Penultimate glacial sea surface temperature and hydrologic variability in the tropical South Pacific from 150 ka Tahiti corals

Abstract. Constraining climate models under extreme boundary conditions of the past on societally-relevant timescales is complicated by a common lack of high-resolution reconstructions of sea surface environmental variability for glacial periods. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope (δ¹⁸O) records from well-preserved and precisely-dated fossil corals of the penultimate glacial and last glacial periods drilled by Integrated Ocean Drilling Program Expedition 310 "Tahiti Sea Level" in the central tropical South Pacific. The proxy records document the mean and seasonality of sea surface temperature (SST) and seawater δ¹⁸O (δ¹⁸O_sw) at 153 ka and 148 ka, during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results show mean SST 2.8–4.0 °C lower than present for MIS 6b, and about 3.8 °C lower for MIS 3a. The MIS 6b SST differences are greater during the austral winter (3.7–4.4 °C lower) than during the austral summer (2.0–3.7 °C lower), indicating a greater SST seasonality relative to today during the penultimate glacial. A reconstructed higher mean δ¹⁸O_sw for both MIS 6b and MIS 3a (+0.41‰ to +0.51‰ relative to today) suggest more saline surface waters in the central tropical South Pacific over the entire year. Our coral-based reconstructions of SST and hydrology may indicate a reduced mixed layer depth around Tahiti during the penultimate glacial and last glacial. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by lower activity and/or displacement of the South Pacific Convergence Zone.