TOWARD AN UNDERSTANDING OF WHY TIDEWATER GLACIERS ADVANCE WHEN ITS WARM - ITS THE DIRT

Detailed investigations of glacier systems in coastal Alaska indicates a phasing contrast between the advance patterns of terrestrial and tidewater glaciers. As might be expected the terrestrial glaciers advance during intervals of cold temperatures likely reflecting the  decrease in surface ablation. However, tide-water glaciers advanced from AD 850 to 1080 at a time when tree-ring and other proxi indicators suggest a warm time interval.

The primary challenge understanding the dynamics of tide-water glaciers is that the more effective calving ablation term must be overcome. Whereas it is possible that increased mass flux from precipitation can achieve this, we offer another possibility based on reducing the effective water depth. At Columbia Glacier the interval of most rapid advance occurred about 1000 AD. At this time near- coastal lake cores and ice cores from Mt. Logan indicate a major isotopic shift possible reflecting a change in circulation patterns. Should that shift bring about changing rainfall patterns, it would change the sedimentation influx of the steep tributaries to the main fjord. Any such sediment would work into the glacial plumbing and eventually end up at the glacier snout where its deposition would fill up the fjord. This would decrease the water depth adjacent to the calving margin and thus decrease the ablation. This sediment mound would be propagated forward until the glacier reached a equilibrium. Subsequently, warming conditions, surface melting, and dynamic calving would force the catastrophic retreat observed today. This processes is likely to be most effective in temperate settings like coastal Alaska.