THE EVOLUTION OF OCEAN SURFACE CONDITIONS IN THE SOUTHWEST PACIFIC OVER THE PLIO-PLEISTOCENE TRANSITION

The Pliocene (5.3 to 2.7Ma) was a time of global warmth relative to the cool Pleistocene (2.7 to 0.01Ma). Because the Pliocene was the most recent period of sustained warmth and had atmospheric carbon dioxide concentrations comparable to today (400ppm), it has been used as a potential analog for future warmth and studied in order to better understand how the climate system behaves during warm climate intervals. Here, we present an orbital resolution (3kyr) alkenone-derived sea surface temperature (SST) record from Ocean Drilling Program Site 1125 (42.55°S, 178.17°W, 1360m water depth). We developed an age model for our record by tuning to the LR04 benthic stack and then refining the model by tuning to the alkenone-derived SST data from ODP Site 846 (3.06°S, 90.49°W). Our record is the first high-resolution (3kyr) SST record from the southwest Pacific Ocean that spans the Plio-Pleistocene transition (3Ma to present). Comparisons of our site 1125 SST record with other Plio-Pleistocene temperature records from the southwest Pacific derived from other proxies (e.g. benthic δ¹⁸O and faunal analysis) suggest that our alkenone-based approach is reliably reflecting past variations in climate conditions from this region. Our data show a ~4°C cooling from the Pliocene into the Pleistocene. Our site 1125 record taken in conjunction with SST datasets from other mid-latitude sites in the southern hemisphere indicate quite different cooling histories. In contrast, SST records from mid-latitude sites in the northern hemisphere indicate a coordinated response to the Plio-Pleistocene transition. The ~2-4°C cooling observed at southern hemisphere sites in contrast to the ~3-5.5°C cooling observed at northern hemisphere sites suggests that the expansive ocean warm pool during the Pliocene contracted in an hemispherically asymmetric manner over the Plio-Pleistocene transition. The effects of orbital forcing are also visible in the ODP 1125 record, characterized by strong 100k power with weaker 41k and 23k beats. Evolutionary spectra reveal that 100k power dominates throughout the 3 Myr interval in the southern hemisphere sites, in contrast to the more classic 41k to 100k transition across the Mid-Pleistocene Transition (MPT) evident from the benthic δ¹⁸O and North Atlantic SST records.