Pre-glacial, Early Glacial, and Ice Sheet Stratigraphy Cored During NBP1402, Sabrina Coast, East Antarctic Margin

Abstract:

Western Wilkes Land provides an unusual setting with regard to passive margin subsidence and exposure of Cenozoic sedimentary units across the continental shelf, due to the unique rift to drift history off of the Australian-Antarctic Discordance and subsequent deep glacial erosion of the evolved continental shelf. The first factor has provided extensive accommodation space for the preservation of stratigraphic sequences that in turn represent critical periods in the climate evolution of Antarctica. Glacial erosion has then provided access to this stratigraphy that is usually inaccessible to all but deep drilling programs. Such stratigraphies are well exposed to within cm of the seafloor off the Sabrina Coast. Cruise NBP1402 investigated this region via a combination of multi-channel seismic imaging and innovative, strategic coring. The geophysical data imaged the geologic evolution of the margin, which exhibits a continuum from non-glacial, partly glaciated, to fully glaciated depo- and erosional systems. Based on the seismic stratigraphy, we collected dredges and one barrel Jumbo Piston Cores (JPCs) across areas of outcropping strata imaged seismically, a unique strategy that allowed us to identify and sample specific reflectors. The stratigraphically deepest coring targeted sections for which the seismic character suggested a pre-glacial context, with non-glaciated continental margin sequences including deltas. Coring recovered dark organic rich siltstones and sandy mudstones, and a large concretion whose center contained a cm-sized plant fossil. In addition, the sediments contain a fossil snail. These fossils provide a glimpse into the pre-glacial terrestrial environment in Antarctica. Overlying this section, coring recovered similar dark siltstones with a 20 cm thick horizon with abundant large angular clasts of variable lithology, interpreted to be ice-rafted debris and indicative of early glacial ice in Antarctica. Finally, JPCs targeting a younger part of the sequence recovered diatomites suitable for biostratigraphic control. We highlight this new mode of operation for JPCs as a relatively inexpensive way to acquire stratigraphic samples at key seafloor outcroppings with the tremendous benefit of being able to conduct this kind of sampling from a mobile platform in icy waters.