Early–Middle Miocene (17–14 Ma) Antarctic ice dynamics reconstructed from the heavy mineral provenance in the AND-2A drill core, Ross Sea, Antarctica

Abstract

The Miocene Climatic Optimum (17–15Ma) and the rapid cooling of the Middle Miocene Climate Transition (15–13Ma) together signal a major change in Earth's climate system. Here we examine the sediment provenance in the AND-2A drill core, located 10km from the East Antarctic coastline, to significantly increase our understanding of Antarctic ice development, glacial erosion, and transport in the Ross embayment during this time. Heavy minerals are very diagnostic of source rock types and assemblages can be used to track changes in the areas of maximum erosion under the margin of an ice sheet. We used a combination of optical mineralogy and SEM-EDS analysis to characterize the heavy mineral fractions of diamictites and sandstones in the upper 650m of AND-2A, which includes an expanded section dated between ~17 and 14Ma. We find four diagnostic heavy mineral assemblages distributed in intervals throughout the core: I. (650–552mbsf) elevated orthopyroxene, titanaugite, and carbonate contents; II. (552–308mbsf) abundant diopside, pigeonite, and orthopyroxene, with sillimanite and kyanite; III. (308–250mbsf) increasing contents of garnet and green hornblende; and IV. (250–20mbsf) abundant green hornblende, titanaugite, green augite, and carbonate. Based on the heavy mineral analysis we demonstrate that (1) the ice sheet was grounded on the shelf at ~17.7–17.1Ma, and it was eroding Cenozoic volcanic rocks to the south of the drillsite; (2) during the early part of the Miocene Climatic Optimum (~17.1–15.5Ma) the East Antarctic Ice Sheet retreated landward into upland regions of the Transantarctic Mountains, where it eroded dolerite sills and high-grade metamorphic rocks; (3) immediately prior to the Middle Miocene Climate Transition (15.5–14.3Ma), the East Antarctic ice advanced and eroded granitic and low-medium grade metamorphic basement rocks in the coastal sections of the Transantarctic Mountains; and (4) following this initial phase of ice growth, the West Antarctic Ice Sheet and the East Antarctic Ice Sheet coalesced into a larger than modern (interglacial) Antarctic Ice Sheet prior to 14.3Ma and eroded Cenozoic volcanic and low- to medium-grade metamorphic basement rocks to the south of the drillsite. Our results suggest that, although East Antarctica may have remained glaciated during the Miocene Climatic Optimum, ice extent was reduced to a configuration within the present interglacial extent, even during orbital-scale glacial maxima. Ice growth during the Middle Miocene Climate Transition commenced at ca. 15.5Ma in the Ross Sea basin, which is slightly earlier than inferred from deep-sea stable isotope records, but in agreement with low-latitude sea-level reconstructions.