Reconstruction of Antarctic Cenozoic Paleoenvironments Through Palynological Analysis of Subglacial Lake and Ice Stream Sediments

Abstract

The West Antarctic Ice Sheet (WAIS), with a large portion being grounded below sea level, is historically known for its instability. Timing and evaluating the magnitude of WAIS advance and retreat is essential to improve our understanding of control mechanisms. The scarcity of accessible subglacial geology in areas such as the Siple Coast in West Antarctica limits our understanding and ability to gauge WAIS response to environmental changes. To evaluate the ice-sheet history around the Siple Coast, i.e., when it was last covered by vegetation, access to sediment immediately below the ice-sheet is essential. In the absence of large-scale drilling, such as ANDRILL, in this region, sediments were sampled by targeting basal tills. Palynological analysis was conducted on ninety-one sample of West Antarctic subglacial till, shallow subsurface sediments, and grounding zone wedge sediments from the Bindschadler Ice Stream (BIS), Kamb Ice Stream (KIS), Whillans Ice Stream (WIS), Subglacial Lake Whillans (SLW), the Whillans Grounding Zone (WGZ), and the Ross Ice Shelf Project (RISP). Based on two key dinoflagellate cysts and pollen and spores recovered, the age of the youngest assemblage (last vegetation) is estimated to be between 60 and 30 Ma. Our results show that the bulk of the vegetation likely disappeared between 33 to 30 Ma. We acknowledge that some of the tundra vegetation recovered could be as young as Oligocene, as the absence of post-Eocene age dinoflagellate cysts could be a result of limiting environmental sea-surface conditions. Palynomorphs recovered indicate differences between sites. BIS and KIS have the highest terrestrial palynomorph concentrations observed by more than double all other sites combined, and also are composed of a high abundance of moss spores. WIS, SLW, and WGZ are primarily composed of a large diversity of Nothofagidites spp., indicating the presence of rich and diverse beech forests – a diversity typical of Eocene times. The difference in assemblages confirms that the catchment boundaries of BIS, KIS, and WIS are distinct from one another. Vegetation recovered offers confirmation of West Antarctic paleotopographic models suggesting that large portions of West Antarctica were above sea level at least until the end of the Eocene.