Coupled Southern Ocean temperatures and East Antarctic hydroclimate during Miocene global warmth

East Antarctica experienced a dramatic transformation in hydroclimate during the Miocene, from warm and wet conditions during the Miocene Climatic Optimum (MCO, ~14-17 Ma) to the establishment of polar desert conditions with cooling and ice sheet expansion during the Middle Miocene Climate Transition (MMCT, ~13.8 Ma) and late Miocene cooling interval (~6-8 Ma). During Miocene warmth, retreat of Antarctic ice sheets allowed for the rise of vascular plant ecosystems on Antarctica, which preserved information on terrestrial hydroclimate through their epicuticular waxes. Here, we investigate molecular biomarkers of plant waxes from East Antarctica preserved in the polar Southern Ocean at Ocean Drilling Program Site 1165. We quantify the isotopic composition of long-chain n-alkanoic acids, providing a record between 5 and 19 Ma. In addition, we present high-resolution (5 kyr) data between 13.2 and 15.1 Ma which span the expansion of the East Antarctic Ice Sheet during the MMCT.

Comparison of plant wax isotopes (δD) with U^k’₃₇, a sea surface temperature (SST) proxy, that we measured in the same samples constrains interactions between polar Southern Ocean temperatures and East Antarctic terrestrial hydroclimate throughout the Miocene. Our records reveal a timescale-dependent relationship between SSTs and plant wax δD, with opposing isotopic responses to temperature forcing on orbital versus longer timescales. Our results demonstrate a complex response of Antarctic precipitation isotopes during the Miocene, with plant wax δD not only reflecting temperature changes, but also changes in vapor transport to Antarctica, aridity, and/or ecology. We conclude that our plant wax constraints on Antarctic precipitation isotopes have broader implications for the past isotopic composition of the East Antarctic Ice Sheet and the resulting deconvolution of global benthic δ¹⁸O into temperature and ice volume components during the Miocene.