Paleoceanographic evolution of the equatorial Indian Ocean during the late Miocene (9-5 Ma)

During the late Miocene (~11-5 Ma), atmospheric pCO2may have been 1 to 3 times pre-industrial values1,2, and mean global temperatures were likely 3−4°C higher than today with reduced latitudinal gradients3. Major ecosystem changes that took place during the late Miocene (e.g., ref 4 and refs therein) are consistent with falling global temperatures and decreasing pCO2, but proxy evidence for these physical climate changes remains sparse because few continuous, orbital-resolution palaeoclimate records cover this interval. Recently, geographically widespread and major (~5-10°C) late Miocene sea surface cooling was revealed for the mid-and high-latitudes based on the Uk'37 sea surface temperature (SST) proxy3. However, existing tropical SST records suggest only minor cooling in this interval (1-2°C). Here we present new multi-proxy geochemical records of surface and deep ocean variability from IODP Site U1443, drilled in the equatorial Indian Ocean (5°N, 90°E) during IODP Expedition 353, which spanthe interval 9 to 5 Ma. Surface ocean records from Site U1443 have the potential to resolve SST change at a “warm pool” tropical site, whilst also providing insight into the long-term evolution in the Indian monsoon subsystem. SST estimates derived from Mg/Ca ratios in the surface-dwelling planktic foraminifer Trilobatus trilobusshow ~4°C of cooling (3.8 -2.5 mmol/mol Mg/Ca) between 8 and 6 Ma. Following a SST minimum at 5.8-6 Ma, reconstructed temperatures increase from 5.8 to 5 Ma, consistent with published extratropical SST records. SST estimates combined with paired foraminiferal δ18O measurements reveal a ~1.5‰ decrease in local seawater δ18O between 8 and 6 Ma, and a subsequent increase between 6 and 5 Ma. Consistent with records from other sites, Site U1443 benthic δ18O data show no long-term increase concurrent with the SST decrease. We compare our results with late Miocene simulations using the IPSL-CM5 earth system model, exploring both pCO2and paleogeographic effects on regional paleoceanography.