Orbital-scale record of Ross Sea ocean temperatures across the Miocene Climatic Optimum and Middle Miocene Climate Transition

The Miocene Climatic Optimum (MCO; ~17-14.5 Ma), the warmest period of the Neogene, preceded the Middle Miocene Climate Transition (MMCT; ~14 Ma), one of the three major Cenozoic intervals of cooling and Antarctic Ice Sheet expansion. The MCO was characterized by reduced meridional thermal gradients, a significant reorganization of Earth's carbon cycle, and orbitally-paced fluctuations in global ice volume. Proxy CO₂ data suggest MCO ice growth and MMCT expansion occurred when atmospheric pCO₂ concentrations were 250 to 600 ppmv, implying enhanced climate sensitivity or additional forcing mechanisms (e.g., changes in poleward heat and moisture transport) were required for cryosphere expansion.

To understand how ocean-ice sheet interactions influenced Miocene Antarctic cryosphere expansion, International Ocean Discovery Program (IODP) Expedition 374 to the Ross Sea recovered an early to middle Miocene (~17-14 Ma) sedimentary sequence from Site U1521 (75°41.0351'S, 179°40.3108'W, 562 m water depth), drilled in the Pennell Basin on the Ross Sea outer continental shelf. The sequence contains an expanded diatom-bearing/rich mudstone unit (~17-16 Ma) indicative of open-marine conditions during the MCO. A hiatus exists between ~16 and ~15 Ma, suggesting ice expansion during the MCO, but an interval of diamict dating to ~15-14 Ma indicates that ice retreated in Pennell Basin immediately before the MMCT.

Here we present the first early to middle Miocene ocean temperature (TEX₈₆) record from Site U1521, which reveals orbitally-paced variations in high-latitude upper ocean temperatures between ~17 and 16 Ma. This record, combined with lithofacies, palynologic, and geochemical analyses from Site U1521 and ANDRILL-2B, on the inner Ross Sea continental shelf provide an ice-proximal view of Ross Sea glaciation and polar amplification during the early to middle Miocene. This integrated view is required to elucidate the timing, mechanisms, and feedbacks associated with Antarctic ice sheet and global climate system development during the early to middle Miocene.