IDENTIFICATION OF ICE-RAFTED DEBRIS IN THE WEDDELL SEA TO CHARACTERIZE GLACIATION AT THE EOCENE-OLIGOCENE TRANSITION

The Eocene-Oligocene transition (EOT) occurred approximately 34 million years ago (Ma) when the Earth’s climate changed from a warm greenhouse to a cooler climate which resulted in rapid ice-sheet growth and stable continental-scale ice-sheets in Antarctica. Hole 696B of the Ocean Drilling Program Leg 113 drilled in 650 m of water depth on the South Orkney Microcontinent spans the entire EOT. Cores 53R and 54R are dated at lower Oligocene (~33.1 to ~33.5 Ma) and total ~14 meters in thickness from ~549 to ~564 mbsf. Ice-rafted debris (IRD) in sediment cores gives insight into the extent of glaciation in West Antarctica, as well as the evolution of global cooling during the EOT. Distinguishing between Sea-ice IRD and Iceberg-IRD provides information about weathering processes and depositional environments. Laser Particle Size Analysis was used to determine the volume percentage of sediment in different size classes, i.e. the grain size distribution of 113 samples. IRD was identified by examining quartz grains ˃150 µm. Samples with large volume percentages of coarse grains were sieved to obtain the sand fraction of grains >150 µm. 5 samples with the greatest yield of grains ˃150 µm were selected for microtexture analysis using the Scanning Electron Microscope (SEM). SEM with Energy Dispersive Spectroscopy (EDS) was used to identify quartz grains. Sea-ice IRD and Iceberg-IRD grains were distinguished by examining physical topography and microtextures in Secondary Electron images. Quartz grains were categorized into 7 groups based on roundness, grain relief, amount of silica dissolution, and presence of grain fractures. The 7 types are further grouped by IRD type where Iceberg-IRD types have little to no silica dissolution, angular/sub-angular grains, with fractures present; Sea-ice IRD types have silica dissolution, rounded/sub-rounded grains, with little to no fractures present. 185 grains from 5 samples were analyzed using EDS and SEM, 105 of which were quartz grains. Based on the IRD groupings, 40% of the quartz grains are Iceberg-IRD and 60% are Sea-IRD. The ratio of Sea-ice IRD to Iceberg-IRD is uniform in 3 samples. Sea-ice IRD is dominant in 2 samples. We conclude that IRD is present in the early Oligocene, originating from different depositional environments based on the variety of quartz grains identified.