Paleoceanographic Evolution of the Equatorial Indian Ocean during the Late Miocene

During the late Miocene ( 11-5 Ma), atmospheric pCO₂ may have been 1 to 3 times pre-industrial values, and mean global temperatures were likely 3-4°C higher than today with reduced latitudinal gradients. Major ecosystem changes that took place during the late Miocene 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 U^k₃₇ sea surface temperature (SST) proxy, however existing tropical SST records suggest only minor cooling in this interval (1-2°C). Here we present new multi-proxy geochemical and micropaleontological records of surface and deep ocean variability from IODP Site U1443, drilled in the equatorial Indian Ocean (5°N, 90°E) during Expedition 353, which span the interval 9.5 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 long-term and orbital-scale changes in the Indian monsoon subsystem. SST estimates derived from Mg/Ca ratios in the surface-dwelling planktic foraminifer Trilobatus trilobus show 4°C of cooling (from 3.8 to 2.5 mmol/mol Mg/Ca) between 8.5 and 6 Ma. Following a SST minimum at 6 Ma, reconstructed temperatures increase from 6 to 5 Ma, consistent with published extratropical SST records. SST estimates combined with paired foraminiferal δ¹⁸O measurements reveal a 2‰ decrease in seawater δ¹⁸O (δ¹⁸O_sw, uncorrected for ice-volume changes) between 8.5 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 Mg/Ca-SST decrease. We also observe dynamic changes in coccolith mass, a proxy for cell size and/or calcification, on both million-year and more abrupt timescales, sometimes correlated to SST changes. On orbital timescales, our records show coeval cyclicity in SST, δ¹⁸O_sw, and export productivity proxies, suggesting that changes in monsoon strength affected upper water column structure and properties on these timescales during the late Miocene.