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32nd Annual Arctic Workshop Abstracts
March 14-16, 2002
INSTAAR, University of Colorado at Boulder

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LATE WISCONSIN GLACIAL LIMITS IN SOUTHERN SOUTHEASTERN ALASKA, AS INDICATED BY A NEW BATHYMETRIC MAP

AUTHORS

CARRARA, PAUL E. U.S. Geological Survey.
Ager, Thomas A. U.S. Geological Survey.
Baichtal, James F. U.S. Forest Service.
Van Sistine, Darren . U.S. Geological Survey.

LATE WISCONSIN GLACIAL LIMITS IN SOUTHERN SOUTHEASTERN ALASKA, AS INDICATED BY A NEW BATHYMETRIC MAP

Carrara, P.E.1, Ager, Thomas A.1, Baichtal, James F.2, Van Sistine, Darren1 , 1U.S. Geological Survey, Mail Stop 980, Box 25046, Denver Federal Center, Denver, CO 80225 (pcarrara@usgs.gov), 2U.S.D.A. Forest Service, Tongass National Forest, P.O. Box 19001, Thorne Bay, AK 99919

An ice extent during the late-Wisconsin glaciation (ca. 26,000 to 13,000 yr BP) in southern southeastern Alaska, more restricted than previous estimates, is indicated by evidence revealed by a new bathymetric map. The map was prepared from a digital database of water depths and contoured, thus allowing topographic depiction of submarine features, including moraines, deep glacially scoured troughs, and submarine slides. During the late-Wisconsin glaciation the Cordilleran glacier complex formed vast ice fields along the crest of the Coast Mountains. As these glaciers flowed west toward the Gulf of Alaska they were joined by local glaciers originating on the higher reaches of the islands of the Alexander Archipelago (Mann and Hamilton, 1995). This extensive volume of ice was channeled into the present-day fjords that formed major outlet glaciers, such as the glaciers that occupied Chatham Strait and Dixon Entrance.
The bathymetric map indicates that major outlet glaciers in Chatham Strait, Sumner Strait, and Dixon Entrance eroded deep (>500 m in places) troughs extending to the western edge of the continental shelf. For instance, the glacier that flowed into the Dixon Entrance trough advanced to the edge of the continental shelf; its retreat began sometime after 15,000 to 16,000 BP and the trough was free of ice by 13,000 BP (Barrie and Conway, 1999). On the shelf the Sumner Strait trough is not a well-defined feature and appears to be partly filled with sediments. It may be pre-late Wisconsin in age. In addition to these major troughs, smaller glacial troughs and moraines are discernable at several locations on the inner continental shelf. Although the exact extent of the late Wisconsin glaciers in southeastern Alaska is poorly known, the new bathymetric map indicates that most late Wisconsin glaciers along the western edge of the Alexander Archipelago did not extend to the shelf edge as some previous studies suggested (e.g. Pewe, 1975).
In addition to the bathymetric map possible ice-free areas in southeast Alaska during the late-Wisconsin glaciation were identified by analyses of aerial photographs, topographic maps, and large-scale nautical charts, as well as a review of previous literature, and reconnaissance fieldwork. These ice-free areas include, 1) parts of the inner continental shelf exposed by the lowering of sea level during the late Wisconsin, 2) parts of the westernmost islands of the Alexander Archipelago (Worley, 1980), 3) unglaciated ocean-facing slopes and forelands, and 4) nunataks, present on many of the high mountains of Admiralty, Baranof, Chichagof, and Prince of Wales Islands.
The bathymetric map shows extensive areas of the inner continental shelf that were subaerially exposed during the last glaciation, when global sea level was about 125 m below present sea level. These ice-free areas on the continental shelf may include; an area adjacent to the southern part of Baranof Island in the vicinity of Whale Bay (830 sq. km), an area west and south of Coronation Island (620 sq. km), and a large area west and adjacent to Noyes and Baker Islands (1150 sq. km). Several of the outer islands apparently were not overridden by continental ice, including Forrester, Coronation, and Warren. Although the latter two islands probably supported small local glaciers, it appears that parts of these islands were ice-free. Unglaciated ocean-facing slopes and forelands were probably present on Noyes, Baker, Sumez, and Dall Islands.
These unglaciated areas may have been important refugia for at least the hardiest elements of the southeastern Alaska flora and fauna during the late Wisconsin and as centers for biotic dispersal upon deglaciation (Heusser, 1960, 1989; Worley, 1980). Furthermore, these refugia may have served as "stepping stones" for human migration along the coast of North America (Heusser, 1960, 1989; Dixon, 2001; Mandryk et al., 2001).
The bathymetric map also shows what may be a massive submarine slide on the edge of the continental shelf west of Baker and Noyes Islands that may have been triggered by a large-magnitude earthquake along the Fairweather-Queen Charlotte fault. Two possible submarine volcanoes can also be seen south of Coronation Island and west of Noyes Island.

REFERENCES
Barrie, J.V., and Conway, K.W., 1999, Late Quaternary glaciation and postglacial stratigraphy of the northern Pacific margin of Canada. Quaternary Research, v. 51, p. 113-123.
Dixon, E.J., 2001, Human colonization of the Americas: timing, technology, and process. Quaternary Science Reviews, v. 20, p. 277-299.
Heusser, C.J., 1960, Late-Pleistocene environments of north Pacific North America.: an elaboration of late-glacial and post-glacial climatic, physiographic, and biotic changes. Special Publication no. 35, American Geographical Society, 308 p.
Heusser, C.J., 1989, North Pacific coastal refugia - the Queen Charlotte Islands in perspective. In Scudder, G.G.E. and Gessler, N., (eds.). The outer shores, proceedings of the Queen Charlotte Islands 1st International Symposium, University of British Columbia, p. 91-106.
Mandryk, C.A.S., Josenhans, H., Fedje, D.W., Mathewes, R.W., 2001, Late Quaternary paleoenvironments of northwestern North America: implications for inland versus coastal migration routes. Quaternary Science Reviews, v. 20, p. 301-314.
Mann, D.H. and Hamilton, T.D., 1995, Late Pleistocene and Holocene paleoenvironments of the north Pacific coast. Quaternary Science Reviews, v. 14, p. 449-471.
Pewe, T.L., 1975, Quaternary geology of Alaska. U.S. Geological Survey Professional Paper 835, 135 p.
Worley, I, 1980, Ancient environments and age of non-glaciated terrain in southeastern Alaska. National Geographic Society Research Reports (1971), p. 733-747.

FIGURES


Figure 1. Map of southern southeastern Alaska showing late Wisconsin ice limit

 

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