Limited Ice in northern Beringia During the LGM by oek76922


									           Limited Ice in northern Beringia During the Last Glacial Maximum

  Lyn Gualiteri1, Sergey Vartanyan2, Patricia M. Anderson1 and Julie Brigham-Grette3
      Quaternary Research Center, University of Washington, Seattle, WA 98195
    Wrangel Island State Reserve, Ushakovskoye, Chukotka Region, Russia 687870
     Department of Geosciences, University of Massachusetts, Amherst, MA 01003

It has been proposed that a 2 km thick marine-based “Arctic Ice Sheet” existed on the Chukchi
Shelf during the Last Glacial Maximum (LGM) (Polyak et al., 2001, Grosswald and Hughes,
2002). Several researchers have strongly questioned the existence of an Arctic Ice Sheet; (Sher,
1995; Heiser, 1997; Mostoller, 1997; Gualtieri et al., 2000; Hopkins et al., 1998 Brigham-Grette
et al., 1999) however, there has not been any investigation of the glacial or sea level history
along the proposed ice sheet’s margin. Based on field evidence from Wrangel Island, presented
here as well as previously published field evidence from Chukotka, Russia and Alaska, we
challenge this hypothesis. Wrangel Island’s strategic geographic position is critical for evaluating
the presence or absence of an Arctic Ice Sheet. Beryllium-10 and 26Al cosmogenic isotope ages
on bedrock indicate that the island has not been covered by ice for at least the last 35 ka. The
lack of glacial landforms, deposits or erratics in major river valleys and the ubiquity of tors,
commonly forming columns 10 m high, further support a limited ice cover on Wrangel Island
and in northern Beringia during the LGM.

There is evidence on Wrangel Island for at least two Pleistocene marine transgressions. Eustatic
shorelines (and not glacioisostatic shorelines) across the northern tundra plain marked by
remnant marine sediments and ancient barrier beaches are all older than the range of radiocarbon
dating (>50 ka). Amino acid and ESR age estimates (n = 116) on fossil mollusks from 20-40 m
asl marine sediment are >500 ka, correlative with the Wainwrightian Transgression, as described
in northern and western Alaska. Amino acid age estimates on marine sediment 3-5 m asl are 73
ka, correlative with the Simpsonian Transgression, also described in northern and western
Alaska. All of the marine sediment is normally magnetized, further constraining its lower age
limit. Microfossil analysis on the marine sediment and δ18O isotopic analysis on shells is being
used to help glean information about the temperature, salinity and sea ice cover of the ocean
water at the time of marine sediment deposition. Similar to the paleoshorelines and marine
deposits on the Alaskan North Slope, the Wrangel Island shorelines are clearly eustatic in origin
and no evidence has been found so far for glacioisostatic unloading.

Radiocarbon dates on mammoth bones, teeth, and tusks and other animals (Rhinoceros, bison)
yield ages that range continuously through time from >38 ka to 3700 years indicating the local
presence of large mammals during the Last Glacial Maximum (LGM) and most of the Holocene
(Vartanyan et al, 1993, Nature). These data preclude the presence of an ice sheet during the
LGM and probably over the past half million years. Glacial ice extent on the island and across
northern Beringia during the LGM was limited to mountain valleys and did not extend offshore.

To top