Archaeological excavations at the coast of A'asu, in Tutuila Island of American Samoa, exposed a depositional sequence spanning the past circa 700 years. With the period represented, sedimentation rates exceeded 10.15 cm per century in the valley floor and 16.34 cm per century along the valley margin. The occupational history may correlate with changes in climate, sea level, and coastal geomorphology. Although the evidence accords with the expected responses to the Little Climatic Optimum (circa 1050 to 690 B.P.) and Little Ice Age (circa 575 to 150 B.P.), the most plausible explanation for the A'asu case is that environmental change accompanied expansion of upland land use. Based on evidence here and elsewhere in Tutuila, it is proposed that the establishment of fortifications, monuments and permanent settlements in the uplands was part of a broader pattern of land-use expansion beginning in the fourteenth century A.D. Keywords: American Samoa, Polynesia, landscape evolution, prehistoric human impacts, geochronology.
Thirty-five years ago, a study of oxygen-isotope ratios from a speleothem (cave formation) in New Zealand demonstrated that a rapid temperature drop occurred in southern Polynesia in the fourteenth century. Wilson et al. (1979) proposed that the violent climatic conditions accompanying this temperature drop would not only have resulted in increased latitudinal temperature variants but would also have had catastrophic effects on agriculture. At the same time, the more violent climate would have made long-distance voyages inherently more dangerous.
More recently, Bridgeman (1983), based on an extensive review of previously published climatological data, proposed that climatic change may have contributed to a general collapse in Polynesian migrations after A.D. 1350. These studies have received renewed discussion since 1995 when Nunn (1995, 1998, 1999, 2000) published sea level data synchronized to climate change, proposing his own theory that changing climatic conditions had a profound regional effect on Polynesian culture.
A better understanding of climate-induced landscape change is necessary in order to better model human response to dynamic ecosystems. Climate change may result in systematic environmental adjustments in an alluvial catchment. Because archaeological sites are found within Holocene-age sediments, it is necessary to understand the late Quaternary history of the region in order to understand prehistoric settlement patterns and the completeness of the archaeological record.
In order to determine the extent to which changes in the late Holocene geological record correlate with climate and ecological changes over the same period, geoarchaeological explorations were undertaken in A'asu on the remote northern shores of Tutuila Island, American Samoa (Fig. 1). (1) As the geochronology became known, it was apparent that the sedimentary record had been highly irregular in the late Holocene, particularly after A.D. 1300. Explaining the causes of that dynamism became the focus of research over the next several years. In this article, geoarchaeological data from A'asu are compared with similar data from coastal settings (Ayres and Eisler 1987; Clark and Michlovic 1996; Hunt and Kirch 1997) and combined with recent research on the establishment of mountain settlements Pearl 2004) to show that changes in climate, settlement patterns, and landscape evolution converged, beginning in the fourteenth century A.D., a time when social complexity was on the rise in Samoa.
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The lush valley of A'asu is situated on the north coast of western Tutuila, with numerous inhabitable valleys undisturbed by roads and contemporary settlements, standing in stark contrast to other more populated and developed areas of Tutuila. The valley floor extends about 0.5 km inland and comprises about 0. …