Numerical Modeling of the Last Glacial Maximum Yellowstone Ice Cap Captures Asymmetry in Moraine Ages

The Last Glacial Maximum (LGM) Yellowstone Ice Cap was the largest continuous ice body in the US Rocky Mountains. Terminal moraine ages derived from cosmogenic radionuclide dating (e.g., Licciardi and Pierce, 2008) constrain the timing of maximum Ice Cap extent. Importantly, the moraine ages vary by several thousand years around the Ice Cap; ages on the eastern outlet glaciers are significantly younger than their western counterparts. In order to interpret these observations within the context of LGM climate in North America, we perform two numerical glacier modeling experiments: 1) We model the initiation and growth of the Ice Cap to steady state; and 2) We estimate the range of LGM climate states which led to the formation of the Ice Cap. We use an efficient semi-implicit 2-D glacier model coupled to a fully implicit solution for flexural isostasy, allowing for transient links between climatic forcing, ice thickness, and earth surface deflection. Independent of parameter selection, the Ice Cap initiates in the Absaroka and Beartooth mountains and then advances across the Yellowstone plateau to the west. The Ice Cap advances to its maximum extent first to the older eastern moraines and last to the younger western and northwestern moraines. This suggests that the moraine ages may reflect the timescale required for the Ice Cap to advance across the high elevation Yellowstone plateau rather than the timing of local LGM climate. With no change in annual precipitation from the present, a mean summer temperature drop of 8-9° C is required to form the Ice Cap. Further parameter searches provide the full range of LGM paleoclimate states that led to the Yellowstone Ice Cap. Using our preferred parameter set, we find that the timescale for the growth of the complete Ice Cap is roughly 10,000 years. Isostatic subsidence helps explain the long timescale of Ice Cap growth. The Yellowstone Ice Cap caused a maximum surface deflection of 300 m (using a constant effective elastic thickness of 4 km (McQuarrie and Rogers, 1998)). Furthermore, subsidence in the foredeep formed by the weight of the Yellowstone Ice Cap may be responsible for the deep alluviation (30+ m) of the Yellowstone valley and Jackson Hole, with subsequent ice retreat and glacial isostatic adjustment leading to exhumation and incision of these fill packages.