Multiple Miocene marine productivity events in West Antarctica as recorded in upper Miocene sediments beneath the Ross Ice Shelf (Site J-9)

Abstract

Considerable debate surrounds the age and origin of sediments recovered from underneath the Ross Ice Shelf, Antarctica at Site J-9. Although diatom biostratigraphy provides the best means of dating these sediments, diatom fragmentation, sediment reworking and microfossil assemblage mixing have led to conflicting interpretations regarding the age and depositional history of sediments collected as part of the Ross Ice Shelf Project (R.I.S.P.). Our biostratigraphic approach differs from previous analyses of R.I.S.P. sediments by differentiating between microfossil assemblages recovered from the sediment matrix and from within abundant semi-indurated sediment clasts. Diatomaceous clasts that were eroded and transported to Site J-9 by glacier ice reveal the age and composition of unknown marine deposits in West Antarctic interior basins. In addition to rare reworked Paleogene and Cretaceous microfossils, three distinct Miocene diatom assemblages are recognized — two from sediment clasts (middle lower Miocene and early middle Miocene) and a third restricted to matrix sediments (middle upper Miocene). Contrary to some published reports, no microfossils unequivocally younger than middle late Miocene are present, other than modern subice-shelf benthic foraminifera and ostracodes found in the uppermost sediments. Whether R.I.S.P. sediments are an in situ glacimarine deposit or a basal till is still uncertain. However, new data presented here support deposition by glacial marine processes during the middle late Miocene; an age indicated by foraminifera and the youngest diatom assemblage in R.I.S.P. Pliocene and Pleistocene marine sediments were probably stripped off by subsequent ice grounding events, most likely in association with the Ross Sea Unconformity. Reworked sedimentary clasts of diatomite suggest a highly productive, open marine environment in West Antarctica with intense productivity and limited glacial ice at sea level in the Ross Sea embayment during the middle early Miocene.