VALLEY GLACIER RECORDS FOR THE ONSET OF THE LITTLE ICE AGE

Moraines and forefield wood deposits in southern Alaska, the Canadian Rockies, and the Swiss Alps record three distinct phases of valley glacier advance in the last millennium. The earliest of these three advances occurred after several centuries of generally retracted ice margins centered on c.1000 A.D., and as such this advance records the end of the Medieval Warm Period (MWP) and the onset of the Little Ice Age (LIA). In southern Alaska, tree-ring crossdates on glacially killed wood show that this early LIA advance was underway from at least the 1210s to 1240s at Princeton Glacier with trees being overrun and buried in till. Outwash at Sheridan Glacier was aggrading possibly at 1156 and definitely from 1240 to 1269, and the advancing Sheridan ice margin crushed trees into a bedrock knob between 1279 and 1284. The terminus of Tebenkof Glacier was advancing and killing trees from at least the 1280s to 1320s.

These data give insight into the potential climatic forcings for the MWP-to-LIA transition. Valley glacier termini typically have a lag time of a few years to decades for a terminus to respond to a climate forcing, and there can be an additional delay before the advancing terminus reaches and begins killing forefield trees. Therefore, the forcing responsible for the early LIA advances of these termini was likely several decades before these dates of forefield tree death. Previous work has suggested that a cluster of large explosive volcanic eruptions in the 1250s to 1280s or a decrease in solar irradiance around the same time may have been responsible for initiating the LIA. However, both of these forcing factors appear to have been too late to explain the timing of the early LIA advances in Alaska.