QUATERNARY GEOLOGY AND PALEOECOLOGY OF SOUTHEASTERN ALASKA: PROGRESS AND OPPORTUNITIES FOR FUTURE RESEARCH
AGER, THOMAS A. U.S. Geological Survey.
The Alexander Archipelago of southeastern Alaska has been largely neglected by Quaternary geologists, perhaps because of logistical challenges, a rainy climate, a dense, jungle-like cover of coastal rainforest and muskeg vegetation, and in many areas, an abundance of brown bears. However, significant progress in unraveling the late Quaternary history of adjacent coastal British Columbia has been made in recent decades, demonstrating that successful research can be done in spite of the region?s challenging environment.
The U.S. Geological Survey began a research project in 1998 aimed primarily at reconstructing the postglacial history of environmental change in southeastern Alaska, by means of pollen analysis of cores obtained from peat deposits and lake sediments. We have also attempted to improve on mapping of the poorly delineated extent of late Wisconsin glacial ice in southeastern Alaska (Ager, 1999; Carrara et al., this volume). Ongoing USGS research on the history of postglacial vegetation development and climate history builds on pioneering studies by Calvin Heusser in the 1950?s. Heusser conducted a study of the vegetation history of the North Pacific coast from northern California to the Aleutians by studying pollen records obtained by coring peat deposits (e.g., Heusser, 1960, 1985). From a modern day perspective, Heusser?s research in southeastern Alaska suffers from a lack of radiocarbon dates for most sites, and the few sites with some age control usually have only a single radiocarbon date.
During the ongoing USGS research project, 6 lakes and 14 peat deposits have been sampled between Ketchikan and Yakutat, and we are presently analyzing and dating many of these sites. These pollen records will provide a detailed regional vegetation and climate reconstruction for the postglacial. Lake sediment records tend to preserve longer histories than peat deposits, thus we have cored as many lakes as we could.
Preliminary results of our USGS study suggests that the major deglaciation of southeastern Alaska occurred between 14,000-13,000 radiocarbon yr B.P. although some ice thinning and retreat may have begun earlier. Only one site we have sampled penetrates to an early (ca.13,000 yr B.P.) lowland tundra vegetation that colonized newly deglaciated lands in the western fringes of the Alexander Archipelago. Most sites we have studied thus far indicate that the earliest recorded vegetation following local deglaciation was open pine forest (probably Pinus contorta, ssp. contorta) with an understory composed mostly of ferns. This vegetation type may have developed as early as 12,500 yr B.P. in some areas. During the Younger Dryas interval of cold, dry climate (ca. 11,000-10,000 yr B.P.), pine-dominated forests declined as alders (Alnus) spread across much of the region, and Sitka spruce (Picea sitchensis) began to replace pine as the dominant tree species. Western hemlock (Tsuga heterophylla) and mountain hemlock (Tsuga mertensiana) soon spread across much of southeastern Alaska during the early Holocene. Early Holocene forests were probably not as dense and as extensive as those which developed in the late Holocene under a wetter climatic regime. Red cedar (Thuja plicata) and yellow cedar (Chamaecyparis nootkatensis) do not appear to have been well established in southern southeastern Alaska until the late Holocene.
A rich record of fossil vertebrates is emerging from cave deposits in karst areas of southeastern Alaska (e.g., Heaton et al., 1996). Radiocarbon dates indicate that the faunas span more than the last 50,000 years of the late Quaternary. Unfortunately, the pollen records obtained thus far are mostly younger than 13,000 yr B.P., and therefore the vegetation types associated with the earlier faunas cannot yet be documented. Future studies may lead to the discovery of older pollen-bearing deposits from the region. There are a number of areas on the western islands of the Alexander Archipelago that appear to have escaped being overridden by glacial ice during the late Wisconsin. In those refugia, deposits of late Wisconsin age, and possibly older fossil-bearing sediments may be preserved. Finding such deposits is proving to be a challenging task.
Recent archeological discoveries in southeastern Alaska and in the Queen Charlotte Islands of northern coastal British Columbia indicate that humans have inhabited the region for at least the past 10,000 years (Dixon et al., 1997; Baichtal et al., 1997; Josenhans et al., 1997). Additional geological studies are needed to help pinpoint areas where other sites of early occupation by humans may be found, in caves, on isostatically and tectonically raised shorelines, or on the now-submerged inner continental shelf.
There are many areas of southeastern Alaska where there are excellent subjects for future research projects by university-based scientists. Some topics for research include: (1) refinement of glacial limits during the late Wisconsin and earlier glacial events; (2) studies of Holocene glacial history of mountainous areas of the Alexander Archipelago (e.g., Baranof Island); (3) sonar and seismic imaging of the inner continental shelf to delineate till deposits, moraines, lake basins, glacial river valleys, and other features submerged during postglacial sea level rise; (4) sediment coring on the inner continental shelf to obtain terrestrial and postglacial (marine) paleoenvironmental records; (5) high resolution paleolimnological studies of sediment cores from lakes in the region (e.g., diatoms); (6) unraveling the complex history of regional postglacial tectonic uplift and isostatic rebound; (7) study of karst landscape formation in a wet, cool temperate environment; (8) searching for evidence of full-glacial paleoenvironments within refugia in the western Alexander Archipelago.
Ager, T.A., 1999. Late Wisconsin glacial and postglacial history of southeastern Alaska: Implications for ecosystem development and colonization by humans: Geological Society of America Abstracts with Programs, v 31, no. 7, p. A-367.
Baichtal, J., Streveler, G., and Fifield, T., 1997. The geological, glacial and cultural history of southern southeast. In Rennick, P. Alaska?s Southern Panhandle: Alaska Geographic, v. 24, no. 1, p. 6-31.
Carrara, P.E., Ager, T.A., Baichtal, J.F., and Van Sistine, D., (this volume), Late Wisconsin glacial limits in southern southeastern Alaska, as indicated by a new bathymetric map.
Dixon, E.J., Heaton, T.H., Fifield, T.E., Hamilton, T.D., Putnam, D.E., and Grady, F., 1997. Late Quaternary regional geoarchaeology of southeast Alaska karst: a progress report: Geoarchaeology, an International Journal, v. 12, p. 689-712.
Heaton, T.H., Talbot, S.L., and Shields, G.F., 1996. An ice age refugium for large mammals in the Alexander Archipelago, southeastern Alaska: Quaternary Research, v. 46, p. 186-192.
Heusser, C.J., 1960, Late Pleistocene Environments of North Pacific North America: American Geographical Society, Special Publication 35, New York, 308 p.
Heusser, C.J., 1985, Quaternary pollen records from the Pacific Northwest coast: Aleutians to the Oregon-California boundary. In Bryant, V.M., and Holloway, R.G. (eds.), Pollen Records of Late-Quaternary North American Sediments: Dallas, American Association of Stratigraphic Palynologists Foundation, p.141-165.
Josenhans, H.W., Fedje, D.W., Pienitz, R., and Southon, J., 1997. Early humans and rapidly changing Holocene sea levels in the Queen Charlotte Islands-Hecate Strait, British Columbia, Canada: Science, v. 277, p. 71-74.
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