A THOUSAND YEAR HISTORY OF GLACIAL CHANGE FROM COLUMBIA GLACIER, ALASKA

The once 60 km-long Columbia Glacier has undergone a drastic retreat of 12 km, starting in AD 1982. As a result, previously overrun mountain hemlock (T. mertensiana) forests at the fjord margins have been uncovered. Tree-ring and radiocarbon dating of over 100 core and disc samples from these trees were used for glacier reconstruction. Samples of those trees killed by direct ice-push and by inundation in ice-dammed lakes reveal multiple ages of ice advance over the past millennium. These results represent the later stages of an extended advance period. Preliminary results from tree-ring dating of sites closest to the present margin suggest ice advance of the Columbia Glacier as early as AD 1057. Evidence of continued advance in AD 1389 is provided by sites farther downvalley. Tree-ring dating at these same sites, but from forests occurring at higher elevations, indicate near continuous thickening through AD 1762, with possible stillstands and minor retreat. Tree-ring dating of a forest inundated by an extensive ice-dammed Nanatuk Lake farther south indicates that the Columbia Glacier advanced through this area near its recent terminal position between AD 1753 and AD 1778. Shortly after, the glacier reached its recent maximum, where it remained until the early 1980’s. Tree-ring dates from overrun forests of the adjoining eastern glacier Land Lobe, which shares the same nevés as the Columbia Glacier, shows advance about AD 1810.

Comparison of the reconstructed intervals of advance at Columbia Glacier with the well established record from land-terminating glaciers in Prince William Sound suggests that pulses of tidewater glacier advance are less sensitive to climate signals and persist for longer intervals than those of land-terminating glaciers. Initial advances appear to be forced by climate cooling associated with the Little Ice Age. However, internal glacial mechanisms, such as water depth and increased flow speed due to steepening of longitudinal gradient, may sustain these advances for longer intervals than depicted by land-terminating glacial records.