Age models and tephrostratigraphy from two lakes on Adak Island, Alaska

Radiocarbon-based age models from two lakes on Adak Island, Alaska, were compared using correlated tephra to check the accuracy of the radiocarbon ages and to develop a regional tephrochronology. Andrew and Heart Lakes are located adjacent to and 5 km south of Mt. Moffett on the northern part of Adak Island in the central Aleutian Arc. The downcore trends in radiocarbon ages of macrofossils, the 1963 spike in plutonium (239+240Pu) activity, and a lead (210Pb) profile from Andrew Lake were modeled with smooth spline fits. Tephras were correlated based on magnetic susceptibility, particle morphology, and internal stratigraphy including color and particle size. Of the 19 pure tephras in Heart Lake and 21 in Andrew Lake, ten could be confidently correlated between the two lakes. The composite tephrostratigraphy from the two lakes includes at least 30 unique tephras deposited over 9600 years. Radiocarbon ages obtained from mixed aquatic and terrestrial macrofossils with ages hundreds of years too old were inferred for Heart Lake by correlated tephra ties to the Andrew Lake age model. The eroding tephra-soil deposits that crop out along the banks of Heart Lake, and its large shallow platform, may facilitate remobilization of previously stored sediments. Previous studies on Adak Island of archeological sites and tephra-soil outcrops described five tephras that are correlated with a subset of tephras from the lakes based on stratigraphic succession, inferred age, and physical description. Age-depth models for Heart and Andrew Lakes as well as tephrochronology for Adak Island provide geochronological context for future studies.