We reconstructed the orbital-scale variability of deep-water mass properties in the equatorial Indian Ocean during the late early to late Miocene (17.9 to 8.2 Ma) using benthic foraminiferal oxygen (δ18O) and carbon (δ13C) isotope records, in combination with X-ray fluorescence scanner elemental records at International Ocean Discovery Program (IODP) Site U1443. These records suggest that hyperthermal-like features (negative excursions in δ18O and δ13C coupled to suboxic and acidic conditions in the deep ocean) developed at eccentricity maxima during the Miocene Climatic Optimum (MCO; 16.9 to 14.7 Ma) on an almost ice-free Earth. Elevated Mn and U concentrations indicate that bottom and pore waters in the Indian Ocean became progressively suboxic during the later phase of the MCO. A fundamental change in deep-water circulation occurred after the expansion of the East Antarctic Ice Sheet during the Middle Miocene Climatic Transition (MMCT; 14.7 to ~13.8 Ma). Stepwise increases in δ18O at ~13.8 and ~13.1 Ma concurred with dampening of deep-water δ18O and δ13C variability, as Antarctic ice cover expanded and became more stable. A marked improvement in deep-water oxygenation after ~13.5 Ma coincided with the end of the last δ13C maximum of the Monterey Excursion. Carbonate dissolution in the equatorial Indian Ocean intensified and prolonged episodes of reduced carbonate deposition during the Carbonate Crash interval (~12.6 to 8.6 Ma) were not restricted to eccentricity maxima as during the MCO. Rising Mn and U concentrations after ~9 Ma indicate decreased oxygenation of bottom and pore waters, associated with the onset of the Biogenic Bloom in the Indian Ocean.
https://doi.org/10.1016/j.palaeo.2021.110511