2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 4
Presentation Time: 8:00 AM-12:00 PM


NELSON, Robert E., Dept. of Geology, Colby College, 5804 Mayflower Hill, Waterville, ME 04901-8858 and VANDERHOEK, Richard, 2927 Glacier Street, Anchorage, AK 99508, renelson@colby.edu

A 3.5-meter coastal bluff exposure of postglacial sediments has produced a complex, detailed record of local vegetation changes over the past >10ka on the central Alaska Peninsula. Postglacial vegetation succession and plant migration patterns have been punctuated by sometimes catastrophic volcanic events.

Stratigraphy in the site includes at least 16 distinct volcanic ashes, including an oxidized airfall pumiceous tephra 80 cm thick (<7350 14C ybp) and the 70-cm-thick Aniakchak pyroclastic flow (PF) of ca. 3500 14C ybp. Volcanic units are interbedded with multiple buried A horizons and thin (<15 cm) peats. One fine-medium sand 15 cm thick bears numerous monocot leaves and stems, and may represent an early Holocene shallow pond. Minor lateral slumping down a paleoslope is suggested by distortion of some bedding surfaces. Initial postglacial vegetation at the site, not yet radiocarbon dated, was dominated by ferns and Cyperaceae, with minor Poaceae, Ericales and Betula. Alnus had become abundant by 7350 14C ybp. However, the dominant theme of the record is recovery from episodic volcanic eruptions that blanketed the area with tephra. Mesic communities have existed only during the late phases of landscape recovery. Early vegetation following eruptions has been dominated by Poaceae (>70% of the basic pollen sum) with associated xeric or disturbed-ground forbs (e.g., Fabaceae, Onagraceae, Artemisia and other Asteraceae). Low pollen accumulation rates and abraded grains suggest landscapes were discontinuously vegetated for prolonged times; part of the surface of the Aniakchak PF remains unvegetated to this day elsewhere on the peninsula.

Repeated disruptions of the biological communities by voluminous volcanic eruptions has major implications for not just postglacial environments, but also for prehistoric human populations in the region, as resource bases were repeatedly altered.