Prelude to the Plio-Pleistocene Glaciations: Southwest Pacific Sea Surface Temperature During the Late Miocene and Pliocene

Cenozoic climate cooling and the concomitant establishment of polar ice sheets, first in the southern hemisphere and then in the northern hemisphere, is well-documented by the deep sea benthic oxygen isotope record and by glaciological evidence. Yet, our knowledge of regional climate evolution over this time interval is still not well-resolved. Further spatial resolution of regional climate trends during the transition from unipolar to bipolar glaciation can help us better understand the respective influences of the northern and southern hemisphere on climate system evolution and probe the relationship between climate and atmospheric CO2 levels. Here we use the alkenone paleothermometer to document southern hemisphere sub-tropical climate behavior over the last 10 Myr, with particular interest in climate conditions during the Pliocene, in the lead up to the intensification of northern hemisphere glaciation. We present a sea surface temperature (SST) record from ODP Site 1125 in the southwest Pacific (42°S, 178°W), which shows a monotonic decline in SST in this region since 10 Ma, from temperatures of around 25°C to the modern mean annual SST of ~15°C. The steady SST decline in the sub-tropical southwest Pacific is not well-correlated with existing reconstructions of pCO2, which imply a major drop in CO2 by ~12 Ma and only modest changes in mean atmospheric CO2 levels between 10 Ma and the present. In the absence of evidence for a significant CO2 change over this interval, we suggest that our SST record is more likely forced by the contraction of southern Pacific gyre circulation in response to the growth of Antarctic ice. If this is true, our regional SST record may provide valuable insight into the southern hemisphere ice volume component of the deep-sea benthic oxygen isotope record. We also present orbitally-resolved Pliocene SST data from Site 1125 in order to assess the frequency and amplitude of southern hemisphere sub-tropical SST cycles during the early Pliocene Warm Period and subsequent intensification of northern hemisphere glaciation. Pliocene glacial-interglacial temperature variations are 2-3°C. Based on a preliminary age model, 41 kyr cyclicity appears to become much more pronounced beginning at ~2.7 Ma, coincident with the amplification of northern hemisphere glacial cycles and in line with the pattern observed in SST records from several other tropical and northern hemisphere localities.