The Pliocene sedimentary record offshore the Amundsen Sea, West Antarctica: initial results from IODP Expedition 379

Abstract:

How greenhouse forcing affects ice sheets and sea level during peak interglacial times is an unresolved question of great societal importance. Here we examine the development of glacial-interglacial sequence motifs in drill cores from the Pacific margin of Antarctica and explore how the West Antarctic Ice Sheet impacted sedimentary systems under different climate states, including during the Pliocene Climatic Optimum (PCO) ~4.5-4.0 Ma. Hundreds of meters of Pliocene sediments were recovered during IODP Expedition 379 in two drill sites within the Resolution Drift. Site U1533 was positioned near a submarine channel originating from the Amundsen Sea continental slope, and Site U1532 was located on the western upper flank of the hemipelagic drift. Abundance and type of sediment transport structures in the cores change gradually in lower Pliocene strata in both drill sites in concert with changes in sedimentation rate. Pliocene strata are characterized with a repetitive facies stacking pattern composed of greenish gray clast-bearing mud with a biosiliceous component, interbedded with dark brownish gray laminated silty clay. The greenish gray clast-bearing muds are tentatively interpreted as “interglacials”, however, many greenish grey units have sharp upward transitions into “glacial” laminated mud and the units are irregularly spaced with depth. Analysis of the red-green channel (a*) in shipboard reflectance spectroscopy and colorimetry (RSC) data for Site U1532 on the shipboard age model demonstrates the variable orbital cyclicity of the facies assemblage. Sequence motifs, which are further explored in half-core magnetic susceptibility and XRF core-scanner data, highlight how ice growth and decay, downslope sediment transport, and the position and intensity of ocean currents affected the depositional systems. The ultimate goal of this work is to combine facies interpretations with the detrital sedimentology and other data sets to assess the behavior of the West Antarctic Ice Sheet in a warmer than present climate scenario.