[M-IS14] Paleoclimatology and paleoceanography

[MIS14-P28] Penultimate glacial sea surface temperature and hydrologic variability in the tropical South Pacific from fossil Tahiti corals

*Ryuji Asami¹, Thomas Felis², Ryuichi Shinjo^3,4, Masafumi MURAYAMA^5,7, Yasufumi Iryu^1,6 (1.Graduate School of Science, Tohoku University, 2.MARUM – Center for Marine Environmental Sciences, University of Bremen, 3.Research Institute for Humanity and Nature, 4.Department of Physics and Earth Sciences, University of the Ryukyus, 5.Center for Advanced Marine Core Research, Kochi University, 6.Advanced Institute for Marine Ecosystem Change (WPI-AIMEC), Tohoku University, 7.Faculty of Agriculture and Marine Science, Kochi University)

Keywords:IODP Expedition 310 “Tahiti Sea Level”, fossil coral, Sr/Ca paleothermometer, seawater oxygen isotope ratio, penultimate glacial period, South Pacific Convergence Zone

Ocean-atmosphere interactions in the tropical Pacific Ocean play a substantial role in global climate on seasonal and interannual timescales. The Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), areas of enhanced precipitation, are prominent features in the tropical-to-subtropical Pacific. Their activity and position change on seasonal and interannual timescales, partly related to the Western Pacific Warm Pool (WPWP) and the El Niño/Southern Oscillation (ENSO). Knowledge of past ocean-atmosphere variability from geological archives is crucial for better understanding the natural range of Earth’s climate system on these societally-relevant timescales. Especially, high-resolution sea surface temperature (SST) reconstructions for extreme climate conditions of the past (e.g., glacial periods) are essential to constrain numerical model simulations of past and future climate. However, widely-distributed foraminiferal Mg/Ca and alkenone records from deep-sea sediments commonly document long-term variability of millennial to multimillennial seawater temperature changes for depths of <50–100 m, due to low sedimentation rates and bioturbation in the open ocean. This precludes seasonal reconstructions of past ocean-atmosphere variability. Furthermore, rare glacial temperature reconstructions from marine sediments in the tropical-to-subtropical South Pacific contain large uncertainties and inconsistencies with climate model simulations. Here, we present subseasonally-resolved Sr/Ca and oxygen isotope 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 oxygen isotope ratio during Marine Isotope Stage (MIS) 6b, and around 30 ka during MIS 3a. Results indicate that penultimate glacial and last glacial climate around Tahiti is characterized by colder and more saline conditions at the sea surface and seasonal differences in thermal and hydrological conditions are more pronounced relative to today. Our coral-based reconstructions of SST and hydrology suggest a reduced mixed layer depth around Tahiti during the glacial periods. A potential explanation is a westward-expanded South Pacific subtropical dry area relative to today, probably accompanied by a lower activity and/or displacement of the South Pacific Convergence Zone.