Hydrologic cycling over Antarctica during the middle Miocene warming

From 20 to 15 million years (Myr) ago, a period of global warmth reversed the previous ice growth on Antarctica, leading to the retreat of the West Antarctic Ice Sheet and the contraction of the East Antarctic Ice Sheet(1,2). Pollen recovered from the Antarctic shelf indicates the presence of substantial vegetation on the margins of Antarctica 15.7 Myr ago(3). However, the hydrologic regime that supported this vegetation is unclear. Here we combine leaf-wax hydrogen isotopes and pollen assemblages from Ross Sea sediments with model simulations to reconstruct vegetation, precipitation and temperature in Antarctica during the middle Miocene. Average leaf-wax stable hydrogen isotope (delta D) values from 20 to 15.5 Myr ago translate to average delta D values of -50 parts per thousand h for precipitation at the margins of Antarctica, higher than modern values. We find that vegetation persisted from 20 to 15.5 Myr ago, with peak expansions 16.4 and 15.7 Myr ago coinciding with peak global warmth(4) and vegetation growth(5). Our model experiments are consistent with a local moisture source in the Southern Ocean(6). Combining proxy measurements with climate simulations, we conclude that summer temperatures were about 11 degrees C warmer than today, and that there was a substantial increase in moisture delivery to the Antarctic coast.