FAUNAL AND ALKENONE RECONSTRUCTIONS OF SUBTROPICAL NORTH-ATLANTIC SURFACE HYDROGRAPHY AND PALEOTEMPERATURE OVER THE LAST 28 KYR

Two techniques for estimating past variations in sea surface temperatu re (SST) have been used to investigate climatic change in Biogeochemic al Oceanic Flux Study (BOFS) core 31K (19 degrees N, 20 degrees W) fro m the eastern subtropical Atlantic, High-resolution SST records for th e last 28 kyr have been produced using planktonic foraminiferal assemb lages, based on the Imbrie-Kipp transfer function technique, and the U -37(K') index derived from abundances of C-37 alkenones biosynthesized by prymnesiophyte algae. Modem observations suggest that these indice s reflect particular hydrographic conditions in the upper ocean: the U -37(K') index corresponds to the temperature at the time of maximum co ccolith productivity, typically late spring-early summer in the study area today, whereas the faunal transfer function is calibrated for sea sonal maximum and minimum temperatures, In general, the faunal and bio marker paleotemperature records display comparable SST variations duri ng the last glacial and degiacial, but although the overall trends are similar, differences exist in the magnitude and timing of these tempe rature changes. Most notably, the faunal T-warm and U-37(K') SST estim ates diverge by 3 degrees C between 8 ka and 6 ka, and this offset per sists through the late Holocene. This difference cannot be adequately explained by uncertainties associated with either the calibration data sets or fluctuating preservation levels. We therefore propose that th e deviation in SST estimates is linked to a switch in the seasonal tim ing of maximum coccolith production from the summer in the glacial oce an to the late spring-early summer in the modem ocean. Our results sug gest that a dual approach to SST estimation based on faunal and biomar ker proxies can provide a valuable means of evaluating mixed layer and productivity changes associated with the movement of oceanographic fr ontal zones during the late Quatemary.