HIGH-RESOLUTION ICE RAFTING RECORD FROM THE ALASKA MARGIN, IODP EXPEDITION 341

A major goal of IODP Expedition 341 is to assess the glacial history of the Northwestern Cordilleran ice sheet in the Pacific. Site U1419 was drilled in the northeast Gulf of Alaska on the continental slope in a water depth of 721 m. This near shore location is in the path of the Alaskan Coastal Current and it receives a large influx of glacimarine sediment and ice rafted debris (IRD).

364 samples were taken from the composite section at Site U1419 to produce an IRD curve based on the coarse sand fraction (250 microns – 2 mm) abundance. Visual inspection of each sample confirmed the presence of terrigenous sand. The sand fraction from the largest peaks was used to prepare epoxy grain mounts for thin sections, which were analyzed for composition and grain roundness. Microtextures of quartz grains were imaged with a scanning electron microscope to determine the glacial transport history.

Based on initial chronology developed by radiocarbon dating of foraminifers, the record to 90 m below sea floor spans the last 60,000 years B.P. Our sample spacing averaged 150 years and sedimentation rates ranged from 25cm/ka to 100cm/ka, making this the highest resolution record in the Pacific. The record contains four broad groups of IRD peaks averaging three to four thousand years duration. Timing of these peaks approximately corresponds with North Atlantic Heinrich Events: H1, H2, H4, and H6. The composition of sand in the peaks is dominated by rock fragments and quartz grains reflecting a provenance of nearby coastal Alaska. Transition from the IRD-dominated sediment occurs abruptly at 14.9 ka yrs B.P., which corresponds with a sharp drop in sediment density identified by Davies et al. (2011). This clearly shows deglaciation of coastal Alaska in the marine record. In addition, the multi-millennial scale sedimentation rates calculated at Site U1419 mimics the broad scale peaks and troughs of the IRD curve showing clear linkage between the overall sediment flux and the advance of glaciers to sea level in the region.