A POSTGLACIAL HISTORY OF THE MATANUSKA VALLEY

The Matanuska Valley in south central Alaska was glaciated several times, although the timing of the most recent deglaciation is poorly known and remains controversial. Samples of peat and marl were taken from an exposure in a lateral moraine near the present-day terminus of the glacier to better define the glacial and climate history of the region. The outcrop is situated near the top of the moraine, approximately 30m above the present Matanuska River. From bottom to top, the exposure consists of a diamicton base, roughly 20m thick, followed by a thin marl layer (15cm), and a diamicton lens sandwiched by peat. The diamicton lens is roughly 115cm thick, with as little as 5cm of peat on the bottom and 70cm at the top. Sediments at this site were deposited into a narrow depression located adjacent and parallel to the former lateral ice margin, with the peat and marl indicating climate change towards warmer summers. Based on this interpretation, we suggest that the diamicton lens is the result of sediment flowing off the glacier and into the peat. Therefore, radiocarbon dates from the peat above and below this lens should be close in age. Preliminary study of ostracodes indicates a wetland environment with warmer than present-day summers. Two radiocarbon dates on mollusk shells from the marl produced uncalibrated ages of 13,060 +/- 40 and 12,640 +/- 40 indicating that marl deposition and warm summer environments occurred during the late Pleistocene. Pollen, ostracode, and additional radiocarbon samples are under study and will provide a more definite post-glacial picture of the Matanuska Valley.