Midlatitude Temperature Variations in the Oligocene to Early Miocene

Antarctic ice sheet margin extent and the sensitivity of benthic delta O-18 to orbital forcing have varied on million-year timescales during the Oligocene to Early Miocene. However, few sea surface temperature (SST) records for this time interval exist to evaluate links between polar processes and mean temperature outside polar regions. Here, we present a new record of SST for the time interval 30 to 17 Ma derived from the long-chain alkenone unsaturation ratio (U-37(k')) at Integrated Ocean Drilling Program Site 1406A in the midlatitude North Atlantic. Results confirm that warm temperatures from 24 degrees C to over 30 degrees C prevailed in midlatitudes in this time and suggest a transition from colder early-middle Oligocene to warmer average conditions after 24.5 Ma. The global significance of this transition is highlighted by the coincidence with changes in the dominance from marine- to terrestrial-terminating ice sheets in the Ross Sea around Antarctica. The longest continuous section of the record (20.6 to 26.6 Ma) contains multiple 2 million-year cycles in SST, potentially paced by long obliquity modulation. Complex and temporally varying relationships are observed between North Atlantic SST and benthic delta O-18 in paired samples; significant covariation is only observed around the Oligocene-Miocene transition, coincident with a lower average marine ice extent. These North Atlantic U-37(k') temperature records provide a new context in which to examine the stability of climate and the Antarctic ice sheet during the Oligocene and early Miocene.