Alkenone unsaturation estimates of late Miocene through late Pliocene sea-surface temperatures at Site 958

Abstract

Samples of pelagic carbonates from Ocean Drilling Program Hole 958A were analyzed for the unsaturation index of C 37 alkenones, a proxy for sea-surface temperature (SST), over a record that ranges from 2.2 to 6.5 Ma. Sufficient alkenones were recovered from all samples to estimate past SST. The profile shows quite stable unsaturation values from 6.5 to 2.5 Ma. Two significant excursions toward lower unsaturation indices occur at ~2.3 and 2.5 Ma, with a more modest decrease at ~3.1 Ma. These indicate that North Atlantic SST decreased during times of increased Northern Hemisphere glaciation. Using the Prahl et al. (1988) calibration to convert unsaturation indices to paleotemperature, we deduce that late Miocene through early late Pliocene SSTs at the location of Hole 958A were about 25.3 ± 1°C, and that the late Pliocene coolings associated with glacia-tion were of the order of 5°C, similar in magnitude to the most recent glacial-interglacial change in SST in the region. Results from Site 958 suggest that, with the initiation of Northern Hemisphere ice sheets, high latitude temperature anomalies were linked to large decreases in ocean temperatures in the subtropical North Atlantic.