Alaskan Tidewater Glacier Response to Climate Change during the Medieval Climate Anomaly

Alaskan tidewater glaciers were expanding through a warm period that spanned over 100 years at the end of the first millennium AD. Ice margins overcame the loss of mass due to increased ablation and continued to expand - the question is: how was this possible?

Glacier Histories of major glacier systems including those in the Kenai Fiords, Columbia Bay, Icy Bay, Yakutat Bay and Glacier Bay have been dated using tree-rings and radiocarbon. These histories show significant expansions between AD 800 and 1100. For the same interval, multi-proxy records from near-coastal sites along the Northeast Pacific suggest a major change in circulation and a possible increase in moisture flux to the glaciated Alaskan coast. Our well-documented advances of tidewater margins through the warming at first appears to be an interval of climatic insensitivity. We suggest that an increase in sediment flux to the fjords also resulted from the elevated precipitation in the region, and thereby maintained glacier expansion despite the higher loss in ice mass as summer temperatures increased between about AD 900 and 1000. The increase in sediment flux nourished the morainal bank and increased sedimentation and infilling proximal to it, thereby decreasing water depth, slowing the calving rate and allowing for continued terminus advance through the warm interval.

While climate is the primary control on tidewater glacier advance, sediment flux provides an important second order control modulating tidewater glacier behavior during warm periods. Further comparisons between glacier histories and climate proxy records during known past warm intervals can provide a perspective on present-day response of tidewater glacier systems to a warming world.