COMPARING THREE NEW IRD MAR RECORDS FROM IODP EXPEDITION 341 IN THE GULF OF ALASKA

Three drill sites from IODP Expedition 341 in the Gulf of Alaska have new radiocarbon dated, high resolution ice-rafted debris (IRD) records. We used IRD mass accumulation rate (MAR) curves from the sites: U1417, U1419, and U1421 to track the activity of marine-ending glaciers in coastal Alaska during the Last Glacial Maximum back to 35 Kyr. These sites were drilled as a transect, with Site U1421 located 10 km from the mouth of the Bering trough, Site U1419 located farther west on the continental slope, and Site U1417 located in the open ocean, approximately 400 km SW of Bering Glacier. Icebergs calved from Bering Glacier were transported to Site U1421 and then Site U1419 on the Alaska Coastal Current, at times reaching Site U1417. IRD MAR decays exponentially with iceberg transport distance: Site U1417 is one order of magnitude lower than Site U1419, which is three orders of magnitude lower than Site U1421. IRD MAR for Site U1417 is also affected by the deep sea sedimentation rate, compared with the slope sites.

We propose that rocky, coarse-grained intervals in the drill core resulting in no recovery at Site U1421 are periods of glacial advance to the shelf edge. This interpretation is supported by analogous IRD MAR drops in Site U1419. Intervals of coincident IRD peaks suggest periods of glacial termini retreat along the Alaskan coast. During a period of Bering Glacier retreat, a short-term reduction in IRD MAR was observed at Site U1421. We propose that this was caused by sea ice expansion which may have trapped Bering Glacier icebergs on the shelf. Inconsistencies in IRD MAR peaks do occur between the three sites, potentially due to contributions from other coastal Alaskan tidewater glaciers, influences of oceanic currents, and varying local sea surface temperatures. Glaciers finally terminated at Site U1421 and U1419 at approximately 15.5 Kyr. Prominent peaks in IRD MAR at Sites U1419 and U1417 are coincident with Heinrich event 1 (H1), but the response to H2 and H3 is less apparent.