Ice volume variations and provenance trends in the Oligocene-early Miocene glaciomarine sediments of the Central Ross Sea, Antarctica (DSDP Site 270)

Since the early Oligocene, the descent into global colder conditions has led to the stepwise growth of the Antarctic Ice Sheets. However, ocean and air temperatures were not sufficiently cold enough to maintain a large and stable continental ice sheet and associated marine ice shelf, and therefore during the Oligocene to Miocene the glaciation waxed and waned. However there is limited geological evidence for the extent of these early glacial episodes. Here we present a multi-proxy single-grain provenance study on the glaciomarine sediments in DSDP Site 270, from the Central High in the central Ross Sea to determine the extent of such ice volume variations. The Ross Sea embayment is a key region in Antarctica for preserving geological evidence on the early glacial events, because glaciation was synchronous with continuous sea-floor subsidence and uplift of the surrounding mountain ranges. The Central High in the central Ross Sea was drilled by the DSDP project in the 1970s (Site 270) and a sequence of c. 400 m of Oligocene to lower Miocene glaciomarine sediments was recovered, underlain by a few meters of granitic talus breccia and gneissic basement rocks. The sediments evolve from a non-glacial, continental environment to a glaciomarine environment characterized by variable ice-rafted debris content and several diamictite units. Our approach combines information on crystallization ages in the source region (U-Pb detrital zircon dating), medium- and low-temperature thermochronology constraints (detrital apatite U-Pb and fission-track dating on the same grains), information on the source bedrock type (detrital apatite trace element analysis) and petrological analysis of gravel-size clasts. We provide new constraints on the ice sheet evolution since the earliest phases of glaciation, and show a transition from local to regional-scale ice sheets in the central Ross Sea region in the late Oligocene and early Miocene. In addition, our data can identify the source of sediment transported by ice streams to the central Ross Sea from different areas in the Transantarctic Mountains (TAM) and within the West Antarctic Rift System (WARS). In particular, the provenance data show the late Oligocene diamictite strata were sourced from a distant region, likely southern West Antarctica, indicating a substantial continental ice sheet already existed in the late Oligocene. These new data provide robust geological constraints for modeling of ice sheet volumes and ice drainage patterns in the late Oligocene-early Miocene.