A Potentially Non-Steady State Pinedale Glacial Maximum, as Indicated by Half Moon Lake Glacial Valley, Wyoming

Details A Potentially Non-Steady State Pinedale Glacial Maximum, as Indicated by Half Moon Lake Glacial Valley, Wyoming A Potentially Non-Steady State Pinedale Glacial Maximum, as Indicated by Half Moon Lake Glacial Valley, Wyoming

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.c21d..08v&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #C21D-08

Physics

0720 Glaciers, 0774 Dynamics, 0798 Modeling

Scientific paper

The greatest extent of glacial ice during MIS2 (Wisconsinan) in the western US may record a short-lived (sub- millennial) cold event rather than an extended Last Glacial Maximum, based on modeling experiments simulating the Pinedale moraines of Half Moon Lake and adjacent valleys near Pinedale, Wyoming. In some locations including the Half Moon Lake valley, Bull Lake (MIS6) moraines lie well down-valley (2 km) of Pinedale moraines, whereas nearby the moraines are much more closely nested (e.g., Fremont Lake valley, 0.5 km). In a simple flow-line glacier model of Half Moon Lake valley, the subglacial topography (steep upper reaches feeding a nearly flat and locally overdeepened region down-glacier) introduces strong hysteresis behavior with abrupt transitions. We have been unable to find any steady conditions that would grow a steady-state glacier ending at the Pinedale moraines. Instead, the ice preferentially terminates either well up-valley, inside modern Half Moon Lake, or advances to the Bull Lake terminal moraines. In the model, advance of the glacier terminus past Half Moon Lake thickens the ice up-valley of the lake, raising more of the glacier into the accumulation zone and causing further advance. If we specify a warming event as the ice reaches the Pinedale moraines, a steady state Pinedale terminus is possible for a narrow range of parameters; smaller warming allows continuing advance, and larger warming triggers retreat. The modeled time-scale for advance from Half Moon Lake to the Pinedale moraines is typically some centuries for climatic perturbations tested, suggesting the hypothesis that the Pinedale maximum at this site records a short-lived event perhaps linked to the Dansgaard-Oeschger or Heinrich oscillations of the North Atlantic. Simulations for the adjacent Fremont Lake valley, in which the Bull Lake terminated up-valley of any prominent flattening of the valley floor, show more-nearly linear dependence of terminus position on snowline elevation, and faster response of the terminus position to climate change. The moraine positions of the two valleys can be explained if the Bull Lake was similar to or only slightly colder than the Pinedale, but the Bull Lake cold persisted longer.

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