PROVENANCE OF ICE RAFTED DEBRIS ALONG THE ALASKAN MARGIN OVER THE LAST 50 KA

The ice rafted debris mass accumulation rate (IRD MAR) from IODP Expedition 341 Site U1419 is a proxy for both iceberg and sea ice transport adjacent to the continental shelf. The lithology of rock fragments within these samples is used to infer the origin of ice along the coastline. The drainage basin of the Bering Glacier cuts across the Chugach terrane, Wrangellia terrane and Yakutat terrane, eroding siltstone, sandstone, argillite and basalt. Marble and granite rock fragments originate from outside of this area and traveled on the Alaskan Coastal Current from further south. The provenance is determined from the frequency percent of these lithologies in each sample. Both icebergs from Bering Glacier and sea ice are considered local and icebergs from Hubbard Glacier and glaciers further south along the Alaskan coast are far traveled.

The samples of up to 300 grains of coarse sand (250 µm-2mm) were embedded in epoxy and thin sectioned. Each grain was mapped and identified on a low magnification image under crossed polars and plane light. Provenance of each grain mount was determined from frequency percent of local lithologies versus far traveled lithologies.

The coarse sand provenance is dominated by lithologies originating from the Bering Glacier drainage basin, although far traveled lithologies are present in every sample. 5 discrete IRD MAR peaks occur within the last 50 ka. The largest peak, dominated by local provenance, occurs between 43-40 ka. Between 37-29 ka IRD MAR gradually increases with far traveled lithologies becoming a major component. Three peaks occur between 27-16 ka that are dominated by local provenance. The timing supports the interpretation that long distance travel by icebergs occurs during warmer conditions when sea ice is at a minimum.