DID ICE EXIST ON THE CHUKCHI SHELF DURING THE LAST GLACIAL MAXIMUM?

Recently discovered glaciogenic bedforms, identified by seafloor mapping, suggest that a grounded ice shelf existed during the Last Glacial Maximum (LGM) on the western Chukchi Shelf (Polyak et al., 2001 Nature, pp.453-457). The orientation of the bedforms led Polyak et al. (2001) to infer that ice from Alaskan/Canadian ice streams was deflected by either the continental slope or by the edge of a large “Chukchi ice sheet ”- located along the lower portions of the Chukchi Shelf. A likely place to find evidence to support the LGM ice sheet hypothesis is Wrangel Island, which forms the highest point on the shelf.

If such an ice mass was present, Wrangel Island would have been directly affected by either being 1) glacioisostatically loaded by the crust; or 2) covered by the ice mass. If the former were true, then raised marine deposits of post-LGM age would be expected on the landscape. Instead, we have found much older raised marine deposits of early Pleistocene age that correlate with similar deposits in Alaska associated with high eustatic sea levels and not glacioisostatic loading on the shelf. If the island were ice-covered, then LGM-age glacial landforms indicative of northerly ice-flow should be found there. The lack of glacial landforms, deposits, or erratics in major river valleys suggests that Wrangel Island was mostly or completely ice-free during the LGM. Furthermore, paired 10Be and 26Al cosmogenic isotope ages on quartz from boulders on the tops of rounded hills suggest the area was unglaciated for at least the last 35 ka and possibly longer. Thus, it is unlikely that a Chukchi ice sheet, grounded or otherwise, existed on the Chukchi Shelf during the LGM based on our geomorphic and geochronologic evidence, and alternative interpretations regarding the age and origin of the bedforms should be considered.