Evidence for West Antarctic Glaciation During the Late Oligocene (~26-23 Ma): 40Ar/39Ar Ages of Ice-rafted Debris from DSDP Site 270, Ross Sea

Abstract:

Ice-rafted biotite and hornblende grains from the Late Oligocene strata of Deep Sea Drilling Program Site 270 were analyzed for ⁴⁰Ar/³⁹Ar derived ages. Characteristic ages are found from both East and West Antarctica, indicating that both of these areas were glaciated and producing icebergs during a time when major West Antarctic glaciation was previously uncertain. Site 270 is located in the central Ross Sea on the southeastern flank of the Central High (77°26.48'S; 178°30.19'W). Approximately 36% of the grain population exhibits ages between 475 and 500 Ma, representing the Ross Orogeny and Transantarctic Mountains of East Antarctica. These ages are slightly older than the bedrock at the base of Site 270, which has been previously dated at 437 ± 6 Ma. Smaller populations of grains are clustered near 100, 200, and 350 Ma, all of which point to a West Antarctic contribution. While it has been previously thought that West Antarctic glaciation could have initiated during the Oligocene, it has remained inconclusive. Our data set presents the first geochemical provenance evidence of a glaciated West Antarctica during this time. We suggest the Ross Sea was not just influenced by the ice caps residing on the uplifted basement highs, but was already supporting the transport of ice bergs from the surrounding coastlines.