Stable oxygen isotope and Mg/Ca data of Globigerinoides ruber white, water stable oxygen isotope data, SST and MAT records of sediment core GeoB6211-2

During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SSTs) of the western South Atlantic, and in mean air temperatures (MATs) over southeastern South America. It has been suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This anti-phase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is so far largely unknown. Here we address this issue, presenting high-temporal-resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling-Allerød, in phase with an existing SST record from the NBC. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a two-step increase during Termination 1, synchronous with atmospheric CO2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental in driving the Earth out of the last glacial.

Alkenone data of sediment core GeoB9508-5 from the continental slope off Senegal

In order to investigate a possible link between tropical Northeast (NE) Atlantic sea-surface temperature (SST), Atlantic meridional overturning circulation (AMOC), and drought in the Sahel during the past 44 thousand years (kyr) we used alkenone paleothermometry and d13C of C. wuellerstorfi of a marine sediment core from the continental slope off Senegal. Our data show periods of low SST and reduced AMOC that coincided with drought in the Sahel during North Atlantic Heinrich stadials (HS). The coldest period was HS1 (ca. 15-18 kyr before present, BP) when SST decreased by more than 2°C. Moreover, the SST off Senegal lagged variations in Sahel aridity, which is in agreement with results from a freshwater hosing experiment. We conclude that variations in tropical NE Atlantic SST were not the initial trigger of millennial-scale Sahel droughts of the past 44 kyr. Instead, it is thought that these droughts were induced by substantial coolings of the extratropical North Atlantic.

(Table 1) Age determination of sediment core A-7

Detailed deglacial and Holocene records of planktonic d18O and Mg/Ca-based sea surface temperature (SST) from the Okinawa Trough suggest that at ~18 to 17 thousand years before present (kyr B.P.), late spring/early summer SSTs were approximately 3°C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.

Composite stratigraphy of ODP Site 177-1094 and sediment core TTN057-13-PC4 (Table 1)

Visual counts of ice-rafted debris (IRD), foraminifera, and radiolaria were made for ~1500 samples in Site 1094 spanning the last four climatic cycles (marine isotope stages 1-11). Most, but not all, of the IRD variability is captured by whole-core physical properties including magnetic susceptibility and Q-ray attenuation bulk density. Glacial periods are marked by high IRD abundance and millennial-scale variability, which may reflect instability of ice shelves in the Weddell Sea region. Each interglacial period exhibits low IRD and high foraminiferal abundance during the early part of the interglacial, indicating relatively warm sea-surface temperatures and reduced influence of sea ice. IRD increases and foraminiferal abundances decrease during the latter part of each interglacial, indicating a return to more glacial-like conditions. Glacial terminations I and V are each characterized by a step-wise reduction in ice-rafting punctuated by a brief pulse in IRD delivery and reversal in delta18O. The coarse fraction of the sediment is dominated by ash and radiolaria, and the relative abundance of these components is remarkably similar to the concentration of Na+ in Vostok. Each of these variables is believed to be controlled mainly by sea-ice cover, thereby providing a means for sediment-ice core correlation.

Benthic foraminiferal assemblages in ODP Hole 167-1017E

A strong oxygen-minimum zone (OMZ) currently exists along the California margin because of a combination of high surface-water productivity and poor intermediate-water ventilation. However, the strength of this OMZ may have been sensitive to late Quaternary ocean-circulation and productivity changes along the margin. Although sediment-lamination strength has been used to trace ocean-oxygenation changes in the past, oxygen levels on the open margin are not sufficiently low for laminations to form. In these regions, benthic foraminifera are highly sensitive monitors of OMZ strength, and their fossil assemblages can be used to reconstruct past fluctuations. Benthic foraminiferal assemblages from Ocean Drilling Program Site 1017, off Point Conception, exhibit major and rapid faunal oscillations in response to late Quaternary millennial-scale climate change (Dansgaard-Oeschger cycles) on the open central California margin. These faunal oscillations can be correlated to and are apparently synchronous with those reported from Santa Barbara Basin. Together they represent major fluctuations in the strength of the OMZ which were intimately associated with global climate change-weakening, perhaps disappearing, during cool periods and strengthening during warm periods. These rapid, major OMZ strength fluctuations were apparently widespread on the Northeast Pacific margin and must have influenced the evolution of margin biota and altered biogeochemical cycles with potential feedbacks to global climate change.

Paleotemperature reconstructions of the Atlantic Sector of the Southern Ocean

Diatom assemblages from ODP Leg 177 sites 1093, 1094 and core PS2089-2, from the present Antarctic sea ice free zone and close to the Polar Front, were analyzed in order to reconstruct the climate development around the Mid-Brunhes Event 400 000 yr ago, as reflected by summer sea surface temperature (SSST) and sea ice distribution. Dense sample spacing allows a mean temporal resolution during Marine Isotope Stage (MIS) 11 (423-362 ka) of 300-400 yr. SSST values were estimated from diatom assemblages using a transfer function technique. The distribution pattern of sea ice diatoms indicates that the present-day ice free Antarctic Zone was seasonally covered by sea ice during the cold MIS 12 and MIS 10. These glacial periods are characterized by sea ice fluctuations with a periodicity of 3 and 1.85 kyr, suggesting the occurrence of Dansgaard-Oeschger-style millennial-scale oscillations in the Atlantic sector of the Southern Ocean during the glacial stages MIS 12 and MIS 10. Termination V (MIS 12/11) is characterized by a distinct temperature increase of 4-6°C, intersected especially at the southern site 1094 and core PS2089-2 by two distinct cooling events reminiscent of the Younger Dryas, which are associated with a northward shift of the winter sea ice edge in the Antarctic Zone. The SSST record is characterized by distinct temperature intervals bounded by stepwise, rapid changes. Maximum temperatures were reached during Termination V and the early MIS 11, exceeding modern values by 2°C over a period of 8 kyr. This pattern indicates a very early response of the Southern Ocean to global climate on Milankovitch-driven climate variability. The SSST optimum is marked by millennial-scale temperature oscillations with an amplitude of ca. 1°C and periodicities of ca. 1.85 and 1.47 kyr, probably reflecting changes in the ocean circulation system. The SSSTs during the MIS 11 temperature optimum do not exceed values obtained from other interglacial optima such as the early periods of MIS 5 or MIS 1 from the Antarctic Zone. However, the total duration of the warmest period was distinctly longer than observed from other interglacials. The comparison of the South Atlantic climate record with a high-resolution record from ODP Leg 162, site 980from the North Atlantic shows a strong conformity in the climate development during the studied time interval.

Stable isotope record and composition of planktonic foraminifera from ODP Site 172-1056

The stable isotopic composition of two planktonic foraminifer species (Globigerinoides sacculifer and Neogloboquadrina dutertrei) and two benthic foraminifer species (Cibicidoides wuellerstorfi and Uvigerina peregrina) was measured at sub-orbital resolution through the marine isotope stages (MISs) 10, 11, and 12 (345-460 ka) at Site 1056 on the Blake-Bahama Outer Ridge. Planktonic foraminifers were counted for the interval 405-450 ka at 2-4-kyr resolution. Site 1056 (32°29'N, 76°20'W) is located on the continental slope (water depth: 2167 m) beneath the Gulf Stream. The average rate of sediment accumulation through the interval is 11.4 cm/kyr, but sediment accumulation is much more rapid during glacial intervals (15-17 cm/kyr). The decline in percent carbonate during glacial intervals, and its rise during interglacials, indicates that the increased sediment supply is of terrigenous origin. Low carbonate values and low benthic delta13C, which are both associated with a weak Western Boundary Undercurrent and low North Atlantic Deep Water production, persist for 6 kyr after the benthic delta18O record indicates that ice volume has begun to decrease. Recovery of carbonate and benthic delta13C values is abrupt and rapid. Millennial-scale variation (~3-4 kyr) is apparent in the glacial intervals of the planktonic delta18O records and is more pronounced in a Delta delta18O record, which represents the temperature range in the photic zone. Semi-precessional (10-12-kyr) cycles are apparent in the planktonic deltadelta13C record. The millennial-scale cycles are largely caused by an increase in G. sacculifer delta18O and represent surface warming. They are interpreted as representing periodic increases in westward intensification of the gyre. The semi-precessional cycles are driven by changes in the N. dutertrei delta13C and represent fluctuations in the Gulf Stream itself and therefore likely have a tropical origin. Planktonic foraminifer census counts did not show an expected response to one of the largest glacial/interglacial transitions of the late Pleistocene. The most obvious change was an increase in faunal diversity during MIS 12.2, the interval of maximum delta18O values. This suggests that cool slope water and warm subtropical gyre water penetrated a more sluggish Gulf Stream with greater frequency at this time. The millennial-scale maxima in the Delta delta18O record are accompanied by decreases in diversity, which is consistent with the interpretation of surface warming during these events.

Dinoflagellate cyst accumulation rates of ODP Hole 146-893A from Santa Barbara Basin

The dinoflagellate cyst record from Ocean Drilling Program Hole 893A, Santa Barbara Basin, southern California, is examined at millennial-scale resolution for the past 40 kyr. Changes in cyst abundance, composition of cyst assemblages, and their diversity reflect major shifts in climate and ocean circulation in the region over this time interval. Throughout the sequence, dinoflagellate cyst assemblages are dominated by heterotrophic dinoflagellates. Brigantedinium spp. and other upwelling-related taxa such as Echinidinium and Protoperidinium americanum are abundant, indicating the continued influence of coastal upwelling on the basin during the late Quaternary. A significant increase in cyst accumulation rates is seen during the Holocene and, to a lesser extent, during shorter warming events such as Bolling/Allerod and Dansgaard-Oeschger interstadials, implying enhanced marine productivity during these periods. Cyst diversity is high during the Holocene. An increase in abundance of cysts produced by autotrophic dinoflagellates in the late Holocene suggests enhanced input of warm, nutrient-rich waters. In contrast, cyst assemblages from the Last Glacial Maximum exhibit a relatively low diversity and an increase in the cysts of heterotrophic dinoflagellates, in particular Selenopemphix nephroides. The presence of this taxon in association with Brigantedinium spp. implies substantial cooling of surface waters in the Santa Barbara Basin at that time.