CONTRASTING PATTERNS OF CLIMATE VARIABILITY IN THE MID-LATITUDE NORTH ATLANTIC DURING THE LAST 3 GLACIAL PERIODS

Numerous investigations suggest that the subpolar ocean has experienced considerable variability in sea surface temperature (SST) associated with iceberg discharge events during the last glacial-interglacial cycle but, at present, little is known about the significance or amplitude of SST climatic fluctuations in previous glacial-interglacial cycles. We have generated a suite of detailed palaeoenvironmental measurements spanning the last 3 glacial periods from cores SU90-03 and MD99-2253, recovered from the northern margin of the subtropical gyre and the subpolar gyre respectively. The chronology for the cores are derived from ¹⁴C dates and d¹⁸O measurements made on the foraminifers Globigerina bulloides, Cibicidoides wuellerstorfi and Uvigerina spp. Past SST conditions were estimated from compositional changes in the planktonic foraminiferal assemblages and compared to a history of ice rafted detritus (IRD) input provided by counts of lithic particles. The impact of the northward surface transfer of heat on the return flow of North Atlantic Deep Water are evaluated from benthic d¹³C measurements.

A series of IRD depositional events are documented throughout the length of both cores, with most intervals of enhanced IRD input readily recognisable at both 40°N and 56°N. In general, short-term coolings appear to match the pattern of IRD variations. However, a major amplification of the IRD signal and an increase in the frequency of IRD events appears to have occurred during the last glacial period compared to the previous glacials. This pattern is matched to a large extent by the characteristics of the palaeotemperature records through MIS 8, 6, and 4-2. Well-defined millennial-scale fluctuations in surface water temperatures are less frequent during MIS 6 and the amplitude of SST variability during MIS 8 appears to be of a lesser magnitude (3-4°C) compared to the last glacial period (6-8°C) at the southern site. Significant differences are also evident when comparing SST variability and patterns of IRD variation through MIS 7 and 5 and the last 3 deglacial transitions. Results from these North Atlantic cores reveal a complex pattern resulting from differences in the timing and the strength of mid latitude temperature gradients which accompanied the phases of ice sheet growth and decay over the last 300,000 years.