LATE HOLOCENE GLACIAL HISTORY OF MUIR INLET, GLACIER BAY NATIONAL PARK AND PRESERVE, ALASKA

Glacier Bay National Park and Preserve of Alaska is an intensively glaciated, maritime coastal environment. Retreat of ice from the Glacier Bay Holocene maximum began about 1750 CE and extended to the mouth of Muir Inlet about 1860 CE. Over the next ~130 years, Muir inlet was progressively deglaciated to reveal a series of complex lacustrine and glacial depositional settings with multiple generations of buried and overrun forests spanning ~8500 to 850 yr BP. Our ongoing investigations of the glacial history of Glacier Bay are utilizing these forest remnants dated with radiocarbon and tree-rings to provide a chronology of the tidewater and terrestrial glacial activity in the fjord. Trees that are directly overrun by advancing glaciers or killed by prograding deltas and outwash are sampled and their ages provide the basis for the glacial chronology. Most of these logs were detrital and within fluvial deposits; others are stumps in growth position buried in lacustrine, diamicton and other deposits.

Our focus is on the most recent advance down Muir Inlet that spanned approximately 2400 to 1500 yr BP and the rates of advance in relation to bathymetric data of the fjord showing a series of discrete basins behind morainal banks. Initial findings based on a tree-ring series tied to radiocarbon ages shows that the glacier was advancing circa 2370 yr BP near the head of Muir Inlet and then extended about 15 kilometers during the next 400 years to a mid-fjord position. Continued advance filled the entire Inlet (~40 km) by ~1500 yr BP. The glacier advanced slowly along the reaches where the water depth was greatest, and thus water depth and fjord geometry influenced the rates of ice expansion.

The dating of samples collected during the summer of 2015 will add detail to the glacial chronology and hopefully will bridge a crucial gap in the tree ring record between about 1850 -2200 years B.P. The bridging of the tree-ring record will then provide a continuous, calendar-dated record of this glacial advance through Muir Inlet.