GULF OF ALASKA CONTINENTAL SHELF – A COMPLICATED GLACIAL TO MARINE TRANSITION IN THE LATE QUATERNARY

Detailed knowledge of sea level rise on the Gulf of Alaska continental shelf has long been of interest to paleoanthropologists studying the migration of indigenous populations into the Americas along the coastal route. Sites containing records of the transition from lacustrine and estuarine to marine conditions provide important evidence of the availability of coastal occupation sites in a complex environment affected by eustatic and relative sea level changes, as well as local tectonic, volcanic, and glacial conditions. Here we evaluate diatom and biogenic silica (opal) records from two sediment cores on the southeastern Alaska margin: EW0408-11JC (Barron et al., 2009, Mar. Micro., 72, 176-195; Gulf of Esquibel, 55.63^oN 133.51^oW, 183 m wd, 1752 cm long) and -40JC (Sitka Sound, 56.99^oN 135.48^oW, 216 m wd, 1161 cm long). Age control is provided by ¹⁴C and tephrochronology.

Both sites preserve a complete Bølling-Allerød (B-A) to Holocene record, although the post-glacial transition to modern conditions is different at each site. In EW0408-11JC, freshwater diatom taxa increase to almost 40% of the assemblage, replacing sea ice and related taxa, and coastal and oceanic species comprise the remainder of the assemblage during the B-A and early Younger Dryas (YD). Freshwater species are then replaced by sea ice-related species during the YD and the former disappear in the Early Holocene (EH), whereas the assemblage is dominated by coastal and oceanic species during the Holocene. Freshwater taxa dominate the assemblage in EW0408-40JC during the B-A and early YD. Brackish and marine species begin replacing freshwater taxa early in the YD and the assemblage is comprised almost entirely of brackish and marine taxa by the EH, with sea ice species and related species never comprising more than a few percent of the assemblage. The results suggest that conditions at 11JC during the B-A and early YD were estuarine, with a gradual reduction in freshwater influence, while those at 40JC were lacustrine. Although the shifts in assemblage composition reflect transition through different environments, they reflect eustatic and relative sea level rise. These data show the ecological effects of rapid sea level rise along the Alaskan margin and offer insight into potential impacts associated with projected sea level rise in the future.