of western North Atlantic dust deposition associated with the Little Ice Age climate anomalies may have been modest compared to prior climatic events of the early Holocene or the last deglaciation.

Article

Oct 2016
EARTH PLANET SC LETT

High resolution paleoclimate records from low latitudes are critical for understanding the role of the tropics in transmitting and generating feedbacks for high-latitude climate change on glacial–interglacial and millennial timescales. Here we present three new records of ²³⁰Thxs,0-normalized ²³²Th-derived dust fluxes from the central equatorial Pacific spanning the last 150 kyr at millennial-resolution. All three dust flux records share the “sawtooth” pattern characteristic of glacial–interglacial cycles in ice volume, confirming a coherent response to global climate forcing on long timescales. These records permit a detailed examination of millennial variability in tropical dust fluxes related to abrupt perturbations in oceanic and atmospheric circulation. Increases in dust flux in association with at least six of the longest Greenland stadials provide evidence that abrupt, high-latitude climate oscillations influenced the atmospheric aerosol load in the equatorial Pacific, with implications for both direct and indirect effects on the tropical energy balance. Our latitudinal transect of cores captures shifts in the position of the Intertropical Convergence Zone (ITCZ) in response to variations in the interhemispheric thermal gradient associated with cooling in Greenland and bipolar seesaw warming in Antarctica. These observations demonstrate that changes in the energy and hydrologic balance of the tropics were repeated features of the penultimate deglaciation, last glacial inception and last glacial cycle, and highlight the role of the tropical atmosphere as a dynamic and responsive component of Earth's climate system.

Plio-Pleistocene glacial-interglacial productivity changes in the eastern equatorial Pacific upwelling system

Article

Full-text available

Mar 2016

The eastern equatorial Pacific Ocean (EEP) upwelling system supports >10% of the present-day global ocean primary production, making it an important component in Earth's atmospheric and marine carbon budget. Traditionally, it has been argued that, since intensification of Northern Hemisphere Glaciation (iNHG, ~2.7 Ma), changes in EEP productivity have predominantly depended on trade-wind strength-controlled upwelling intensity. An alternative hypothesis suggests that EEP productivity is primarily controlled by nutrient supply from the high southern latitudes via mode-waters. Here we present new high-resolution data for the latest Pliocene/early Pleistocene from Ocean Drilling Program Site 849, located within the equatorial divergence system in the heart of the EEP upwelling regime. We use carbon isotopes in benthic and planktic foraminiferal calcite and sand accumulation rates to investigate glacial-interglacial (G-IG) productivity fluctuations between 2.65 and 2.4 Ma (marine isotope stages (MIS) G1 to 94). This interval includes MIS 100, 98 and 96, three large-amplitude glacials (~1‰ in benthic δ18O) representing the culmination of iNHG. Our results suggest that latest Pliocene/early Pleistocene G-IG productivity changes in the EEP were strongly controlled by nutrient supply from Southern Ocean-sourced mode-waters. Our records show a clear G-IG cyclicity from MIS 100 onwards with productivity levels increasing from full glacial conditions and peaking at glacial terminations. We conclude that enhanced nutrient delivery from high southern latitudes during full glacial conditions together with superimposed intensified regional upwelling towards glacial terminations strongly regulated primary productivity rates in the EEP from MIS 100 onwards.

McGee et al-2016-GCA

Data

Mar 2016

Large deglacial shifts of the Pacific Intertropical Convergence Zone

Article

Full-text available

Jan 2016

The position of the Intertropical Convergence Zone (ITCZ) is sensitive to changes in the balance of heat between the hemispheres which has fundamental implications for tropical hydrology and atmospheric circulation. Although the ITCZ is thought to experience the largest shifts in position during deglacial stadial events, the magnitude of shifts has proven difficult to reconstruct, in part because of a paucity of high-resolution records, particularly those including spatial components. Here we track the position of the ITCZ from 150 to 110 ka at three sites in the central equatorial Pacific at sub-millennial time resolution. Our results provide evidence of large, abrupt changes in tropical climate during the penultimate deglaciation, coincident with North Atlantic Heinrich Stadial 11 (∼136-129 ka). We identify this event both as a Northern Hemisphere increase in aeolian dust and as a shift in the mean position of the ITCZ a minimum of 4° southwards at 160° W.

Tracking eolian dust with helium and thorium: Impacts of grain size and provenance

Article

Nov 2015
GEOCHIM COSMOCHIM AC

Reconstructions of the deposition rate of windblown mineral dust in ocean sediments offer an important means of tracking past climate changes and of assessing the radiative and biogeochemical impacts of dust in past climates. Dust flux estimates in ocean sediments have commonly been based on the operationally defined lithogenic fraction of sediment samples. More recently, dust fluxes have been estimated from measurements of helium and thorium, as rare isotopes of these elements (He-3 and Th-230) allow estimates of sediment flux, and the dominant isotopes (He-4 and Th-232) are uniquely associated with the lithogenic fraction of marine sediments. In order to improve the fidelity of dust flux reconstructions based on He and Th, we present a survey of He and Th concentrations in sediments from dust source areas in East Asia, Australia and South America. Our data show systematic relationships between He and Th concentrations and grain size, with He concentrations decreasing and Th concentrations increasing with decreasing grain size. We find consistent He and Th concentrations in the fine fraction (<5μm) of samples from East Asia, Australia and Central South America (Puna-Central West Argentina), with Th concentrations averaging 14μg/g and He concentrations averaging 2μccSTP/g. We recommend use of these values for estimating dust fluxes in sediments where dust is dominantly fine-grained, and suggest that previous studies may have systematically overestimated Th-based dust fluxes by 30%. Source areas in Patagonia appear to have lower He and Th contents than other regions, as fine fraction concentrations average 0.8μccSTP/g and 9μg/g for 4He and 232Th, respectively. The impact of grain size on lithogenic He and Th concentrations should be taken into account in sediments proximal to dust sources where dust grain size may vary considerably. Our data also have important implications for the hosts of He in long-traveled dust and for the 3He/4He ratio used for terrigenous He in studies of extraterrestrial He in sediments and ice.We also investigate the use of He/Th ratios as a provenance tracer. Our results suggest differences in fine fraction He/Th ratios between East Asia, Australia, central South America and Patagonia, with ratios showing a positive relationship with the geological age of source rocks. He/Th ratios may thus provide useful provenance information, for example allowing separation of Patagonian sources from Puna-Central West Argentina or Australian dust sources. He/Th ratios in open-ocean marine sediments are similar to ratios in the fine fraction of upwind dust source areas. He/Th ratios in mid-latitude South Atlantic sediments suggest that dust in this region primarily derives from the Puna-Central West Argentina region (23-32°S) rather than Patagonia (>38°S). In the equatorial Pacific, He/Th ratios are much lower than in extratropical Pacific sediments or potential source areas measured as a part of this study (East Asia, South America, Australia) for reasons that are at present unclear, complicating their use as provenance tracers in this region.

Twelve thousand years of dust: The Holocene global dust cycle constrained by natural archives

Article

Full-text available

Jun 2015

Mineral dust plays an important role in the climate system by interacting with radiation, clouds, and biogeochemical cycles. In addition, natural archives show that the dust cycle experienced variability in the past in response to global and local climate change. The compilation of the DIRTMAP (Dust Indicators and Records from Terrestrial and MArine Palaeoenvironments) paleodust data sets in the last 2 decades provided a benchmark for paleoclimate models that include the dust cycle, following a time slice approach. We propose an innovative framework to organize a paleodust data set that builds on the positive experience of DIRTMAP and takes into account new scientific challenges by providing a concise and accessible data set of temporally resolved records of dust mass accumulation rates and particle grain size distributions. We consider data from ice cores, marine sediments, loess–paleosol sequences, lake sediments, and peat bogs for this compilation, with a temporal focus on the Holocene period. This global compilation allows the investigation of the potential, uncertainties, and confidence level of dust mass accumulation rate reconstructions and highlights the importance of dust particle size information for accurate and quantitative reconstructions of the dust cycle. After applying criteria that help to establish that the data considered represent changes in dust deposition, 45 paleodust records have been identified, with the highest density of dust deposition data occurring in the North Atlantic region. Although the temporal evolution of dust in the North Atlantic appears consistent across several cores and suggests that minimum dust fluxes are likely observed during the early to mid-Holocene period (6000–8000 years ago), the magnitude of dust fluxes in these observations is not fully consistent, suggesting that more work needs to be done to synthesize data sets for the Holocene. Based on the data compilation, we used the Community Earth System Model to estimate the mass balance of and variability in the global dust cycle during the Holocene, with dust loads ranging from 17.2 to 20.8 Tg between 2000 and 10 000 years ago and with a minimum in the early to mid-Holocene (6000–8000 years ago).

Albani 2014 Twelve thousand years of dust the Holocene global dust cycle constrained by natural archives

Data

Full-text available

Dec 2014

Albani 2014 Twelve thousand years of dust

Data

Full-text available

Nov 2014

Twelve thousand years of dust: The Holocene global dust cycle constrained by natural archives

Article

Full-text available

Nov 2014
CPD

clim-past-discuss.net/10/4277/2014/ doi:10.5194/cpd-10-4277-2014 © Author(s) 2014. CC Attribution 3.0 License. This discussion paper is/has been under review for the journal Climate of the Past (CP).

Postglacial North Pacific

Chapter

Jan 2013

J.I. Martínez

From this compilation, it is apparent that the influence of the ENSO system extends all over the North Pacific during the Holocene, and that for centuries to thousands of years, El Niño-like and La Niña-like conditions can become the dominant state. Apart from a general pattern where the Kuroshio Current is weakened and the California Current is strengthened during El Nin˜ o- and La Nin˜ a-like states, respectively, there are significant regional differences. These are imposed by the interaction between the EAM and AIM in the IPWP and the northwest Pacific, and the PDO and the NPIW in the northeast Pacific. Of major interest is the recognition that the AISM is not only controlled by solar insolation, but also by sealevel change. Similarly, more robust evidence is building for teleconnections between the EAM, the California Current, and the AMOC, modulated by insolation and ice-sheet dynamics on suborbital time scales. The general paleoceanographicpicture for the North Pacific in the Holocene is one of a La Niña-like (negative PDO) state during the early–middle Holocene to one of El Niñoo-like (positive PDO) state during the late Holocene. Therefore, the ITCZ changed from a northern to a southern location from the middle Holocene to the late Holocene. This might have reduced the strength of the Kuroshio and California Currents.

The Flux and Provenance of Dust Delivered to the SW Pacific During the Last Glacial Maximum

Article

Full-text available

Dec 2020

Atmospheric dust is a primary source of iron (Fe) to the open ocean, and its flux is particularly important in the high nutrient, low chlorophyll (HNLC) Southern Ocean where Fe currently limits productivity. Alleviation of this Fe limitation in the Subantarctic Zone of the Atlantic by increased dust‐borne Fe supply during glacial periods has been shown to increase primary productivity. However, previous work has found no such increase in productivity in the Pacific sector. In order to constrain the relative importance of Southern Ocean Fe fertilization on glacial‐interglacial carbon cycles, records of dust fluxes outside of the Atlantic sector of the Southern Ocean at the Last Glacial Maximum (LGM) are required. Here we use grain size and U‐series analyses to reconstruct lithogenic and CaCO3 fluxes and Nd, Sr, and Pb isotopes to ascertain the provenance of terrigenous material delivered to four deep water cores in the SW Pacific Ocean over the last ~30 kyr. We find evidence for an increase in the relative proportion of fine‐grained (0.5–12 μm) terrigenous sediment and higher detrital fluxes during the LGM compared to the Holocene. The provenance of the LGM dust varied spatially, with an older, more “continental” signature (low εNd, high ⁸⁷Sr/⁸⁶Sr) sourced from Australia in the northern cores, and a younger, more volcanogenic source in the southern cores (high εNd, low ⁸⁷Sr/⁸⁶Sr), likely sourced locally from New Zealand. Given this increase in lithogenic flux to the HNLC subantarctic Pacific Southern Ocean during the LGM, factors besides Fe supply must have regulated the biological productivity here.

西太平洋暖池晚第四纪黏土矿物的冰期旋回和岁差周期

Article

Aug 2012

The Penultimate Glacial Termination and Variability of the Pacific Intertropical Convergence Zone

Article

Full-text available

May 2019
GEOPHYS RES LETT

Plain Language Summary In this study we investigate the changes in dust origin to the Central Equatorial Pacific that happened between 160 and 105 kyr before present. This time period is interesting because it includes the end of an ice age which terminated between 136 and 129 kyr. This transitional period was long enough to allow us to look at changes in dust sources as the ice age was ending. We used changes in the chemistry of the dust fraction found in marine sediments and calculated what percentage of this dust originated in the Northern and the Southern Hemispheres. We found that more dust originated in South America than was previously thought. We also found that the dust pattern deposited across out sites suggests that the dominant tropical hydroclimate feature, coincident with the thermal equator, was south of its modern position during the ice age and that it traveled further south during the transition from cold to warm conditions, finally settling near its modern position once the climate transition was complete.

41561 2018 205 MOESM1 ESM

Data

Full-text available

Aug 2018

Dissolved and particulate 230Th–232Th in the Central Equatorial Pacific Ocean: Evidence for far-field transport of the East Pacific Rise hydrothermal plume

Article

Dec 2015
EARTH PLANET SC LETT

Surface δ11B-pH reconstructions and insights into the ocean-atmosphere carbon exchange during the last deglaciation

Conference Paper

Aug 2013

Identifying imprints of externally derived dust and halogens in the sedimentary record of an Iberian alpine lake for the past ~13,500 years – Lake Peixão, Serra da Estrela (Central Portugal)

Article

Aug 2023
SCI TOTAL ENVIRON

Iberian lacustrine sediments are a valuable archive to document environmental changes since the last glacial termination, seen as key for anticipating future climate/environmental changes and their far-reaching implications for generations to come. Herein, multi-proxy-based indicators of a mountain lake record from Serra da Estrela were used to reconstruct atmospheric (in)fluxes and associated climatic/environmental changes over the last ~13.5 ka. Depositions of long-range transported dust (likely from Sahara) and halogens (primarily derived from seawater) were higher for the pre-Holocene, particularly in the late Bølling-Allerød-Younger Dryas period, compared to the Holocene. This synchronous increase could be related to a recognized dust-laden atmosphere, along with the combined effect of (i) an earlier proposed effective transport of Sahara dust for higher latitudes during cold periods and (ii) the progressive Polar Front expansion southwards, with the amplification of halogen activation reactions in lower latitudes due to greater closeness to snow/sea ice (halide-laden) surfaces. Additionally, the orographic blocking of Serra da Estrela may have played a critical role in increasing precipitation of Atlantic origin at higher altitudes, with the presence of snow prompting physical and chemical processes involving halogen species. In the late Holocene, the dust proxy records highlighted two periods of enhanced input to Lake Peixão, the first (~3.5-2.7 ka BP) after the end of the last African Humid Period and the second, from the 19th century onwards, agreeing with the advent of commercial agriculture, and human contribution to land degradation and dust emission in the Sahara/Sahel region. The oceanic imprints throughout the Holocene matched well with North Atlantic rapid climatic changes that, in turn, coincided with ice-rafted debris or Bond events and other records of increased storminess for the European coasts. Positive parallel peaks in halogens were found in recent times, probably connected to fire extinction by halogenated alkanes and roadway de-icing.

The pteropod species Heliconoides inflatus as an archive of late Pleistocene to Holocene environmental conditions on the Northwest Shelf of Australia

Article

Full-text available

Oct 2022

There is growing interest in the use of pteropods as potential archives of past changes in ocean chemistry. However, pteropods have rarely been used in studies of millennial-scale sedimentary records, especially in shallow-marine environments. This study obtained core data for the last 16 kyr from the Northwest Shelf of Australia (NWS). Changes in the carbonate saturation state were assessed based on the carbon isotope ratios (δ13C) of shells and the Limacina dissolution index (LDX) measured on the planktonic pteropod species Heliconoides inflatus. In addition, the calcification depth of the pteropods was estimated based on oxygen isotope values (δ18O) of pteropod shells and seawater. Our findings indicate that H. inflatus calcifies at a depth of 95–140 m. This confirms that H. inflatus records a shallow-marine signal on the NWS. The δ13C values of the pteropods record a notable decrease in carbonate ion concentrations after 8.5 ka. This decrease is associated with the post-glacial onset of humid conditions on the NWS. The studied pteropod shells are pristine throughout the 16 kyr section and have low LDX values. Therefore, the LDX proxy appears to lack the sensitivity to be applicable in this highly supersaturated, shallow-marine environment. Until this study, proxies derived from H. inflatus have been exclusively utilized in open-marine settings. Our results indicate that the δ13C values of H. inflatus also represent a useful proxy for carbonate ion concentrations in shallow-marine environments.

Pb isotopes Pacific dust

Article

Apr 2021

Assessing Sedimentary Detrital Pb Isotopes as a Dust Tracer in the Pacific Ocean

Article

Full-text available

Apr 2021

Mineral dust particles from different source regions typically have distinct Pb isotope ratios. Theoretically, Pb isotopic composition of terrigenous minerals isolated from open‐ocean sediments should allow for dust provenance reconstructions. However, Pb isotopes of terrigenous fractions of sediments have frequently been inconsistent with expected source region signatures. This study investigates the reason(s) for offsets between the Pb isotope values of the dust component in sediment cores and those expected from source regions with focus on changes in sediment composition, sediment age, and sediment processing for analysis. Pb isotope ratios from Pacific Ocean core‐top sediments show a general delineation of the Intertropical Convergence Zone (ITCZ). Isotope mixing models support these general trends, though similarity in Pb isotope ratios of disparate source regions makes constraining specific sources challenging. Pb isotope ratios in downcore samples varied on glacial/interglacial time scales, being less radiogenic during the last glacial maximum, suggesting either a weakened ITCZ or the addition of a new, less radiogenic, source to the system. Finally, Pb isotope ratios in some source region samples yielded different Pb isotope signatures in bulk source sample than in the insoluble terrigenous fraction of the source sample, indicating that differential mineral preservation within the terrigenous component in sediments may cause offsets from source signatures. Overall, while Pb isotopes show distinct basin‐scale variations, high‐resolution spatial reconstructions require tight age controls and consistency in analytical treatment if used to define ocean sediment source regions.

An Assessment of xsBa Flux as a Paleoproductivity Indicator and Its Water‐Depth Dependence in the Easternmost Equatorial Pacific Ocean

Article

Full-text available

Dec 2020

We present the first regional‐scale records of biogenic Barium (xsBa) fluxes in the Panama basin of the eastern‐equatorial margin of the Pacific Ocean in order to assess xsBa as a paleoproductivity proxy. Measurements of xsBa from 13 cores that range in water depths from about 700 to 3,000 m show an increase in ²³⁰Th‐normalized xsBa mass accumulation rates (MARs) with increasing water depth during both marine oxygen isotope stages (MIS) 1 and 2. The correlation of xsBa MARs with depth are strong despite differences in bulk sediment MARs and differing degrees of sediment redistribution. We interpret the increasing xsBa with water depth as likely due to the continued decomposition and remineralization of falling and/or resuspended biogenic particles. xsBa does not seem to be affected by diagenetic sulfate reduction in most of the cores. Calculated estimates of xsBa preservation in the sediment pile are high and fluctuate between 45% and 52% throughout the last 25 kyr. Although xsBa fluxes can be a robust indicator of paleoproductivity, caution is needed if (a) there is evidence of sulfate reduction in sediments being analyzed, and (b) one is trying to quantify differences in paleoproductivity among sites that are located at different depths in the water column.

Dissolved Thorium Isotope Evidence for Export Productivity in the Subtropical North Pacific During the Late Quaternary

Article

Full-text available

May 2020
GEOPHYS RES LETT

The vast area of the North Pacific subtropical gyre (NPSG) makes it an important region in exporting biogenic particles and carbon to the deep ocean. Nevertheless, we remain largely ignorant on its past changes in export productivity, leaving an important gap in the understanding of ocean‐climate interactions during the late Quaternary. Here, we take a novel approach to study the particle export via reconstruction of dissolved ²³⁰Th/²³²Th based on ferromanganese crusts in deep waters of the NPSG at 3.8‐ and 2.8‐km water depth, respectively. The U‐series data indicate stable growth rates of both samples and a rather small variability of their initial (²³⁰Th/²³²Th) in the last ca. 0.7 Myr. These glacial–interglacial integrated values are only about half of the modern dissolved (²³⁰Th/²³²Th) in the deep NPSG, reflecting increased dust dissolution and also particle scavenging related to ~10–40% higher average export productivity than today.

Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

Article

Full-text available

Feb 2020

Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of ²³⁰Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of ²³⁰Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of ²³⁰Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm²kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm²kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of ²³⁰Th as a constant flux proxy. Anomalous ²³⁰Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that ²³⁰Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).

Dating Deep‐Sea Sediments With Th Excess Using a Constant Rate of Supply Model

Article

Full-text available

Dec 2019

We present a dating method for deep‐sea sediments that uses the natural radionuclide ²³⁰Th (half‐life 75,380 years) in analogy to ²¹⁰Pb with the constant rate of supply (CRS) model. Using an example from the western Indian sector of the Southern Ocean, we demonstrate how sets of values of ²³⁰Th, ²³²Th, and U isotopes activities can supply absolute age information for the last ~450,000 years, given a sufficient precision, resolution, and depth coverage of the analytical data in a suitable core. An assessment of age uncertainties resulting from analytical errors using a Monte Carlo approach and an analytical solution for error propagation shows good agreement. We also investigate errors due to a violation of model assumptions by variable focusing of deep‐sea sediments by means of a simulated core. Finally, we use real examples from independently dated sediment cores containing carbonate, using previously existing ²³⁰Th data, to test the approach. The consideration of the systematic errors and the examples suggests that the uncertainties are smallest in the central part of the record and that the variability of focusing conditions controls the accuracy of the ²³⁰Th CRS dates. Our own example demonstrates an excellent agreement of the ²³⁰Th CRS method with independent age constraints, adding an important tool for dating marine records that does not depend on the presence of carbonate. The obtained values are also suitable to calculate ²³⁰Th‐normalized preserved vertical rain rates of various sedimentary compounds, permitting an improved quantitative comparison of marine paleorecords with other archives like ice cores.

Highly bioavailable dust-borne iron delivered to the Southern Ocean during glacial periods

Article

Full-text available

Oct 2018

Significance Dust-borne iron fertilization of Southern Ocean phytoplankton contributes to lower glacial atmospheric CO 2 . Previous studies evaluating the impact of dust on climate estimate bioavailable iron using total iron fluxes in sediment cores. Thus, all iron is considered equally bioavailable over geologic time, despite evidence that glaciers mobilize highly bioavailable iron from bedrock, which winds can deliver to the Southern Ocean. Here we reconstruct dust-borne iron speciation over the last glacial cycle, showing that highly bioavailable iron(II) silicate minerals are a greater fraction of total iron reaching the Southern Ocean during glacial periods. The abundance of iron(II) silicates likely controls the bioavailable iron supply to the Southern Ocean and contributes to the previously observed increase in glacial productivity and CO 2 drawdown.

The Evolution of Deep Ocean Chemistry and Respired Carbon in the Eastern Equatorial Pacific Over the Last Deglaciation: EEP Deep Ocean Chemistry Evolution

Article

Dec 2017

It has been shown that the deep Eastern Equatorial Pacific (EEP) region was poorly ventilated during the Last Glacial Maximum (LGM) relative to Holocene values. This finding suggests a more efficient biological pump, which indirectly supports the idea of increased carbon storage in the deep ocean contributing to lower atmospheric CO2 during the last glacial. However, proxies related to respired carbon are needed in order to directly test this proposition. Here we present Cibicides wuellerstorfi B/Ca ratios from Ocean Drilling Program Site 1240 measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) as a proxy for deep water carbonate saturation state (Δ[CO32−], and therefore [CO32−]), along with δ13C measurements. In addition, the U/Ca ratio in foraminiferal coatings has been analyzed as an indicator of oxygenation changes. Our results show lower [CO32−], δ13C, and [O2] values during the LGM, which would be consistent with higher respired carbon levels in the deep EEP driven, at least in part, by reduced deep water ventilation. However, the difference between LGM and Holocene [CO32−] observed at our site is relatively small, in accordance with other records from across the Pacific, suggesting that a “counteracting” mechanism, such as seafloor carbonate dissolution, also played a role. If so, this mechanism would have increased average ocean alkalinity, allowing even more atmospheric CO2 to be “sequestered” by the ocean. Therefore, the deep Pacific Ocean very likely stored a significant amount of atmospheric CO2 during the LGM, specifically due to a more efficient biological carbon pump and also an increase in average ocean alkalinity.

Geochemical Composition of Aeolian Dust and Surface Deposits from the Qatar Peninsula

Article

Oct 2017
CHEM GEOL

The trace metal geochemistry of atmospheric dust and terrestrial surface particles were studied on the Qatar Peninsula from November 21, 2013 to November 01,2015. We included samples of the mega dust-storm event on 01-02 April, 2015. Atmospheric dust samples were collected using passive dust traps. Terrestrial surface deposits of recent dust accumulation and traffic particulate from roads were also sampled. All samples were total acid digested and analyzed for major and trace elements using ICP-OES analyses. The concentration of thirteen elements (Ca, Mg, Ag, As, Cd, Cr, Cu, Mo, Ni, Se, Sn, Sr, Zn) were enriched in atmospheric dust samples, relative to Upper Continental Crust (UCC). Calcium was especially enriched by up to 435% relative to UCC. About 33% of the total sample mass was CaCO3, reflecting the composition of surface rocks and soils in the source areas. Of the elements typically associated with anthropogenic activity, Ag, Ni, and Zn were most enriched relative to UCC, with enrichment factors (EF) of 182%, 233%, and 209%, respectively. Other metals, which normally reflect anthropogenic sources, including Pb and V, were not significantly enriched, with enrichment factors of 25% and 3%, respectively. Major elements (Al, Mn, and Fe) were depleted (-58%, -35%, and -5%, respectively) relative to UCC due to the large dilution effect of the enrichment of CaCO3. Back trajectories were determined at the date of sampling for each sample using the NOAA HYSPLIT model. These showed that the source of the dust particles was almost equally divided between northerly and southerly sources, except one sample, which appeared to originate from the west. More variability in particle source locations were observed during the winter months (October to March). Samples from the mega-dust storm were solubilized using an acetic acid-hydroxylamine hydrochloride leach procedure to obtain an upper estimate of the potential contribution of bioactive elements to surface seawater. The leach procedure solubilized a significant fraction of almost all elements. Ca was the element most affected (81% removed) because of the carbonate minerals present. Bioactive elements like Fe (25%) and P (58%) were also significantly solubilized. Because river input is so small to the Arabian Gulf, this solubilized fraction of dust is likely a major source of nutrients to surface seawater. Enrichment factors were also calculated with respect to the average composition of Terrestrial Surface Deposits (TSD). Samples are not enriched significantly with respect to major components (EF<2), with a depletion in Ca, K, Na in dust storm samples, reflecting a different origin. A significant enrichment of the same trace metals is evident in dust deposits and in traffic samples, but not in dust storms: Cu, Mo, Ni, Zn, possibly deriving from local atmospheric sources (traffic, industries). Samples with northern and southern origins were compared to see if the composition could be used to identify source. Only three elements were observed to be statistically different. Pb and Na were higher in samples from the south, while Cr was higher in those from the north.

Millennial-scale iron fertilization of the eastern equatorial Pacific over the past 100,000 years

Article

Full-text available

Sep 2017

The eastern equatorial Pacific Ocean plays a crucial role in global climate, as it is a substantial source of CO2 to the atmosphere and accounts for a significant portion of global new export productivity. Here we present a 100,000-year sediment core from the eastern equatorial Pacific, and reconstruct dust flux, export productivity and bottom-water oxygenation using excess-230Th-derived fluxes of 232Th and barium, and authigenic uranium concentrations, respectively. We find that during the last glacial period (71,000 to 41,000 years ago), increased delivery of dust to the eastern equatorial Pacific was coeval with North Atlantic Heinrich stadial events. Millennial-scale pulses of increased dust flux coincided with episodes of heightened biological productivity, suggesting that dissolution of dust released iron that promoted ocean fertilization. We also find that periods of low atmospheric CO2 concentrations were associated with suboxic conditions and increased storage of respired carbon in the deep eastern equatorial Pacific. Increases in CO2 concentrations during the deglaciation are coincident with increases in deep Pacific and Southern Ocean water oxygenation levels. We suggest that deep-ocean ventilation was a primary control on CO2 outgassing in this region, with superimposed pulses of high productivity providing a negative feedback.

Latitudinal change in benthic foraminiferal fauna by ITCZ movement along the ~131°W transect in the equatorial Pacific Ocean

Article

Nov 2016

Modern and fossil benthic foraminifera were examined from nine surface sediments and two piston cores along the ~131°W transect in the equatorial Pacific Ocean. This study was conducted to clarify the biotic response of abyssal benthic foraminifera during the last 220 ka to changes in the seasonal extent of the Intertropical Convergence Zone (ITCZ). The abundance of modern benthic foraminifera was high at stations between the equator and 6°N, whereas it was low at stations north of 6°N, which is generally consistent with the latitudinal CaCO3 distribution of surface sediments. The northward increase of Epistominella exigua from the equator to ~6°N is similar to the seasonal variations in chlorophyll-a concentrations in the surface water and ITCZ position along ~131°W. This species was more common at core PC5103 (~6°N) than at core PC5101 (~2°N) after ~130 ka, when the Shannon-Wiener diversity (H’) between the two cores started to diverge. Hence, the presentday latitudinal difference in benthic foraminifera (E. exigua and species diversity) between ~2°N and ~6°N along ~131°W has been generally established since ~130 ka. According to the modern relationship between the seasonality of primary production and seasonal ITCZ variations in the northern margin of the ITCZ, the latitudinal divergence of benthic foraminiferal fauna between ~2°N and ~6°N since ~130 ka appear to have been induced by more distinct variations in the seasonal movement of ITCZ.

Paleodust variability since the Last Glacial Maximum and implications for iron inputs to the ocean

Article

Full-text available

Mar 2016
GEOPHYS RES LETT

Changing climate conditions affect dust emissions and the global dust cycle, which in turn affects climate and biogeochemistry. In this study we use observationally-constrained model reconstructions of the global dust cycle since the Last Glacial Maximum, combined with different simplified assumptions of atmospheric and sea ice processing of dust-borne iron, to provide estimates of soluble iron deposition to the oceans. For different climate conditions, we discuss uncertainties in model-based estimates of atmospheric processing and dust deposition to key oceanic regions, highlighting the large degree of uncertainty of this important variable for ocean biogeochemistry and the global carbon cycle. We also show the role of sea ice acting as a time buffer and processing agent, which results in a delayed and pulse-like soluble iron release into the ocean during the melting season, with monthly peaks up to ~17 Gg/month released into the Southern Oceans during the LGM.

Geochemical composition of dust from Qatar peninsula

Article

Full-text available

Nov 2015

Atmospheric dust samples have been collected from different areas in Qatar and analyzed for major and trace element composition. This region of the Arabian Gulf represents an area largely affected by dust from natural and anthropogenic sources. Twenty one samples were collected during 2014 and 2015 from Al-Khor, Katara, Sealine, Al-Waab, and Qatar University by passive and active sampling techniques. Some bulk samples were collected during the massive megastorm that occurred in April 2015. Back trajectories were determined for each sample set using the NOAA HYSPLIT model over a 50 hour time interval. Samples were equally divided between northerly (n=12; northern Saudi Arabia, Kuwait or Iraq) and southerly (n=8; SE Saudi Arabia, United Arab Emirates and Oman) sources. One sample is expected to originate from westward, in Saudi Arabia. There is more variability noticed in source locations throughout the winter months (October to March), with more of them coming from the south (9 times) compared to summer mo...

Export Production Fluctuations in the Eastern Equatorial Pacific during the Pliocene-Pleistocene: Reconstruction Using Barite Accumulation Rates

Article

Oct 2015
PALEOCEANOGRAPHY

Export production is an important component of the carbon cycle, modulating the climate system by transferring CO2 from the atmosphere to the deep-ocean via the biological pump. Here we use barite accumulation rates to reconstruct export production in the Eastern Equatorial Pacific over the past 4.3 Ma. We find that export production fluctuated considerably on multiple time scales. Export production was on average higher (51 gC m-2 yr-1) during the Pliocene than the Pleistocene (40 gC m-2 yr-1), decreasing between 3 to 1 Ma (from 60 to 20 gC m-2 yr-1) followed by an increase over the last million years. These trends likely reflect basin scale changes in nutrient inventory and ocean circulation. Our record reveals decoupling between export production and temperatures on these long (million years) time scale. On orbital time scales export production was generally higher during cold periods (glacial maxima) between 4.3 and 1.1 Ma. This could be due to stronger wind stress and higher upwelling rates during glacial periods. A shift in the timing of maximum export production to deglaciations is seen in the last ~1.1 million years. Results from this study suggest that in the EEP mechanisms that affect nutrient supply and/or ecosystem structure and in turn carbon export on orbital time scales differ from those operating on longer time scales and that processes linking export production and climate modulated oceanic conditions changed about 1.1 million years ago. These observations should be accounted for in climate models to ensure better predictions of future climate change.

Evaluation of Similarity of Water Column Properties and Sinking Particles between Impact and Preserved Sites for Environmental Impact Assessment in the Korea Contracted Area for Manganese Nodule Development, NE Pacific

Article

Full-text available

Dec 2014

Verifying the similarity of environmental characteristics between an artificial impact site and a preserved or reference site is necessary to quantitatively and qualitatively evaluate the environmental impact of mining activity. Although an impact site (BIS station) and a preserved site (called KOMO station) that have been selected in the Korea manganese nodule contract area may share similar environmental characteristics, similarities in terms of the water column environment between both sites has not been investigated. In this study, we compared the chemical properties of the water columns and sinking particle fluxes between BIS and KOMO stations through two observations (August 2011 and September 2012). Additionally, we observed particle fluxes at the KOMO station for five years (July 2003 ~ July 2008) to understand long-term natural variability. Vertical distributions of water column properties such as dissolved oxygen, inorganic nutrients (N, P, Si), total organic carbon below surface layer (within the depth range of 200 m) were not considerably different between the two sites. Especially, values of water column parameters in the abyssopelagic zone from 4000 m to bottom layer (~5000 m) were very similar between the BIS and KOMO sites. Sinking particle fluxes from the two sites also showed similar seasonality. However, natural variation of particle flux at the KOMO site varied from 3.5 to 129.9 mg m-2 day-1, with a distinct temporal variation originating from ENSO events (almost forty times higher than a minimum value). These results could provide valuable information to more exactly evaluate the environmental impact of mining activity on water columns. © 2014, Korea Ocean Research and Development Institute. All rights reserved.

Stable carbon isotopic variation of long chain n-alkanoic acids in the equatorial Pacific sediments over the last 40 Ma: Implications for expansion of C4 grassland in South America

Article

Full-text available

Nov 2014
ORG GEOCHEM

The stable carbon isotope ratio (δ13C) values of sedimentary long chain (> C20) n-alkanoic acids from the equatorial Pacific (IODP Sites U1331–1338) were measured for historical reconstruction of C4 plant contribution from South America over the last 40 Ma. The values for C30 and C32 acids ranged between –36.9 and –25.4‰, suggesting that they likely originated from higher plant wax associated with C3-dominated vegetation. Molecular distribution of alkanoic acids in the sediments showed a marked change across the Intertropical Convergence Zone (ITCZ), indicating a potential contribution from the Northern Hemisphere to the sediments located at > 10˚N. In contrast, the abundances of n-alkanoic acids significantly decrease in the sediments at < 10˚N. These results suggest that the ITCZ may have likely acted as an effective barrier for the atmospheric transport of terrigenous biomarkers from the Northern Hemisphere, and the records presumably reflect the plant wax signal from the northern part of South America. The latitudinal variations in the δ13C values suggest that the differences in the core locations possibly cause north- and southward shifts of ca. –1.0‰/˚N and ca. +0.5‰/˚S from the equator, respectively. The corrected δ13C values of C30 and C32 acids did not cross the C3 end member threshold until 1.7 Ma, despite the onset of C4 plants in the central South America at ca. 7 Ma. This implies that vegetation in the low latitudes of South America is less affected by hydrological changes induced by the uplift of the Andes during the late Miocene.

Biological response to millennial variability of dust supply in the Subantarctic South Atlantic Ocean

Article

Full-text available

Jul 2014
PHILOS T R SOC A

One contribution of 12 to a Theo Murphy Meeting Issue 'New models and observations of the Southern Ocean, its role in global climate and the carbon cycle' . Fluxes of lithogenic material and fluxes of three palaeo-productivity proxies (organic carbon, biogenic opal and alkenones) over the past 100 000 years were determined using the 230 Th-normalization method in three sediment cores from the Subantarctic South Atlantic Ocean. Features in the lithogenic flux record of each core correspond to similar features in the record of dust deposition in the EPICA Dome C ice 2014 The Author(s) Published by the Royal Society. All rights reserved. fluxes correlate with lithogenic fluxes in each sediment core. Our preferred interpretation is that South American dust, most probably from Patagonia, constitutes a major source of lithogenic material in Subantarctic South Atlantic sediments, and that past biological productivity in this region responded to variability in the supply of dust, probably due to biologically available iron carried by the dust. Greater nutrient supply as well as greater nutrient utilization (stimulated by dust) contributed to Subantarctic productivity during cold periods, in contrast to the region south of the Antarctic Polar Front (APF), where reduced nutrient supply during cold periods was the principal factor limiting productivity. The anti-phased patterns of productivity on opposite sides of the APF point to shifts in the physical supply of nutrients and to dust as cofactors regulating productivity in the Southern Ocean.

Postglacial North Pacific

Chapter

Jan 2024

Revisit of thorium-based dust fluxes and their implications for the iron fertilization hypothesis

Article

Nov 2021

Aeolian dust has a crucial impact on the marine carbon cycle, through its leverage on marine export production. To reconstruct dust depositions, 232Th in marine sediment is widely used as a proxy, with different 232Th concentrations of upper continental crust (UCC). Here, by comparing a novel compilation of 230Th-normalized 232Th in globally distributed marine sediment and the directly measured dust flux, we show that 14 ppm of 232Th concentration in UCC is the optimized parameter for deriving dust flux. This concentration was subsequently applied to recalculate the thorium-based dust flux during both the Holocene and the last glacial maximum (LGM), which yielded elevated LGM dust deposition over Holocene in most cores. A closer look at the dust comparison reveals a marked increase in the LGM dust flux at regions between 45°S to 55°S of the Southern Ocean owing to the variation in the dynamics of the Antarctic Circumpolar Current. Furthermore, with a compilation of the 230Th-normalized excess barium flux, these thorium-derived dust depositions suggest that no dust-induced iron fertilization occurred in the equatorial Pacific and the Antarctic zone of the Southern Ocean, as upwelling-derived iron was much greater than dust-derived iron, while the same case in the north Pacific Ocean was caused by the reduced vertical supply of nutrients. In contrast, dust-born iron boosted export productivity in most regions of the Subantarctic zone of the Southern Ocean.

Productivity and sediment focusing in the Eastern Equatorial Pacific during the last 30,000 years

Article

Apr 2019
DEEP-SEA RES PT I

The Eastern Equatorial Pacific (EEP) affects the ocean-atmosphere exchange of CO_2 on seasonal and interannual time scales through a balance of upwelling of CO_2-rich waters and the drawdown of CO_2 by biological productivity in the surface waters. The EEP accounts for almost 3/4ths of the global oceanic outgassing of CO_2 to the atmosphere, and it is known that the size of this EEP source of CO_2 varies significantly during El Nino events (Feely et al., 1999). There has been much effort to determine the El Nino Southern Oscillation (ENSO) state of the Equatorial Pacific during the past, particularly at the Last Glacial Maximum (LGM) when the global atmospheric [CO_2] was low, yet the glacial ENSO state remains a source of considerable controversy (Ford et al., 2015; Herguera, 2000; Koutavas et al., 2002; Loubere, 2001; Loubere et al., 2004; Lyle, 1988; Paytan et al., 1996; Pedersen, 1983; Sarnthein et al., 1988). Reconstructing past changes in equatorial productivity could help establish the prevailing ENSO state of the Pacific during the LGM, as the El Nino-related deepening of the thermocline in the East Pacific reduces productivity in the EEP and increases it in the Western Equatorial Pacific. Here we investigate changes in productivity in four cores from the equatorial Pacific, in the heart of the modern equatorial cold tongue. We determine changes in productivity using measurements of ^(231)Pa, ^(230)Th, ^(232)Th, ^(235)U and ^(238)U along with sedimentary fluxes. We also compare our findings to other sediment cores in the Pacific. We find elevated (^(231)Pa/^(230)Th)_(xs_ values (higher than production values) in general across the cores, indicating a net sink for oceanic ^(231)Pa in the EEP. We also find evidence for low levels of lateral sediment focusing, as well as lower productivity during the glacial in reduced ^(230)Th-normalized opal fluxes and decreased (^(231)Pa/^(230)Th)_(xs) at multiple sites. Examination of authigenic uranium at our sites in conjunction with previous work (Jacobel et al., 2017) shows that between 2 and 3.5 km depth in the Equatorial Pacific, there was a floating pool of respired carbon associated with the southward return flow of North Pacific Deep Water, sequestering CO_2 from the atmosphere during the LGM. We also compile Pacific basin wide records of productivity and Pa/Th during the Holocene (0-11kya) and LGM (18-22kya) and find evidence consistent with a more frequent or persistent glacial El Niño state throughout much of the Pacific (North Pacific, Western Equatorial Pacific and EEP).

Central Equatorial Pacific benthic foraminifera during the mid-Brunhes dissolution interval: Ballasting of particulate organic matter by biogenic silica and carbonate

Article

Apr 2019
QUATERNARY SCI REV

We evaluated the response of Quaternary abyssal benthic foraminifera in cores PC5101 (2º00.86′N, 131º34.32′W) and PC5103 (6º00.10′N, 131º28.57′W) of the Central Equatorial Pacific Ocean to the environmental changes over the past ∼520 kyrs, focusing on the mid-Brunhes dissolution interval (∼533–191 ka). We used multi-dimensional scaling (MDS) to derive MDS axis 1, reflecting food supply from low (negative scores) to high (positive scores) amounts, and MDS axis 2, reflecting variability in the food supply. From ∼120 ka on, Epistominella exigua, an indicator of variable food supply, was more abundant in core PC5103 (∼6oN) than in core PC5101 (∼2oN), but this was reversed from ∼300 to 250 ka. In core PC5101, MDS axis 1 scores are negatively correlated to the biogenic opal mass accumulation rates (MAR) after 249.6 ka, i.e., lower food supply at higher opal-MAR. In contrast, MDS axis 1 scores are positively correlated to the CaCO3-MAR from 520.8 to 331.2 ka. Both carbonate and opal skeletons might ballast particulate organic matter (POM) to enhance food supply to the benthos, but our data indicate that carbonate is more efficient and that changes in dominant ballasting of POM by different biominerals thus may have significantly affected the biological pump. During the transitional period (∼327.5–257.1 ka), ballasting of POM changed from control by calcareous plankton to control by siliceous plankton, with a transient period during which the latitudinal pattern of the Intertropical Convergence Zone was opposite to its modern pattern, with the more variable food supply at ∼ 6oN.

Sediment redistribution and grainsize effects on 230Th-normalized mass accumulation rates and focusing factors in the Panama Basin

Article

Nov 2017
EARTH PLANET SC LETT

Here, we examine how redistribution of differing grain sizes by sediment focusing processes in Panama Basin sediments affects the use of ²³⁰Th as a constant-flux proxy. We study representative sediments of Holocene and Last Glacial Maximum (LGM) time slices from four sediment cores from two different localities close to the ridges that bound the Panama Basin. Each locality contains paired sites that are seismically interpreted to have undergone extremes in sediment redistribution, i.e., focused versus winnowed sites. Both Holocene and LGM samples from sites where winnowing has occurred contain significant amounts (up to 50%) of the ²³⁰Th within the >63 μm grain size fraction, which makes up 40–70% of the bulk sediment analyzed. For sites where focusing has occurred, Holocene and LGM samples contain the greatest amounts of ²³⁰Th (up to 49%) in the finest grain-sized fraction (<4 μm), which makes up 26–40% of the bulk sediment analyzed. There are slight underestimations of ²³⁰Th-derived mass accumulation rates (MARs) and overestimations of ²³⁰Th-derived focusing factors at focused sites, while the opposite is true for winnowed sites. Corrections made using a model by Kretschmer et al. (2010) suggest a maximum change of about 30% in ²³⁰Th-derived MARs and focusing factors at focused sites, except for our most focused site which requires an approximate 70% correction in one sample. Our ²³⁰Th-corrected ²³²Th flux results suggest that the boundary between hemipelagically- and pelagically-derived sediments falls between 350 and 600 km from the continental margin.

Investigating the use of 232 Th/ 230 Th as a dust proxy using co-located seawater and sediment samples from the low-latitude North Atlantic

Article

Jul 2017
GEOCHIM COSMOCHIM AC

The thorium isotope ratio ²³²Th/²³⁰Th can be measured in seawater and sediment samples, and has been used as a proxy to reconstruct lithogenic fluxes to the oceans for the modern day and the Pleistocene. There has not yet been a study testing the proxy using the ²³²Th/²³⁰Th ratio in seawater and the ratio recorded in the underlying sediment. In this study we use co-located core-top sediments and seawater samples from five seamount sites spanning the tropical North Atlantic to investigate the link between seawater and sediment ²³²Th/²³⁰Th ratios across a range of water depths. Our results indicate that a broad correlation exists between seawater and sedimentary ²³²Th/²³⁰Th ratios. Both seawater and sedimentary ²³²Th/²³⁰Th ratios record a signal consistent with decreasing lithogenic input east to west, from Africa to South America. However, calculated ²³²Th fluxes for the core-top sediment samples indicate a strong dependence on depth, with up to a factor of ∼4 difference from shallow (<600 m) to deep sites (>2900 m). This depth dependence is likely caused by either a deficit of ²³⁰Th burial at depth compared to the production in the overlying water column, through addition of ²³²Th, or by a combination of the two processes. By comparing seawater and sedimentary ²³²Th/²³⁰Th ratios we derive an apparent fractional solubility of ²³²Th of 29 ± 3%, in reasonable agreement with the upper end of existing estimates.

Tracing Dust Input to the Global Ocean using Thorium Isotopes in Marine Sediments: ThoroMap

Article

Sep 2016
GLOBAL BIOGEOCHEM CY

Continental dust input into the ocean-atmosphere system has significant ramifications for biogeochemical cycles and global climate, yet direct observations of dust deposition in the ocean remain scarce. The long-lived isotope thorium-232 (232Th) is greatly enriched in upper continental crust compared to oceanic crust and mid-ocean ridge basalt (MORB)-like volcanogenic material. In open ocean sediments, away from fluvial and ice-rafted sources of continental material, 232Th is often assumed to be of predominantly eolian origin. In conjunction with flux normalization based on the particle reactive radioisotope thorium-230 (230Th), 232Th measurements in marine sediments are a promising proxy for dust accumulation in the modern and past ocean. Here we present ThoroMap, a new global data compilation of 230Th-normalized fluxes of 232Th. After careful screening, we derive dust deposition estimates in the global ocean averaged for the late Holocene (0-4 ka) and the Last Glacial Maximum (LGM, 19-23 ka). ThoroMap is compared with dust deposition estimates derived from CCSM3 and CCSM4, two coupled atmosphere, land, ocean, and sea-ice models. Model-data correlation factors are 0.63 (CCSM3) and 0.59 (CCSM4) in the late Holocene and 0.82 (CCSM3) and 0.83 (CCSM4) in the LGM. ThoroMap is the first compilation that is built on a single, specific proxy for dust and that exclusively uses flux-normalization to derive dust deposition rates.

No iron fertilization in the equatorial Pacific Ocean during the last ice age

Article

Jan 2016

The equatorial Pacific Ocean is one of the major high-nutrient, low-chlorophyll regions in the global ocean. In such regions, the consumption of the available macro-nutrients such as nitrate and phosphate is thought to be limited in part by the low abundance of the critical micro-nutrient iron(1). Greater atmospheric dust deposition(2) could have fertilized the equatorial Pacific with iron during the last ice age-the Last Glacial Period (LGP)-but the effect of increased ice-age dust fluxes on primary productivity in the equatorial Pacific remains uncertain(3-6). Here we present meridional transects of dust (derived from the Th-232 proxy), phytoplankton productivity (using opal, Pa-231/Th-230 and excess Ba), and the degree of nitrate consumption (using foraminifera-bound delta N-15) from six cores in the central equatorial Pacific for the Holocene (0-10,000 years ago) and the LGP (17,000-27,000 years ago). We find that, although dust deposition in the central equatorial Pacific was two to three times greater in the LGP than in the Holocene, productivity was the same or lower, and the degree of nitrate consumption was the same. These biogeochemical findings suggest that the relatively greater ice-age dust fluxes were not large enough to provide substantial iron fertilization to the central equatorial Pacific. This may have been because the absolute rate of dust deposition in the LGP (although greater than the Holocene rate) was very low. The lower productivity coupled with unchanged nitrate consumption suggests that the subsurface major nutrient concentrations were lower in the central equatorial Pacific during the LGP. As these nutrients are today dominantly sourced from the Subantarctic Zone of the Southern Ocean, we propose that the central equatorial Pacific data are consistent with more nutrient consumption in the Subantarctic Zone, possibly owing to iron fertilization as a result of higher absolute dust fluxes in this region(7,8). Thus, ice-age iron fertilization in the Subantarctic Zone would have ultimately worked to lower, not raise, equatorial Pacific productivity.

Boron isotope evidence for oceanic carbon dioxide leakage during the last deglaciation

Article

Full-text available

Feb 2015

Atmospheric CO2 fluctuations over glacial–interglacial cycles remain a major challenge to our understanding of the carbon cycle and the climate system. Leading hypotheses put forward to explain glacial–interglacial atmospheric CO2 variations invoke changes in deep-ocean carbon storage1, 2, probably modulated by processes in the Southern Ocean, where much of the deep ocean is ventilated3. A central aspect of such models is that, during deglaciations, an isolated glacial deep-ocean carbon reservoir is reconnected with the atmosphere, driving the atmospheric CO2 rise observed in ice-core records4, 5, 6. However, direct documentation of changes in surface ocean carbon content and the associated transfer of carbon to the atmosphere during deglaciations has been hindered by the lack of proxy reconstructions that unambiguously reflect the oceanic carbonate system. Radiocarbon activity tracks changes in ocean ventilation6, but not in ocean carbon content, whereas proxies that record increased deglacial upwelling4, 7 do not constrain the proportion of upwelled carbon that is degassed relative to that which is taken up by the biological pump. Here we apply the boron isotope pH proxy in planktic foraminifera to two sediment cores from the sub-Antarctic Atlantic and the eastern equatorial Pacific as a more direct tracer of oceanic CO2 outgassing. We show that surface waters at both locations, which partly derive from deep water upwelled in the Southern Ocean8, 9, became a significant source of carbon to the atmosphere during the last deglaciation, when the concentration of atmospheric CO2 was increasing. This oceanic CO2 outgassing supports the view that the ventilation of a deep-ocean carbon reservoir in the Southern Ocean had a key role in the deglacial CO2 rise, although our results allow for the possibility that processes operating in other regions may also have been important for the glacial–interglacial ocean–atmosphere exchange of carbon.

Hydrological evolution of the eastern tropical Pacific during the last 430,000 years

Thesis

Full-text available

Jul 2013

Hasrizal Shaari

The eastern tropical Pacific (ETP) is a key region in paleocenography to understand the response of upwelling, the position of the intertropical convergence zone (ITCZ), and the zonal and meridional atmospheric circulations to orbital forcing. In this study, the author reconstructed the hydrological evolution of the ETP by analyzing glycerol dialkyl glycerol tetraethers (GDGTs), alkenones and the oxygen isotopes of foraminifera and bulk carbonate in sediments from Ocean Drilling Program Sites 1237, 1239, and 1241 during the last 430,000 years. In the first chapter, the author describes the outline of modern oceanography of the ETP and the history of paleoceanographic studies for the ETP. He also presents the purpose and strategy of this study. In the third chapter, the author proposes that the difference between TEX86H- and UK37′-derived temperatures (∆T) and the abundance ratio of GDGTs to alkenones (GDGT/alkenone ratio) are potential upwelling indices which show consistent results with other upwelling indices, and discusses changes in upwelling intensity in the eastern equatorial Pacific (EEP) at the offshore of Ecuador (Site 1239) over the past 430 ka. The ΔT and GDGT/alkenone ratio were maximal during the last five deglaciations, suggesting intensified upwelling. The intensification of upwelling in the EEP coincided with those at the Peru margin and in the Southern Ocean. This coincidence suggests that the reorganization of the Southern Hemisphere atmospheric circulation induced the intensification of the subtropical high-pressure cell, causing stronger southeast trade winds along the west coast of South America and the southern westerlies over the Southern Ocean, enhancing upwelling in both regions. In the fourth chapter, the author discusses the hydrological evolution of the eastern Pacific warm pool region (EPWP) at the offshore of Panama (Site 1241) during the last 150,000 years. GDGTs and alkenone concentrations showed higher values in MIS 2 and MIS 6, which suggest the enhancement of primary production at glacial periods. The TEX86H- and UK37′- derived temperature depicted the different surface temperature evolution. UK37′-derived temperature was marked by small variation during the glacial- interglacial cycles, whereas TEX86H- showed a pronounced variation that was similar to Mg/Ca-derived temperature records obtained at a nearby core in the EPWP. Given that enhanced primary productivity during glacials suggest nutricline shoaling, unchanged UK37′ over glacial-interglacial cycles can be interpreted to be related to the shift of alkenone production depth. TEX86H seems not to be influenced by glacial-interglacial changes in nutricline depths, recording an integrated temperature in surface and thermocline water. The shallow nutricline in the EPWP during glacial maxima most likely reflected the intense formation of Antarctic intermediate water. In the fifth chapter, the author describes the SST evolution in the Peru margin (Site 1237) and discusses changes in the intensity of the Peru Coastal Current (PCC) by reconstructing the latitudinal gradient of SST along the western margin of Central and South America during the last 90,000 years. GDGTs and alkenones were analyzed forsediment samples retrieved from ODP Site 1237 (Peru margin) and compared the SST record with the records at Site 1241 (off Panama) and Site 1239 (off Ecuador). A decrease of temperature gradient between the Peru margin (Site 1237) and the EEP (Site 1239) during the last deglaciation suggests that the PCC intensified over the coastal boundary region. The intensification of the PCC coincided with the thermocline shoaling in the EEP and the enhanced upwelling in the EEP and the intensified anoxia at the Peru margin during the last deglaciation. This coincidence suggests that the intensification of the PCC is a part of regional hydrological change in the eastern Pacific during the last deglaciation. In the last chapter, the author summarise the results and discussion this study. Both the enhanced upwelling in the EEP and the intensification of the PCC, along with other evidence from published records, suggest the intensification of the South Pacific subtropical gyre circulation during deglaciations. In the EPWP, thermocline shoaled in glacial maxima, likely reflected the intense formation of the AAIW.

Particle sorting during sediment redistribution processes and the effect on 230Th-normalized mass accumulation rates

Article

Aug 2014
GEOPHYS RES LETT

The 230Th method of determining mass accumulation rates (MARs) assumes that little to no fractionation occurs during sediment redistribution processes at the seafloor. We examine 230Th inventories in radiocarbon-dated multicore sediments from paired winnowed and focused sites at Cocos and Carnegie Ridges, Panama Basin. Radiocarbon-derived sand MARs, which likely represent the vertical rain of particles poorly transported by bottom currents, are similar at each of the paired sites, but are different using 230Th normalization. 230Th-normalized MARs are 60% lower focused sites, and likely underestimate vertical MARs, while the reverse is true for winnowed sites. We hypothesize that size fractionation occurs most frequently at lower current velocities, resulting in the coarse fraction being left behind and primarily the fine 230Th-rich grains being transported downslope. 230Th-normalization works well for recording fine-grained (detrital and opal), but not coarse-grained (carbonate), fluxes in regions that have undergone sediment redistribution.

Leg 199 summary

Chapter

Full-text available

Jan 2002

Ocean Drilling Program (ODP) Leg 199, "The Paleogene Equatorial Transect" (Sites 1215-1222), was designed to study the evolution of the equatorial Pacific current and wind system as Earth went from maximum Cenozoic warmth to initial Antarctic glaciations. The drilling program was primarily devoted to a transect along the 56- to 57-Ma crust, old enough to capture the Paleocene/Eocene boundary in the basal, more carbonate-rich sediments. The Leg 199 transect extends from a paleolatitude of ~4°N- ~4°S to encompass a relatively thick lower Eocene sediment section perhaps 8° north of the paleoequator. One site (1218) was also drilled on ~40-Ma crust to collect a near-equatorial sediment sequence from the middle Eocene to the Oligocene in order to investigate the transition in global climate from the Eocene "greenhouse" to Oligocene "icehouse." The Pacific plate has drifted northward through Cenozoic time transporting biogenic sediments deposited under the high-productivity equatorial belt into a zone of extremely slow sediment (red clay) accumulation. Thus, the central tropical North Pacific Ocean is an ideal region in which to sample shallowly buried Paleogene sequences of equatorially deposited biogenic sediments. The thin Neogene cover of red clay in the area means that the entire Paleogene sediment section is potentially drillable by ODP advanced piston coring and extended core barrel methods.

Late Cenozoic Atmospheric Circulation Intensity and Climatic History Recorded by Eolian Deposition in the Eastern Equatorial Pacific Ocean, Leg 138

Article

Full-text available

Aug 1995

Steve Hovan

Sources and global distributions of dust aerosols simulated with the GOCART model

Article

Full-text available

Jan 2001

The global distribution of dust aerosol is simulated with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GO-CART) model. In this model all topographic lows with bare ground surface are assumed to have accumulated sediments which are potential dust sources. The uplifting of dust particles is expressed as a function of surface wind speed and wetness. The GOCART model is driven by the assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS) which facilitates direct comparison with observations. The model includes seven size classes of mineral dust ranging from 0.1-6 pm radius. The total annual emission is estimated to be between 1604 and 1960 Tg yr-I in a 5-year simulation. The model has been evaluated by comparing simulation results with ground-based measurements and satellite data. The evaluation has been performed by comparing surface concentrations, vertical distributions, deposition rates, optical thickness, and size distributions. The comparisons show that the model results generally agree with the observations without the necessity of invoking any contribution from anthropogenic disturbances to soils. However, the model overpredicts the transport of dust from the Asian sources to the North Pacific. This discrepancy is attributed to an overestimate of small particle emission from the Asian sources.

A New Late Neogene Time Scale: Application to Leg 138 Sites

Article

Full-text available

Aug 1995

The sediments recovered during Leg 138 provide a remarkable opportunity to improve the geological time scale of the late Neogene. We have developed new time scales in the following steps. First, we constructed age models on the basis of shipboard magnetostratigraphy and biostratigraphy, using the time scale of Berggren, Kent, and Flynn (1985). Second, we refined these age models using shipboard GRAPE density measurements to provide more accurate correlation points. Third, we calibrated a time scale for the past 6 m.y. by matching the high-frequency GRAPE density variations to the orbital insolation record of Berger and Loutre (1991); we also took into account δ 18 θ records, where they were available. Fourth, we generated a new seafloor anomaly time scale using our astronomical calibration of C3A.n (t) at 5.875 Ma and an age of 9.639 Ma for C5n.ln (t) that is based on a new radiometric calibration (Baksi, 1992). Fifth, we recalibrated the records older than 6 Ma to this new scale. Finally, we recon-sidered the 6-to 10-Ma interval and found that this could also be partially tuned astronomically.

The atmospheric input of trace species to the World ocean

Article

Full-text available

Sep 1991

Trace elements considered were Pb, Cd, Zn, Cu, Ni, As, Hg, Sn, Al, Fe, Si, and P. Oxidized and reduced forms of nitrogen were considered, including nitrate and ammonium ions and the gaseous species NO, NO2, HNO3, and NH3. Synthetic organic compounds considered included polychlorinated biphenyls (PCBs), hexachlorocyclohexanes (HCHs), DDTs, chlordane, dieldrin, and hexachlorobenzenes (HCBs). Making this assessment was difficult because there are very few actual measurements of deposition rates of these substances to the ocean. However, there are considerably more data on the atmospheric concentrations of these species in aerosol and gaseous form. Mean concentration data for 10° × 10° ocean areas were determined from the available concentration data or from extrapolation of these data into other regions. These concentration distributions were then combined with appropriate exchange coefficients and precipitation fields to obtain the global wet and dry deposition fluxes. -from Authors

Oceanic primary production: 2. Estimation at global scale from satellite (Coastal Zone Color Scanner) chlorophyll

Article

Full-text available

Mar 1996

The purpose of this study was to assess the oceanic seasonal evolution and spatial distribution of photosynthetic carbon fixation. Computation of primary production from the upper ocean chlorophyll-like pigment concentrations were made from monthly global maps from the coastal zone color scanner data archive. Relative contributions of various oceans and zonal belts were identified. Depending on the ratio used for active pigments to total pigments, the calculated global annual production ranges from 36.5 and 45.6 G tons (metric) carbon per year. These values are among the highest estimates proposed to date; although the absolute values may be somewhat questionable, the relative contribution of the various zonal belts and oceans are considered to have a high degree of accuracy. 33 refs., 4 figs., 2 tabs.

The “African humid period” and the record of marine upwelling from excess 230Th in Ocean Drilling Program Hole 658C

Article

Full-text available

Dec 2006

Using a high-resolution 230Th normalized record of sediment flux, we document the deglacial and Holocene history of North African aridity and coastal upwelling at Ocean Drilling Program Hole 658C. At both the end of the Younger Dryas and after the 8.2 ka event, there are significant drops in terrigenous accumulation at our site, indicating an increase in the monsoon moisture flux over Africa at this time. At 5.5 ka, there is an abrupt end to the “African humid period” and a return to stronger upwelling conditions. For carbonate and opal fluxes the 230Th normalization completely changes the shape of each record based on percentage variations alone. This site is a clear example of how variations in one sediment component can obscure changes in the others, and it demonstrates the need for radionuclide measurements more generally in paleoceanography. By taking our new records and a large amount of previous data from this site we conclude that increases in African moisture are tightly coupled to decreases in coastal upwelling intensity.

Patterns of CaCO3 deposition in the eastern tropical Pacific Ocean for the last 150 Kyr: Evidence for a southeast Pacific depositional spike during marine isotope stage (MIS) 2

Article

Full-text available

May 2002

We constructed biogenic mass accumulation rate (MAR) time series for eastern Pacific core transects across the equator at ~105° and ~85°W and along the equator from 80° to 140°W. We used empirical orthogonal function (EOF) analysis to extract spatially coherent patterns of CaCO3 deposition for the last 150 kyr. EOF mode 1 (51% variance) is a CaCO3 MAR spike centered in marine oxygen isotope stage 2 (MIS 2) found under the South Equatorial Current. EOF mode 2 (19% of variance) is high north of the equator. EOF mode 3 (9% of variance) is an east-west mode centered along the North Equatorial Counter Current. The MIS 2 CaCO3 spike is the largest event in the eastern Pacific for the last 150 kyr: CaCO3 MARs are 2-3 times higher at 18 ka than elsewhere in the record, including MIS 6. It is caused by high CaCO3 production rather than minimal dissolution. EOF 2, while it resembles deep water flow patterns, nevertheless, shows coherence to Corg deposition and is probably also driven by CaCO3 production.

Sensitivity study of meteorological parameters on mineral aerosol mobilization, transport, and distribution

Article

Full-text available

Aug 2003

Predicting mineral aerosol distributions is a difficult task due to the episodic nature of the sources and transport. Here we show comparisons between a 22-year simulation of mineral aerosols and satellite and in situ observations. Our results suggest that the model does a good job of predicting atmospheric mineral aerosol distributions, with some discrepancies. In addition, there are differences between our model results and previously published results [e.g., Ginoux et al., 2001]. We conduct several tests of the sensitivity of mineral aerosol simulations to the meteorological data sets and mobilization parameterizations in order to understand the differences. Comparisons between model simulations using National Center for Atmospheric Research/National Center for Environmental Prediction (NCEP/NCAR) and National Aeronautics and Space Administration Data Assimilation Office (NASA DAO) reanalysis data sets show that the model results with the two data sets are fairly consistent but with some important differences. The sensitivity analysis shows that differences between simulated dust near Australia are likely due to differences in both source parameterization and surface winds. Differences over East Asia are dominated by differences in meteorology. The sensitivity analysis also shows that we cannot tell from comparisons with observations whether the cultivation source is active nor eliminate it because of the large uncertainty in meteorology and source parameterization.

Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data

Article

Full-text available

Jul 1997

Global distributions of UV-absorbing aerosols are obtained using measured differences between the 340 and the 380 nm radiances from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) for the years 1979-1993. Time series are shown for major sources of biomass burning and desert dust giving the frequency of occurrence and areal coverage over land and oceans. Minor sources of UV-absorbing aerosols in the atmosphere are also discussed (volcanic ash and oil fires). Relative values of year-to-year variability of UV-absorbing aerosol amounts are shown for major aerosol source regions: (1) central South America (Brazil) near 10°S latitude; (2) Africa near 0°-20°S and 0° to 10°N latitude; (3) Saharan Desert and sub-Saharan region (Sahel), Arabian Peninsula, and the northern border region of India; (4) agricultural burning in Indonesia, Eastern China, and Indochina, and near the mouth of the Amazon River; and (5) coal burning and dust in northeastern China. The first three of these regions dominate the injection of UV-absorbing aerosols into the atmosphere each year and cover areas far outside of their source regions from advection of UV-absorbing particulates by atmospheric wind systems. During the peak months, smoke and dust from these sources are transported at altitudes above 1 km with an optical depth of at least 0.1 and can cover about 10% of the Earth's surface. Boundary layer absorbing aerosols are not readily seen by TOMS because the small amount of underlying Rayleigh scattering leads to a small signal. Significant portions of the observed dust originate from agricultural regions frequently within arid areas, such as in the Sahel region of Africa, especially from the dry lake-bed near Lake Chad (13.5°N, 14°E), and intermittently dry drainage areas and streams. In addition to drought cycle effects, this suggests there may be an anthropogenic component to the amount of dust injected into the atmosphere each year. Detection of absorbing aerosols and calculation of optical depths are affected by the presence of large-scale and subpixel clouds in the TOMS field of view.

Sources and distributions of dust aerosols simulated with the GOCART model

Article

Full-text available

Sep 2001

The global distribution of dust aerosol is simulated with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. In this model all topographic lows with bare ground surface are assumed to have accumulated sediments which are potential dust sources. The uplifting of dust particles is expressed as a function of surface wind speed and wetness. The GOCART model is driven by the assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS) which facilitates direct comparison with observations. The model includes seven size classes of mineral dust ranging from 0.1-6 mum radius. The total annual emission is estimated to be between 1604 and 1960 Tg yr-1 in a 5-year simulation. The model has been evaluated by comparing simulation results with ground-based measurements and satellite data. The evaluation has been performed by comparing surface concentrations, vertical distributions, deposition rates, optical thickness, and size distributions. The comparisons show that the model results generally agree with the observations without the necessity of invoking any contribution from anthropogenic disturbances to soils. However, the model overpredicts the transport of dust from the Asian sources to the North Pacific. This discrepancy is attributed to an overestimate of small particle emission from the Asian sources.

Sediment focusing in the central equatorial Pacific Ocean

Article

Full-text available

Jun 2001

At four sites in the central equatorial Pacific Ocean the flux of extraterrestrial 3He, determined using the excess 230Th profiling method, is 8×10-13cm3STPcm-2ka-1. This supply rate is constant to within 30%. At these same sites, however, the burial rate of 3He, determined using chronostratigraphic accumulation rates, varies by more than a factor of 3. The lowest burial rates, which occur north of the equator at 1°N, 139°W are lower than the global average rate of supply of extraterrestrial 3He by 20% and indicate that sediment winnowing may have occurred. The highest burial rates, which are recorded at the equator and at 2°S, are higher than the rate of supply of extraterrestrial 3He by 100%, and these provide evidence for sediment focusing. By analyzing several proxies measured in core PC72 sediments spanning the past 450 kyr we demonstrate that periods of maximum burial rates of 230Th,3He,10Be, Ti, and barite, with a maximum peak-to-trough amplitude of a factor of 6, take place systematically during glacial time. However, the ratio of any one proxy to another is constant to within 30% over the entire length of the records. Given that each proxy represents a different source (234U decay in seawater, interplanetary dust, upper atmosphere, continental dust, or upper ocean), our preferred interpretation for the covariation is that the climate-related changes in burial rates are driven by changes in sediment focusing.

Thorium-230 normalized particle flux and sediment focusing in the Panama Basin region during the last 30,000 years

Article

Full-text available

Jun 2007

Application of the 230Th normalization method to estimate sediment burial fluxes in six cores from the eastern equatorial Pacific (EEP) reveals that bulk sediment and organic carbon fluxes display a coherent regional pattern during the Holocene that is consistent with modern oceanographic conditions, in contrast with estimates of bulk mass accumulation rates (MARs) derived from core chronologies. Two nearby sites (less than 10 km apart), which have different MARs, show nearly identical 230Th-normalized bulk fluxes. Focusing factors derived from the 230Th data at the foot of the Carnegie Ridge in the Panama Basin are >2 in the Holocene, implying that lateral sediment addition is significant in this part of the basin. New geochemical data and existing literature provide evidence for a hydrothermal source of sediment in the southern part of the Panama Basin and for downslope transport from the top of the Carnegie Ridge. The compilation of core records suggests that sediment focusing is spatially and temporally variable in the EEP. During oxygen isotope stage 2 (OIS 2, from 13-27 ka BP), focusing appears even higher compared to the Holocene at most sites, similar to earlier findings in the eastern and central equatorial Pacific. The magnitude of the glacial increase in focusing factors, however, is strongly dependent on the accuracy of age models. We offer two possible explanations for the increase in glacial focusing compared to the Holocene. The first one is that the apparent increase in lateral sediment redistribution is partly or even largely an artifact of insufficient age control in the EEP, while the second explanation, which assumes that the observed increase is real, involves enhanced deep sea tidal current flow during periods of low sea level stand.

Characterization of tropospheric aerosols over the oceans with the NOAA AVHRR optical thickness operational product

Article

Full-text available

Jul 1997

The National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) is an instrument on a polar orbiting satellite that provides information on global aerosol distributions. The remote sensing algorithm is based on measurements of backscattered solar radiation which yield a measure of the "radiatively equivalent" aerosol optical thickness Z^sat (EAOT) over the oceans. Seasonally composited EAOT data for the period July 1989 to June 1991 reveal many spatially coherent plume-like patterns that can usually be interpreted in terms of known (or reasonably hypothesized) sources in association with climatological wind fields. The largest and most persistent areas of high EAOT values are associated with wind-blown dust and biomass burning sources; especially prominent are sources in Africa, the middle East, and the Asian subcontinent. Prominent plumes over the midlatitude North Atlantic are attributed to pollution emissions from North America and Europe. Large plumes attributed to pollution aerosols and dust from sources in Asia are clearly visible over the western and central North Pacific. On a global scale the annually averaged northern hemisphere EAOT values are about 1.7 times greater than those in the southern hemisphere. Considering each hemisphere separately, EAOT values in summer are about twice those in winter. Within the midlatitude band 300-60 ø (i.e., where anthropogenic emissions are greatest) the summer/winter ratio is about 3. The temporal variability of monthly mean EAOT in specific ocean regions often shows characteristic seasonal patterns that are usually consistent with aerosol measurements made in the marine boundary layer. Nonetheless, there are features in the EAOT distributions that can not be readily interpreted at this time. The AVHRR EAOT distributions demonstrate that satellite products can serve as a useful tool for the planning and implementation of focused aerosol research programs and that they will be especially important in studies of climate- related processes.

Continental Dust in the Atmosphere of the Eastern Equatorial Pacific

Article

Full-text available

Jun 1969

Atmospheric dust samples were collected on a cruise in the eastern equatorial Pacific during the spring of 1967 in the general area of 8°N to 17°S and 80°W to 110°W. The measured dust loads ranged from 0.04 to 1.2 μg/m3 air, the highest concentrations occurring principally along a track between 5° and 8°N. The mineralogical composition of the samples collected north of the equator differs markedly from the more southerly samples. The northern dust contains a plagioclase as the dominant mineral and subordinate amounts of quartz; in the southerly samples, quartz is prevalent and appreciable quantities of micas, chlorites, and smectites are also present. All samples contain significant amounts of organic material. The relatively high dust loadings encountered along the northern track suggest that windborne dust may be of significance to sedimentation in large areas of the Pacific to the west of Central America.

Glacial-interglacial dynamics of the eastern equatorial Pacific cold tongue-ITCZ system reconstructed from oxygen isotope records

Article

Full-text available

Dec 2003

We use planktonic oxygen isotope (delta18O) records spanning the last 30,000 years (kyr) to constrain the magnitude and spatial pattern of glacial cooling in the upwelling environment of the eastern equatorial Pacific (EEP). Fourteen new downcore delta18O records were obtained from surface-dwelling planktonic foraminifera Globigerinoides sacculifer and Globigerinoides ruber in eight cores from the upwelling tongue of the EEP. All sites have sedimentation rates exceeding 5 cm/kyr and, with one exception, lie above the modern depth of the foraminiferal lysocline. Sites directly underlying the cool band of upwelling immediately south of the equator record mean late Holocene (LH)-Last Glacial Maximum (LGM) delta18O amplitudes ranging between 1.0 and 1.30/00. We estimate that mean sea surface temperatures (SST) in this region during the LGM were on average 1.5 +/- 0.5°C lower than the LH. Larger delta18O amplitudes are observed in sites north of the equator, indicating a spatial pattern of reduced meridional SST gradient across the equator during the LGM. This result is supported by comparison of Mg/Ca SST reconstructions from two sites straddling the equator. We interpret the reduction of this gradient during the LGM as evidence for a less intense cold tongue-Intertropical Convergence Zone (ITCZ) frontal system, a more southerly position of the ITCZ, and weaker southeast equatorial trades in the EEP.

24. SEISMIC STRATIGRAPHY OF THE EASTERN EQUATORIAL PACIFIC OCEAN: PALEOCEANOGRAPHIC IMPLICATIONS 1

Article

Full-text available

The collection of Leg 138 well-log and shipboard physical-property data, in conjunction with high-resolution seismic profiles, provides an opportunity to understand the paleoceanographic significance of seismic reflectors and to gain insight into the pale- oceanographic evolution of the eastern equatorial Pacific Ocean. A series of eight reflectors or reflector packages were traced between two transects connecting five Leg 138 sites. By generating synthetic seismograms at each of these sites and comparing these to the field records, the origin of these seismic reflectors was determined in terms of physical-property variations and other core measurements. In particular, these reflectors were usually associated with sharp variations in density, which in turn, are related to variations in carbonate content. Intervals with moderate or poor nannofossil preservation indices were generally restricted to intervals below the reflectors traced in this study, suggesting that dissolution played little role in producing these reflectors. However, intervals with T. longissima mats were associated with many of the reflectors (R3-b, R4, R5-t, R5-b, R6) at the three sites (847, 849, and 850) where this diatom was encountered. This suggests that the reflectors found in this study are related to productivity events, although these events manifested themselves in a different way at the sites in which T. longissima mats were not observed. Interpreted seismic stratigraphic sections were compiled from the reflector horizon data for the two transects. Along the western transect, the section between reflectors R3 and R8 thins abruptly north of l°40' to l°50'N, suggesting that this marks the northern limit of high equatorial productivity at that time (3.9-9.5 Ma), because the seafloor is reasonably constant in depth along this transect. Unfortunately, statements about sharp productivity gradients cannot be made for the eastern transect where sediment thinning corresponds to a deepening of the seafloor and thus may be related to variations in dissolution. Finally, six reflectors were found to be associated with major paleoceanographic events; three of these reflectors correspond to those found by Mayer et al. (1985,1986) in the central equatorial Pacific Ocean, suggesting that these correspond to Pacific-wide Oceanographic events. One of these reflectors (R8-b) is caused by a pervasive dissolution event as is its central equatorial counter- part. The others (R3-b and R5-t), however, appear to be the result of productivity events in the eastern equatorial Pacific that are synchronous with dissolution events in the central equatorial Pacific. We suggest that while localized high productivity creates low carbonate intervals (and thus reflectors) in the eastern equatorial Pacific, steep gradients in the CCD result in enhanced dissolution and low carbonate intervals (and thus reflectors) in the deeper central equatorial Pacific.

Surface radiative forcing by soil dust aerosols and the hydrologic cycle

Article

Full-text available

Feb 2004

For absorbing aerosols like soil (or "mineral") dust, radiative forcing at the surface differs substantially from the value at the top of the atmosphere (TOA). The climate response depends not only upon the TOA forcing, but its difference with respect to the surface value, which represents radiative heating within the atmosphere. Surface forcing alters evaporation and the hydrologic cycle, which feeds back upon the aerosol burden through the efficiency of wet deposition. We calculate the surface forcing by soil dust aerosols and its global sensitivity by varying aspects of the dust distribution that are poorly constrained by observations. Ignorance of the global dust burden corresponds to a forcing uncertainty of over a factor of two, with smaller uncertainties due to imprecise knowledge of particle optical properties and the particle size distribution. While global evaporation and precipitation are reduced in response to surface radiative forcing by dust, precipitation increases locally over desert regions, so that dust emission can act as a negative feedback to desertification. The effect of the global reduction in precipitation is to lengthen the particle lifetime by reducing the efficiency of wet deposition, representing a positive feedback upon the global dust burden. For the current climate, we calculate the reduction in wet deposition by dust radiative forcing and find that the aerosol burden is increased only modestly. However, the global dust burden and associated radiative forcing are substantially higher during glacial climates, so that the amplification of the dust load by this feedback is larger. By extrapolating from its radiative forcing in the current climate, we estimate that dust reduces precipitation during glacial times by as much as half the reduction due to the colder climate alone.

Oceanic primary production 2. Estimation at global scale from satellite (CZCS) chlorophyll

Article

Full-text available

Jan 1996

Seasonal and interannual variability of the mineral dust cycle under present and glacial climate conditions

Article

Full-text available

Aug 2002

1] We present simulations of the dust cycle during present and glacial climate states, using a model, which explicitly simulates the control of dust emissions as a function of seasonal and interannual changes in vegetation cover. The model produces lower absolute amounts of dust emissions and deposition than previous simulations of the Last Glacial Maximum (LGM) dust cycle. However, the simulated 2-to 3-fold increase in emissions and deposition at the LGM compared to today, is in agreement with marine-and ice-core observations, and consistent with previous simulations. The mean changes are accompanied by a prolongation of the length of the season of dust emissions in most source regions. The increase is most pronounced in Asia, where LGM dust emissions are high throughout the winter, spring and summer rather than occurring primarily in spring as they do today. Changes in the seasonality of dust emissions, and hence atmospheric loading, interact with changes in the seasonality of precipitation, and hence of the relative importance of wet and dry deposition processes at high northern latitudes. As a result, simulated dust deposition rates in the high northern latitudes show high interannual variability. Our results suggest that the high dust concentration variability shown by the Greenland ice core records during the LGM is a consequence of changes in atmospheric circulation and precipitation locally rather than a result of changes in the variability of dust emissions. interannual variability of the mineral dust cycle under present and glacial climate conditions, J. Geophys. Res., 107(D24), 4744, doi:10.1029/2002JD002365, 2002.

Dust sources and deposition during the last glacial maximum and current climate: A comparison of model results with paleodata from ice cores and marine sediments

Article

Full-text available

Jul 1999

Mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean. Ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2-20 times greater during glacial periods, raising the possibility that mineral aerosols might have contributed to climate change on glacial-interglacial time scales. To address this question, we have used linked terrestrial biosphere, dust source, and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum (LGM) climates. We obtain a 2.5-fold higher dust loading in the entire atmosphere and a twenty-fold higher loading in high latitudes, in LGM relative to present. Comparisons to a compilation of atmospheric dust deposition flux estimates for LGM and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations; differences suggest where further improvements in the simple dust model could be made. The simulated increase in high-latitude dustiness depends on the expansion of unvegetated areas, especially in the high latitudes and in central Asia, caused by a combination of increased aridity and low atmospheric [CO2]. The existence of these dust source areas at the LGM is supported by pollen data and loess distribution in the northern continents. These results point to a role for vegetation feedbacks, including climate effects and physiological effects of low [CO2], in modulating the atmospheric distribution of dust.

Mineral Dust Entrainment And Deposition (DEAD) model: Description and 1990s dust climatology

Article

Full-text available

Jul 2003

We describe a model for predicting the size-resolved distribution of atmospheric dust for climate and chemistry-related studies. The dust distribution from 1990–1999 is simulated with our mineral aerosol entrainment and deposition module embedded in a chemical transport model. Mobilization processes include entrainment thresholds for saltation, moisture inhibition, drag partitioning, and saltation feedback. For mobilization we assume soil texture is globally uniform and is replete with saltators. Soil erodibility is prescribed by a new physically based geomorphic index which is proportional to the runoff area upstream of each source region. Dry deposition processes include sedimentation and turbulent mix-out. Nucleation scavenging and size-resolved washout in both stratiform and convective cloud types are represented. Simulations of the 1990s broadly agree with station observations and satellite-inferred dust distributions. Without invoking anthropogenic mechanisms, the model captures the seasonal migration of the trans-Atlantic African dust plume, and the spring maximum in Asian dust outflow and concentration over the Pacific. We estimate the 1990s global annual mean and variability of D < 10 µm dust to be: Emissions, 1490 ± 160 Tg yr −1 ; Burden, 17 ± 2 Tg; Optical depth at 0.63 µm, 0.030 ± 0.004. This emission, burden, and optical depth are significantly lower than some recent estimates. The model underestimates transport and deposition of East Asian and Australian dust to some regions of the Pacific Ocean. An underestimate of long range transport of particles larger than 3 µm contributes to this bias. Our experiments support the hypothesis that dust emission "hot spots" exist in regions where alluvial sediments have accumulated and may be disturbed.

Change in atmospheric mineral aerosols in response to climate: Last glacial period, preindustrial, modern, and doubled carbon dioxide climates

Article

Full-text available

May 2006

1] Desert dust simulations generated by the National Center for Atmospheric Research's Community Climate System Model for the current climate are shown to be consistent with present day satellite and deposition data. The response of the dust cycle to last glacial maximum, preindustrial, modern, and doubled-carbon dioxide climates is analyzed. Only natural (non-land use related) dust sources are included in this simulation. Similar to some previous studies, dust production mainly responds to changes in the source areas from vegetation changes, not from winds or soil moisture changes alone. This model simulates a +92%, +33%, and À60% change in dust loading for the last glacial maximum, preindustrial, and doubled-carbon dioxide climate, respectively, when impacts of carbon dioxide fertilization on vegetation are included in the model. Terrestrial sediment records from the last glacial maximum compiled here indicate a large underestimate of deposition in continental regions, probably due to the lack of simulation of glaciogenic dust sources. In order to include the glaciogenic dust sources as a first approximation, we designate the location of these sources, and infer the size of the sources using an inversion method that best matches the available data. The inclusion of these inferred glaciogenic dust sources increases our dust flux in the last glacial maximum from 2.1 to 3.3 times current deposition., Change in atmospheric mineral aerosols in response to climate: Last glacial period, preindustrial, modern, and doubled carbon dioxide climates, J. Geophys. Res., 111, D10202, doi:10.1029/2005JD006653.

Climate response and radiative forcing from mineral aerosols during the last glacial maximum, pre-industrial, current and doubled-carbon dioxide climates

Article

Full-text available

Oct 2006
GEOPHYS RES LETT

1] Mineral aerosol impacts on climate through radiative forcing by natural dust sources are examined in the current, last glacial maximum, pre-industrial and doubled-carbon dioxide climate. Modeled globally averaged dust loadings change by +88%, +31% and À60% in the last glacial maximum, pre-industrial and future climates, respectively, relative to the current climate. Model results show globally averaged dust radiative forcing at the top of atmosphere is À1.0, À0.4 and +0.14 W/m 2 for the last glacial maximum, pre-industrial and doubled-carbon dioxide climates, respectively, relative to the current climate. Globally averaged surface temperature changed by À0.85, À0.22, and +0.06 °C relative to the current climate in the last glacial maximum, pre-industrial and doubled carbon dioxide climates, respectively, due solely to the dust radiative forcing changes simulated here. These simulations only include natural dust source response to climate change, and neglect possible impacts by human land and water use. Citation: (2006), Climate response and radiative forcing from mineral aerosols during the last glacial maximum, pre-industrial, current and doubled-carbon dioxide climates, Geophys. Res. Lett., 33, L20705, doi:10.1029/2006GL026126.

Dust impact on marine biota and atmospheric CO2 in glacial periods

Article

Aug 2002
GEOCHIM COSMOCHIM AC

Proceedings of the Ocean Drilling Program, 162 initial reports

Article

Jan 1996

Leg 199

Article

Jan 2002

Global distribution of UV-absorbing aerosols from Nimbus7/TOMS data

Article

Jan 1997

Quantitative resolution of eolian continental crustal material and volcanic detritus in North Pacific surface sediment

Article

Feb 1996

Proxy records of continental climate and atmospheric circulation provided by analyses of eolian continental material extracted from marine sediment have resulted in significant new information concerning the behavior of these climate systems on various timescales. These studies, however, currently are limited to certain geographic areas because no chemical or physical extraction procedure provides an unambiguous separation of eolian continental crustal material from other contaminants like volcanic detritus. We employ a combined analytical and statistical procedure in an effort to extract a more refined eolian "signal" from areas that may be affected by volcanic detritus. Bulk surface sediment samples from 33 locations in the North Pacific were treated using a conventional sequential extraction procedure to remove the carbonate, silica and oxyhydroxide components, and the residue was analyzed by instrumental neutron activation analysis (INAA) for La, Ce, Sm, Eu, Yb, Lu, Hf, Sc, and Th. Q-mode factor analysis of these data shows that > 99% of the variance is explained by two end-members, which we interpret to be continental crustal material and volcanic detritus. Five least squares normative analysis models were evaluated to estimate the relative amount of these end-members in each sample. The continental crustal component was approximated using a fine-grained fraction of China loess and bulk loess. The volcanic component was approximated using the compositions of average Kurile-Kamchatka volcanic material, a median Ocean Drilling Program (ODP) Leg 145 Ash, and a median Kurile basalt. The model based on average Kurile-Kamchatka volcanic material and the fine loess fraction gives the most accurate results. Central and east Pacific samples typically contain up to 100% of the eolian continental crustal component, while samples near Japan, Kamchatka, and the Aleutians contain a majority of volcanic detritus. However, the highly variable composition of volcanic material can result in systematic errors up to 25% in samples dominated by volcanic detritus. The geographic distribution of the compositional end-members is consistent with a continental dust source originating in Asia and being diluted by ash from the volcanic arcs of the Pacific rim. This improved identification and resolution of the eolian continental component realized in this approach should permit paleoclimatic reconstructions to be developed from sediments in significant portions of the world that were previously precluded from analysis because of limitations with chemical or physical extraction procedures.

Latitudinal variations in benthic processes in the abyssal equatorial Pacific: control by biogenic particle flux

Article

Jan 1997
DEEP-SEA RES PT II

The equatorial Pacific forms a band of high, globally significant primary production. This productivity drops off steeply with distance from equatorial upwelling, yielding large latitudinal gradients in biogenic particle flux to the abyssal seafloor. As part of the US JGOFS Program, we studied the translation of these particle-flux gradients into the benthic ecosystem from 12°S to 9°N along 135–140°W to evaluate their control of key benthic processes, and to evaluate sediment proxies of export production from overlying waters. In October–December 1992 the remineralization rates of organic carbon, calcium carbonate and biogenic opal roughly matched the rain rates of these materials into deep sediment traps, exhibiting peak values within 3° of the equator. Rates of bioturbation near the equator were about ten-fold greater than at 9°N, and appeared to exhibit substantial dependence on particulate-organic-carbon flux, tracer time scale (i.e. age-dependent mixing), and pulsed mixing from burrowing urchins. Organic-carbon degradation within sediments near the equator was dominated by a very labile component (reaction rate constant, k approximately 15 per year) that appeared to be derived from greenish phytodetritus accumulated on the seafloor. Organic-carbon degradation at the highest latitudes was controlled by a less reactive component, with a mean k of approximately 0.075 per year. Where measured, megafaunal and macrofaunal abundances were strongly correlated with annual particulate-organic carbon flux; macrofaunal abundance in particular might potentially serve as a proxy for export production in low-energy abyssal habitats. Sedimentary microbial biomass also was correlated with the rain rate of organic carbon, but less strongly than larger biota and on shorter time scales (i.e. approximately 100 days). We conclude that the vertical flux of biogenic particlues exerts tight control on the nature and rates of benthic biological and chemical processes in the abyssal equatorial Pacific, and suggest that global changes in productivity on decadal or greater time scales could yield profound changes in deep-sea benthic ecoystems.

Seismic Stratigraphy of the Eastern Equatorial Pacific Ocean: Paleoceanographic Implications

Chapter

Aug 1995

The collection of Leg 138 well-log and shipboard physical-property data, in conjunction with high-resolution seismic profiles, provides an opportunity to understand the paleoceanographic significance of seismic reflectors and to gain insight into the paleoceanographic evolution of the eastern equatorial Pacific Ocean. A series of eight reflectors or reflector packages were traced between two transects connecting five Leg 138 sites. By generating synthetic seismograms at each of these sites and comparing these to the field records, the origin of these seismic reflectors was determined in terms of physical-property variations and other core measurements. In particular, these reflectors were usually associated with sharp variations in density, which in turn, are related to variations in carbonate content. Intervals with moderate or poor nannofossil preservation indices were generally restricted to intervals below the reflectors traced in this study, suggesting that dissolution played little role in producing these reflectors. However, intervals with T. longissima mats were associated with many of the reflectors (R3-b, R4, R5-t, R5-b, R6) at the three sites (847, 849, and 850) where this diatom was encountered. This suggests that the reflectors found in this study are related to productivity events, although these events manifested themselves in a different way at the sites in which T. longissima mats were not observed. Interpreted seismic stratigraphic sections were compiled from the reflector horizon data for the two transects. Along the western transect, the section between reflectors R3 and R8 thins abruptly north of l°40' to l°50'N, suggesting that this marks the northern limit of high equatorial productivity at that time (3.9-9.5 Ma), because the seafloor is reasonably constant in depth along this transect. Unfortunately, statements about sharp productivity gradients cannot be made for the eastern transect where sediment thinning corresponds to a deepening of the seafloor and thus may be related to variations in dissolution. Finally, six reflectors were found to be associated with major paleoceanographic events; three of these reflectors correspond to those found by Mayer et al. (1985,1986) in the central equatorial Pacific Ocean, suggesting that these correspond to Pacific-wide Oceanographic events. One of these reflectors (R8-b) is caused by a pervasive dissolution event as is its central equatorial counterpart. The others (R3-b and R5-t), however, appear to be the result of productivity events in the eastern equatorial Pacific that are synchronous with dissolution events in the central equatorial Pacific. We suggest that while localized high productivity creates low carbonate intervals (and thus reflectors) in the eastern equatorial Pacific, steep gradients in the CCD result in enhanced dissolution and low carbonate intervals (and thus reflectors) in the deeper central equatorial Pacific.

Causes and implications of the middle rare earth element depletion in the eolian component of North Pacific sediment - Its Composition and Evolution

Article

May 1998
GEOCHIM COSMOCHIM AC

Previous studies clearly demonstrate that the detrital fraction of central North Pacific sediment is derived almost exclusively from wind-born particles from the arid and semi-arid regions in Asia. These conclusions are based, in part, on trajectories observed for westerly wind systems and on a grainsize distribution for central North Pacific sediment which can only be explained by eolian transport. These observations have been verified by comparing the mineralogy, neodymium isotopic composition, and trace element geochemistry of the sediment with its Asian source region. However, recent geochemical investigations consistently highlight compositional differences exemplified by a MREE (middle rare earth element) depletion and a lower Th/Sc ratio in eolian material extracted from North Pacific sediment when compared to bulk China loess, the continental analog of the eolian material. These geochemical differences persist even when the bulk loess is subjected to the same extraction procedure. Here we present experiments demonstrating that these compositional differences are caused by a combination of grainsize fractionation during transport and partial dissolution of REE- and Th-bearing phosphatic phases during the extraction procedure. Two bulk loess samples from China were separated into several different grain size fractions and a split of each fraction was subjected to the extraction procedure commonly used to isolate terrigenous material from marine sediment. All extracted and unextracted sample pairs were analyzed for P, Th, Sc, Fe, La, Ce, Sm, Eu, Yb, and Lu. The amount of P removed by the extraction procedure correlates well with both the observed MREE depletion and the decrease in Th/Sc and is most pronounced in the finest grained samples. These correlations strongly suggest that the extraction procedure dissolves inorganic phosphate phases which are rich in MREEs and Th. A comparison of the compositions of the 2–3 mm loess size fractions with central North Pacific sediment shows that both of these materials are identical.

Eolian inputs of lead to the North Pacific

Article

Apr 2000
GEOCHIM COSMOCHIM AC

Chemical composition of aerosols collected over the tropical North Atlantic Ocean

Article

Jun 2000
J GEOPHYS RES

Ambient aerosol samples were collected over the tropical northern Atlantic Ocean during the month of April 1996 onboard the R/V Seward Johnson. Dichotomous high-volume collector samples were analyzed for ferrous iron immediately after collection, while trace metals, anions, and cations were determined upon return to the laboratory. Data are analyzed with the aid of enrichment factor, principal component, and weighted multiple linear regression analyses. Average mineral aerosol concentrations amounted to 19.3±16.4 μg m−3 whereby the chemical characteristics and air mass back trajectories indicated the dust to be of a typical shale composition and Saharan origin. Calcite accounted for 3.0 and 7.9% of the mineral aerosol during the first and second halves of the cruise, respectively. Total iron concentrations (averaging 0.84±0.61 μg m−3) are crustally derived, of which 0.51±0.56% is readily released as Fe(II). Eighty-six percent of this Fe(II) is present in the fine (

Quantitative resolution of eolian continental crustal material and volcanic detritus in North Pacific surface sediment

Article

Feb 1996

Strategies for successful U-Th dating of paleolake carbonates: An example from the Bolivian Altiplano

Article

May 2006
GEOCHEM GEOPHY GEOSY

We report over 90 U-Th dates from carbonates deposited around paleolakes on the Bolivian Altiplano. Petrographic and chemical data for tufas and the siliciclastic detritus contained within them allow (1) assessment of possible diagenetic effects, (2) a development of a strategy for selection of carbonate samples with low initial Th contents, and (3) assessment of the uncertainty due to initial Th. This strategy allows us to produce precise U-Th dates from lacustrine carbonates. The principal consideration in dating such carbonates is the composition and quantity of initial Th incorporated into the carbonate, and additional uncertainty is introduced because this initial Th may have two sources in lacustrine deposits. Isochron plots, measured (230Th/232Th), and X-ray diffraction and trace element chemistry of silicic residues all favor regional soil and dust as the sole source of initial Th in carbonates from the Bolivian Altiplano, a situation that simplifies single-sample dating of Altiplano carbonates.

Global Precipitation: A 17Year Monthly Analysis Based on Gauge Observations, Satellite Estimates, and Numerical Model Outputs

Article

Nov 1997

Gridded fields (analyses) of global monthly precipitation have been constructed on a 2.5° latitude-longitude grid for the 17-yr period from 1979 to 1995 by merging several kinds of information sources with different characteristics, including gauge observations, estimates inferred from a variety of satellite observations, and the NCEP-NCAR reanalysis. This new dataset, which the authors have named the CPC Merged Analysis of Precipitation (CMAP), contains precipitation distributions with full global coverage and improved quality compared to the individual data sources. Examinations showed no discontinuity during the 17-yr period, despite the different data sources used for the different subperiods. Comparisons of the CMAP with the merged analysis of Huffman et al. revealed remarkable agreements over the global land areas and over tropical and subtropical oceanic areas, with differences observed over extratropical oceanic areas. The 17-yr CMAP dataset is used to investigate the annual and interannual variability in large-scale precipitation. The mean distribution and the annual cycle in the 17-yr dataset exhibit reasonable agreement with existing long-term means except over the eastern tropical Pacific. The interannual variability associated with the El Niño-Southern Oscillation phenomenon resembles that found in previous studies, but with substantial additional details, particularly over the oceans. With complete global coverage, extended period and improved quality, the 17-yr dataset of the CMAP provides very useful information for climate analysis, numerical model validation, hydrological research, and many other applications. Further work is under way to improve the quality, extend the temporal coverage, and to refine the resolution of the merged analysis.

Fluxes of 230Th and 231Pa to the deep sea: Implications for the interpretation of excess 230Th and 231Pa/230Th profiles in sediments

Article

Sep 2001
EARTH PLANET SC LETT

Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230Th scavenged to the seafloor remains close to its production rate from the decay of 234U in the overlying water column, and generally validates the use of 230Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230Th measured in the low flux regions amounts to 90±6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231Pa and the 231Pa/230Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231Pa fluxes and 231Pa/230Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231Pa/230Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231Pa/230Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231Pa/230Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231Pa/230Th is more difficult and requires a more quantitative understanding of the influence of particle composition.

Climate Response to Soil Dust Aerosols

Article

Dec 1998

The effect of radiative forcing by soil dust aerosols upon climate is calculated. Two atmospheric GCM (AGCM) simulations are compared, one containing a prescribed seasonally varying concentration of dust aerosols, and the other omitting dust. Each simulation includes a mixed layer ocean model, which allows SST to change in response to the reduction in surface net radiation by dust. Dust aerosols reduce the surface net radiation both by absorbing and reflecting sunlight. For the optical properties of the dust particles assumed here, the reflection of sunlight is largely offset by the trapping of upwelling longwave radiation, so that the perturbation by dust to the net radiation gain at the top of the atmosphere is small in comparison to the surface reduction. Consequently, the radiative effect of soil dust aerosols is to redistribute heating from the surface to within the dust layer.Beneath the dust layer, surface temperature is reduced on the order of 1 K, typically in regions where deep convection is absent. In contrast, surface temperature remains unperturbed over the Arabian Sea during Northern Hemisphere (NH) summer, even though the dust concentration is highest in this region. It is suggested that the absence of cooling results from the negligible radiative forcing by dust at the top of the atmosphere, along with the frequent occurrence of deep convection, which ties the surface temperature to the unperturbed value at the emitting level.Where convection is absent, cooling at the surface occurs because radiative heating by dust reduces the rate of subsidence (and the corresponding mass exchange with the convecting region). Thus, the temperature contrast between these two regions must increase to maintain the original transport of energy, which is unperturbed by dust. It is suggested that cooling over the Arabian Sea during NH winter, despite the much smaller dust loading, is permitted by the absence of convection during this season. Thus, the change in surface temperature forced by dust depends upon the extent of overlap between the dust layer and regions of deep convection, in addition to the magnitude of the radiative forcing.Surface temperature is also reduced outside of the dust cloud, which is unlikely to result solely from natural variability of the AGCM.It is suggested that the perturbation by dust to Indian and African monsoon rainfall may depend upon the extent to which ocean dynamical heat transports are altered by dust.

Mineralogical and geochemical compositions of the eolian dust from the northeast equatorial Pacifc and their implications on paleolocation of the Intertropical Convergence Zone

Article

Mar 2005

Eolian components of a 328-cm-long piston core collected from the northeast equatorial Pacific at 16°12′N and 125°59′W were investigated for mineral and geochemical compositions in order to constrain the sources of dust and determine the latitudinal position of the Intertropical Convergence Zone (ITCZ) recorded in the core. The eolian components below 250 cm are characterized by smectite- and phillipsite-rich mineral composition, depleted rare earth elements (REEs), and high Eu/Sm ratios, indicative of volcanic-rich composition. These characteristics are found in equatorial and south Pacific surface sediments, of which eolian particles are supplied from Central and South America. In contrast, eolian components above 250 cm are characterized by quartz- and illite-rich mineralogy, and more shale-like REE and trace element compositions, which are common in surface sediments of the central Pacific north of the ITCZ, where eolian particles are sourced from the Asia and North America. The observed changes are attributed to the shifting of its eolian sources from the Central and South America to the China and North America across the hemispheric dust barrier of the ITCZ. This result suggests that smectite-illite transition, a phenomenon that smectite amount increases over illite at a depth, can be used as a tracking tool for the paleolocation of the ITCZ in the northeast and central Pacific. Backtrack path construction of Pacific plate indicates paleolocation of the ITCZ north of 12°N (±2°) prior to late Miocene.

Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures

Article

Jun 1998

The primary focus of this review is tropical-extratropical interactions and especially the issues involved in determining the response of the extratropical atmosphere to tropical forcing associated with sea surface temperature (SST) anomalies. The review encompasses observations, empirical studies, theory and modeling of the extratropical teleconnections with a focus on developments over the Tropical Oceans-Global Atmosphere (TOGA) decade and the current state of understanding. In the tropical atmosphere, anomalous SSTs force anomalies in convection and large-scale overturning with subsidence in the descending branch of the local Hadley circulation. The resulting strong upper tropospheric divergence in the tropics and convergence in the subtropics act as a Rossby wave source. The climatological stationary planetary waves and associated jet streams, especially in the northern hemisphere, can make the total Rossby wave sources somewhat insensitive to the position of the tropical heating that induces them and thus can create preferred teleconnection response patterns, such as the Pacific-North American (PNA) pattern. However, a number of factors influence the dispersion and propagation of Rossby waves through the atmosphere, including zonal asymmetries in the climatological state, transients, and baroclinic and nonlinear effects. Internal midlatitude sources can amplify perturbations. Observations, modeling, and theory have clearly shown how storm tracks change in response to changes in quasi-stationary waves and how these changes generally feedback to maintain or strengthen the dominant perturbations through vorticity and momentum transports. The response of the extratropical atmosphere naturally induces changes in the underlying surface, so that there are changes in extratropical SSTs and changes in land surface hydrology and moisture availability that can feedback and influence the total response. Land surface processes are believed to be especially important in spring and summer. Anomalous SSTs and tropical forcing have tended to be strongest in the northern winter, and teleconnections in the southern hemisphere are weaker and more variable and thus more inclined to be masked by natural variability. Occasional strong forcing in seasons other than winter can produce strong and identifiable signals in the northern hemisphere and, because the noise of natural variability is less, the signal-to-noise ratio can be large. The relative importance of tropical versus extratropical SST forcings has been established through numerical experiments with atmospheric general circulation models (AGCMs). Predictability of anomalous circulation and associated surface temperature and precipitation in the extratropics is somewhat limited by the difficulty of finding a modest signal embedded in the high level of noise from natural variability in the extratropics, and the complexity and variety of the possible feedbacks. Accordingly, ensembles of AGCM runs and time averaging are needed to identify signals and make predictions. Strong anomalous tropical forcing provides opportunities for skillful forecasts, and the accuracy and usefulness of forecasts is expected to improve as the ability to forecast the anomalous SSTs improves, as models improve, and as the information available from the mean and the spread of ensemble forecasts is better utilized.

The Continental Crust: Its Composition and Evolution

Book

Jan 1985

This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.

Patchy deposits of Cenozoic pelagic sediments in the central Pacific

Article

Jan 2005
GEOLOGY

Export of pelagic carbonate tests from surface waters and their deposition at the seafloor plays a significant role in the CO2 cycle and ability of the oceans to absorb atmospheric CO2. Sediment 230Th and 3He measurements have been interpreted as evidence that significant lateral advection of pelagic material occurs in the water column, leading to marked spatial variations in deposition rates and, in particular, to significant focusing of deposits on the Pacific equator. We report spatially continuous stratigraphy from two 1000 km seismic lines that show evidence of depositional anomalies near the equator. Accumulation rates were apparently enhanced locally by a factor of two, similar to the proposed modern sediment-focusing factors, but the anomalies are surprisingly patchy over the 20 m.y. period analyzed—they are not confined to an equatorial region, and they are not necessarily found on adjacent seismic profiles. These intermediate-scale anomalies are >˜100 km across and represent areas of seafloor that received more deposits for one period, less in following periods, and vice versa. Variogram analysis was used to determine how the spatial scales of deposition changed over the Neogene. The period when the spatial scale of depositional variability was largest correlates with hiatuses in drill cores, a correlation that we interpret as caused by enhanced and spatially heterogeneous carbonate dissolution at that time. The study suggests that seismic stratigraphy has the potential to reveal spatial patterns related to unsteady bottom-water flow and chemistry.

The paleoclimatic record provided by Eolian deposition in the deep sea: The geologic history of wind

Article

May 1994

David K Rea

This article looks at the paleoclimatic record of winds recorded in the deposits of dust grains in the deep sea. Such grains are carried to the sea by the wind. The strength of the wind is indicated by the size of dust grains observed, larger grains requiring stronger winds to effect transport from arid regions. The number of the dust grains provides an indication of the aridity of land masses at the times in question. The record indicates that wind varies on the Milankovitch cycles of orbital variability, but also on time scales shorter than the 100kyr cycles associated with glaciation and aridity.

Climatic Change and CaCO3 Preservation: an 800,000 Year Bathaymetric Reconstruction from the Central Equatorial Pacific Ocean

Article

Aug 1989

Sixteen deep-sea cores from the central equatorial Pacific are used to reconstruct a continuous 800,000-year (800-kyr) record of bathymetric variations in carbonate preservation as measured by calcium carbonate (CaCO3) content. The depth of the sedimentary lysocline has fluctuated markedly in conjunction with late Pleistocene climate cycles while the carbonate critical depth (CCrD), which is the water depth where the sediments contain 10% CaCO3, has remained relatively constant. As a result, the depth of the lysocline controls the bathymetric position and thickness of the CaCO3 transition zone, defined as the depth from the lysocline to the CCrD. Modern and interglacial-aged sediments show poor CaCO3 preservation and thick CaCO3 transition zones. Glacial-aged sediments show good preservation and deep, thin zones due to the deepening of the lysocline. Detailed comparison of the CaCO3 preservation and oxygen isotope records from the central equatorial Pacific confirms the observation that preservation maxima and minima tend to occur during the latter half of glacial and interglacial stages and on climate transitions rather than during the middle of climatic stages. During the nine major glacial stages of the last 800 kyr, the lysocline deepened by at least 400 to 800 m. This deepening indicates an increase in the abyssal carbonate ion concentration ([CO3=]) and a depression of the calcite saturation horizon best explained by the deeper presence of a more carbonate-saturated water mass. The bottom of the transition zone has remained at a relatively constant depth during the Brunhes Chron, indicating a balance between CaCO3 sedimentation and dissolution in the deepest waters of the central equatorial Pacific.

The distribution of clay minerals in the World Ocean

Article

Aug 1968
Deep Sea Res Oceanogr Abstr

The relative amounts of chlorite, montmorillonite, kaolinite and illite in the less than 2 micron size fraction of pelagic sediments are related to the sources and transport paths of solid phases from the continents to the oceans and to injections of volcanic materials to the marine environment. Three modes of entry of solid phases from the lands to the seas are considered: by glaciers, by rivers and by atmospheric winds. The compositions of the clay size fraction are also related to rates of accumulation of the non-biogenous phases.

Rapid bioturbation in equatorial Pacific sediments: evidence from excess 234Th measurements

Article

Dec 1996
DEEP-SEA RES PT II

In order to evaluate bioturbation in equatorial Pacific sediments, profiles of excess 234Th were measured in cores collected during November and December 1992, at six abyssal sites along the JGOFS Equatorial Pacific 140°W transect, from 5°S to 9°N. Excess 234Th (half-life=24.1 days) was detected at all sites. Mean inventories ranged from a low of 0.25 dpm cm−2 at 9°N, to a high of 2.6 dpm cm−2 at 2°N. Eddy-diffusive models of the excess 234Th profiles suggest that, on 100-day timescales, particle mixing is slow at 5°S and 2°S (eddy diffusion coefficient, Db, < 0.4 cm2 year−1), faster at 0° and 2°N (Db=0.34–4.8 cm2 year−1), and fastest at 5°N (Db=4.7−11 cm2 year−1). At 9°N, 234Th is found almost entirely in the upper 5 mm of the seabed, and Db values are <0.1 cm2 year−1. Db values generally increase as POC flux increases, although the linear relationship (r2=0.334, p=0.365) displays considerable scatter. However, if the only sites included in the regression are those where 234Th profiles were measured in at least two core sets (i.e. 0°, 2°N, 5°N and 9°N), the linear regression improves substantially (r2=0.754, p=0.0012). These results suggest that measurements on a single core may not fully characterize the sediment mixing regime in a particular region, and highlight the need for multiple samples from an individual site.

Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?

Article

Mar 2005

1] The paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing (bioturbation and horizontal sediment transport). Diagenesis and bioturbation are relatively well-studied, but the effects of physical sedimentation have been largely ignored. Modeling U series isotopes (e.g., 230 Th) can potentially quantify horizontal sediment movement, but model horizontal sediment focusing often equals or exceeds the vertical particle rain. We find no evidence of this level of sediment focusing in the equatorial Pacific from geophysical data or sediment core comparisons. The overestimate of sediment focusing by 230 Th is probably caused by poor model assumptions: that sediment does not fractionate (does not sort according to size during transport) and that 230 Th cannot leak from slowly accumulating sediments. Both assumptions are weak. U series methods do hold promise to quantify sediment focusing if properly calibrated. With calibration the trade-offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed. (2005), Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?, Paleoceanography, 20, PA1005, doi:10.1029/2004PA001019.

Ice-core palaeoclimate records in tropical South America since the Last Glacial Maximum

Article

May 2000

2000. Ice-core palaeoclimate records in tropical South America since the Last Glacial Maximum. ABSTRACT: Ice-core records spanning the last 25 000 yr from the tropical Andes of South America are reviewed. These records from Quelccaya, Huascarán and Sajama present a high temporal resolution picture of both the Late Glacial Stage (LGS) and the Holocene climatic and environmental conditions in the South American Andes. Late Glacial Stage conditions at high elevations appear to have been cooler than today, although the magnitude of the inferred cooling differs with the particular proxy used (e.g. snowline depression, pollen, ice cores). Insoluble dust and anion concentrations in the ice cores reveal that LGS hydrological conditions in the tropics (9°S) were much drier than today, whereas in the subtropics (18°S) LGS conditions were much wetter. This probably reflects the migration of the tropical Hadley Cell in response to a different meridional temperature gradient. Low nitrate concentrations in the LGS ice from both Huascarán and Sajama suggest that the Amazon Basin forest cover may have been much less extensive. Discussed is the conundrum surrounding the use of 18 O as a palaeothermometer in the tropics, where temperatures exhibit little seasonal variation yet the ice-core records suggest that 18 O records temperature variations on decadal to millennial time-scales. Finally evidence is presented for a strong twentieth century warming.

Chemical characteristics of Pacific tropospheric air in the region of the Intertropical Convergence Zone and South Pacific Convergence Zone

Article

Mar 1999

The Pacific Exploratory Mission (PEM)-Tropics provided extensive aircraft data to study the atmospheric chemistry of tropospheric air in Pacific Ocean regions, extending from Hawaii to New Zealand and from Fiji to east of Easter Island. This region, especially the tropics, includes some of the cleanest tropospheric air of the world and, as such, is important for studying atmospheric chemical budgets and cycles. The region also provides a sensitive indicator of the global-scale impact of human activity on the chemistry of the troposphere, and includes such important features as the Pacific "warm pool," the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), and Walker Cell circulations. PEM-Tropics was conducted from August to October 1996. The ITCZ and SPCZ are major upwelling regions within the South Pacific and, as such, create boundaries to exchange of tropospheric air between regions to the north and south. Chemical data obtained in the near vicinity of the ITCZ and the SPCZ are examined. Data measured within the convergent zones themselves are not considered. The analyses show that air north and south of the convergent zones have different chemical signatures, and the signatures are reflective of the source regions and transport histories of the air. Air north of the ITCZ shows a modest urban/industrialized signature compared to air south of the ITCZ. The chemical signature of air south of the SPCZ is dominated by combustion emissions from biomass burning, while air north of the SPCZ is relatively clean and of similar composition to ITCZ south air. Chemical signature differences of air north and south of the zones are most pronounced at altitudes below 5 km, and, as such, show that the ITCZ and SPCZ are effective low-altitude barriers to the transport of tropospheric air. At altitudes of 8 to 10 km, chemical signatures are less dissimilar, and air backward trajectories (to 10 days) show cross-convergent-zone flow. At altitudes below about 5 km, little cross-zonal flow is observed. Chemical signatures presented include over 30 trace chemical species including ultrafine, fine, and heated-fine (250øC) aerosol.

Deglacial changes in dust flux in the eastern equatorial Pacific

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