Peter J. Müller

Universität Bremen, Bremen, Bremen, Germany

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Publications (33)163.86 Total impact

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    ABSTRACT: We present well-dated high-resolution Holocene records of sea-temperature (SST) and upwelling intensity off northwest (NW) Africa. We identify long-term cooling trends over the Holocene in the subtropical North Atlantic in response to boreal summer insolation. A pronounced cooling event of ∼1 °C ca. 8.5 cal ka indicates a large-scale reorganization of the ocean current system possibly induced by meltwater from the northern North Atlantic. Our alkenone SST record off Cape Ghir provides strong evidence for the impact of ocean circulation changes on subtropical North Atlantic SSTs. It is likely that cold waters were propagated to the subtropics via the Canary Current in a way similar to Heinrich events and the Younger Dryas off Cape Blanc. We find 2-3 k.y. periodic variations in SST and upwelling intensity off NW Africa superimposed on the cooling trend. Such a cycle has been documented in various paleoclimate archives in phase with solar forcing. We show that these variations on millennial time scales are linked to the North Atlantic subtropical gyre circulation and the Northern Hemisphere atmospheric circulation, and in particular to changes in the pressure gradient between the Icelandic Low and the Azores High. This suggests that oceanic circulation, in response to solar forcing, played a more important role in the generation of 2-3 k.y. cyclicity than has been previously considered.
    No preview · Article · May 2007 · Geology
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    ABSTRACT: Recent studies have revealed that lateral transport and focusing of particles strongly influences the depositional patterns of organic matter in marine sediments. Transport can occur in the water column prior to initial deposition or following sediment re-suspension. In both cases, fine-grained particles and organic-rich aggregates are more susceptible to lateral transport than coarse-grained particles (e.g., foraminiferal tests) because of the slower sinking velocities of the former. This may lead to spatial and, in the case of redistribution of re-suspended sediments, temporal decoupling of organic matter from coarser sediment constituents. Prior studies from the Argentine Basin have yielded evidence that suspended particles are displaced significant distances (100–1000 km) northward and downslope by strong surface and/or bottom currents. These transport processes result in anomalously cold alkenone-derived sea-surface temperature (SST) estimates (up to 6 °C colder than measured SST) and in the presence of frustules of Antarctic diatom species in surface sediments from this area. Here we examine advective transport processes through combined measurements of compound-specific radiocarbon ages of marine phytoplankton-derived biomarkers (alkenones) from core tops and excess 230Th (230Thxs)-derived focusing factors for late Holocene sediments from the Argentine Basin. On the continental slope, we observe 230Thxs-based focusing factors of 1.4–3.2 at sites where alkenone-based SST estimates were 4–6 °C colder than measured values. In contrast, alkenone radiocarbon data suggest coeval deposition of marine biomarkers and planktic foraminifera, as alkenones in core tops were younger than, or similar in age to, foraminifera. We therefore infer that the transport processes leading to the lateral displacement of these sediment components are rapid, and hence probably occur in the upper water column (<1500 m).
    Full-text · Article · Jul 2006 · Deep Sea Research Part I Oceanographic Research Papers
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    ABSTRACT: The carbon isotopic fractionation (εp) of the C37:2 alkenone was analysed for 19 South Atlantic sediment samples from the Last Glacial Maximum (LGM). Our study covers the equatorial and subtropical ocean including the coastal upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western basins. The results were compared to the Holocene εp records from the respective core locations ( [2] and [8]). Generally, alkenone εp was lower during the LGM compared to the Holocene. Higher glacial εp values were only found in sediments from the Angola Basin and in one sample from the eastern crest of the Walvis Ridge. Considering present understanding of LGM–Holocene changes in surface-water conditions (i.e. nutrient level, primary productivity, phytoplankton assemblages), the observed glacial/interglacial difference in εp indicates that multiple factors controlled the isotopic fractionation in alkenone producing algae depending on the regional setting. In the oligotrophic areas of the South Atlantic the lower than Holocene glacial εp values can be partly explained with a decrease in surface-water PCO2 during the LGM. In contrast, the Holocene to LGM decrease in εp values in the coastal upwelling areas as well as in the eastern tropical Atlantic most probably reflects much higher glacial haptophyte growth rates induced by an increase in surface-water nutrient concentrations. The exceptional opposite trend of the εp differences in the Angola Basin can be explained by a shift in the phytoplankton community towards a greater dominance of diatoms under glacial conditions, thus leaving less nutrients available for haptophytes. In this way, the isotopic record of alkenones indicate lower haptophyte growth rates during the LGM although other palaeoceanographic proxies point to enhanced productivity and higher nutrient levels.
    No preview · Article · May 2005 · Palaeogeography Palaeoclimatology Palaeoecology
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    ABSTRACT: 1] We compared ocean atlas values of surface water [PO 4 3À ] and [CO 2 (aq)] against the carbon isotopic fractionation (e p) of alkenones obtained from surface sediments of the South Atlantic and the central Pacific (Pacific data are from Pagani et al. [2002]). We observed a positive correlation between e p and 1/[CO 2 (aq)], which is opposite of what would be expected if the concentration of CO 2 (aq) were the major factor controlling the carbon isotopic fractionation of C 37:2 alkenones. Instead, we found inverse relationships between e p and [PO 4 3À ] for the two ocean basins (for the Atlantic, e p = À4.6*[PO 4 3À ] + 15.1, R = 0.76; for the Pacific, e p = À4.1*[PO 4 3À ] + 13.7, R = 0.64), suggesting that e p is predominantly controlled by growth rate, which in turn is related to nutrient concentration. The similarity of the slopes implies that a general relationship between both parameters may exist. Using the relationship obtained from the South Atlantic, we estimated surface water nutrient concentrations for the past 200,000 years from a deep-sea sediment core recovered off Angola. Low e p values, indicating high nutrient concentrations, coincide with high contents of total organic carbon and C 37 alkenones, low surface water temperatures, and decreased bulk d 15 N values, suggesting an increased upwelling of nutrient-rich cool subsurface waters as the main cause for the observed e p decrease. INDEX TERMS: 1050 Geochemistry: Marine geochemistry (4835, 4850); 1040 Geochemistry: Isotopic composition/chemistry; 1615 Global Change: Biogeochemical processes (4805); 1055 Geochemistry: Organic geochemistry; 1635 Global Change: Oceans (4203); KEYWORDS: alkenones, carbon isotopic composition, paleonutrient proxy, biomarker Citation: Schulte, S., A. Benthien, P. J. Müller, and C. Rülemann (2004), Carbon isotopic fractionation (e p) of C 37 alkenones in deep-sea sediments: Its potential as a paleonutrient proxy, Paleoceanography, 19, PA1011, doi:10.1029/2002PA000811.
    Full-text · Article · Mar 2004 · Paleoceanography
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    ABSTRACT: U37K' sea surface temperature (SST) estimates for the Last Glacial Maximum (LGM) are discussed and compared to coupled general circulation model (GCM) results. We conclude that U37K' estimates and most numerical models agree in the magnitude of the large scale SST cooling features of the glacial ocean. There was a larger cooling in the northern than in the Southern Hemisphere. The eastern boundary currents and equatorial regions were also colder. There is a consensus between models and alkenone data for a tropical cooling of about -2°C, which is in disagreement with both a smaller cooling as suggested by the CLIMAP project reconstruction, or a more pronounced cooling as suggested from coral proxy records. The comprehensive HadCM3 simulation examined reproduces a puzzling feature of the U37K'-SST reconstruction, i.e., the warming of the glacial north Atlantic, which needs further investigation using numerical modelling and proxy data reconstruction.
    No preview · Article · Feb 2004 · Geophysical Research Letters
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    ABSTRACT: A compilation of 1118 surface sediment samples from the South Atlantic was used to map modern seafloor distribution of organic carbon content in this ocean basin. Using new data on Holocene sedimentation rates, we estimated the annual organic carbon accumulation in the pelagic realm (>3000 m water depth) to be approximately 1.8×1012 g C year−1. In the sediments underlying the divergence zone in the Eastern Equatorial Atlantic (EEA), only small amounts of organic carbon accumulate in spite of the high surface water productivity observed in that area. This implies that in the Eastern Equatorial Atlantic, organic carbon accumulation is strongly reduced by efficient degradation of organic matter prior to its burial.During the Last Glacial Maximum (LGM), accumulation of organic carbon was higher than during the mid-Holocene along the continental margins of Africa and South America (Brazil) as well as in the equatorial region. In the Eastern Equatorial Atlantic in particular, large relative differences between LGM and mid-Holocene accumulation rates are found. This is probably to a great extent due to better preservation of organic matter related to changes in bottom water circulation and not just a result of strongly enhanced export productivity during the glacial period. On average, a two- to three-fold increase in organic carbon accumulation during the LGM compared to mid-Holocene conditions can be deduced from our cores. However, for the deep-sea sediments this cannot be solely attributed to a glacial productivity increase, as changes in South Atlantic deep-water circulation seem to result in better organic carbon preservation during the LGM.
    No preview · Article · Feb 2004 · Global and Planetary Change
  • Jung-Hyun Kim · Ralph R. Schneider · Stefan Mulitza · Peter J. Müller
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    ABSTRACT: A prominent feature in the Southeast Atlantic is the Angola-Benguela Front (ABF), the convergence between warm tropical and cold subtropical upwelled waters. At present, the sea-surface temperature (SST) gradient across the ABF and its position are influenced by the strength of southeasterly (SE) trade winds. Here, we present a record of changes in the ABF SST gradient over the last 25 kyr. Variations in this SST contrast indicate that periods of strengthened SE trade-wind intensity occurred during the Last Glacial Maximum, the Younger Dryas, and the Mid to Late Holocene, while Heinrich Event 1, the early part of the Bølling-Allerød, and the Early Holocene were periods of weakened SE trade-winds.
    No preview · Article · Nov 2003 · Geophysical Research Letters
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    ABSTRACT: We present high-resolution paleoceanographic records of surface and deep water conditions within the northern Red Sea covering the last glacial maximum and termination I using alkenone paleothermometry, stable oxygen isotopes, and sediment compositional data. Paleoceanographic records in the restricted desert-surrounded northern Red Sea are strongly affected by the stepwise sea level rise and appear to record and amplify well-known millennial-scale climate events from the North Atlantic realm. During the last glacial maximum (LGM), sea surface temperatures were about 4°C cooler than the late Holocene. Pronounced coolings associated with Heinrich event 1(∼2°C below the LGM level) and the Younger Dryas imply strong atmospheric teleconnections to the North Atlantic. Owing to the restricted exchange with the Indian Ocean, Red Sea salinity is particularly sensitive to changes in global sea level. Paleosalinities exceeded 50 psu during the LGM. A pronounced freshening of the surface waters is associated with the meltwater peaks MWP1a and MWP1b owing to an increased surface-near inflow of "normal" saline water from the Indian Ocean. Vertical δ18O gradients are also increased during these phases, indicating stronger surface water stratification. The combined effect of deglacial changes in sea surface temperature and salinity on water column stratification initiated the formation of two sapropel layers, which were deposited under almost anoxic condition in a stagnant water body.
    Preview · Article · Jun 2003 · Paleoceanography
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    ABSTRACT: We infer variations in paleoproductivity and eolian input at ODP Site 1082 in the Walvis Basin from stable oxygen isotope compositions of the planktonic foraminifera Globorotalia inflata, total organic carbon mass accumulation rates (TOC MAR), and X-ray fluorescence analyses of Fe content. The most pronounced paleoclimatic changes correspond to the time at about 0.9 Ma, when glacial conditions in the northern hemisphere (NH) led to the onset of pronounced 100-kyr glacial–interglacial cycles. We used Fe intensity as a proxy for eolian terrigenous input, and TOC MAR as a paleoproductivity indicator. Paleoproductivity and eolian input show generally higher-amplitude variations of glacial–interglacial cyclicity from 1.5 to 0.58 Ma, indicating pronounced variations in upwelling-favorable winds in this area. At 0.58 Ma, paleoproductivity and eolian input shifted abruptly to lower-amplitude variations with a periodicity of 100 kyr while δ18O values show a trend toward more negative isotope values for the past 0.65 Myr. Especially during glacial periods, oxygen isotope values indicate increasingly warmer sea-surface temperatures toward the end of the Pleistocene. To evaluate the relative influences of NH glaciation and southern hemisphere (SH) insolation as potential forcing mechanisms for variations of eolian input and productivity in the northern Benguela system, we filtered our proxy records at orbital frequencies. The filtered records of Fe intensity and TOC MAR indicate a strong influence of the 100-kyr and 41-kyr frequency bands, supporting our assumption that strong ice buildup in the NH is the dominant trigger for climate changes on the continent and probably in trade-wind intensity. SH insolation and low-latitude precession-related insolation changes were important for paleoproductivity variations in the northern Benguela system, modifying the nutrient supply by southern ocean intermediate waters and the zonal direction of upwelling-inducing trades by the African monsoon system, respectively.
    No preview · Article · May 2003 · Palaeogeography Palaeoclimatology Palaeoecology
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    ABSTRACT: Modern sedimentary total organic carbon (TOC) content as a proxy for surface water export production was mapped on the shelf and on the upper continental slope of the Benguela upwelling system using 137 core tops. Shelf maxima in TOC can be correlated with maxima in surface water productivity. On the slope, high TOC contents are observed offshore from sites of strong modern upwelling. Estimates of modern TOC mass accumulation rates (MAR) show that approximately 85% of the total is accumulating on the shelf. TOC MAR were calculated, mapped, and budgeted for the Holocene and for the Last Glacial Maximum (LGM) using 19 sediment cores from the continental slope. During the LGM, centers of deposition and production have migrated offshore with respect to their Holocene positions. TOC accumulation on the continental slope was approximately 84% higher during the LGM than during the Holocene, possibly reflecting enhanced productivity. The TOC distribution patterns and sediment echo sounding data suggest that undercurrents strongly influence the sedimentation off Namibia. Winnowing and focusing result in great lateral heterogeneity of sedimentation rates and sediment properties. Individual cores therefore do not necessarily reflect general changes in export production. These results highlight the need for detailed regional studies based on a large number of sediment cores for highly heterogeneous high-productivity areas in order to derive general statements on total fluxes.
    Full-text · Article · Dec 2002 · Global Biogeochemical Cycles
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    Jung-Hyun Kim · Ralph R. Schneider · Peter J. Müller · Gerold Wefer
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    ABSTRACT: Sediment core GeoB 1023-5 from the eastern South Atlantic was investigated at high temporal resolution for variations of sea-surface temperature (SST) during the past 22 kyr, using the alkenone (UK′37) method. SSTs increased by 3.5°C from about 18°C during the Last Ice Age (21±2 cal kyr BP) to about 21.5°C at 14.5 cal kyr BP. This warming trend associated with the deglaciation phase was followed by a cooling event with lowest SSTs near 20°C, persisting for about 1000 years between 13 and 12 cal kyr BP. The SSTs then continued to increase to about 22.5°C at the Holocene climatic optimum at 7 cal kyr BP, and decreased again during the Late Holocene to a core-top value of 19.8°C that is comparable to modern annual mean SST values. When compared with alkenone SST records from the eastern North Atlantic, our SST record indicates continuous warming throughout the deglaciation phase in the Benguela Current, while its northern counterpart, the Canary Current, experienced prominent cooling during ‘Heinrich Event 1’ (H1). On the other hand, for the time period corresponding to the ‘Younger Dryas’ (YD) cooling event, the Benguela SST record exhibits a cold-temperature interval that corresponds to that observed in the eastern North Atlantic SST records. This observation suggests that interhemispheric climate response in Atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with Termination I. For the H1, the eastern South Atlantic SST record strongly supports the hypothesis that an ‘anti-phase’ thermal behavior in South Atlantic surface waters was forced by the slowdown of the North Atlantic Deep Water formation during cold spells in the North Atlantic. In contrast, the abrupt cooling in the eastern South Atlantic coincident with the YD period was probably induced by more vigorous global atmospheric circulation, enhancing the upwelling intensity in both eastern boundary current systems. This atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the South Atlantic SSTs during the YD, which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the YD than during the H1.
    Full-text · Article · Sep 2002 · Earth and Planetary Science Letters
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    ABSTRACT: We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone-based carbon isotopic fractionation (εp) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found εp positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43-] and [NO3-]. However, the relationship between εp and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. [1997]that the isotopic fractionation in haptophytes is related to nutrient-limited growth rates. The relatively high variability of the εp-[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth-limiting resources such as light intensity or micronutrient concentrations.
    Full-text · Article · Mar 2002 · Global Biogeochemical Cycles
  • Jung-Hyun Kim · Ralph R Schneider · Peter J Müller · Gerold Wefer
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    ABSTRACT: Sediment core GeoB 1023-5 from the eastern South Atlantic was investigated at high temporal resolution for variations of sea-surface temperature (SST) during the past 22 kyr, using the alkenone (UK′37) method. SSTs increased by 3.5°C from about 18°C during the Last Ice Age (21±2 cal kyr BP) to about 21.5°C at 14.5 cal kyr BP. This warming trend associated with the deglaciation phase was followed by a cooling event with lowest SSTs near 20°C, persisting for about 1000 years between 13 and 12 cal kyr BP. The SSTs then continued to increase to about 22.5°C at the Holocene climatic optimum at 7 cal kyr BP, and decreased again during the Late Holocene to a core-top value of 19.8°C that is comparable to modern annual mean SST values. When compared with alkenone SST records from the eastern North Atlantic, our SST record indicates continuous warming throughout the deglaciation phase in the Benguela Current, while its northern counterpart, the Canary Current, experienced prominent cooling during ‘Heinrich Event 1’ (H1). On the other hand, for the time period corresponding to the ‘Younger Dryas’ (YD) cooling event, the Benguela SST record exhibits a cold-temperature interval that corresponds to that observed in the eastern North Atlantic SST records. This observation suggests that interhemispheric climate response in Atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with Termination I. For the H1, the eastern South Atlantic SST record strongly supports the hypothesis that an ‘anti-phase’ thermal behavior in South Atlantic surface waters was forced by the slowdown of the North Atlantic Deep Water formation during cold spells in the North Atlantic. In contrast, the abrupt cooling in the eastern South Atlantic coincident with the YD period was probably induced by more vigorous global atmospheric circulation, enhancing the upwelling intensity in both eastern boundary current systems. This atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the South Atlantic SSTs during the YD, which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the YD than during the H1.
    No preview · Article · Jan 2002 · Earth and Planetary Science Letters
  • Peter J Müller · Gerhard Fischer
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    ABSTRACT: We analysed long-chain alkenones in sinking particles and surface sediments from the filamentous upwelling region off Cape Blanc, NW Africa, to evaluate the transfer of surface water signals into the geological record. Our study is based on time-series sediment trap records from 730 m (1990–1991) to 2195–3562 m depth (1988–1991). Alkenone fluxes showed considerable interannual variations and no consistent seasonality. The average flux of C37 and C38 alkenones to the deep traps was 1.9 μg m−2 d−1 from March 1988 to October 1990 and sevenfold higher in the subsequent year. Alkenone fluxes to the shallower traps were on average twice as high and showed similar temporal variations. The alkenone unsaturation indices UK′37, UK38Me and UK38Et closely mirrored the seasonal variations in sea-surface temperature (weekly Reynolds SST). Time lags of 10–48 days between the SST and unsaturation maxima suggest particle sinking rates of about 80 and 280 m d−1 for the periods of low and high alkenone fluxes, respectively. The average flux-weighted UK′37 temperature for the 4-year time series of the deeper traps was 22.1°C, in perfect agreement with the mean weekly SST for the same period. This and the comparison with seasonal temperature variations in the upper 100 m of the water column suggests that UK′37 records principally the yearly average of the mixed-layer temperature in this region. A comparison between the average annual alkenone fluxes to the lower traps (2400 μg m−2 yr−1) and into the underlying sediments (4 μg m−2 yr−1) suggests that only about 0.2% of the alkenones reaching the deep ocean became preserved in the sediments. The flux-weighted alkenone concentrations also decreased considerably, from 2466 μg gC−1 in the water column to 62 μg gC−1 in the surface sediments. Such a low degree of alkenone preservation is typical for slowly accumulating oxygenated sediments. Despite these dramatic diagenetic alkenone losses, the UK′37 ratio was not affected. The average UK′37 value of the sediments (0.796±0.010 or 22.3±0.3°C) was identical within error limits to the 4-year average of the lower traps. The unsaturation indices for C38 alkenones and the ratio between C37 and C38 alkenones also revealed a high degree of stability. Our results do not support the hypothesis that UK′37 is biased towards higher values during oxic diagenesis.
    No preview · Article · Aug 2001 · Deep Sea Research Part I Oceanographic Research Papers
  • Sonja Schulte · Peter J. Müller
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    ABSTRACT: Sea surface temperatures (SST) and primary productivities have been reconstructed for the northeastern Arabian Sea during the past 65,000 years, using C37-alkenones. Comparison of this SST record with &#3918O from Greenland ice core GISP2 shows striking similarities, indicating an apparent linkage between the climate of the Arabian Sea with that of the northern North Atlantic, most probably via atmospheric and/or oceanic circulation. These rapid SST changes are in the long term overlain by insolation changes at 30N.
    No preview · Article · Jan 2001 · Geo-Marine Letters
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    ABSTRACT: We here present records of total organic carbon (TOC) and C37 alkenones, used as indicators for past primary productivity, from the western (WAS) and eastern Arabian Sea (EAS). New data from an open ocean site of the WAS upwelling area are compared with similar records from Ocean Drilling Program (ODP) Site 723 from the continental margin off Oman and MD 900963 from the EAS. These records together with other proxies used to reconstruct upwelling intensity, indicate periods of high productivity in tune with precessional forcing. On the basis of their phase relationship to boreal summer insolation they can be divided into three groups: in the WAS differences between monsoonal proxies (1) and productivity (2) document a combined signal of moderate SW monsoon winds and of strengthened and prolonged NE monsoon winds, whereas in the EAS phasing indicates maximum productivity (3) at times of stronger NE monsoon winds associated with precession-related maxima in ice volume.
    No preview · Article · Jun 2000 · Paleoceanography
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    ABSTRACT: Three sediment cores on a transect across the continental slope off Namibia at about 23°S were investigated for alkenone-derived past sea-surface temperature (SST) and total organic carbon (TOC) content. These records are used to reconstruct variations of surface circulation, coastal upwelling, and paleoproductivity in the northern Benguela Current System for the last 150,000 yr. The SST record most distant from the coast resembles a SST pattern typical of the pelagic ocean, with the lowest SST at full-glacial periods and the highest SST during the Eemian and the Holocene. In contrast to the modern conditions where annual mean SST decreases toward the coast, the shelf-edge SST record has the most prominent warm anomalies of about 2°C during isotope stages 2 and 6 compared with the open ocean. The glacial SST minimum in the record close to the shelf is observed between 50,000 and 35,000 yr B.P., while the record midway along the transect shows intermediate temperature conditions between the offshore and nearshore records. The causal process for the warm anomalies under full ice-age conditions close to the coast may be similar to that of recent “Benguela Niño events” that originate from perturbations in the tradewind system over the western tropical Atlantic. During these events the Angola–Benguela Front, located at about 16°S, weakens and intensive southward protrusions of tropical water masses extend into the nearshore upwelling area as far as 25°S. Thus, the two nearshore records primarily responded to variations in the time-integrated balance between upwelling intensity and southward protrusions of anomalously warm and nutrient-poor Angolan surface waters, as indicated by the good anticorrelation of SST and TOC content. Accordingly, surface water cooling off Namibia over the last 150,000 yr was most intense during stage 3 due to strong winds that worked in favor of upwelling and a decrease of Angolan warm water influence.
    No preview · Article · Jul 1999 · Quaternary Research
  • Ralph R. Schneider · Peter J. Müller · Ruth Acheson
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    ABSTRACT: The variability of sea-surface temperatures (SST’s) in the Atlantic Ocean is examined for the past 370 kyr BP in order to unravel differences in climate change in low latitudes versus high latitudes for both hemispheres. For this purpose, special emphasis is given to variations in alkenone-derived SST estimates at time scales of 10 to 100kyr. Tropical SST records show relatively warm temperature values during the penultimate glacial marine oxygen isotope stage (MIS) 6, and further indicate that warming in the equatorial Atlantic began 10 to 15kyr prior to deglacial sea-level rise and melting of continental ice caps at the last three glacial terminations. The periodic glacial to interglacial changes in the temperature pattern closely resemble the 100 and 400kyr amplitude modulation of precessional insolation variations at low latitudes. This implies that, to a great extent, variations in surface-ocean temperatures are a direct response to low-latitude insolation forcing by eccentricity, and not just a result of eccentricity-related climate change in high-latitudes, e. g., changes in ice volume, thermohaline circulation and atmospheric CO2 concentration. Alkenone SST records from mid latitudes in the North and South Atlantic reveal SST patterns that are more congruent with marine oxygen-isotope records. Here, the coldest glacial SST’s occur simultaneously with maxima in ice volume at the end of glaciation periods rather than about 10 to 15kyr earlier as is observed in the tropical SST records. Nonetheless, the notably warm temperatures characteristic of the glacial MIS 6 in the tropics are also exhibited in SST curves off South Africa and in the North Atlantic. This indicates that part of the alkenone SST signal typical for the tropics is observable at mid latitudes, at sites influenced by surface currents originating in the tropical Atlantic or Indian Ocean. Comparison of alkenone SST records from low and mid latitudes therefore hints at a potential role for the tropics as an amplifier of the insolation forcing related to eccentricity, which may help to explain the strong response of global climate change to an orbital forcing relatively low in strength. Longer alkenone SST records are needed from tropical oceans to assure that the characteristic 100 and 400kyr periodicity in tropical SST variations is real for more than just the last three glacial-interglacial cycles.
    No preview · Article · Jan 1999
  • Peter J. Müller · Martin Čepek · Götz Ruhland · Ralph R. Schneider
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    ABSTRACT: Sea surface temperatures (SSTs) derived from the alkenone U37κ′) record of Quaternary sediments may be subject to bias if algae with different temperature sensitivities have contributed to the sedimentary alkenone record. The alkenone-derived SST records are usually based on a U37κ′-temperature relationship which was measured in culture experiments using the coccolithophorid Emiliania huxleyi (F.G. Prahl, L.A. Muehlhausen and D.L. Zahnle, 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochim. Cosmochim. Acta 52, 2303–2310). To assess possible effects of past species changes on the U37κ′-temperature signal, we have analysed long-chain alkenones and coccolithophorids in a late Quaternary sediment core from the Walvis Ridge and compared the results to SST estimates extracted from the δ18O record of the planktonic foraminifer Globigerinoides ruber. Alkenones and isotopes were determined over the entire 400-kyr core record while the coccolithophorid study was confined to the last 200 kyr when the most pronounced changes in alkenone content occurred. Throughout oxygen-isotope stages 6 and 5, species of the genus Gephyrocapsa were the predominating coccolithophorids. E. huxleyi began to increase systematically in relative abundance since the stage transition, became dominant over Gephyrocapsa spp. during stage 3 and reached the highest abundances in the Holocene. Carbon-normalized alkenone concentrations are inversely related to the relative abundances of E. huxleyi, and directly related to that of Gephyrocapsa spp., suggesting that species of this genus were the principal alkenone contributors to the sediments. Nevertheless, SST values obtained from the U37κ′-temperature relationship for E. huxleyi compare favourably to the isotope-derived temperatures. The recently reported U37κ′-temperature relationship for a single strain of Gephyrocapsa oceanica (J.K. Volkman, S.M. Barrett, S.I. Blackburn and E.L. Sikes, 1995. Alkenones in Gephyrocapsa oceanica: Implications for studies of paleoclimate. Geochim. Cosmochim. Acta 59, 513–520) produces unrealistically high SST values indicating that the temperature response of the examined strain is not typical for the genus Gephyrocapsa. This is supported by the C37:C38 alkenone ratios of the sediments which are comparable to average ratios reported for E. huxleyi, but significantly higher than for the G. oceanica strain. Most notably, the general accordance of the alkenone characteristics between sediments and E. huxleyi persists through stages 8 to 5 and even in times that predate the first appearance of this species (268 ka; H.R. Thierstein, K.R. Geitzenauer and B. Molfino, 1977. Global synchroneity of late Quaternary coccolith datum levels: Validation by oxygen isotopes. Geology 5, 400–404). Our results suggest that U37κ′-temperature relationships based on E. huxleyi produce reasonable paleo-SST estimates even for late Quaternary periods when this species was scarce or absent because other alkenone-synthesizing algae, e.g. of the genus Gephyrocapsa, responded similarly to temperature changes.
    No preview · Article · Dec 1997 · Palaeogeography Palaeoclimatology Palaeoecology
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    ABSTRACT: Sedimentary δ15N contains a record of biogeochemical processes occurring in the water column. Stable nitrogen isotopes were measured in the bulk sedimentary matter of 18 surface sediment samples collected in three transects in the eastern Angola Basin, perpendicular to the coast, at 6 °S, 12 °S and 17 °S and two samples from the Zaire estuary. Relative enrichment in 15N in sediments with distance from the coast was seen in each of the three transects off northern, middle and southern Angola. Values in the Zaire Fan (northern Angola) and southern Angola increased by 1.9%. and 1.6%., respectively, from shallow (~100 m) to deep (~4500 m) water sediments and in middle Angola by 2.5%. (water depth range 73–3809 m). We propose that in the Angola region the degree of fractionation of nitrogen isotopes in organic matter is a function of nutrient supply to sunlit waters. The shift towards lighter isotopic values in shallow water signifies that the larger nitrate pool is not utilized as extensively as in water farther away from the coastal upwelling centers. Southern Angola sediments were more than 1%. lower (average δ15N, 5.4%.) than the middle and northern transects (average δ15N, 7.2%. and 6.5%., respectively). This difference is attributed to higher nitrate concentrations in the surface water at 17 °S, for which there are two reasons. One is that these sediments are located south of the Angola/Benguela front at ~16 °S which demarcates the convergence of warm equatorial water with cold, nutrient-rich Benguela Current water. The second cause is the intense Namibian coastal upwelling zone extending northward up to the front. Comparisons of bulk δ15N with δ13Corg and ratios show that the observed sedimentary δ15N variations with water depth are due neither to water column diagenesis nor to mixing of terrestrial material with marine-derived organic matter. The two estuary sediment samples were mainly terrigenous in origin, based on δ13Corg and ratios. δ15N values of these two samples were 7.0%. and 7.6%.
    No preview · Article · Sep 1996 · Marine Geology