LINKING TERRESTRIAL-MARINE RECORDS OF GLACIATION, NEOTECTONISM, CLIMATE CHANGE, AND VOLCANISM USING TEPHROCHRONOLOGY

James Beget

Dept. of Geology and Geophysics

and Geophysical Institute

University of Alaska

Fairbanks, AK.  99775

Abstract

     Explosive eruptions are frequent at Aleutian arc volcanoes, where at least 12 caldera-forming eruptions have occurred just since deglaciation.  Tephras from these eruptions can be traced thousands of kilometers downwind. Volcanoes in the Wrangell Volcanic Complex and the Mt. Edgecumbe Volcanic Complex near Sitka have also dispersed tephra across Alaska, NW Canada, and the NE Pacific Ocean.

        Tephra layers are important tools for age-dating and correlating widely separated sediment sequences.   Some of the major tephras have been dated by fission track, Ar/Ar, radiocarbon, thermoluminesence, and by orbital tuning of proxy climate records, but many others remain undated.  The electron microprobe, ion microprobe, and laser ICP-MS have been the most successful geochemical approaches to geochemical characterization of the tephras.

        About 70 major regional tephras layers are currently known from eastern Beringia.The tephras are most often found in thick loess deposits, sometimes associated with active or relict permafrost, or with ancient interglacial buried forests.  More rarely, tephras are preserved on outwash terraces or incorporated in glacial moraines. Less is known about ttephras in the NW Pacific Ocean.  The  Dawson tephra (25 kyr)has just been identified in a marine core, and is the first Pleistocene ash to link terrestrial and marine records (Beget, unpub. data).  The identification of additional tephras in marine sediments will provide more direct links between terrestrial and marine sediments, and help reduce age uncertainties for some tephras.

        The earliest glaciation in eastern Beringia is thought have occurred ca. 2.6 million years ago.  Loess deposition near Fairbanks and along the Yukon River is dated by the same teprhas, and began about the same time.  The oldest fossil permafrost in Alaska is associated with 2.0 myr PA tephra at the Palisades of the Yukon, while tephras at Dawson are found with fossil permafrost ca. 2.6 million years old.

          Tephrochronology suggests glaciations in interior Alaska were in-phase with global Milankovitch cycles at least since MIS 7. However, differential neotectonic uplift  within the Alaska Range apparently produced varying glacial histories. The maximum ice advance in the eastern Alaska Range is recorded by the Delta Glaciation (MIS 6), which is bracketed by the Old Crow tephra (140 kyr) and the Sheep Creek Ash (190 kyr).  The anomalously short glacial record in the eastern portion of the Alaska Range suggests this area has been uplifted significantly since middle Pleistocene time (Beget and Keskinen, in press). In contrast, moraine sequences of the western Alaska Range and in the Yukon Territory record numerous ice advances since ca. 2.0 myr, and require much earlier uplift in these areas.