YOUNGER DRYAS MARINE ECOSYSTEM RECORD FROM SITKA SOUND ALASKA, USA: RAPID TRANSITION FROM PROGLACIAL LAKE TO OPEN OCEAN

The deglacial history of the Northeast Pacific Ocean margin is complex, reflecting abrupt environmental changes that include sea level rise, enhanced volcanic activity, and the retreat of the Cordilleran Ice Sheet. Against this complex backdrop, sediment core EW0408-40JC (56.99^o N, 135.48^o W, 216 m water depth, 1161 cm core length) recovered from Sitka Sound on the southeastern Alaska continental margin, contains a new record of changing aquatic ecosystems. The age model for the core is based on radiocarbon dates as well as correlation of a tephra at 900 cm with tephra erupted from the Mount Edgecumbe volcanic field. These show that the core spans the period ~14,400 to 11, 600 cal yr BP and contains a full record of the Younger Dryas. Diatom assemblage data reveal a multifaceted paleoenvironmental history. Between ~14,400 and 12,500 years ago (1,080-750 cm), the diatom assemblage is dominated by Aulacoseira spp. and Stephanodiscus spp., indicating a freshwater lake. During this interval, biogenic silica (opal) concentrations decrease from more than 25 to around 15 wt%. The beginning of the Younger Dryas (885 cm) is marked by an abrupt increase followed by an abrupt decrease and then a gradual decline in freshwater diatoms and biogenic silica. The local salinity appears to increase in the middle part of the Younger Dryas (~12,500) with a decrease in freshwater planktic taxa and an increase in salt-tolerant benthic species, as well as the appearance of marine species. The abundance of freshwater taxa fluctuates between 12 and 30 percent of the diatom assemblage through the end of the Younger Dryas at 11,700 cal yr B.P., while the remainder of the assemblage becomes increasingly dominated by open ocean and benthic marine taxa. Opal values fluctuate between 10 and 15 wt% during this time, and finally stabilize at about 10 wt% at the end of the Younger Dryas. The silicoflagellate flora is less diverse than those found elsewhere in the Gulf of Alaska, reflecting less favorable conditions. The sediments in the upper 20 cm of the core contain typical North Pacific Ocean diatom and silicoflagellate assemblages, and opal values reach 15 wt%.