MEDIEVAL WARM PERIOD ADVANCES OF ALASKAN ICEBERG-CALVING GLACIERS

Tree-ring and radiocarbon dated advances from five major iceberg-calving glaciers along the Gulf of Alaska show ice expansions during the interval considered to be the Medieval Warm Period (MWP). Advances occurred along a 500km transect on the southern coast of Alaska during the 9th through 12th centuries.

Shortly after AD 850, the freshwater-calving margin of Yakutat Glacier was advancing across tributary valleys inundating forest. Radiocarbon and tree-ring dating of stumps in the tributary show advance at this time. Tree-ring series from the extra-tropics of the Northern Hemisphere suggest that the warmest interval of the MWP spanned AD 950-1045. At this time, in Prince William Sound, Columbia Glacier was advancing through mature mountain hemlock forest at about AD 1050. Similarly, radiocarbon and preliminary tree-ring dating at the tidewater McCarty Glacier and glaciers in Northwestern Fjord from the Kenai Fjords show expansions about AD 990 and 1045, respectively. Radiocarbon dates on buried forests from the Bering Glacier forefield also suggests an advance of this surging, freshwater-iceberg-calving margin at about AD 1080. MWP advances are not yet recognized for the land-terminating glaciers from the region.

The forcing factors responsible for these ice expansions are likely a combination of climate cooling within the MWP and the dynamics of iceberg-calving glaciers. These advances appear to have been sustained as general cooling associated with the early Little Ice Age occurred beginning about AD 1250. The glacial histories of these ice margins show a relatively insensitive stage of general advance for several hundred years followed by catastrophic retreat during recent centuries.