The formation and Late Quaternary palaeoenvironmental history of sediment mounds in the Amundsen Sea, West Antarctica
This thesis presents the first high-resolution palaeoceanographic study of environmental changes in the Amundsen Sea sector of the West Antarctic continental margin during the Late Quaternary. This part of the West Antarctic Ice Sheet (WAIS) is currently experiencing rapid mass loss and longer-term records can provide important context for these changes.
Four piston cores, covering the last c. 375 kyrs, have been studied from two of the five large sediment mounds which stand on the continental rise of the eastern Amundsen Sea. Four of the mounds have been previously been identified in the literature as sediment drifts. The cores were analysed for sedimentology (grain size, physical properties, spectrophotometry), mineralogy (clay minerals, sand fraction composition) and geochemistry (XRF, biogenic silica content, TOC, CaCO3). These data were used to infer the supply of terrigenous material from the West Antarctic Ice Sheet, the amount of biological productivity and the nature of the bottom current. Age constraints for the ≤375 kyr records are derived from relative palaeomagnetic intensity, diatom biostratigraphy, AMS 14C dates, tephrochronology and lithostratigraphy.
Analysis of the sediments together with new geophysical and bathymetric data suggests the mounds are mixed contourite-turbidite drifts. Turbidity currents were initiated at the margins of, and between, the mouths of Pine Island Trough East and West and Abbot Trough. The turbidity currents eroded channels in the slope, some of which connect to the deeply incised, maximum 20 km wide and 400 km long channels separating the drifts. The fine-grained fraction of the turbidity currents was pirated and deposited on the drift crests by the weak, eastwards-flowing bottom current, which may be Antarctic Bottom Water or Lower Circumpolar Deep Water. The coarse-grained component of the turbidity currents was largely constrained to the channels, with occasional spill-over depositing sand and sandy muds on the drift flanks. The drifts are long (250-433 km), narrow (38-130 km), stand up to 900 m above the sea floor and are asymmetric, with a gently-sloping western flank and steeper eastern flank resulting from sediment interaction with the bottom current.
The sediments exhibit strong cyclicity corresponding to glacial-interglacial cycles. During glacial periods, deposition was mostly of grey, terrigenous, typically laminated contourites with dispersed ice rafted debris and locally-sourced kaolinite and illite. Sedimentation rates ranging from 0.1 to 17.2 cm/kyr reflect WAIS advance and retreat across the shelf. Bottom currents captured the fine grained fraction of turbidity currents and deposited laminated sediments, similar to those reported in the drifts west of the Antarctic Peninsula, attributed to a steady bottom current and the absence of bioturbation under perennial sea-ice. Manganese contents suggest suboxic conditions during glacial periods. A laminated sand and sandy mud turbidite deposit is present in a drift flank core.
Olive-brown, bioturbated, diatom-bearing and often calcareous-foraminifera-bearing, mixed contourite and hemi-pelagic muds were deposited in interglacial periods. Sedimentation rates range from 0.2 to ≥3.7 cm/kyr reflecting changes in productivity that were mostly controlled by sea-ice coverage. The smectite content of surface samples from the drifts are larger than any other known sample from the Amundsen Sea shelf or rise and suggest that the bottom current also deposits far-travelled clay.
There are no major depositional anomalies or thick IRD layers in the drift cores that might indicate collapse of the WAIS. However, millennial-scale cyclical variations in the provenance of terrigenous material in PS58/255-2 during mid-late MIS 6 may reflect changes in ice dynamics.
Authors: Horrocks, Jennifer Rose
5 July, 2018