Interhemispheric comparison of deglacial sea-surface temperature patterns in Atlantic eastern boundary currents

摘要

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 (U_(37)~(K')) method. SSTs increased by 3.5 ℃ from about 18 ℃ during the Last Ice Age (21 ± 2 cal kyr BP) to about 21.5 ℃ 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 ℃, persisting for about 1000 years between 13 and 12 cal kyr BP. The SSTs then continued to increase to about 22.5 ℃ 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 ℃ 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 l' (Hl). 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.