LIFE ON THE EDGE: HOLOCENE HISTORY AND TEPHRA STRATIGRAPHY OF
TANGINAK ANCHORAGE, SITKALIDAK ISLAND, KODIAK ARCHIPELAGO,
ALASKA
Elizabeth Mahrt1, Joanne Bourgeois2, J. Benjamin Fitzhugh3
Earth & Space Sciences, University of Washington. Seattle, Washington, USA
2
ESS, University of Washington, Seattle, Washington, USA
3
Anthropology, University of Washington. Seattle, Washington, USA
bmahrt@u.washington.edu, jbourgeo@u.washington.edu, fitzhugh@u.washington.edu
1
Comprehending the implications of human-environment relationships is important in the
North Pacific, where climate change has altered both atmospheric patterns and ocean circulation,
affecting ecosystems already sensitive to change (Hayward, 1997). This region is also subject to
geologic hazards associated with the subducting Pacific plate, which can have profound, if
usually temporary, effects on local ecosystems. Historical observations of climatic changes and
geologic hazards have only begun to be systematically recorded in the 20th century, so available
data are sparse. For this reason scientists have turned to modeling to determine the effects of
these changes on both the environment and human populations (e.g. IPCC 2001). Another way in
which we can help predict the response of modern populations to these changes is to use the
archaeological and geological record to elucidate ways in which similar changes have affected
ancient populations. This approach allows us to extend our knowledge beyond the reach of
modern instrumentation.
The Kodiak Archipelago is a prime area for studying the effects of geologic hazards and
climate changes on prehistoric populations in the North Pacific. This mountainous island chain is
located only ~200 km northwest of the Aleutian Trench, and its close proximity to this actively
subducting margin subjects it to many geologic hazards. The islands have also experienced
marked climate change since deglaciation, allowing us to look at climate variation on a long time
scale. Finally human occupation of this region, which began in the early Holocene, has been
well documented; therefore, we can study thousands of years of human interaction with the
environment (Fitzhugh 2003).
This study focuses on the Tanginak Spring Site. Located on Sitkalidak Island it is the
earliest known human occupation in the Kodiak archipelago. Historically Sitkalidak Island has
experienced both earthquakes and ash falls, so we must assume that such events also occurred
prehistorically (Gilpin 1995). The site was first colonized around 7000 14C yr BP and continued
to be intermittently occupied until 6000 14C yr BP. Nothing in the archaeological record suggests
an obvious reason for the initial occupation of this site and its subsequent permanent
abandonment. It is possible that environmental changes may have made the area more or less
desirable allowing occupation or abandonment to occur.
In this study we are documenting Holocene environmental changes on Sitkalidak Island
and relating these changes to the archaeological record. Studies in the Kodiak Archipelago
suggest that large subduction zone earthquakes occur in this region on about a 500 year interval
(Gilpin 1995, Saltonstall and Carver 2002). Ash falls from volcanic eruptions in both the
Aleutian Islands (Carson et al 2002, Miller and Smith 1987) and the southwestern region of
Alaska (Child et al, 1998) also occur on a regular basis but little is known about the frequency
with which these ash falls may have reached the Kodiak Archipelago. Almost nothing has been
published on the tephrachronology of the region (Gilpin 1995, Tannenbaum 1996, Peteet and
Mann 1994). The palynological record has also sparsely documented, especially between 4,500
and 9,000 yrs BP (Peteet and Mann 1994, Nelson and Jordan 1988, Heusser 1985).
In order to create an accurate Holocene paleo-environmental reconstruction of the
Tanginak Spring Site we are employing several field and lab techniques, including stratigraphic
correlation, tephra stratigraphy, and pollen, diatom, petrographic and microprobe analyses.
Herein we report a preliminary tephra chronology in peat excavations dating back to the earliest
Holocene. Dates are radiocarbon years BP on peat directly below tephra. Marker tephra present
in our reference sections are Katmai 1912, light gray (historic?), medium gray (3370), medium
gray (3720), beige 1 (4340), apricot (5390), beige 3 (6790), black (9280), and white (11,520).
Currently the tephra are being described petrographically and their glass sampled for microprobe
analyses. It is important to establish the tephra chronology in this area not only for local
correlation but also to help established a tephra stratigraphy of the Kodiak Archipelago and
beyond. The frequency of tephra in Tanginak Anchorage sections suggests that tephra will be a
very useful stratigraphic tool in this region.
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