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Constraints on the magnitude of the deglacial migration of the ITCZ in the Central Equatorial Pacific Ocean
... The Central Equatorial Pacific (CEP) is an ideal location to study the coupled ocean-atmosphere climate phenomena of the Intertropical Convergence Zone (ITCZ) (e.g., Jacobel et al., 2017aJacobel et al., , 2016Reimi et al., 2019;Reimi and Marcantonio, 2016). ...
... The bathymetric range of the ML1208 cores means that it is possible to retrieve carbonate-rich sediments from the ridge flanks, spanning a variety of depths that intersect with abyssal Pacific water masses including Pacific Deep Water (PDW) at intermediate depths (1,800 m) and Lower Circumpolar Deep Water (LCDW) at the deepest points (3,800 m). Sediments from the ridge thus have significant potential to capture past changes in deep 105 watermass characteristics and geometry (e.g., Jacobel et al., 2020Jacobel et al., , 2017aLynch-Stieglitz et al., 2015), in addition to surface ocean-atmosphere hydroclimate phenomena (Costa et al., 2017(Costa et al., , 2016Jacobel et al., 2017aJacobel et al., , 2016Monteagudo et al., 2021;Reimi et al., 2019;Reimi and Marcantonio, 2016;Rongstad et al., 2019;Rustic et al., 2015;White et al., 2018). In contrast with other CEP core sites (Fig. 1, inset) where reconstructions only extend to the last glacial maximum, records from the Line Islands have been shown to cover the last several glacial cycles (Jacobel et al., 2017a;Lynch-Stieglitz et al., 2015) and here 110 we expand the available temporal perspective. ...
... Ocean-atmosphere phenomena previously investigated at the ML1208 core sites include millennial shifts of the Intertropical Convergence Zone during deglacial transitions Reimi et al., 2019;Reimi and Marcantonio, 2016), glacialinterglacial and millennial scale variations in dust deposition (Jacobel et al., 2017a), glacial-interglacial changes in nutrient utilization and export production (Costa et al., 2017(Costa et al., , 2016, abyssal ocean respired carbon storage (Jacobel et al., , 2017b, 140 variations in ENSO strength across the modern, Holocene and glacial period (Rongstad et al., 2019;Rustic et al., 2020;White et al., 2018), and last glacial sea surface temperatures (Monteagudo et al., 2021). ...
The equatorial Pacific is a nexus of key oceanic and atmospheric phenomena, and its regional climate has critical implications for hydroclimate, the partitioning of CO2, and temperature on a global scale. The spatial complexity of climate signals across the basin has long posed a challenge for interpreting the interplay of different climate phenomena including changes in the Intertropical Convergence Zone (ITCZ) and El Niño Southern Oscillation. Here, we present new, millennially resolved sediment core chronologies and stable isotope records from three sites in the equatorial Pacific’s Line Islands region, as well as updated chronologies for four previously studied cores. Age constraints are derived from 14C (n=17) and δ18O (n=610), which are used as inputs to a Bayesian software package (BIGMACS) that constructs age models and uncertainty bounds via correlation with the global benthic δ18O stack (Lee et al., 2023). We also make use of the new planktonic δ18O data to draw inferences about surface water salinity and to infer a southward-shifted position for the ITCZ at the Last Glacial Maximum (18–24 ka) and Marine Isotope Stage 6 (138–144 ka). These new chronologies and related datasets improve our understanding of equatorial Pacific climate and show strong promise for further surface and deep ocean paleoclimate reconstructions over the last several glacial cycles.
... A consistent southward shift of the ITCZ during the LGM and HS1 has been supported by both proxy records and model simulations (Arbuszewski et al., 2013;Bahr et al., 2018;Broccoli et al., 2006;Carolin et al., 2013;Denniston et al., 2013;Donohoe et al., 2013;Gibbons et al., 2014;Koutavas & Lynch-Stieglitz, 2003;Li & Liu, 2022;McGee et al., 2014;Partin et al., 2007;Peterson et al., 2000;Wang et al., 2004), but the estimated magnitude of the ITCZ shift varies across studies. While climate models and theoretical calculations have estimated a global mean southward displacement of <1°for the LGM and HS1 (Atwood et al., 2020;Donohoe et al., 2013;McGee et al., 2014), some marine proxy records indicate much greater displacements of up to 7°for these intervals (e.g., Arbuszewski et al., 2013;Reimi & Marcantonio, 2016;Seo et al., 2016). Yet there are also proxy data that support much smaller ITCZ shifts even in the ocean (e.g., McGee et al., 2007;Rowland et al., 2021;Xie & Marcantonio, 2012). ...
... Red circles mark the cores analyzed in this study. Yellow rectangles are the 16 cores whose surface layers were analyzed by Seo et al. (2021); white circles are the cores from other previous studies discussed in the text (from left to right: Seo et al., 2016;Reimi & Marcantonio, 2016;Ziegler et al., 2008;Xie & Marcantonio, 2012). this "geochemically defined mean ITCZ position" from its modern location at 7°N allow us to quantitatively estimate the magnitude of the ITCZ shift during the two periods of NH high latitude cooling. ...
... The dominant influence of Asian dust appears to have reached ∼3°farther south during the LGM and ∼1°-3°farther south during HS1 than present. The greater penetration of NH dust to the south could result from one of the following four mechanisms: (a) symmetrical contraction of the ITCZ range (Collins et al., 2010); (b) weakened role of the ITCZ as a barrier to interhemispheric dust transport (McGee et al., 2007;Xie & Marcantonio, 2012); (c) enhanced hemispheric asymmetry in dust flux; or (d) southward shift of the mean ITCZ position (Reimi & Marcantonio, 2016). The first mechanism is improbable because the present ∼6°seasonal range (4°N-10°N) of the ITCZ in the study area is too narrow to explain the ∼3°latitudinal shift of the provenance boundary. ...
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.
... However, hemispheric dust source signals in surface sediments do not show systematic latitudinal changes across the ITCZ in the central Pacific, where the position of the ITCZ varies seasonally between 4°N and 10°N. For instance, surface sediments at latitudes of 5°N and 9°N along a 140°W transect indicate a NH source (Ziegler et al., 2008), whereas sediments from sites at 6°N along longitudes of 110°W and 158°W show SH and mixed NH-SH source signals, respectively (Reimi & Marcantonio, 2016;Xie & Marcantonio, 2012). Moreover, on the basis of geochemical analysis of seafloor sediments, Ziegler et al. (2008) estimated that Asian dust comprises more than 60% of terrigenous material deposited much further south (to 3°S) of the boreal winter ITCZ position (4°N). ...
... Oceanic ε Nd data are from Jones et al. (1994), Nakai et al. (1993), Smith et al. (2003), and Stancin et al. (2006). The solid red rectangle denotes the latitudinal range covered by the 16 cores collected in this study (equator to 16°N) and black dots show the locations of cores from previous studies (from left to right: Reimi & Marcantonio, 2016;Xie & Marcantonio, 2012;Ziegler et al., 2008). Red solid circles show the locations of cores used in the dust flux study (Janecek & Rea, 1983;Rea & Bloomstine, 1986). ...
... Surface sediments at sites MC148 (9°N, 140°W) and 37BB (7°N, 158°W) show mixed ε Nd source signals (−7.7 and −6.6, respectively; Figure 2a; Reimi & Marcantonio, 2016;Ziegler et al., 2008). In contrast, ε Nd values in surface sediments at ODP site 853 (7°N, 110°W) indicate a SH source (ε Nd = −3.86; Figure 2a; Xie & Marcantonio, 2012). ...
Plain Language Summary
The Intertropical Convergence Zone (ITCZ), a band of heavy precipitation in the tropics, migrates latitudinally towards the warmer hemisphere on seasonal and longer timescales. The past and future latitudinal position of the ITCZ is of great interest to the scientific community as the ITCZ influences global temperature and precipitation patterns as well as the hydroclimate of the tropics. The position of the ITCZ can be identified by tracking changes in dust sources and fluxes between hemispheres, as the ITCZ acts as a barrier to cross‐equatorial dust transport. Here, we use geochemical analysis to investigate the regional sources of dust on the seafloor in various sites in the tropical central Pacific to define the mean present‐day position of the ITCZ. We find that dust deposited north of 7°N is predominantly from Asia and dust deposited south of 6°N is mostly from Asia and South America. We propose the southernmost latitude with an intact Asian source signal (7°N) as the present mean ITCZ position in the central Pacific. This geochemically defined mean ITCZ position can be used as a reference for the quantitative assessment of ITCZ displacement in the past using sedimentary down‐core records.
... Pb isotopes have been used in a range of studies to trace aeolian dust flux sources (Jones et al., 2000;Pettke et al., 2000;Pichat et al., 2014;Stancin et al., 2006) and to reconstruct changes in the location of the ITCZ (Reimi et al., 2019;Reimi & Marcantonio, 2016;Xie & Marcantonio, 2012). The premise is that since different dust source regions are of different ages and have different lithologies, and therefore distinct stable Pb isotope ratios, it should be possible to discern the relative contribution of these different sources using source apportion mixing models. ...
... Kalnay et al. (1996) and ITCZ locations are from Liu et al. (2015). Site 17PC represents downcore studies from Reimi et al. (2016Reimi et al. ( , 2019, Site 849 includes work by Pichat et al. (2014) and Xie and Marcantonio (2012), and PC72 is presented in this study. Source region sample locations used in this study are shown in yellow. ...
... Given that the location of the ITCZ is presumed to have a prevailing influence on the sources of dust reaching the EEP and that the ITCZ location is known to vary both seasonally and over longer-term climate cycles (Liu et al., 2015), we have further focused our study on samples from the EEP. Although the large-scale source signals are captured in the EEP as noted above, on a regional to local scale significant variability was observed in Pb isotope signatures of the detrital fractions among cores in the EEP, consistent with other studies (Reimi et al., 2016;Xie & Marcantonio, 2012). ...
... Pb isotopes have been used in a range of studies to trace aeolian dust flux sources (Jones et al., 2000;Pettke et al., 2000;Pichat et al., 2014;Stancin et al., 2006) and to reconstruct changes in the location of the ITCZ (Reimi et al., 2019;Reimi & Marcantonio, 2016;Xie & Marcantonio, 2012). The premise is that since different dust source regions are of different ages and have different lithologies, and therefore distinct stable Pb isotope ratios, it should be possible to discern the relative contribution of these different sources using source apportion mixing models. ...
... Kalnay et al. (1996) and ITCZ locations are from Liu et al. (2015). Site 17PC represents downcore studies from Reimi et al. (2016Reimi et al. ( , 2019, Site 849 includes work by Pichat et al. (2014) and Xie and Marcantonio (2012), and PC72 is presented in this study. Source region sample locations used in this study are shown in yellow. ...
... Given that the location of the ITCZ is presumed to have a prevailing influence on the sources of dust reaching the EEP and that the ITCZ location is known to vary both seasonally and over longer-term climate cycles (Liu et al., 2015), we have further focused our study on samples from the EEP. Although the large-scale source signals are captured in the EEP as noted above, on a regional to local scale significant variability was observed in Pb isotope signatures of the detrital fractions among cores in the EEP, consistent with other studies (Reimi et al., 2016;Xie & Marcantonio, 2012). ...
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.
... Climate proxies are clustered near the coasts of the western and eastern tropical Pacific (Kiefer and Kienast, 2005;Partin et al., 2007;Gray et al., 2020), but few deglacial records are available for the central Pacific region (Lyle et al., 1992;Lee et al., 2001;Lynch-Stieglitz et al., 2015;Reimi and Marcantonio, 2016). Quaternary deposits from the Hawaiian Islands thus provide a unique opportunity to fill the hydroclimate data gap over the central Pacific for the last glacial period. ...
... The mid Holocene rainfall at Kaʻala (8e6 ka) exhibits a clear isotopic signature of storm-induced precipitation, while dust influx in the Line Islands south of Hawaiʻi indicate a northward migration of the ITCZ~9 ka (Reimi and Marcantonio, 2016). The Late Holocene displays the opposite trend, with decreased storm rainfall and a meridional shift of the ITCZ after~7 ka (Reimi and Marcantonio, 2016;Sachs et al., 2018) that reached its southern limit in the Central Tropical Pacific during the last millennium (Sachs et al., 2009;Higley et al., 2018). ...
... The mid Holocene rainfall at Kaʻala (8e6 ka) exhibits a clear isotopic signature of storm-induced precipitation, while dust influx in the Line Islands south of Hawaiʻi indicate a northward migration of the ITCZ~9 ka (Reimi and Marcantonio, 2016). The Late Holocene displays the opposite trend, with decreased storm rainfall and a meridional shift of the ITCZ after~7 ka (Reimi and Marcantonio, 2016;Sachs et al., 2018) that reached its southern limit in the Central Tropical Pacific during the last millennium (Sachs et al., 2009;Higley et al., 2018). This opposed rainfall-ITCZ association, relative to the Late Pleistocene, suggests that another mode of climate variability prevailed during the Holocene epoch. ...
Long-term hydroclimate variability at low latitudes exerts a strong influence on global climate, yet its timing and mechanisms are poorly documented for the central Pacific region. Here we present the molecular and hydrogen isotopic compositions of long-chain n-alkanes in a montane wetland deposit at Mount Kaʻala on the Island of Oʻahu, Hawaiʻi, which document hydrological conditions and related vegetation changes over the late Pleistocene and the Holocene. To aid the interpretation of fossil n-alkanes, we also determine chain-length distributions of the most common bog plants growing locally. Organic matter accumulation started around 45 ka BP, one of the oldest reported dates for peat-forming wetland ecosystems in Hawaiʻi. Sedimentary n-alkane distributions suggest vegetation shifts driven by changes in rainfall regime, especially with enhanced abundances of shorter chain length homologues during wet periods. Depleted values of n-alkane δ²H dated to ca 45–40, 24–22.5, 19–16.5, and 10–6 ka BP generally coincide with faster carbon accumulation rates, and may indicate enhanced winter cyclonic storm precipitation, as suggested by modern rainwater isotope data. For the Late Pleistocene, the consistency in timing and direction of hydroclimate variability at Kaʻala, dynamics of the Mauna Kea ice cap, precipitation in the Cariaco basin, and East Asian monsoon intensity suggest that southward shifts in the mean position of the ITCZ contributed to a greater frequency of extratropical cyclonic storms, increasing precipitation at leeward and high-elevation sites. During the Holocene, the ITCZ-hydroclimate linkage at Kaʻala is inverted, consistent with a Pacific dominated by ENSO variability.
... Previous studies have used dust provenance to track ITCZ position over glacial-interglacial timescales (Abouchami et al., 2013;Abouchami & Zabel, 2003;Xie & Marcantonio, 2012). In the CEP, the ITCZ was found to be further south during the last glacial period than during the Holocene (Reimi & Marcantonio, 2016). Here, we are able to resolve dust provenance changes during the penultimate termination, consistent with the timing of Heinrich Stadial 11 (HS11) and consider whether these changes are due to movement of the ITCZ precipitation centroid (i.e., changes in the position of the interhemispheric dust transport barrier) or to decreases in the convective intensity of the ITCZ (which would allow interhemispheric leakage of dust). ...
... Pb and Nd isotope ratios were measured via Thermal Ionization Mass Spectrometry at Texas A&M University (Table S1). The operationally defined detrital fraction of the sediment was isolated by sequential leaching following the procedure outlined in Reimi and Marcantonio (2016; see supporting information Text S1). The data have been corrected with a 0.1%/amu linear fractionation correction based on multiple runs of the NIST981 Pb standard. ...
... Previous work determined that the four most dominant sources of dust to the Line Islands were three South American volcanic regions and Chinese Loess (Reimi & Marcantonio, 2016). The most northern South American source is the North Central Volcanic Zone (N-CVZ), between 14.5°S and 19.5°S, as defined by Pichat et al. (2014a). ...
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.
... The ITCZ, which marks the convergence of southeasterly and northeasterly trade winds, forms a powerful convective boundary for dust transportation and deposition to the EEP Ocean 35 . It has been suggested that the ITCZ shifts in response to global cooling, which would affect EEP dust transportation and deposition patterns [34][35][36] . ...
... The ITCZ, which marks the convergence of southeasterly and northeasterly trade winds, forms a powerful convective boundary for dust transportation and deposition to the EEP Ocean 35 . It has been suggested that the ITCZ shifts in response to global cooling, which would affect EEP dust transportation and deposition patterns [34][35][36] . An EEP dust record from Site U1334 (palaeolati-tude~0.38°S ...
Stimulation of the biological pump by iron-bearing dust in the eastern equatorial Pacific Ocean plays an important role in long-term carbon sequestration, yet past dust fertilization and its impact on CO2 perturbations over major climate transitions remain debated. Here, we integrate proxies of dust input, source-region weathering, and biological pump activity from late Eocene to early Miocene sediments of Integrated Ocean Discovery Program Hole U1333, which includes the Eocene-Oligocene Transition (~34 million years ago) when a major ice sheet was first established on Antarctica. We find that intensified chemical weathering in the large central Asian dust source region enhanced atmospheric CO2 removal at ~34 Ma. Superimposed dust fertilization and biological pump action amplified this CO2 removal before ~34 Ma, while weakening of this amplification process helped to moderate the CO2 decline after that time. The observed inter-linked, counteracting processes with different timescales illustrate the complexity of carbon cycle feedbacks associated with major climate changes.
... Marine sediment records suggest that during times of increasing Northern Hemisphere dust load the ITCZ moved southward in both the Atlantic (Adkins et al., 2006) and Pacific (Jacobel et al., 2016) basins. Conversely, when African dust emission is reduced, precipitation migrates northward in both models (Evans et al., 2019;Pauseta et al., 2016) and observations Reimi & Marcantonio, 2016). In this study, we develop simulations to model the effect of hemispheric dust asymmetries by multiplying dust emission in one hemisphere or both by 0, 2, and 5 times preindustrial levels and examine the effects on tropical precipitation. ...
... Ocean sediment cores from the central Pacific show dust fluxes three times greater than the preindustrial during the Last Glacial Maximum (Jacobel et al., 2017). Our results imply a 1.11°s outhward shift of the Pacific ITCZ and 1.05°in the Atlantic, during that time due to dust forcing, compared to observations of at least 2.5°in the Pacific (Reimi & Marcantonio, 2016) and 7°in the Atlantic (Arbuszewski et al., 2013). The magnitude of the dust-induced shifts is secondary to the large shifts due to orbital and ice sheet forcing during the LGM but still contributes roughly 15% of the Atlantic shift and possibly more than that in the Pacific. ...
Plain Language Summary
Dust aerosols in the climate system come from desert regions that are unevenly distributed across the globe. Currently, this unevenness produces much more dust in the Northern Hemisphere than in the Southern, but in previous climates, the degree of unevenness has varied. The presence of dust affects atmospheric energy balance by reflecting sunlight and absorbing infrared radiation. One consequence of an atmospheric energy imbalance is that the band of intense tropical rainfall near the equator shifts northward or southward. Using a global climate model, we increased and decreased the amount of dust in each hemisphere independently, creating a range of uneven dustiness and energy imbalance, and studied the tropical rainfall response. We found that tropical rainfall shifts away from the dustier hemisphere in a predictable way over both the Atlantic and Pacific oceans. The shift is especially strong over the Atlantic because it is downwind of the Sahara desert, the largest source of dust. Comparing our findings to records from the last ice age shows that dust accounts for roughly 15% of the shift of rainfall over the Atlantic since then. This dust‐rainfall relationship may also help predict changes to the location of tropical rainfall in the future.
... Observations of ocean chemistry in the tropical Atlantic Ocean show a linear correlation between ITCZ precipitation and dissolved iron from mineral dust (Schlosser et al., 2014). Existing paleorecords confirm that sites within the ITCZ are characterized by latitudinal gradients in dust flux (McGee et al., 2007) and provenance (Xie and Marcantonio, 2012;Reimi and Marcantonio, 2016). Thus, we can use the scavenging of dust by the ITCZ to determine its position, taking into consideration that dust at the northern edge of the ITCZ will be primarily sourced from the Northern Hemisphere (NH) and that which is deposited at its southern edge will be predominately sourced from the Southern Hemisphere (SH). ...
... However, during MIS 2, from approximately 29 to 14 ka, and during MIS 4 from 71 to 57 ka, site 37BB displays the highest dust fluxes of any of the three sites (Fig. 4) despite having the lowest overall sedimentation rates. This observation indicates that the ITCZ likely delivered relatively more precipitation and dust to the northernmost site (7.0 • N), pointing to a Pacific ITCZ at a latitude similar to its present day position (Fig. 1) consistent with work from the Eastern Equatorial Pacific (Xie and Marcantonio, 2012), dust provenance data from the same ML1208 sites studied here (Reimi and Marcantonio, 2016), and modeling results (McGee et al., 2014). Conclusions about the average position of the ITCZ during MIS 3 and MIS 5 are much harder to draw because of the frequency of millennial events in both hemispheres, which are not always well resolved by our records. ...
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.
... Such variations in precipitation across the Neotropics correlate with the southward migration of the ITCZ. While some studies suggest a modest southward displacement of 1.11° in the Pacific and 1.05° in the Atlantic (Evans et al., 2020), more extensive multi-site analyses across marine basins indicate displacements of at least 2.5° in the Pacific and up to 7° in the Atlantic (Arbuszewski et al., 2013;Reimi and Marcantonio, 2016). ...
Lake Petén Itzá preserves a sedimentary record spanning the last 413 ka BP, offering insights into past climate and environmental changes of lowland Central America. This record is crucial for understanding past climate changes in the region due to its extensive temporal coverage and the lake’s position at the convergence of various climate drivers, such as the Intertropical Convergence Zone (ITCZ) and the Caribbean Low-Level Jet (CLLJ). The lake also holds significant cultural, environmental, and ecological value, but currently is highly vulnerable to climate change. In 2006, the International Continental Scientific Drilling Program (ICDP) drilled seven cores at different lake locations, whose variability in length and temporal coverage provide a comprehensive dataset for analyzing paleoclimate data across different timescales. This doctoral dissertation investigates the hydroclimate response of lowland Central America using Lake Petén Itzá sediments combined with inorganic (XRF data) and organic (bulk-geochemical, Rock-Eval, and biomarker data) proxies and supported by pollen and mineralogy data, to understand environmental changes and their potential climate drivers across geological time. Firstly, a new chronology established for the oldest sediments allowed us to detect glacial-interglacial cycles corresponding to Marine Isotope Stages (MIS) 11 to 5 (413-70 ka BP). Our data suggest a clear response of wet environments during the warm periods of MIS11 and MIS9 and dry conditions during MIS10, both influenced by the latitudinal migration of the ITCZ and the CLLJ. A hiatus between 304 and 147 ka BP indicates the absence of sediments deposited during MIS8, MIS7, and the beginning of MIS6. After 147 ka BP, wet conditions with high lake levels and Pinus and Quercus forests were reconstructed for the end of MIS6, linked to moisture input from cold fronts originating in higher latitudes. During MIS5 and part of MIS4 (130-70 ka BP), an increase in Moraceae and relatively dry environments marked the beginning of the penultimate interglacial, characterized by potential millennial-scale moisture variability. Secondly, this doctoral work focused on the MIS3 to MIS2 (59-15 ka BP) to analyze millennial-scale hydroclimate variations associated with Dansgaard-Oeschger (D-O) and Heinrich Stadials (HS). During the cold phases of the D-O (Greenland Stadials; GS), the environment was dominated by dry conditions, which intensified during the HS, aligning with high fire activity and previous temperature reductions of 5 to 10°C compared to the present. Conversely, during the warm phases of the D-O (Greenland Interstadials; GI), the environment experienced wet conditions, low evaporation, and anoxic bottom waters. In this study, we propose that the ITCZ had a greater influence on millennial-scale precipitation variations in lowland Central America, while the CLLJ more significantly affected precipitation on orbital scales, causing reduced rainfall during the Last Glacial Maximum (LGM). Thirdly, the first comprehensive record of organic matter content and lipid-based temperature over the late Pleistocene to Holocene (last 27 kyr) is presented, detailing changes in primary productivity and the influence of lake level variations and runoff on Lake Petén Itzá. Enhanced aquatic primary productivity is observed during humid periods like the LGM and Early Holocene, while dry periods such as HS1 and the Younger Dryas show low organic matter content dominated by terrestrial sources. Human settlement led to increased aquatic productivity around 3 ka ago. Additionally, temperature reconstructions using the MBT’5Me lipid paleothermometer suggest average temperatures of 20.8°C during the LGM and 25.4°C during the warmest Early Holocene peak with a deglacial temperature increase by 4°C. Finally, an extensive review of 155 paleoclimate sites in the northern Neotropics was conducted for the first time to understand the role of different climate drivers and the response of various regions from MIS11 to LGM. This review highlights the importance of obtaining more continental records from MIS11 to MIS6, as well as the complexity of the interaction of different climate drivers from MIS5 to LGM. It also emphasizes the high regionalization in environmental responses, potentially influenced by the continentality and altitude of the records. This work significantly advances the understanding of Lake Petén Itzá’s climate history and its broader implications for the northern Neotropics, laying a foundation for future paleoclimate studies in the region.
... Reconstructions of past ENSO variability from coral skeletons (Cobb et al., 2013;Emile-Geay et al., 2016;Grothe et al., 2020;McGregor & Gagan, 2004;Tudhope et al., 2001), marine bivalves (Carré et al., 2014(Carré et al., , 2021Emile-Geay et al., 2016), lake sediments (Conroy et al., 2008;Moy et al., 2002;Rodbell et al., 1999), and marine sediment cores (Ford et al., 2015;Koutavas et al., 2006;Koutavas & Joanides, 2012;Leduc et al., 2009;Rein et al., 2005;Rustic et al., 2020;Sadekov et al., 2013;White et al., 2018) have been instrumental in constraining ENSO behavior over the Holocene and the LGM. However, there is a scarcity of ENSO-resolvable records spanning the last deglaciation-a period of global warming characterized by millennial-scale fluctuations in surface temperatures (Shakun et al., 2012), atmospheric CO 2 concentrations (Bereiter et al., 2015;Petit et al., 1999), ice sheet extent (Bentley et al., 2010;Carlson et al., 2008), and oceanic and atmospheric circulation (Broccoli et al., 2006;McManus et al., 2004;Reimi & Marcantonio, 2016;Schmidt et al., 2012;Them et al., 2015). There is little agreement as to how ENSO responded to deglacial climate change among the few paleo-ENSO records available from this interval. ...
Plain Language Summary
The El Niño‐Southern Oscillation (ENSO) is one of the planet's largest and most influential recurring climate patterns. The fluctuation between warm (El Niño) and cold events (La Niña) every several years has a substantial impact on global weather patterns that carry important socioeconomic consequences. How the frequency and severity of ENSO events may change in response to present and future climate change is largely uncertain, although lessons from past abrupt climate events can help inform our projections of future ENSO. To that end, we analyzed the chemical composition of small, individual zooplankton shells buried in marine sediments to reconstruct “snapshots” of subsurface temperature variability in the eastern Pacific, a feature closely linked to ENSO, over the last 25,000 years. During this time, two abrupt climate events induced by melting glacial ice (“Heinrich Stadial 1” and the “Younger Dryas”) were characterized by different ENSO responses (stronger and weaker, respectively). We suggest that the average climate conditions in the tropical Pacific (temperature, wind strength, etc.) played an important role by either priming the ENSO system for disruption by meltwater (Heinrich Stadial 1) or buffering against it (Younger Dryas).
... A well-established energetic framework, which links ITCZ migration with atmospheric net energy input, cross-equatorial energy transport, and ocean coupling (2,3), also suggests generally small ITCZ movements in the past (4). However, transects of paleoclimate proxy records from the central Pacific and Atlantic oceans hint a larger latitudinal shift (up to 5 to 7°) in ITCZ position during past time intervals such as the Last Glacial Maximum (LGM), Heinrich stadials (HS), and the Little Ice Age (5)(6)(7). The discrepancy raises concerns on whether paleo-proxy records are seasonal biased (4,6), and/or the energetic theory has limited applicability on ITCZ movements (1,2,8). ...
The intertropical convergence zone (ITCZ) plays a key role in regulating tropical hydroclimate and global water cycle through changes in its convection strength, latitudinal position, and width. The long-term variability of the ITCZ, along with the corresponding driving mechanisms, however, remains obscure, mainly because it is difficult to separate different ITCZ variables in paleoclimate proxy records. Here, we report a speleothem oxygen isotope (δ ¹⁸ O) record from southwestern Sulawesi, Indonesia, and compile it with other speleothem records from the Maritime Continent. Using the spatial gradient of speleothem δ ¹⁸ O along a transect across the ITCZ, we constrain ITCZ variabilities over the Maritime Continent during the past 30,000 y. We find that ITCZ convection strength overall intensified from the last glacial period to the Holocene, following changes in climate boundary conditions. The mean position of the regional ITCZ has moved latitudinally no more than 3° in the past 30,000 y, consistent with the deduction from the atmospheric energy framework. However, different from modern observations and model simulations for future warming, the ITCZ appeared narrower during both the late Holocene and most part of the last glacial period, and its expansion occurred during Heinrich stadials and the early-to-mid Holocene. We also find that during the last glacial and deglacial period, prominent millennial-scale ITCZ changes were closely tied to the variability of the Atlantic meridional overturning circulation (AMOC), whereas during the Holocene, they were predominantly modulated by the long-term variability of the Walker circulation.
... The eastward flow of the NECC develops from the offset between the Equator and the intertropical convergence zone (ITCZ; Li & Fedorov, 2022;McCreary et al., 2002), intensifying seasonally as the ITCZ shifts northward during summer (Wyrtki, 1974). During the LGM, asymmetric cooling in the northern hemisphere should induce a southward shift in the ITCZ (Broccoli et al., 2006), consistent with proxy reconstructions (Reimi & Marcantonio, 2016), thereby weakening the NECC. Zonal gradients in surface temperature and thermocline depth, which power the EUC, also appear to have slackened during the LGM (Ford et al., 2018;Monteagudo et al., 2021). ...
The expansion of oxygen deficient zones (ODZs) within the ocean's interior is anticipated to be a major consequence of anthropogenic climate change, but past changes in ODZs are poorly defined. Recent mapping efforts have revealed plumes of the redox‐active metal cobalt within ODZs, driving a basin‐scale correlation between high cobalt and low O2. Here, we investigate the cobalt flux to Equatorial Pacific sediments along the Line Islands Ridge as a novel record of basin‐scale fluctuations in ODZ extent. After accounting for remobilization by diagenesis, we document a ∼40% increase in cobalt accumulation over the last glacial period, with a more pronounced peak during the Last Glacial Maximum, indicative of larger ODZs compared to the Holocene. Our results link ODZ expansion with colder climates and lend support to model‐based assertions that ongoing deoxygenation may reflect a transient response to warming.
... Therefore, the IPCC AR6 has changed its assessment of AMOC's abrupt change before 2100 to medium confidence (Fox-Kemper et al., 2021) and categorized AMOC collapse as a low-probability, high-impact event, known as a black swan event. Such a collapse can cause sudden changes in regional weather patterns and water cycles (Chiang and Bitz, 2005;Chiang and Friedman, 2012;Jackson et al., 2015;Renssen et al., 2018;Sandeep et al., 2020), for example, shifting the tropical ITCZ southward (Peterson et al., 2000;McGee et al., 2014;Schneider et al., 2014;Mohtadi et al., 2016;Reimi and Marcantonio, 2016), weakening the Asian-African monsoon (Tierney and deMenocal, 2013;Otto-Bliesner et al., 2014;Cheng et al., 2016;Grandey et al., 2016;Wurtzel et al., 2018), strengthening the monsoon in the SH (Cruz Jr et al., 2005;Ayliffe et al., 2013;Stríkis et al., 2015;Stríkis et al., 2018), cooling in North America , increasing precipitation in the mid-latitudes of North America (Polyak et al., 2004;Grimm et al., 2006;Wagner et al., 2010;Voelker et al., 2015), and drying in Europe (Genty et al., 2006;Haarsma et al., 2015;Jackson et al., 2015;Rach et al., 2017;Naughton et al., 2019). ...
Based on modern observations, historical proxy data, and climate model simulations, this paper provides a comprehensive overview of the past, present and future evolution characteristics of the Atlantic Meridional Overturning Circulation (AMOC), as well as its impact on the surface air temperature (SAT) at regional and hemispherical scales. The reconstruction results based on the proxy data indicate that the AMOC has weakened since the late 19th century and experienced overall weakening throughout the 20th century with low confidence. Direct observations show that the AMOC weakened during 2004–2016, but it is not possible to distinguish between its decadal variability and long-term trend. Climate models predict that if greenhouse gas emissions continue to increase, AMOC will weaken in the future, but there will not be a sudden collapse before 2100. For the thermodynamic effects of AMOC, the increased surface heat flux release and meridional heat transport (MHT) over the North Atlantic associated with the strong AMOC cause an increase in the hemispherical SAT. At the millennial scale, climate cooling (warming) periods correspond to a weakened (strengthened) AMOC. The enhanced MHT of a strong AMOC can affect Arctic warming and thus influence regional SAT anomalies and SAT extremes through mutual feedback between Arctic sea ice and AMOC. In terms of dynamic effects, a strong AMOC modulates the Rossby wave trains originating from the North Atlantic and spreading across mid-to-high latitudes in the Northern Hemisphere and causes an increase in the variabilities in the circulation anomalies over the Ural and Siberian regions. Ultimately, a strong AMOC significantly affects the frequencies of extreme cold and warm events in the mid-to-high latitude regions over Eurasia. In addition, AMOC can also influence regional and global SAT anomalies through its dynamic adjustment of planetary-scale circulation. Decadal variation in AMOC is closely related to the Atlantic Multidecadal Oscillation (AMO). During positive phases of AMO and AMOC, enhanced surface heat fluxes over the North Atlantic lead to abnormal warming in the Northern Hemisphere, while during negative phases, the reverse case occurs. Under high emission scenarios in the future, the possibility of AMOC collapse increases due to freshwater forcing. However, most advanced climate models underestimate the strength of the AMOC and its impact on the AMO and relevant climate change, which presents a major challenge for future understanding and prediction of the AMOC and its climate effects.
... Therefore, the IPCC AR6 has changed its assessment of AMOC's abrupt change before 2100 to medium confidence (Fox-Kemper et al., 2021) and categorized AMOC collapse as a low-probability, high-impact event, known as a black swan event. Such a collapse can cause sudden changes in regional weather patterns and water cycles (Chiang and Bitz, 2005;Chiang and Friedman, 2012;Jackson et al., 2015;Renssen et al., 2018;Sandeep et al., 2020), for example, shifting the tropical ITCZ southward (Peterson et al., 2000;McGee et al., 2014;Schneider et al., 2014;Mohtadi et al., 2016;Reimi and Marcantonio, 2016), weakening the Asian-African monsoon (Tierney and deMenocal, 2013;Otto-Bliesner et al., 2014;Cheng et al., 2016;Grandey et al., 2016;Wurtzel et al., 2018), strengthening the monsoon in the SH (Cruz et al., 2005;Ayliffe et al., 2013;Stríkis et al., 2015;Stríkis et al., 2018), cooling in North America , increasing precipitation in the mid-latitudes of North America (Polyak et al., 2004;Grimm et al., 2006;Wagner et al., 2010;Voelker et al., 2015), and drying in Europe (Genty et al., 2006;Haarsma et al., 2015;Jackson et al., 2015;Rach et al., 2017;Naughton et al., 2019). ...
... Nearby on the South American continent, precipitation records from Santiago Cave indicate a reduction in SASM strength at the LGM 13 (Fig. 2e), consistent with evidence for a southward displacement of the ITCZ over the South American continent 25 and the equatorial Pacific 26,27 . The effect of this displacement in the EEP, specifically, was a contraction of the EEP cold tongue caused by reduced cross-equatorial flow of the southern trade winds and a deeper thermocline 7 , which reduced upwelling and surface cooling at the equator. ...
Fully and accurately reconstructing changes in oceanic productivity and carbon export and their controls is critical to determining the efficiency of the biological pump and its role in the global carbon cycle through time, particularly in modern CO2 source regions like the eastern equatorial Pacific (EEP). Here we present new high-resolution records of sedimentary 230Th-normalized opal and nannofossil carbonate fluxes and [231Pa/230Th]xs ratios from site MV1014-02-17JC in the Panama Basin. We find that, across the last deglaciation, phytoplankton community structure is driven by changing patterns of nutrient (nitrate, iron, and silica) availability which, in turn, are caused by variability in the position of the Intertropical Convergence Zone (ITCZ) and associated changes in biogeochemical cycling and circulation in the Southern Ocean. Our multi-proxy work suggests greater scrutiny is required in the interpretation of common geochemical proxies of productivity and carbon export in the EEP.
... The LGM has been well recognised as ISM weakening and NEM strengthening period from the BoB records (Govil and Naidu, 2011;Panmei et al., 2018) with southward migration of ITCZ during LGM (Reimi and Marcantonio, 2016). Ideally, the precession minima gives rise to boreal monsoon intensification as the northern hemisphere summer occurs in perihelion which induces maximum peak in the summer insolation thereby amplifying the land-sea pressure difference and intensifying the monsoonal circulation (Ziegler et al., 2010). ...
The Bay of Bengal (BoB) is a semi-enclosed basin located in the north-eastern Indian Ocean that receives enormous fluvial sediment input primarily from the Himalayan and the peninsular rivers depending on the Indian monsoon intensities. The seasonal reversal of monsoonal currents over the northern Indian ocean leads to the Indian monsoon which varies as a function of lateral migration of the intertropical convergence zone (ITCZ) which in turn leads to both Indian summer monsoon (ISM) and Northeast Monsoon (NEM) over the BoB. Despite being such a dynamic region, the instrumental and oceanographic data for the BoB seldom goes beyond the last century which invigorated its long-term reconstruction of paleoclimate and paleoceanography, especially since last 30 ka, as it includes the most recent glacial period i.e. the Last Glacial Maxima (LGM: ~19-23 ka). Further, the last 30 ka period also consists of several other globally recognized cold climatic events such as Henrich events (H1: ~15-17 ka; H2: ~24-25 ka) and Younger Dryas (YD: ~12.9-11.8 ka) and warm climatic events such as Bølling-Allerød (B/A: ~14.8-12.9 ka) and Holocene epoch (~11.8 ka -Present).
... However, the global data shows that the Equatorial Pacific authigenic records are anomalous relative to the global authigenic patterns of change or to records from elsewhere in the deep Pacific ( Fig. 6B; upper panel). The authigenic ε Nd records of the Equatorial Pacific track detrital ε Nd , suggesting a local imprint of changing detrital sources (Reimi and Marcantonio, 2016;Xie and Marcantonio, 2012). To illustrate the potential for correcting authigenic ε Nd records impacted by local detrital changes, we subtracted the change of local detrital ε Nd (with respect to Holocene) from authigenic ε Nd data. ...
The Global Overturning Circulation is linked to climate change on glacial-interglacial and multi-millennial timescales. The understanding of past climate-circulation links remains hindered by apparent conflicts among proxy measures of circulation. Here we reconstruct circulation changes since the Last Glacial Maximum (LGM) based on a global synthesis of authigenic neodymium isotope records (εNd). We propose the bottom-up framework of interpreting seawater and authigenic εNd considering not only conservative watermass mixing, but also the preformed properties and the non-conservative behavior of εNd , both subject to sedimentary influences. We extract the major spatial-temporal modes of authigenic εNd using Principal Component Analysis, and make a first-order circulation reconstruction with budget-constrained box model simulations. We show that during the LGM, the source region of North Atlantic overturning shifted southward, which led to more radiogenic preformed εNd of glacial Northern Source Water (NSW). Considering this preformed effect, we infer that glacial deep Atlantic had a similar proportion of NSW as today, although the overall strength of glacial circulation appears reduced from both North Atlantic and Southern Ocean sources, which increased the relative importance of non-conservative behavior of εNd and may have facilitated accumulation of respired carbon in the deep ocean. During the deglaciation, we find that Southern Ocean overturning increased, which offset suppressed North Atlantic overturning and resulted in a net stronger global abyssal circulation. Faster global scale deglacial circulation reduced the relative importance of non-conservative effects, resulting in Atlantic-Pacific convergence of abyssal εNd signatures. Variations of Southern Ocean overturning likely drove a significant fraction of deglacial changes in atmospheric CO2 and oceanic heat budget.
... As mentioned, the main modern dust-supply region to our study site is the western South America, which dominates the particles flux arriving to the EEP region south of the ITCZ front (Pichat et al., 2014;Saukel et al., 2011;Xie & Marcantonio, 2012); however, the EEP region north of the ITCZ front is mainly influenced by arid regions from western and southern Mexico (Prospero & Bonatti, 1969;Saukel et al., 2011). This is so because the ITCZ acts as a barrier to cross-hemispheric dust transport (Jacobel et al., 2016;Prospero & Bonatti, 1969;Reimi & Marcantonio, 2016;Saukel et al., 2011;Xie & Marcantonio, 2012). However, with an ITCZ mean position located further south, regions from the southern Mexico could have represented an additional northern dust sources influencing the ODP Site 1240. ...
Modern biogeochemical conditions of the Eastern Equatorial Pacific (EEP) region are characterized by high macronutrient concentrations but low phytoplankton abundance due to both iron and silicic acid limitation. Since primary producers significantly impact the global carbon cycle, paleoproductivity in relation to climate change and nutrient availability in this region has been a topic of a number of studies. However, the complex dynamics of this region, especially east of the Galapagos Islands, has led to some discrepancies when linking reconstructed paleoproductivity with potential mechanisms for higher primary productivity. Here we focus on reconstructing primary productivity of haptophyte algae and diatoms, as well as continental material input, sea surface salinity, and sea surface temperature, and compare these reconstructions with existing records for the period comprised between 150 and 110 ka (the penultimate deglaciation period) with the aim to understand the mechanisms that most significantly influence phytoplankton growth over the EEP region east of the Galapagos Islands. Our results suggest enhanced upwelling in the EEP system during the penultimate deglaciation and increased phytoplankton abundance mainly as the result of both the increasing influence of nutrient‐rich Southern Ocean sourced waters through the Equatorial Undercurrent and a higher input of iron through atmospheric deposition. The highest phytoplankton abundances recorded at the study site during the penultimate deglaciation also suggest that maximum input of nutrients might have occurred during the millennial‐scale event Heinrich Event 11 in the North Atlantic as a result of global atmospheric and oceanic reorganizations.
... However, at Pēpē'ōpae the peat layer would have been relatively thin at this time, suggesting that summer (dry) season rainfall was also sufficient to maintain saturated soil conditions year-round in the absence of a thicker peat layer of low hydraulic conductivity that can buffer OM decomposition from summer drought conditions. Shifts in the ITCZ position in response to orbital forcing could have played a role for the overall water table balance during the Holocene (McGee et al., 2014;Reimi and Marcantonio, 2016). ...
The Hawaiian Islands are the only high land in a vast stretch of the North Pacific where past climatological and ecological processes can be reconstructed from terrestrial Earth system archives. We measured hydroclimatic proxies and carbon accumulation in an organic sediment core from the windward montane peatland Pēpē‘ōpae on the Island of Moloka‘i, Hawai‘i using radiocarbon, leaf wax geochemistry, and stable isotopes of carbon and hydrogen in addition to historical pollen records. Following a period of soil development, substantial carbon accumulation began around 10 ka BP (thousands of years before present) under wet conditions. Peat formation was continuous but variable throughout the Holocene, including maxima in carbon accumulation around 9 and 3 ka and a minimum around 1.5 ka that has resulted in a belowground carbon storage today of 144 kg C m⁻². From this core we generated a new chronology for previously published pollen spectra from the study site and a Wetness Index that shows increases in dry-adapted taxa in upwind forests during periods of decreased carbon accumulation in the peatland. Shifts in the distribution of sedimentary n-alkane chain lengths in the context of 14 species of modern bog plant n-alkanes suggests litter inputs have been derived from a diverse plant community that changed in dominant species in response to climate. Hydrogen stable isotope ratios of sedimentary C29 n-alkanes show negative departures around 9 and 3 ka consistent with increases in storm-derived rainfall likely related to the position and strength of the northern jet stream. This study is the first to provide a continuous organic sedimentary record of links between hydroclimate, vegetation, and montane belowground carbon sequestration for this part of the North Pacific.
... ago may reflect southward migration of the ITCZ following expansion of Northern Hemisphere glaciation at ca. 2.9 Ma (Bartoli et al., 2005). While Hovan (1995) observed no evidence for this event in eolian sediments from the eastern equatorial Pacific, there is strong evidence that increased glaciation (vis-à-vis differences between today and the last glacial maximum [LGM]) pushed the ITCZ southward in the past (e.g., Deplazes et al., 2013;Reimi and Marcantonio, 2016). Importantly, the southward position of the ITCZ during the LGM resulted in aridity in the environs of Costa Rica, Panama, and northern South America (e.g., Bush and Colinvaux, 1990;Islebe et al., 1995;Deplazes et al., 2013). ...
Caribbean biota underwent major ecological and evolutionary transformation in the Pliocene–Pleistocene, but a lack of detailed paleoenvironmental reconstruction prevents thorough resolution of cause and effect. We quantify levels of upwelling and freshwater input into Caribbean coastal shelf ecosystems over the last ~6 m.y. with >3300 stable isotope measurements from 74 fossil serially sampled gastropods by normalizing δ18O values to open-ocean δ18O from planktonic foraminifera. We find that the influence of Pacific-like upwelling in the southwestern Caribbean was low after 4.25 Ma but coastal ecosystems were heavily influenced by seasonal freshening until ca. 2.5 Ma, after which time low-freshwater conditions were established. The origination of modern oligotrophic coastal conditions was therefore a result of oceanographic change causing declining upwelling, and declining nutrients from terrestrial sources. We speculate that a southward shift of the Intertropical Convergence Zone, associated with Northern Hemisphere glaciation, reduced rainfall and terrestrial nutrient input and contributed to biotic turnover in the southwestern Caribbean, including the proliferation of modern reef communities.
... In these modeling studies, the ITCZ is southerly positioned when the Northern Hemisphere cools. In response to the interhemispheric thermal gradient, the ITCZ in the Pacific Ocean was reported to shift southward across a large latitudinal band, i.e., 4°south of its modern position in Heinrich Stadial (HS) 11 (Jacobel et al., 2016), 5°during the Little Ice Age (Sachs et al., 2009), and about 2.5°to 7°in the last glaciation (Reimi and Marcantonio, 2016). However, the ITCZ movement in these studies is traced by the rainfall maximum, not by the precipitation centroid (McGee et al., 2018), hampering a full understanding of past ITCZ changes and the dynamics involved. ...
A 20-year-resolution speleothem δ ¹⁸ O record from southern China reveals a detailed Asian summer monsoon (ASM) history between 73.6 and 62.3 ka. ASM changes during Interstadial 19 and late MIS 4 matched Greenland temperature variations but were antiphased with Antarctic temperatures. However, long-term strengthening of the ASM in early MIS 4 agrees well with the gradual Antarctic warming, when Greenland remained in a stable cold state. More specifically, the ASM was less variable during peak interstadials in contrast to striking instabilities during stadials. These observations suggest that the factors dominating ASM variability change through time. During early MIS 4, negligible freshwater perturbations occurred in the North Atlantic, and sea-surface temperatures in the low- to midlatitude Pacific Ocean reached the modern level. Thus, an expansion of the Intertropical Convergence Zone (ITCZ) was likely important for the long-term ASM rise. In late MIS 4, the antiphase correlation between ASM and Antarctic temperature could be attributed to freshwater inputs into the North Atlantic and a southerly positioned ITCZ. Consequently, meridional ITCZ shifts, although within a limited latitudinal band, would result in an antiphase relationship between interhemispheric climate changes. Otherwise, an in-phase correlation could be expected if the centroid of ITCZ is stable along the equator.
... Under the assumption that paleo-δ 18 O P and paleo-δD P represent local P, P cent or P max could be interpreted to have shifted as far south as 8.5°S during the last glacial period and as far north as 20°N during the early Holocene (Fleitmann et al., 2007), excursions of nearly −10°and more than +15°, respectively, relative to modern P cent (McGee et al., 2014). Such excursions far exceed the average~1°shift predicted by climate models under glacial forcings (Donohoe et al., 2013;McGee et al., 2014) or the 2.5-7°shift over the open ocean reconstructed by non-water isotope proxy data (Reimi & Marcantonio, 2016). Additionally, such excursions are often not supported by co-located or adjacent proxy records for vegetation, surface erosion, and other proxies for P or P-E (Dubois et al., 2014;Konecky et al., 2016;Niedermeyer et al., 2014;Wicaksono et al., 2017). ...
Plain Language Summary
The oxygen isotopic composition of tropical precipitation is a powerful tool for “fingerprinting” the history of evaporation, condensation, and transport that water was subjected to in the atmosphere before it reached the ground as precipitation. For this reason, the oxygen isotopic composition of precipitation is commonly employed as a water cycle tracer, both in modern‐day contexts and in geologic archives. Translating oxygen isotope ratios into metrics of circulation and climate is not always straightforward, however, due to the range of processes that can affect precipitation. Here we introduce a novel conceptual and statistical framework for interpreting oxygen isotope ratios in tropical precipitation by deconvolving its multiple competing influences. We find that the relative importance of each factor varies geographically. Moisture source is particularly important around the Indo‐Pacific Warm Pool, while cloud type exerts strong influence in regions where stratiform clouds are abundant. These results help to reconcile conflicting interpretations of how the Intertropical Convergence Zone and other key features of tropical circulation respond to climate forcings, which are critical questions for past climate reconstructions as well as future climate projections.
... The hypothesized connection between the central Pacific and western U.S. lake levels is supported by recent reconstructions indicating large (~5°) southward shifts of the central Pacific ITCZ during the Heinrich stadials of the last two deglaciations (HS1 and HS11) (56,57), substantially larger than estimates of global mean ITCZ displacements (17). Our analysis suggests that these southward shifts would have driven jet and Aleutian Low responses that, superimposed on the circulation effects of large remnant North American ice sheets, could have provided the winter precipitation increases necessary to fill basins to their highstand levels. ...
Lake and cave records show that winter precipitation in the southwestern United States increased substantially during millennial-scale periods of Northern Hemisphere winter cooling known as Heinrich stadials. However, previous work has not produced a clear picture of the atmospheric circulation changes driving these precipitation increases. Here, we combine data with model simulations to show that maximum winter precipitation anomalies were related to an intensified subtropical jet and a deepened, southeastward-shifted Aleutian Low, which together increased atmospheric river–like transport of subtropical moisture into the western United States. The jet and Aleutian Low changes are tied to the southward displacement of the intertropical convergence zone and the accompanying intensification of the Hadley circulation in the central Pacific. These results refine our understanding of atmospheric changes accompanying Heinrich stadials and highlight the need for accurate representations of tropical-extratropical teleconnections in simulations of past and future precipitation changes in the region.
... The less radiogenic glacial detrital εNd was not only found in the Panama Basin, but also in the Philippine Sea 3 , central 4 and eastern 5 equatorial Pacific regions, in accordance with higher Northern Hemisphere continental dust loads during the ice ages than during interglacials 6,7 . Unlike the western and central North Pacific, where most of the dust deposited originates in Asia 3,4,8,9 , the dust provenance of EEP might be different. Previous studies suggest its unradiogenic particles could also come from South America and Africa 5, 10 . ...
... Specifically in the Northeast of Brazil, the summer has a wide variation in pluviometric parameters due to the interactions of two main factors: the Inter-Tropical Convergence Zone (ITCZ) position and the location of subsiding currents of air of Walker-Hadley circulations. The ITCZ is a low-pressure zone located along the tropical region, where there is convergence of the winds (Reimi and Marcantonio, 2016). This zone has an annual north-south displacement that directly influences the incidence of the winds (changes in direction and speed) and precipitation of the affected region. ...
... Sachs et al. (2009) found evidence consistent with a 5 southward shift of the ITCZ in the central Pacific during the Little Ice Age (1400-1850 CE), and Jacobel et al. (2016) suggest that the central Pacific ITCZ shifted south by at least 4 during HS11 (~136-129 ka). Reimi and Marcantonio (2016) also present evidence that the central Pacific ITCZ was between 2.5 and 7 south of its current position during the last glacial period and deglaciation. If the magnitudes of these shifts are taken to represent the zonal-mean ITCZ change during these events, they challenge the arguments put forward above and in McGee et al. (2014). ...
The atmospheric Hadley cells, which meet at the Intertropical Convergence Zone (ITCZ), play critical roles in transporting heat, driving ocean circulation and supplying precipitation to the most heavily populated regions of the globe. Paleo-reconstructions can provide concrete evidence of how these major features of the atmospheric circulation can change in response to climate perturbations. While most such reconstructions have focused on ITCZ-related rainfall, here we show that trade wind proxies can document dynamical aspects of meridional ITCZ shifts. Theoretical expectations based on angular momentum constraints and results from freshwater hosing simulations with two different climate models predict that ITCZ shifts due to anomalous cooling of one hemisphere would be accompanied by a strengthening of the Hadley cell and trade winds in the colder hemisphere, with an opposite response in the warmer hemisphere. This expectation of hemispherically asymmetric trade wind changes is confirmed by proxy data of coastal upwelling and windblown dust from the Atlantic basin during Heinrich stadials, showing trade wind strengthening in the Northern Hemisphere and weakening in the Southern Hemisphere subtropics in concert with southward ITCZ shifts. Data from other basins show broadly similar patterns, though improved constraints on past trade wind changes are needed outside the Atlantic Basin. The asymmetric trade wind changes identified here suggest that ITCZ shifts are also marked by intensification of the ocean’s wind-driven subtropical cells in the cooler hemisphere and a weakening in the warmer hemisphere, which induces cross-equatorial oceanic heat transport into the colder hemisphere. This response would be expected to prevent extreme meridional ITCZ shifts in response to asymmetric heating or cooling. Understanding trade wind changes and their coupling to cross-equatorial ocean cells is key to better constraining ITCZ shifts and ocean and atmosphere dynamical changes in the past, especially for regions and time periods for which few paleodata exist, and also improves our understanding of what changes may occur in the future.
... A decrease in Southern Hemisphere insolation during the mid-Holocene drives coupled oceanatmosphere simulations to minimum values at subtropical latitudes during the austral summer, which result in a northward position of the ITCZ at millennial timescales (Braconnot et al., 2007). Multiple evidence from the equatorial Pacific Ocean (Reimi and Marcantonio, 2016;Seill es et al., 2016), the Cariaco Basin (Haug et al., 2001) and the Amazonian basins (Wang et al., 2017) lends further support to a northwardly displaced ITCZ during the early Holocene ($9 ka), which would have reached its position furthest north during the mid-Holocene ($7 ka). As a consequence, a weakened tropical rainfall system together with a northward shift of the mid-level westerly flow and more frequent cold surges in summer and autumn at 20˚S (Vuille and Keimig, 2004) could be one of the mechanisms that led to the mid-Holocene dry conditions in the Central Andes and adjacent Atacama Desert. ...
In the past two decades, much has been learned about the late Quaternary climate history of the Atacama Desert with some details still unclear about the seasonality, timing and extent of wet and dry phases. Modern climate studies reveal that, far from exhibiting a unique pattern, seasonal precipitation originates from many sources and mechanisms. For the last 16 ka, we attempt to sort out these complexities in pollen records from four fossil rodent midden series spanning 22˚–25˚S in northern Chile. Widespread wet conditions prevailed during the late Pleistocene and early Holocene, particularly between 13 and 9 ka, evidenced by <400 m lowering of pollen zones (plant communities) compared to today. Regional differences in the timing and magnitude of this displacement may be related to the prevailing source (tropical/extra-tropical) or mode (NNW/SE) of tropical precipitation through time. Wet conditions persisted well into the early Holocene, lasting 8 ka, possibly associated with a northward shift of the Inter Tropical Convergence Zone, tracking minimum insolation values at subtropical latitudes during the austral summer. The establishment of conditions similar to today happened $4 ka.
... Of the three modes, Mode 3 has the greatest uncertainty ( Figure 5), but it can be interpreted as either three half-precessional peaks at 0, 10, and 20 ka or one precessional peak at 10 ka, with a gradual increase in productivity from 25 ka to 10 ka followed by an abrupt decline at~8 ka. The more robust productivity maximum at 10 ka coincides with the Northern Hemisphere Holocene Thermal Maximum, which fueled the Green Sahara of the African Humid Period with a mean ITCZ shifted substantially to the north both on land [de Menocal et al., 2000] and in the ocean [Reimi and Marcantonio, 2016]. A similar deflection of the ITCZ in the EEEP would have promoted La Niña-like conditions, vitalizing surface productivity, especially in the relatively nutrient-replete conditions inferred from Mode 1 at this time. ...
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.
... ago may reflect southward migration of the ITCZ following expansion of Northern Hemisphere glaciation at ca. 2.9 Ma (Bartoli et al., 2005). While Hovan (1995) observed no evidence for this event in eolian sediments from the eastern equatorial Pacific, there is strong evidence that increased glaciation (vis-à-vis differences between today and the last glacial maximum [LGM]) pushed the ITCZ southward in the past (e.g., Deplazes et al., 2013;Reimi and Marcantonio, 2016). Importantly, the southward position of the ITCZ during the LGM resulted in aridity in the environs of Costa Rica, Panama, and northern South America (e.g., Bush and Colinvaux, 1990;Islebe et al., 1995;Deplazes et al., 2013). ...
The Panama Paleontology Project (PPP) broke new ground in the rigorous quantitative analysis of evolutionary and ecological change over geological time in a highly biodiverse tropical setting. Many advances were made but most studies within the PPP have focused on a single taxonomic group or a distinct metric of environmental or ecological change. Our aims is to produce a synthetic analysis of temporal and spatial changes across entire biological communities using the combined efforts of over 100 workers in the PPP over the last 30 years. To this goal we have designed the PPP database (PPPD). This relational database will compile quantitative taxonomic, ecological
and functional biotic data within stratigraphic and geochronological context for comparison with environmental data as derived from high resolution geochemistry (δ18O and δ13 C isotopes). PPPD users will have rapid access to a large range of standardized information; will allow downloads and direct analyses to be performed on its contents, interactive access for browsing the contents and formulating specific queries. The PPPD is linked to R to facilitate exploration of patterns of biotic and environmental
change in tropical marine ecosystems over geological time.
The evolution of aridity and its forcing mechanisms in the Arabian Peninsula are of significance for better understanding the environmental response of the region to global change and the impact of past climate shifts in human evolution. Paleoclimatic reconstructions for the region are predominantly derived from fragmentary terrestrial records. Here, we present a detailed environmental magnetic record for marine sediments from the western equatorial Indian Ocean, which constitute the terminal sink for eolian material originated from the Arabian Peninsula. The dominant magnetic minerals identified in the studied sediments include magnetite, maghemite, and hematite. The hard isothermal remanent magnetization (HIRM), widely used as a proxy for the abundances of the high coercivity mineral hematite, has been used to track variations in the concentration of Arabian dust over the last 200 kyr. Dust concentrations are lowest during interglacial periods of maximum boreal summer isolation, and coincide with intervals during which lake and fluvial systems developed and speleothems formed across most of southern and central Arabia. Our results point to a sharp decrease in the production of dust due to the expansion of vegetation cover up to 30°N during these periods. West African Monsoon appears to be the major source of increased rainfall across Arabia, while the Indian Summer monsoon likely contributed to enhance rainfall in the south and southeastern parts of the peninsula. Variation in the supply of Arabian dust were also driven by changes in high-latitude ice volume and atmospheric CO2, as indicated by highest dust concentration found during glacial periods. We interpret that the expansion of high-latitude ice sheets increased the global meridional temperature gradient, weakened the monsoon system, and resulted in a more southerly, colder and descending subtropical westerly jet, leading to the retreat of vegetation cover and the enhanced production of dust from the Arabian deserts. Our results highlight a link between the production of Arabian dust and orbital-scale climate variability, including both monsoon and northern hemisphere ice sheets dynamics.
Pb isotope ratios are used for apportioning the sources of Pb in the blood of children (ages 1–6) screened for high blood Pb levels (>5 μg/dL) surrounding urban areas of Kansas City, MO. We compared Pb isotope ratios measured in the child's blood with those of the most likely sources of Pb in that child's home environment. The environmental sources sampled consisted of topsoils, paints, occupational sources (e.g., oil rig workers' uniforms, mechanics' clothes), indoor air filters, dusts, and dietary sources (e.g., spices). Blood lead levels (BLL) ranged from 2.9 to 12.7 μg/dL in children from the five homes participating in this study. Measurements of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotope ratios were made by multi-collector ICP-MS. Comparison of the Pb isotope ratios in home environment samples versus those in the child's blood in each home allowed the identification of possible sources of a child's Pb exposure in three homes. In five homes investigated, children's blood Pb levels were most likely to be derived from dusts inside, and topsoil outside, the homes, or a mixture thereof. In one case, blood Pb was derived from turmeric spice and, in another, the Pb was derived from paint. It is not always possible to directly link high BLLs to the environmental sources collected when Pb isotope ratios of the environmental samples did not overlap with those of the blood.
Early Paleogene neodymium (Nd) isotopic reconstructions of ocean circulation in the Pacific suggest a bipolar mode of overturning circulation characterized by convection of deep-water masses from 70 to at least 40 Ma. This mode is fundamentally different from the “sluggish, diffusive” mode that characterizes the modern North Pacific. Therefore, major reorganization of Pacific circulation must have occurred between 40 Ma and the modern. To investigate the potential timing of such a reorganization, we present new Nd isotope data from drill sites spanning 40 to 10 Ma. Here, we use Nd isotopic analyses to identify the sources of dissolved Nd to the Pacific in order to evaluate the application of the Nd proxy for ancient water mass composition. We combine analyses of fossil fish debris, sequentially leached oxide coatings and leach residues to further refine our understanding of Nd marine geochemical cycling and to contribute new constraints on the paleoceanographic history of the Pacific. Overall, these data support previous interpretations of the convection of deep water in the Pacific sector of the Southern Ocean and surface-ventilated North Pacific Deep Water formation from ∼70 to 40 Ma. The new Nd isotope data suggest a gradual transition between 36 and 25 Ma, a time period characterized by global cooling. This transition consists of a change from North Pacific Nd isotopic signatures controlled by convection and water mass mixing to values heavily influenced by overprinting from other sources of Nd to the deep Pacific. Such a transition is consistent with a change in overturning circulation from a “Paleogene mode” to a mode dominated by sluggish diffusive mixing. After 25 Ma the data indicate a significant contribution of radiogenic dissolved Nd to the deep waters of the North Pacific. The most likely source of radiogenic dissolved Nd would have been labile volcanic ash that altered the composition of the water mass as it slowly advected through the North Pacific. A slowing of overturning circulation in the North Pacific could have resulted in diminished poleward heat transport, increasing equator to pole thermal gradients and contributing to a cooling global climate.
There is evidence for greater carbon storage in the glacial deep Pacific, but it is uncertain whether it was caused by changes in ventilation, circulation, and biological productivity. The spatial εNd evolution in the deep Pacific provides information on the deepwater transit time. Seven new foraminiferal εNd records are presented to systematically constrain glacial to interglacial changes in deep Pacific overturning and two different εNd evolution regimes occur spatially in the Pacific with reduced meridional εNd gradients in glacials, suggesting a faster deep Pacific overturning circulation. This implies that greater glacial carbon storage due to sluggish circulation, that is believed to have occurred in the deep Atlantic, did not operate in a similar manner in the Pacific Ocean. Other mechanisms such as increased biological pump efficiency and poor high latitude air-sea exchange could be responsible for increased carbon storage in the glacial Pacific.
Changes in dust flux, export productivity, and bottom-water oxygenation in the equatorial Pacific Ocean have been tightly linked with variations in North Atlantic climate over the past 100,000 years, according to analyses of marine sediments.
The interpretation of sedimentary records in terms of glacial-interglacial changes in particle flux in the eastern equatorial Pacific (EEP) has been controversial. Here, we analyze disaggregated inorganic grain size (DIGS) distributions of three marine sediment cores from this region, focusing on the last 21 ka, to investigate evidence of sediment redistribution on the sea floor. Grain size sorting coefficients show that sediments in the EEP are moderately to well sorted, indicating sediment reworking in this region due to bottom currents. Furthermore, a systematic correlation between focusing factors and sorting coefficients at two sites shows that more focused sediments are also better sorted. We conclude that grain size based sedimentary records are consistent with the ²³⁰Th-based evidence of lateral sediment redistribution on the sea floor in the EEP.
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.
The Southern Westerly Winds (SWW) exert a crucial influence over the world ocean and climate. Nevertheless, a comprehensive understanding of the Holocene temporal and spatial evolution of the SWW remains a significant challenge due to the sparsity of high-resolution marine archives and appropriate SWW proxies. Here, we present a north-south transect of high-resolution planktonic foraminiferal oxygen isotope records from the western South Atlantic. Our proxy records reveal Holocene migrations of the Brazil- Malvinas Confluence (BMC), a highly sensitive feature for changes in the position and strength of the northern portion of the SWW. Through the tight coupling of the BMC position to the large-scale wind field, the records allow a quantitative reconstruction of Holocene latitudinal displacements of the SWW across the South Atlantic. Our data reveal a gradual poleward movement of the SWW by about 1-1.5° from the early to the mid-Holocene. Afterwards variability in the SWW is dominated by millennial-scale displacements in the order of 1° in latitude with no recognizable longer-term trend. These findings are confronted with results from a state-of-the-art transient Holocene climate simulation using a comprehensive coupled atmosphere–ocean general circulation model. Proxy-inferred and modeled SWW shifts compare qualitatively, but the model underestimates both orbitally forced multi-millennial and internal millennial SWW variability by almost an order of magnitude. The underestimated natural variability implies a substantial uncertainty in model projections of future SWW shifts.
The Intertropical Convergence Zone (ITCZ) is manifested as a circum-global atmospheric belt of intense, moist convection and rainfall, marking the confluence of the northern and southern trades and the rising branch of the Hadley cell. It regulates the hydrologic cycle over the tropical continents and interacts tightly with the tropical oceans, notably with the seasonal appearance of the equatorial cold tongues of the Atlantic and Pacific. While it undergoes a regular seasonal migration, today the ITCZ maintains a nearly permanent Northern Hemisphere bias. Here we address the question of variability in the mean latitude of the marine ITCZ over the eastern Pacific on time scales of 100–10,000 years, with emphasis on the past 30,000 years. Our strategy relies on reconstructing the intensity of the prominent oceanographic front of the cold tongue–ITCZ complex, using oxygen isotope and magnesium thermometry techniques. We show that a weaker cold tongue–ITCZ front prevailed during the last glacial maximum (LGM), which indicates a more southerly ITCZ at that time. We further show that the Holocene history of sea surface temperature (SST) near the Galapagos Islands is consistent with progressive southward migration of the ITCZ during the last ~7,000 years, in accord with records from South America and the tropical Atlantic. In the more recent past, evidence from eastern Pacific corals supports a northward ITCZ shift since the end of the Little Ice Age (LIA), in agreement with the hydrologic record of the nearby Cariaco Basin. Collectively, the evidence points to coherent behavior of the Pacific, Atlantic, and South American ITCZs over a broad range of time scales. All regions have responded to Northern Hemisphere cooling by southward (equatorward) ITCZ displacements. In the Pacific, such displacements are likely to have been unfavorable to divergent upwelling at the equator, resulting in weaker zonal and meridional SST gradients and more uniform equatorial SSTs, analogous to modern El Niño conditions.
The intertropical convergence zone is a near-equatorial band of intense rainfall and convection. Over the modern Atlantic Ocean, its annual average position is approximately 5N, and it is associated with low sea surface salinity and high surface temperatures. This average position has varied since the Last Glacial Maximum, in response to changing climate boundary conditions. The nature of this variation is less clear, with suggestions that the intertropical convergence zone migrated north-south away from the colder hemisphere or that it contracted and expanded symmetrically around its present position. Here we use paired Mg/Ca and δ 18 O measurements of planktonic foraminifera for a transect of ocean sediment cores to reconstruct past changes in tropical surface ocean temperature and salinity in the Atlantic Ocean over the past 25,000 years. We show that the low-salinity, high-temperature surface waters associated with the intertropical convergence zone migrated southward of their present position during the Last Glacial Maximum, when the Northern Hemisphere cooled, and northward during the warmer early Holocene, by about ±7of latitude. Our evidence suggests that the intertropical convergence zone moved latitudinally over the ocean, rather than expanding or contracting. We conclude that the marine intertropical convergence zone has migrated significantly away from its present position owing to external climate forcing during the past 25,000 years.
Climate simulations, using models with different levels of complexity,
indicate that the north-south position of the intertropical convergence
zone (ITCZ) responds to changes in interhemispheric temperature
contrast. Paleoclimate data on a variety of timescales suggest a similar
behavior, with southward displacements of the ITCZ and associated
changes in tropical atmospheric circulation during cold periods in the
Northern Hemisphere. To identify a mechanism by which ITCZ displacements
can be forced from the extratropics, we use a climate model with
idealized geography and a simple slab ocean. We cool the northern
extratropics and warm the southern extratropics to represent the
asymmetric temperature changes associated with glacial-interglacial and
millennial-scale climate variability. A southward shift in the ITCZ
occurs, along with changes in the trade winds and an asymmetric response
of the Hadley circulation. Changes in atmospheric heat exchange between
the tropics and midlatitudes are the likely cause of this response,
suggesting that this mechanism may play an important role in ITCZ
displacements on timescales from decadal to glacial-interglacial.
Compositional variations of Central Andean subduction-related igneous rocks reflect the plate-tectonic evolution of this active continental margin through time and space. In order to address the effect on magmatism of changing subduction geometry and crustal evolution of the upper continental plate during the Andean orogeny, we compiled more than 1500 major- and trace-element data points, and 650 Sr-, 610 Nd-, and 570 Pb-isotopic analyses of Mesozoic-Cenozoic (190-0 Ma) magmatic rocks in southern Peru and northern Chile (Central Andean orocline), mostly from new data and the literature. This data set documents compositional variations of magmas since Jurassic time, with a focus on the Neogene period, when major crustal thickening developed and its influence on magma composition was most pronounced. We relate the observed variations in Sr/Y, La/Yb, La/Sm, Sm/Yb, and Dy/Yb ratios, as well as in Sr-, Nd-, and Pb-isotopic ratios, to the crustal structure and evolution of the Central Andean orocline. In particular, the evolution of Dy/Yb and Sm/Yb ratios, which track the presence of the higher-pressure minerals amphibole and garnet, respectively, in the lower crust, documents that crustal thickness has grown through time. Spatial variations in trace elements and isotopic ratios further suggest that crustal domains of distinct composition and age have influenced magma composition through some assimilation. The crustal input in Quaternary magmas is quantified to have been between 7% and 18% by simple two-components mixing. When comparing our geochemical data set to the geological record of uplift and crustal thickening, we observe a correlation between the composition of magmatic rocks and the progression of Andean orogeny. In particular, our results support the interpretation that major crustal thickening and uplift were initiated in the mid-Oligocene (30 Ma) and that crustal thickness has kept increasing until present day. Our data do not support delamination as a general cause for major late Miocene uplift in the Central Andes and instead favor continued crustal thickening.
Present-day ratios of Pb isotopes (324 published samples, 435 new) and Nd-Sr isotopes (150 published, 180 new) on Proterozoic to Holocene igneous, metamorphic, and sedimentary rocks define (at high spatial resolution) distinct isotopic domains of the crust in the central Andes. These domains correlate with the internal compositional structure of the crust as revealed by a three-dimensional density model. Pb-Nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect Proterozoic mafic-dominated and Paleozoic felsic-dominated crustal lithologies. Age and composition (mafic versus felsic) of these domains have controlled the rheology of the Andean crust, have influenced crustal deformation patterns, and correlate with the central Andean plateau segmentation.
Although the distribution of sunshine is symmetrical about the equator, the earth's climate is not. Climatic asymmetries are prominent in the eastern tropical Pacific and Atlantic Oceans where the regions of maximum sea surface temperature, convective cloud cover, and rainfall are north of the equator. This is the result of two sets of factors: interactions between the ocean and atmosphere that are capable of converting symmetry into asymmetry, and the geometries of the continents that determine in which longitudes the interactions are effective and in which hemisphere the warmest waters and the intertropical convergence zone are located. 'The Ocean-atmosphere interactions are most effective where the thermocline is shallow because the winds can readily affect sea surface temperatures in such regions. The thermocline happens to shoal in the eastern equatorial Pacific and Atlantic, but not in the eastern Indian Ocean, because easterly trade winds prevail over the tropical Atlantic and Pacific whereas monsoons, with a far larger meridional component are dominant over the Indian Ocean. That is how the global distribution of the continents, by determining the large-scale wind patterns, causes climatic asymmetries to be prominent in some bands of longitude but not others. The explanation for asymmetries that favor the Northern rather than Southern Hemisphere with the warmest waters and the ITCZ- involves the details of the local coastal geometries: the bulge of western Africa to the north of the Gulf of Guinea and the slope of the western coast of the Americas relative to meridians. low-level stratus clouds over cold waters are crucial to the maintenance of the asymmetries.
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.
The Global Precipitation Climatology Project (GPCP) Version-2 Monthly Precipitation Analysis is described. This globally complete, monthly analysis of surface precipitation at 2.5 8 latitude 3 2.58 longitude resolution is available from January 1979 to the present. It is a merged analysis that incorporates precipitation estimates from low-orbit satellite microwave data, geosynchronous-orbit satellite infrared data, and surface rain gauge obser- vations. The merging approach utilizes the higher accuracy of the low-orbit microwave observations to calibrate, or adjust, the more frequent geosynchronous infrared observations. The dataset is extended back into the prem- icrowave era (before mid-1987) by using infrared-only observations calibrated to the microwave-based analysis of the later years. The combined satellite-based product is adjusted by the rain gauge analysis. The dataset archive also contains the individual input fields, a combined satellite estimate, and error estimates for each field. This monthly analysis is the foundation for the GPCP suite of products, including those at finer temporal resolution. The 23-yr GPCP climatology is characterized, along with time and space variations of precipitation.
The tropics are the major source of heat and moisture for the Australasian region. Determining the tropics' response over time to changes in climate forcing mechanisms, such as summer insolation, and the effects of relative sea level on exposed continental shelves during the Last Glacial period, is an ongoing process of re-evaluation. We present a synthesis of climate proxy data from tropical Australasia spanning the last 30,000 years that incorporates deep sea core, coral, speleothem, pollen, charcoal and terrestrial sedimentary records.Today, seasonal variability is governed largely by the annual migration of the inter-tropical convergence zone (ITCZ), influencing this region most strongly during the austral summer. However, the position of the ITCZ has varied through time. Towards the end of Marine Isotope Stage (MIS) 3, conditions were far wetter throughout the region, becoming drier first in the south. Universally cooler land and sea-surface temperature (SST) were characteristic of the Last Glacial Maximum, with drier conditions than previously, although episodic wet periods are noted in the fluvial records of northern Australia. The deglacial period saw warming first in the Coral Sea and then the Indonesian seas, with a pause in this trend around the time of the Antarctic Cold Reversal (c. 14.5ka), coincident with the flooding of the Sunda Shelf. Wetter conditions occurred first in Indonesia around 17ka and northern Australia after 14ka. The early Holocene saw a peak in marine SST to the northwest and northeast of Australia. Modern vegetation was first established on Indonesia, then progressively south and eastward to NE Australia. Flores and the Atherton Tablelands show a dry period around 11.6ka, steadily becoming wetter through the early Holocene. The mid-late Holocene was punctuated by millennial-scale variability, associated with the El Niño-Southern Oscillation; this is evident in the marine, coral, speleothem and pollen records of the region.
Recent studies suggest the existence of a global atmospheric teleconnection of extratropical cooling to the tropical rainfall climate, mediated through the development of a thermal contrast between the hemispheres—an interhemispheric thermal gradient. This teleconnection has been largely motivated by studies that show a global synchronization of rapid climate change during abrupt climate changes of the last glacial period, in addition to attribution studies of twentieth-century Sahel drought and studies that examined the climate impacts of anthropogenic aerosols. This research has led to interesting developments in atmospheric dynamics of the underlying mechanisms and in applications toward understanding past and present tropical climate change. The emerging teleconnection hypothesis promises to offer new insights into understanding future patterns of tropical rainfall changes due to interhemispheric thermal gradients from greenhouse warming, aerosols, and land-use change.
The Ekman divergence around Antarctica raises a large amount of deep water to the ocean's surface. The regional Ekman transport moves the upwelled deep water northward out of the circumpolar zone. The divergence and northward surface drift combine, in effect, to remove deep water from the interior of the ocean. This wind-driven removal process is facilitated by a unique dynamic constraint operating in the latitude band containing Drake Passage. Through a simple model sensitivity experiment we show that the upwelling and removal of deep water in the circumpolar belt may be quantitatively related to the formation of new deep water in the northern North Atlantic. These results show that stronger winds in the south can induce more deep water formation in the north and more deep outflow through the South Atlantic. The fact that winds in the southern hemisphere might influence the formation of deep water in the North Atlantic brings into question long-standing notions about the forces that drive the ocean's thermohaline circulation.
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.
We investigate the causes for a strong high latitude imposed ice (land or sea) influence on the marine Intertropical Convergence
Zone (ITCZ) in the Community Climate Model version 3 coupled to a 50-m slab ocean. The marine ITCZ in all the ocean basins
shift meridionally away from the hemisphere with an imposed added ice cover, altering the global Hadley circulation with an
increased tropical subsidence in the hemisphere with imposed ice and uplift in the other. The effect appears to be independent
of the longitudinal position of imposed ice. The anomalous ice induces a rapid cooling and drying of the air and surface over
the entire high- and midlatitudes; subsequent progression of cold anomalies occurs in the Pacific and Atlantic northeasterly
trade regions, where a wind-evaporation-sea surface temperature (SST) feedback initiates progression of a cold SST ‘front’
towards the ITCZ latitudes. Once the cooler SST reaches the ITCZ latitude, the ITCZ shifts southwards, aided by positive feedbacks
associated with the displacement. The ITCZ displacement transports moisture away from the colder and drier hemisphere into
the other hemisphere, resulting in a pronounced hemispheric asymmetric response in anomalous specific humidity; we speculate
that the atmospheric humidity plays a central role in the hemispheric asymmetric nature of the climate response to high latitude
ice cover anomalies. From an energy balance viewpoint, the increased outgoing radiative flux at the latitudes of the imposed
ice is compensated by an increased radiative energy flux at the tropical latitudes occupied by the displaced ITCZ, and subsequently
transported by the altered Hadley and eddy circulations to the imposed ice latitudes. The situation investigated here may
be applicable to past climates like the Last Glacial Maximum where hemispheric asymmetric changes to ice cover occurred. Major
caveats to the conclusions drawn include omission of interactive sea ice physics and ocean dynamical feedback and sensitivity
to atmospheric physics parameterizations across different models.
Antarctic ice preserves an ~800 ky record of dust activity in the southern hemisphere. Major efforts have been dedicated to elucidate the origin of this material in order to gain greater insight into the atmospheric dust cycle. On the basis of Pb isotopes in Antarctic dust samples and potential sources, this contribution demonstrates for the first time that Patagonia is the main contributor of dust to Antarctica during interglacial periods as well as glacials; although the potential importance of Tierra del Fuego remains unclear because of its geochemical similarities to Patagonia. An important new finding is that the Puna-Altiplano sector of the continent is a second important dust source to eastern Antarctica during both glacials and interglacials, being more prominent during interglacials. The new findings are not consistent with Australia as an interglacial dust contributor to the Antarctic continent.
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.
Oxygen isotope records of five stalagmites from Hulu Cave near Nanjing bear a remarkable resemblance to oxygen isotope records from Greenland ice cores, suggesting that East Asian Monsoon intensity changed in concert with Greenland temperature between 11,000 and 75,000 years before the present (yr. B.P.). Between 11,000 and 30,000 yr. B.P., the timing of changes in the monsoon, as established with 230Th dates, generally agrees with the timing of temperature changes from the Greenland Ice Sheet Project Two (GISP2) core, which supports GISP2's chronology in this interval. Our record links North Atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer East Asian Monsoon originates.
Material sources of the northern deserts and the Loess Plateau in China are always one of the critical focuses in Quaternary and paleoclimate fields. In this paper, the method of Sr-Nd isotope geochemistry is applied to explore the relationship in material source between the Erdos desert and Chinese Loess Plateau. The εNd(0) value of the <75 μm silicate fraction between -11.8 and -17.2 is more positive in the west than in the east, and its 87Sr/86Sr ratio varies from 0.719218 to 0.714824 without similar characteristics in the Erdos desert. In addition, there are Sr-Nd isotopic differences between different grain-size fractions of eolian sand in the Erdos desert. The coarse-grained fractions mainly originated from local parent rock weathering, but the fine-grained fractions were probably affected by the input of foreign materials. Geographical distribution of Sr-Nd isotopes (especially Nd isotopes) of <75 μm silicate fractions indicates that the input of foreign materials into the Erdos desert decreases gradually from its west to east. The εNd(0) and 87Sr/86Sr values of eolian silicate fractions in the Chinese Loess Plateau vary from -9.2 to -13.56 and from 0.719957 to 0.714424, respectively in this study. The εNd(0) and 87Sr/86Sr values of eolian silicate fractions are different between the western-central and eastern Loess Plateau. According to Sr-Nd isotopic data from previous and present studies, the εNd(0) values of eolian silicate fractions in the western-central Loess Plateau mostly fall in a very narrow range of -9∼-11.5, suggesting a uniform source region. However, there is a large Sr isotope variation resulting from different pretreating methods employed by researchers. The εNd(0) and 87Sr/86Sr values of eolian silicate fractions in the eastern Loess Plateau both change markedly. The differences in Sr-Nd isotopes of eolian silicate fractions between the eastern and western-central Loess Plateau indicate that there are different sources for eolian silicate materials in the two sub-areas. Sr-Nd isotopes, especially Nd isotopes of the <75 μm silicate fractions in the Erdos desert are clearly different from those of eolian silicate fractions in the western-central part, but roughly close to those of eolian silicate fractions in the eastern part of the Chinese Loess Plateau, which shows that the Erdos desert was perhaps a main source for the eastern Loess Plateau but not for the western-central Loess Plateau.
Deciphering the sources of eolian dust on the Chinese Loess Plateau (CLP) is fundamental to reconstruct paleo-wind patterns and paleo-environmental changes. Existing datasets show contradictory source evolutions of eolian dust on the CLP, both on orbital and tectonic timescales. Here, the silicate Sr and Nd isotopic compositions of a restricted grain size fraction (28–45 μ m) were measured to trace the source evolution of the CLP since ~2.7 Ma. Our results revealed an unchanged source on orbital timescales but a gradual source shift from the Qilian Mountains to the Gobi Altay Mountains during the past 2.7 Ma. Both tectonic uplift and climate change may have played important roles for this shift. The later uplift of the Gobi Altay Mountains relative to the Qilian Mountains since 5 ± 3 Ma might be responsible for the increasing contribution of Gobi materials to the source deserts in Alxa arid lands. Enhanced winter monsoon may also facilitate transportation of Gobi materials from the Alxa arid lands to the CLP. The shifting source of Asian dust was also reflected in north Pacific sediments. The finding of this shifting source calls for caution when interpreting the long-term climate changes based on the source-sensitive proxies of the eolian deposits.
The mechanisms underlying alterations in brain functions in response to physical exercise are not fully understood. The present study examined the central effect of irisin, a 112 amino acid polypeptide hormone secreted from the skeletal muscle after exercise, on the locomotion in rats. Central administration of irisin significantly increased the locomotion. Relative to control animals treated with IgG Fc peptide, rats receiving irisin demonstrated a marked increase in total travel distance, ambulatory counts and time, and vertical counts and time. These changes were associated with a significant decrease in resting time. Central treatment of irisin also induced significant increases in oxygen consumption, carbon dioxide production and heat production, indicating an increase in metabolic activity. Our study suggests that physical activity may signal to the central nervous system to coordinate locomotion with metabolic activity via irisin.
Copyright © 2015. Published by Elsevier Ireland Ltd.
Much uncertainty exists about the state of the oceanic and atmospheric circulation in the Tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the Western and Eastern parts of the Tropical Pacific, with little information from the Central Tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the Central Tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8 degrees north is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the Central Tropical Pacific).
Rainfall on Earth is most intense in the intertropical convergence zone (ITCZ), a narrow belt of clouds centred on average around six degrees north of the Equator. The mean position of the ITCZ north of the Equator arises primarily because the Atlantic Ocean transports energy northward across the Equator, rendering the Northern Hemisphere warmer than the Southern Hemisphere. On seasonal and longer timescales, the ITCZ migrates, typically towards a warming hemisphere but with exceptions, such as during El Niño events. An emerging framework links the ITCZ to the atmospheric energy balance and may account for ITCZ variations on timescales from years to geological epochs.
Tropical paleoclimate records provide important insights into the response of precipitation patterns and the Hadley circulation to past climate changes. Paleo-records are commonly interpreted as indicating north–south shifts of the Intertropical Convergence Zone (ITCZ), with the ITCZ’s mean position moving toward the warmer hemisphere in response to changes in cross-equatorial temperature gradients. Though a number of records in tropical Central and South America, North Africa, Asia and the Indo-Australian region are consistent with this interpretation, the magnitudes and regional variability of past ITCZ shifts are poorly constrained. Combining estimates of past tropical sea surface temperature (SST) gradients with the strong linear relationship observed between zonally averaged ITCZ position and tropical SST gradients in the modern seasonal cycle and in models of past climates, we quantify past shifts in zonally averaged ITCZ position. We find that mean ITCZ shifts are likely less than 1◦ latitude during the Last Glacial Maximum (LGM), Heinrich Stadial 1 (HS1) and mid-Holocene (6 ka) climates, with the largest shift during HS1. The ITCZ’s position is closely tied to heat transport between the hemispheres by the atmosphere and ocean; accordingly, these small mean ITCZ shifts are associated with relatively large (∼0.1–0.4 PW) changes in cross-equatorial atmospheric heat transport (AHTEQ). These AHTEQ changes point to changes in cross-equatorial ocean heat transport or net radiative fluxes of the opposite sign. During HS1, the increase in northward AHTEQ is large enough to compensate for a partial or total shutdown in northward heat transport by the Atlantic Ocean’s meridional overturning circulation. The large AHTEQ response for small changes in mean ITCZ position places limits on the magnitude of past shifts in the globally averaged ITCZ. Large (>=5◦) meridional displacements of the ITCZ inferred from regional compilations of proxy records must be limited in their zonal extent, and ITCZ shifts at other longitudes must be near zero, for the global mean shift to remain <=1◦ as suggested by our results. Our examination of model results and modern observations supports variable regional and seasonal changes in ITCZ precipitation. This work thus highlights the importance of a dense network of tropical precipitation reconstructions to document the regional and seasonal heterogeneity of ITCZ responses to past climate changes.
Recent studies with climate models have demonstrated the power of extratropical forcing in causing the intertropical convergence zone (ITCZ) to shift northward or southward, and paleoclimate data support the notion that there have been large shifts in the ITCZ over time. It is shown that similar notions apply to slab ocean simulations of global warming. Nine slab ocean model simulations from different modeling centers show a wide range of ITCZ shifts in response to doubling carbon dioxide concentrations, which are experienced in a rather zonally symmetric way in the tropics. Using an attribution strategy based on fundamental energetic constraints, it is shown that responses of clouds and ice in the extratropics explain much of the range of ITCZ responses. There are also some positive feedbacks within the tropics due to increasing water vapor content and high clouds in the new ITCZ location, which amplify the changes driven from the extratropics. This study shows the clear importance of simulating extratropical climate responses with fidelity, because in addition to their local importance, the impacts of these climate responses have a large nonlocal impact on rainfall in the tropics.
The authors quantify the relationship between the location of the intertropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHT[subscript EQ]) in climate models and in observations. The observed zonal mean ITCZ location varies from 5.3°S in the boreal winter to 7.2°N in the boreal summer with an annual mean position of 1.65°N while the AHT[subscript EQ] varies from 2.1 PW northward in the boreal winter to 2.3 PW southward in the boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the ITCZ location and AHT[subscript EQ] are highly anticorrelated in the observations and in a suite of state-of-the-art coupled climate models with regression coefficients of −2.7° and −2.4° PW[superscript −1] respectively. It is also found that seasonal variations in ITCZ location and AHT[subscript EQ] are well correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer depths. However, the regression coefficient between ITCZ location and AHT[subscript EQ] decreases with decreasing mixed layer depth as a consequence of the asymmetry that develops between the winter and summer Hadley cells as the ITCZ moves farther off the equator.
The authors go on to analyze the annual mean change in ITCZ location and AHT[subscript EQ] in an ensemble of climate perturbation experiments including the response to CO[subscript 2] doubling, simulations of the Last Glacial Maximum, and simulations of the mid-Holocene. The shift in the annual average ITCZ location is also strongly anticorrelated with the change in annual mean AHT[subscript EQ] with a regression coefficient of −3.2° PW[superscript −1], similar to that found over the seasonal cycle.
The SiO{sub 2}-undersaturated lavas from Lihir island, Papua New Guinea, like most arc lavas are highly enriched in Sr, Ba, K, Rb, and Cc and depleted in Hf, Ta, Nb, and Ti relative to ocean floor basalts and oceanic island basalts. These alkali-rich lavas have arc trace element signatures and Nd, Sr, and Pb isotopic systematics. However, they are not a product of present-day subduction, as this volcanism has tapped mantle which was enriched by prior subduction episodes. The narrow range of Pb isotopic compositions suggest a cogenetic origin for these lavas. During the fractionation of the primitive Lihir lavas, elements normally considered incompatible (i.e., the light rare earth elements (LREE), Rb, Th, and P) have high bulk solid/melt partition coefficients (0.15-1.5). Relatively higher partition coefficients during formation of the evolved lavas produced crossing rare earth element (REE) patterns, and primitive lavas have higher incompatible elements abundances than evolved lavas. The Lihir lavas have lower alkali, Sr, Ba, K, Rb, Cs, and LREE abundances than other Tabar-Feni lavas. They are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands. The similarity of Sm/Nd ratios of these undersaturated arc lavas to those of tholeiitic and calc-alkaline arc lavas and the moderate chondrite-normalized La/Yb (la/Yb{sub cn} = 3-7) indicates that there has been limited enrichment of the LREE relative to the heavy REE during generation of the arc-modified source mantle. The alkaline nature of these lavas reflects their generation, in a tensional tectonic environment, from a fossil arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes.
The provenance of eolian dust supplied to deep-sea sediments has the potential to offer insights into changes in past atmospheric circulation. Specifically, measuring temporal changes in dust provenance can shed light on changes in the mean position of the Intertropical Convergence Zone (ITCZ), a region acting as a barrier separating wind-blown material derived from northern versus southern hemisphere sources. Here we have analyzed Nd, Sr, and Pb isotope ratios in the operationally-defined detrital component extracted from deep-sea sediments in the eastern equatorial Pacific (EEP) along a meridional transect at 110°W from 3°S to 7°N (ODP Leg 138, sites 848–853).
The Manus Basin to the northeast of Papua New Guinea is an actively spreading/rifting back-arc basin in the Bismarck Sea located between the inactive Manus-Kilinailau trench on the Pacific-plate side and the active New Britain trench on the Solomon-plate side. Spreading/rifting in the Manus Basin takes place in the last 0.78Myr or so. We present major and trace elements, and Sr–Nd–Pb isotope compositions of rock samples taken from the South East Rift (SER) at the eastern end of the Manus Basin. The strong enrichment of Pb and LILE (large ion lithophile elements) relative to HFSE (high field strength elements) and REE (rare earth elements) in the SER lava is also quite similar to other island arc lavas, suggesting that substantial amount of subduction components were present in its source mantle. To investigate the origin of the subduction components in SER lavas, we compare the geochemical data of SER lavas to published data from New Britain Arc (NBA) and Tabar–Lihir–Tanga–Feni (TLTF) island chain. The volcanism in NBA is related to presently active subduction of the Solomon slab, whereas the TLTF volcanism is located in the forearc area of New Ireland arc which was formed during a former subduction of the Pacific slab. In other words, the NBA and TLTF lavas were influenced by subduction components from the present and former subduction, respectively. We argue that the subduction components in SER lava were incorporated in the mantle lithosphere during the active arc volcanism on New Ireland because the amount of the subduction component in SER decreases with increasing in distance from New Ireland. On the other hand, no relationships are found with respect to New Britain. The Sr–Nd–Pb isotopes indicate that SER lavas contain little sediment component and less amount of fluid component derived from altered oceanic crust compared to the TLTF lavas. This is probably due to the fact that SER is located in backarc settings in contrast to TLTF which is located in forearc setting with respect to the Pacific slab. Thus it is likely that the sediment was removed from the slab in the forearc and/or arc areas, and therefore little or none was introduced in the backarc mantle, which is the source region for SER magmas at present. Fluid derived from altered oceanic crust also may have made its way into the sub-forearc region more effectively than backare region by shallow dehydration process.
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.
Strontium and neodymium isotopic data are reported for barite samples chemically separated from Late Miocene to Pliocene sediments from the eastern equatorial Pacific. At a site within a region of very high productivity close to the equator, 87Sr/86Sr ratios in the barite separates are indistinguishable from those of foraminifera and fish teeth from the same samples. However, at two sites north of the productivity maximum barite separates have slightly, but consistently lower (averaging 62 × 10−6) ratios than the coexisting phases, although values still fall within the total range of published values for the contemporaneous seawater strontium isotope curve. We examine possible causes for this offset including recrystallization of the foraminifera, fish teeth or barite, the presence of non-barite contaminants, or incorporation of older, reworked deep-sea barite; the inclusion of a small amount of hydrothermal barite in the sediments seems most consistent with our data, although there are difficulties associated with adequate production and transportation of this phase. Barite is unlikely to replace calcite as a preferred tracer of seawater strontium isotopes in carbonate-rich sediments, but may prove a useful substitute in cases where calcite is rare or strongly affected by diagenesis. In contrast to the case for strontium, neodymium isotopic ratios in the barite separates are far from expected values for contemporary seawater, and appear to be dominated by an (unobserved) eolian component with high neodymium concentration and low 143Nd/144Nd. These results suggest that the true potential of barite as an indicator of paleocean neodymium isotopic ratios and REE patterns will be realized only when a more selective separation procedure is developed.
The geostrophic balance of the mean Pacific Equatorial Undercurrent (EUC) has been demonstrated by using current and density measurements obtained during the 16-month NORPAX Hawaii-to-Tahiti Shuttle Experiment (February 1979 to June 1980). The computed core depth, maximum speed, and boundaries of the EUC agree remarkably with those measured by profiling current meters.
Editor: P. DeMenocal Keywords: eolian dust provenance Loess Plateau glacial history geochemistry The Pb isotopes and trace element compositions of the silicate fraction of airborne dust from the Chinese Loess Plateau were analyzed to infer provenance change during the past 8 Ma. The results indicate that the composition of eolian dust changed at 2.6 Ma, coincident with initiation of the Northern Hemisphere ice sheet. The change in trace element composition at 2.6 Ma indicates that a larger component of the eolian dust was derived from felsic rocks after this time. Pb isotopic evidence demonstrates that the source material of Tertiary Red Clay differs to some extent from that of Quaternary eolian deposits. Although chemical weathering may result in compositional changes in eolian deposits, such a scenario is not supported by the present evidence. Given that glacial grinding and frost-weathering processes have been active in the peaks of high mountains during the Quaternary, eolian dust of this age contains a large proportion of material derived from areas of high topographic relief and relatively little material from low-lying cratonic regions. Such alpine processes played an important role in controlling the distinct changes in Pb isotopes and trance element concentrations recorded at around 2.6 Ma. © 2010 Published by Elsevier B.V.
A suite of altered climate experiments for the Paleocene, the last glacial maximum (LGM), and a 2 x CO2 climate were compared
to assess the factors responsible for producing variations in Hadley cell intensity and extent. The climate simulations used
best-guess topography and marine surface fields, as well as feasible alternative sea surface temperature (SST) patterns. The
individual contributions to the circulations were quantified, and compared among the different simulations. The results show
that the Hadley cell intensity is associated with the gradient in latent heat release from the tropics to the subtropics,
driven in the model by the gradient in sea surface temperature. It is not related to the absolute warmth of the climate, or
of the tropical sea surface temperatures. Eddy forcing, primarily through transient eddy heat transport, amplified the subtropical
portion of the cell, as well as the mid-latitude Ferrel cell. The poleward extent of the Hadley cell is affected by numerous
processes, including the influence of topography in the extratropics. It also does not vary systematically with global mean
temperature. Only the strongest Hadley cell changes are longitudinally homogeneous; there is little relationship between the
change in Hadley cell intensity and the change in strength of the Walker cell, and the Pacific Ocean is the most important
basin for the zonal average Hadley cell response. Although the latitudinal average precipitation does respond interactively
with Hadley cell intensity and extent, the soil moisture variations are less correlated, due to differing seasonal effects
and the influence of temperature/evaporation changes. The importance of the Hadley cell variations for assessing past and
future water availability changes should not be overestimated, although it is a contributing factor.
The zonal climate pattern associated with the Hadley cell circulation is best exemplified in northern Africa, with its Mediterranean northern tip, subtropical Sahara desert, and belts of monsoonal and equatorial climates related to the seasonal migration of the Intertropical Convergence Zone (ITCZ). In the past, astronomical forcing has been the prime factor driving the meridional shifts of these climate belts, but feedback processes from oceans and land surfaces have amplified and modified the direct effects of insolation changes.
A comparison is made of the Hadley cell response to altered external forcing and climatic boundary conditions in two different
climate models. The experiments performed cover past and future climatic forcings that range from the last glacial maximum
(LGM) to a doubling in atmospheric CO2. Both models have a consistent response to all forcings in the depth and meridional
extent of the Hadley cell in both solstitial seasons. The strength of the December–January– February (DJF) cell varies consistently
between the two models, increasing in colder climates and weakening in warmer climates. For June–July–August (JJA), however,
the sign of the response is opposite between models for the range of forcing. It is suggested that the origin of this different
response in the JJA cell is related to the type of ocean model used. Nevertheless, the results from the LGM and doubled CO2
experiments show a consistent correlation between the surface temperature gradient between the winter and summer hemisphere
tropics and Hadley cell strength: When the temperature contrast is larger (smaller), the Hadley cell is stronger (weaker).
The Holocene, with the response to the forcing peaking over the subtropical and mid-latitude continents, has temperature gradients
and Hadley cell responses that deviate from this relationship. The implications of these results to paleoclimate studies are
discussed.
Continental dust transported through the atmosphere can be a tracer of atmospheric circulation and transport, as well as being a source of information about climate in the dust's source area. This information about the provenance of dust transported from desert areas and deposited elsewhere on the oceans and continents can be evaluated using several observational techniques. Satellite imagery has been used to this end in recent decades, and air-mass back trajectories have been calculated to trace dust deposits back to their source area. Another method consists of analysis of the concentration and composition of certain tracers in the dust itself, and comparing them to soils and sediments from possible source areas. This method has the advantage of being useable for paleo-atmospheric circulation and paleo-dust transport studies, whereas satellite imagery and air-mass back trajectories are restricted to relatively recent times. We review here different kinds of isotopic tracers that have been used to trace dust provenance, but special attention is focused on the use of Sr, Nd and Pb isotopes. We evaluate their use in studies of both spatially as well as temporally varying continental dust source and transport.
Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs vs. Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from sediment along a cross-equatorial transect in the central equatorial Pacific at 140°W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash, Papua New Guinea, and ocean island basalt) varies through time, latitude, and climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no connection between a particular Fe source or pathway (eolian vs. Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene “Biogenic Bloom”. This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, may be responsible for past changes in total export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes. Additionally, perhaps a change in Fe source or flux is related to a change in a particular component of the total productivity (e.g., the production of organic matter, calcium carbonate, or biogenic opal).
143Nd/144Nd and 87Sr/86Sr ratios of the acid-insoluble residues of the red clay and overlying loess–paleosols from the Lingtai profile of the Loess Plateau, China, were investigated. The results show that variations of the 87Sr/86Sr ratios of the Lingtai profile can be divided into two stages. From ∼ 7 Ma B.P. to 2.5 Ma B.P., the acid-insoluble residues in the Red Clay (RC) Formation, are characterized by higher 87Sr/86Sr ratios with an average of 0.7230. From 2.5 Ma B.P. to the present, the acid-insoluble residues in the Wuchen Loess (WL4-WS1), Lishi Loess (L15-S1), Malan Loess (L1) and Holocene Loess (S0) have relatively lower 87Sr/86Sr ratios and display an overall descending trend from 0.7223 at ∼ 2.5 Ma B.P. to 0.7182 at the present. Among three possible interpretations for the variations of 87Sr/86Sr ratios of acid-insoluble residues of the loess (provenance change, chemical weathering change, or grain-size distribution change), only grain-size distribution change provides a satisfactory interpretation. This implies that the East Asian winter monsoon strength was weak and relatively stable from ∼ 7 Ma B.P. to ∼ 2.5 Ma B.P., but became continuously enhanced from ∼ 2.5 Ma B.P. to the present. All the red clay and the overlying loess-paleosols in the Lingtai profile have generally identical εNd (0), implying that the source regions of these Eolian deposits of the Chinese Loess Plateau may be relatively stable and has not changed since 7 Ma B.P.