Microfabric Analysis of Upper Pliocene Amundsen Sea Interglacial Sediments (IODP Exp. 379) Demonstrate a Link Between Iceberg Melt and Diatom Productivity

Abstract:

Most of the West Antarctic Ice Sheet (WAIS) is grounded below sea level and is historically unstable and susceptible to retreat, causing dramatic sea level rise. Establishing rates of retreat once the tipping point has been reached is critical to forecasting future behavior of the WAIS. Past studies have been unable to establish the rates of ice sheet collapse in part because shelf records are impacted by erosional hiatuses (Naish et al., 2009). In 2019, the International Ocean Discovery Program (IODP) Expedition 379 obtained records from the Amundsen Sea continental rise, to document WAIS history in an area currently experiencing the largest ice loss in Antarctica. Site U1532 is located on the western upper flank of a large sediment drift, called Resolution Drift. U1532 penetrated 794 m, obtaining a nearly complete Upper Miocene to recent stratigraphic record. The recovered sections include intervals characterized by intense diatom productivity coincident with deposition of pulses of ice rafted debris (IRD). We use low vacuum SEM imagery to capture several instances of soft-sediment deformation associated with dropstones falling into diatomaceous sediment. Nearly mono-specific assemblages of the pelagic diatom Thalassiothrix antarctica, as well as plankton fecal pellets with concentrated clusters of barite grains, indicate high primary productivity directly linked with increasing iceberg melt. We interpret this as clear evidence of continually sourced nutrient seeding of the Southern Ocean by ice sheet retreat, with pulses of IRD coinciding with marine ice sheet collapse that triggered intense diatom productivity. In these Pliocene intervals, there is a notable absence of sea-ice diatoms. Marine snow events represent faster accumulation rates, whereas diatomites consisting of mixed pelagic species show slower accumulation rates and extensive bioturbation. Further analysis may allow interpretation of rates of marine ice sheet retreat during certain Late Pliocene interglacials.