Paleoceanographic Variability in the Central South Pacific during the Late Miocene-Pliocene: A Benthic Foraminiferal Proxy

The Southern Ocean (SO) is an essential part of the world ocean's thermohaline circulation due to the production of bottom waters such as Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW). These deep-waters transfer physical water properties across the major ocean basins. However, the behaviour of the bottom water conditions in the Central South Pacific (CSP) and associated changes to CDW and AABW under warmer than present conditions are poorly understood. Hence, we assess this by analyzing Late Miocene to Pliocene marine sediment cores from IODP Site U1541 in the CSP (54°12.756'S, 125°25.540'W). At 3604 m water depth, the site is bathed by the Lower Circumpolar Deep Water (LCDW) and influenced by the periodic variability of AABW. During the early Pliocene epoch, the Earth was warmer than at the present day, and the study site is on an ideal location for reconstructing paleoceanographic turnovers during that period using benthic foraminiferal assemblages. Based on the coherent result of R-mode factor and Q-mode cluster analysis on the major benthic foraminifera species that are present in relative abundances of ≥5% in 5 or more samples, four biofacies associations have been identified (biofacies Melonis spp., biofacies Laticarinina pauperata, biofacies Uvigerina spp., biofacies Oridosalis umbonatus). These biofacies suggest a remarkable shift in bottom water oxygenation and organic matter flux. This study suggests that from 5.6 to 5.3 Ma, mostly oxic bottom water conditions with moderate organic flux prevailed in the CSP, which changed to suboxic to dysoxic conditions with moderate to high organic flux between 5.3 and 4.6 Ma. The enhanced influx of AABW caused suboxic to oxic conditions with low organic export flux during 4.6 - 3.3 Ma and is supported by the oxic biofacies association Oridosalis umbonatus.