Timing, frequency and nature of sedimentary processes operating on the eastern Ross Sea continental slope during the Pleistocene- a record from IODP Expedition 374

Abstract

<p>The repeated proximity of West Antarctic Ice Sheet (WAIS) ice to the Ross Sea continental shelf break has been inferred to directly influence sedimentary processes occurring on the continental slope. Sediment delivery to the shelf edge by grounded ice sheets during past glacials may have influenced turbidity current and debris flow activity, thus the records of these processes can be used to study the past history of the WAIS. However, the continental slope record may also be affected by density-driven or geostrophic oceanic bottom currents, therefore additionally providing an archive on their history and interplay with depositional mechanisms that are driven by ice sheets. Here, we investigate the upper 120.94m of one sediment core (length: 208.58mbsf) from Hole U1525A collected by International Ocean Discovery Program (IODP) Expedition 374 in 2018. Hole U1525A is located on the south-western levee of the Hillary Canyon (Ross Sea, Antarctica) and the depositional lobe of the nearby trough-mouth fan. Using core descriptions, grain size analysis, and physical properties datasets, we develop a lithofacies scheme that allows construction of a detailed depositional model and environmental history of past ice sheet-ocean interaction at the eastern Ross Sea continental shelf break/slope for the past 2.4 Ma. The earliest Pleistocene interval (2.4-1.35 Ma) is interpreted as a largely hemipelagic environment dominated by ice-rafting and reworking/deposition by relatively persistent bottom current activity. Microfossil barren, finely interlaminated sediments are interpreted as contourites deposited under the presence of multi-year sea-ice. During the latter part of the early Pleistocene (1.35-0.8 Ma), bottom current activity was weaker and turbiditic processes more common, likely related to the increased proximity of grounded ice at the shelf edge. Much of the fine-grained sediments were probably deposited via gravitational settlement from turbid plumes, and a sustained nepheloid layer. The thickest interval of turbidite interlamination occurs after ~1 Ma, following the onset of the “Mid-Pleistocene Transition” (MPT), interpreted as a time when most terrestrial ice sheets increased in size and glacial periods were longer and more extreme. Sedimentation in the mid-late Pleistocene (< ~0.8 Ma) was dominated by glacigenic debris flow deposition, as the trough mouth fan that dominates the eastern Ross Sea continental shelf prograded and expanded over the site. More frequent and longer-lasting fully-extended glacial conditions allowed the continued progradation of the trough-mouth fan across the core site. These findings will help to improve estimations of WAIS ice extent in future Ross Sea shelf-based modelling studies, and provide a basis for more detailed analysis of the formation and growth of the WAIS under distinct oceanographic conditions.</p>