Uwe Pflaumann

East Asian monsoon climate during the Late Pleistocene: high-resolution sediment records from the south China Sea

Abstract Based on the study of 10 sediment cores and 40 core-top samples from the South China Sea (SCS) we obtained proxy records of past changes in East Asian monsoon climate on millennial to bidecadal time scales over the last 220,000 years. Climate proxies such as global sea level, estimates of paleotemperature, salinity, and nutrients in surface water, ventilation of deep water, paleowind strength, freshwater lids, fluvial and/or eolian sediment supply, and sediment winnowing on the sea floor were derived from planktonic and benthic stable-isotope records, the distribution of siliciclastic grain sizes, planktonic foraminifera species, and the UK37 biomarker index. Four cores were AMS-14C-dated. Two different regimes of monsoon circulation dominated the SCS over the last two glacial cycles, being linked to the minima and maxima of Northern Hemisphere solar insolation. (1) Glacial stages led to a stable estuarine circulation and a strong O2-minimum layer via a closure of the Borneo sea strait. Strong northeast monsoon and cool surface water occurred during winter, in part fed by an inflow from the north tip of Luzon. In contrast, summer temperatures were as high as during interglacials, hence the seasonality was strong. Low wetness in subtropical South China was opposed to large river input from the emerged Sunda shelf, serving as glacial refuge for tropical forest. (2) Interglacials were marked by a strong inflow of warm water via the Borneo sea strait, intense upwelling southeast of Vietnam and continental wetness in China during summer, weaker northeast monsoon and high sea-surface temperatures during winter, i.e. low seasonality. On top of the long-term variations we found millennial- to centennial-scale cold and dry, warm and humid spells during the Holocene, glacial Terminations I and II, and Stage 3. The spells were coeval with published variations in the Indian monsoon and probably, with the cold Heinrich and warm Dansgaard–Oeschger events recorded in Greenland ice cores, thus suggesting global climatic teleconnections. Holocene oscillations in the runoff from South China centered around periodicities of 775 years, ascribed to subharmonics of the 1500-year cycle in oceanic thermohaline circulation. 102/84-year cycles are tentatively assigned to the Gleissberg period of solar activity. Phase relationships among various monsoon proxies near the onset of Termination IA suggest that summer-monsoon rains and fluvial runoff from South China had already intensified right after the last glacial maximum (LGM) insolation minimum, coeval with the start of Antarctic ice melt, prior to the δ18O signals of global sea-level rise. Vice versa, the strength of winter-monsoon winds decreased in short centennial steps only 3000–4000 years later, along with the melt of glacial ice sheets in the Northern Hemisphere.

SIMMAX: A modern analog technique to deduce Atlantic sea surface temperatures from planktonic foraminifera in deep‐sea sediments

We present a data set of 738 planktonic foraminiferal species counts from sediment surface samples of the eastern North Atlantic and the South Atlantic between 87°N and 40°S, 35°E and 60°W including published Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP) data. These species counts are linked to Levituss [1982] modern water temperature data for the four caloric seasons, four depth ranges (0, 30, 50, and 75 m), and the combined means of those depth ranges. The relation between planktonic foraminiferal assemblages and sea surface temperature (SST) data is estimated using the newly developed SIMMAX technique, which is an acronym for a modern analog technique (MAT) with a similarity index, based on (1) the scalar product of the normalized faunal percentages and (2) a weighting procedure of the modern analogs SSTs according to the inverse geographical distances of the most similar samples. Compared to the classical CLIMAP transfer technique and conventional MAT techniques, SIMMAX provides a more confident reconstruction of paleo-SSTs (correlation coefficient is 0.994 for the caloric winter and 0.993 for caloric summer). The standard deviation of the residuals is 0.90°C for caloric winter and 0.96°C for caloric summer at 0-m water depth. The SST estimates reach optimum stability (standard deviation of the residuals is 0.88°C) at the average 0– to 75-m water depth. Our extensive database provides SST estimates over a range of −1.4 to 27.2°C for caloric winter and 0.4 to 28.6°C for caloric summer, allowing SST estimates which are especially valuable for the high-latitude Atlantic during glacial times. An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGUs FTP account using ANONYMOUS as the username and GUESTas the password. Go to the right directory by typing CD APPEND. TypeLS to see what files are available. Type GET and the name of the file toget it. Finally type EXIT to leave the system.) (Paper 95PA01743,SIMMAX: A modern analog technique to deduce Atlantic sea surfacetemperatures from planktonic foraminifera in deep-sea sediments, UwePflaumann, Josette Duprat, Claude Pujol, and Laurent D. Labeyrie).Diskette may be ordered from American Geophysical Union, 2000Florida Avenue, N.W., Washington, DC 20009; Payment mustaccompany order.

Glacial North Atlantic: Sea‐surface conditions reconstructed by GLAMAP 2000

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX-28) modern analog technique with 947 modern analog samples and 119 well-dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice-free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45degrees and 60degreesN, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12degreesC. In the subtropical ocean gyres the new reconstruction supports the glacial-to-interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2degrees and 6degreesN suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6degrees-8degreesC during Northern Hemisphere summer. Differences between these SIMMAX-based and published U37(k)- and Mg/Ca-based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.

Variations in Atlantic surface ocean paleoceanography, 50°‐80°N: A time‐slice record of the last 30,000 years

Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (>8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bolling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGUs FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95PA01453, Variations in Atlantic surface ocean paleoceanography, 50°-80°N: A time-slice record of the last 30,000 years, M. Sarnthein et al.) Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009;

Atmospheric and Oceanic Circulation Patterns off Northwest Africa During the Past 25 Million Years

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Fundamental Modes and Abrupt Changes in North Atlantic Circulation and Climate over the last 60 ky — Concepts, Reconstruction and Numerical Modeling

The sediments of the eastern Atlantic contain excellent historical records of the patterns of oceanic and atmospheric circulation in the subtropics. This is particularly the case at the low relief northwest African continental margin which favors unrestrained interaction of the land- sea climatic system and which forms a schematic, vertical cross section through the ocean from the equator to Mediterranean latitudes. Our synthesis of the Neogene and Quaternary evolution of paleoenvironments along this margin tries to show that oceans and land respond in a complementary way to global climatic events and mechanisms.

Atlantic core-top calibration of the U37K index as a sea-surface palaeotemperature indicator

Centennial- to millennial-scale changes in global climate over the last 60 ky were first documented in ice cores from Greenland, with ice sheets around the North Atlantic and its thermohaline circulation (THC) as prime candidates for a potential trigger mechanism. To reach a new quality in understanding the origin and causal links behind these changes, two strategies were intimately tied together in this synthesis, high-resolution 3-D ocean modeling and paleoceanographic reconstructions. Here, five time series with a time resolution of several decades and various time slices of surface and deep-water paleoceanography were established from hundreds of deep-sea cores for the purpose of monitoring rapid changes across the North Atlantic and testing or initiating model results. Three fundamental modes were found to operate Atlantic THC. Today, mode I shows intensive formation of North Atlantic Deep Water (NADW) and strong heat and moisture fluxes to the continents adjacent to the North Atlantic. Peak glacial mode II leads to a reduction in NADW formation by 30-50%, in line with a clear drop in heat flux to Europe. The glacial Nordic Seas, however, remain ice-free during summer and little influenced by meltwater, in contrast to the sea west ofIreland, where iceberg meltwater blocks an eastbound flow into the Norwegian Sea and induces a cold longshore current from Faeroe to the Pyrenees. The subsequent Heinrich 1 (HI) meltwater mode III leads to an entire stop in NADW and intermediate-water production as well as a reversed pattern of THC, stopping any heat advection from the central and South Atlantic to the north. In contrast to earlier views, the Younger Dryas, possibly induced by Siberian meltwater, began with mode I and ended with mode III, continuing into the Preboreal. Modeling the impact of modes I to III on the global carbon budget, we find that the atmosphere has lost 34-54 ppmv CO2 from interglacial to glacial times, but has gained 23-62 ppmv CO2 at the end of HI within a few decades, equivalent to 33-90% of modem, man-made CO2 release. The robust 1500-y Dansgaard- Oeschger (D-O) cycles and their multiples of as much as 7200 years, the Heinrich event cycles, are tied to periodical changes between THC modes I/II and II/III. In the Irminger Sea rapid D-O coolings are in phase with initial meltwater injections from glaciers on East Greenland, here suggesting an internal trigger process in accordance with binge-purge models. Ice rafting from East Greenland and Iceland occurs only 240-280 y later, probably inducing a slight sea-level rise and, in tum, Heinrich ice rafting from the Laurentian ice sheet during H1, H2, H4, H5. At H1 a major surge from the Barents shelf has lagged initial cooling by 1500 y and entails the most prominent and extended reversal in Atlantic THC over the last 60 ky (probably also at the end of glacial stage 4, at H6). Meltwater stratification in the Inninger Sea reaches its maximum only 640 y after initial meltwater injection and induces, via seasonal sea-ice formation, brine-water injections down to 4 km water depth, signals leading the classic D-O jump to maximum warmth by only 125 y. It may be inferred from this short-phase lag that brine water-controlled deep-water formation probably entrains warm water from further south, thereby forming the key trigger mechanism for the final tum-on of the Atlantic THC mode II roughly within a decade (or mode I, in case of favorable Milankovitch forcing).

Palaeoclimatic signals recognized by chemometric treatment of molecular stratigraphic data

Abstract A field calibration of the UK index with sea surface temperature is discussed, through analysis of an extensive suite of surface sediments ( n = 109) from the northeastern Atlantic (2°S–75°N). Values of U 37 K are compared with sea surface temperatures for overlying waters measured at different depths and seasons, to obtain a correlation suitable for palaeotemperature reconstructions. The best fit is obtained using surface (0 m) temperatures corresponding to caloric winter and autumn months. However, the annual average, spring and summer surface temperature equations also have high correlation coefficients, and are also appropriate for climatic studies. The results further validate the general applicability of the U 37 K as a climatic proxy, because the calibration equations are valid over a wide range of surface water temperatures (0–28°C) for different algal populations and are representative of the average contribution of alkenones to sediments, as found in sediment cores.

Modern distribution patterns of planktonic foraminifera in the South China Sea and western Pacific: a new transfer technique to estimate regional sea-surface temperatures

Abstract The high-resolution stratigraphy of various marker compounds has been studied, using GC, HPLC and GC-MS, in a 13 m gravity core recovered from the Kane Gap region, eastern equatorial Atlantic, which provides a record of the glacial/interglacial episodes over the last million years. Downhole variations in many presumed source-specific components are observed (e.g. in n -alkanes from terrigenous land plants and dinosterol from dinoflagellates), which may be due to perturbations or cyclicities resulting from climatic change. Fluctuations in the unsaturation of alkenones attributable to variations in water temperatures show correlations with the glacial/interglacial cycles recorded in the δ 18 O values for planktonic foraminifera, thereby providing a potential organic geochemical measure of past climates. These molecular abundance data can be linked to the palaeotemperature record, following computer treatments using principal component and spectral analyses. Molecular stratigraphy shows promise as a new chemostratigraphical tool where other means of stratigraphy fail, for example, through calcium carbonate dissolution.

Overview of Glacial Atlantic Ocean Mapping (GLAMAP 2000)

Abstract We present 30 new planktonic foraminiferal census data of surface sediment samples from the South China Sea, recovered between 630 and 2883 m water depth. These new data, together with the 131 earlier published data sets from the western Pacific, are used for calibrating the SIMMAX-28 transfer function to estimate past sea-surface temperatures. This regional SIMMAX method offers a slightly better understanding of the marginal sea conditions of the South China Sea than the linear transfer function FP-12E, which is based only on open-ocean data. However, both methods are biased toward the tropical temperature regime because of the very limited data from temperate to subpolar regions. The SIMMAX formula was applied to sediment core 17940 from the northeastern South China Sea, with sedimentation rates of 20–80 cm/ka. Results revealed nearly unchanged summer temperatures around 28°C for the last 30 ky, while winter temperatures varied between 19.5°C in the last glacial maximum and 26°C during the Holocene. During Termination 1A, the winter estimates show a Younger Dryas cooling by 3°C subsequent to a temperature optimum of 24°C during the Bolling/Allerod. Estimates of winter temperature differences between 0 and 100 m water depth document the seasonal variations in the thickness of the mixed layer and provide a new proxy for estimating past changes in the strength of the winter monsoon.