Michael Sarnthein

Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing

A detailed (ca. 100 yr resolution) and well-dated (18 AMS ^(14)C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.

Changes in east Atlantic deepwater circulation over the last 30,000 years: eight time slice reconstructions

Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution of east Atlantic deepwater and intermediate water masses over the last 30,000 years. Our results show that there have been three distinct modes of deepwater circulation: Near the stage 3-2 boundary, the origin of North Atlantic Deep Water (NADW) was similar to today (mode 1). However, after late stage 3 the source region of the NADW end-member shifted from the Norwegian-Greenland Sea to areas south of Iceland (mode 2). A reduced NADW flow persisted during the last glacial maximum, with constant preformed δ13C values. The nutrient content of NADW increased markedly near the Azores fracture zone from north to south, probably because of the mixing of upwelled Antarctic Bottom Water (AABW) from below, which then advected with much higher flux rates into the northeast Atlantic. Later, the spread of glacial meltwater over the North Atlantic led to a marked short-term ventilation minimum below 1800 m about 13,500 14C years ago (mode 3). The formation of NADW recommenced abruptly north of Iceland 12,800–12,500 years ago and reached a volume approaching that of the Holocene, in the Younger Dryas (10,800–10,350 years B.P.). Another short-term shutdown of deepwater formation followed between 10,200 and 9,600 years B.P., linked to a further major meltwater pulse into the Atlantic. Each renewal of deepwater formation led to a marked release of fossil CO2 from the ocean, the likely cause of the contemporaneous 14C plateaus. Over the last 9000 years, deepwater circulation varied little from today, apart from a slight increase in AABW about 7000 14C years ago. It is also shown that the oxygenated Mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30,000 years.

Astronomic timescale for the Pliocene Atlantic δ18O and dust flux records of Ocean Drilling Program site 659

High-resolution benthic oxygen isotope and dust flux records from Ocean Drilling Program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the Atlantic from 2.85 Ma back to 5 Ma. Spectral analysis of the δ18O record indicates that the 41-kyr period of Earths orbital obliquity dominates the Pliocene record. This is shown to be true regardless of fundamental changes in the Earths climate during the Pliocene. However, the cycles of Sahelian aridity fluctuations indicate a shift in spectral character near 3 Ma. From the early Pliocene to 3 Ma, the periodicities were dominantly precessional (19 and 23 kyr) and remained strong until 1.5 Ma. Subsequent to 3 Ma, the variance at the obliquity period (41 kyr) increased. The timescale tuned to precession suggests that the Pliocene was longer than previously estimated by more than 0.5 m.y.

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.

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.

Correlation of marine 14C ages from the nordic seas with the GISP2 isotope record : Implications for 14C calibration beyond 25 ka BP

We present two new high-resolution sediment records from the southwestern Iceland and Norwegian Seas that were dated by numerous (super 14) C ages up to 54 (super 14) C ka BP. Based on various lines of evidence, the local (super 14) C reservoir effect was restricted to 400-1600 yr. The planktic stable isotope records reveal several meltwater spikes that were sampled with an average time resolution of 50 yr in PS2644 and 130 yr in core 23071 during isotope stage 3. Most of the delta (super 18) O spikes correlate peak-by-peak to the stadials and cold rebounds of the Dansgaard-Oeschger cycles in the annual-layer counted GISP2 ice core, with the major spikes reflecting the Heinrich events 1-6. This correlation indicates large fluctuations in the calibration of (super 14) C ages between 20 and 54 (super 14) C ka BP. Generally the results confirm the (super 14) C age shifts as predicted by the geomagnetic model of Laj, Mazaud and Duplessy (1996). However, the amplitude and speed of the abrupt decrease and subsequent major increase of our (super 14) C shifts after 45 (super 14) C ka BP clearly exceed the geomagnetic prediction near 40-43 and 32-34 calendar (cal) ka BP. At these times, the geomagnetic field intensity minima linked to the Laschamp and the Mono Lake excursions and confirmed by a local geomagnetic record, probably led to a sudden increase in cosmogenic (super 14) C and (super 10) Be production, giving rise to excess (super 14) C in the atmosphere of up to 1200 per mil.

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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Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca

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.

Onset of permanent stratification in the subarctic Pacific Ocean

Abstract We have generated benthic foraminiferal Mg/Ca records from eastern tropical Atlantic core M16772 (3.9 km) and eastern tropical Pacific core TR163-31P (3.2 km) to assess the potential for using benthic Mg-paleothermometry to reconstruct Quaternary bottom water temperature histories. Variations in Mg/Ca records from both the Atlantic and Pacific show a significant correlation with climatic oscillations of the last 330 kyr. Shell Mg/Ca peaks during interglacial episodes, with marine isotope stage (MIS) 5e Mg/Ca nearly matching Holocene values in both cores. Lower Mg/Ca values occur during glacial intervals. To estimate temperatures from downcore Mg/Ca, we have augmented the published Cibicidoides sp. Mg/Ca–temperature dataset [Rosenthal et al., Geochim. Cosmochim. Acta 61 (1997) 3633–3643] with new data to include the temperature range of deep waters. Applying the 10.9% change in Mg/Ca per °C defined by the expanded calibration to the deep Atlantic data implies glacial–interglacial shifts in deep water temperature of 2–4°C over the last 300 kyr. Temperature estimates are comparable to deep Atlantic temperature changes proposed by Labeyrie et al. [Nature 327 (1987) 477–482]. There is greater uncertainty in deriving temperatures from the tropical Pacific core due to a limited Uvigerina spp. core top calibration set; however, glacial–interglacial temperature oscillations appear to be on the order of 2–3°C. In addition, core TR163-31P records clear millennial-scale Mg/Ca and δ 18 O oscillations in MIS 3 corresponding to temperature fluctuations of >0.5°C. Additional calibration studies are needed to address potential secondary effects on Mg/Ca.