MODELING VAPOR DIFFUSION THROUGH SUPRAGLACIAL TILLS IN MULLINS VALLEY, ANTARCTICA

We examine the spatial variability of sublimation rates and associated net annual ice loss across a buried alpine glacier in the McMurdo Dry Valleys of Antarctica, purportedly one of the oldest active alpine glaciers in the world. Sublimation of Mullins Glacier is controlled by rates of vapor diffusion through Mullins till, a dry supraglacial till that rests directly on the buried glacier ice. Like most supraglacial tills in cold-desert environments, Mullins till contains three characteristic facies: a surface weathered facies (physically and chemically altered); a sand-wedge facies associated with contraction-crack polygons; and an underlying fresh facies representing the addition of englacial debris (sourced from rock fall) as overlying ice sublimes.

Using a 1D model for Fickian diffusion through porous media and applying site-specific meteorological data collected over a four-year period we show that the rate of subsurface-ice sublimation varies by ~5.5% beneath till facies, and that over timescales of 10⁵ years, preferential diffusion through sand-wedges contributes to the development of deep troughs surrounding high-centered polygons. We show that current rate of ice loss at the stagnant terminus of Mullins Glacier is extremely low but is consistent with complete ice loss under present-day environmental forcing in ~2.5 Myr. Sensitivity tests indicate that sublimation at the terminus asymptotically approaches zero with either a drop in summertime soil and ice-surface temperatures of ~6°C or an increase in atmospheric relative humidity of ~30%, either of which could arise from a slight increase in cloud cover over Mullins Glacier. Sublimation responses to meteorological forcing are not uniform across Mullins Glacier. An increase in subsurface temperature of 2°C results in negligible change in ice sublimation at Mullins terminus, but a 27% increase in ice loss in upper Mullins Valley; the key factor is the Mullins till thickness.

The results highlight the subtle relations among changes in till texture, till thickness, and meteorological forcing on the rate of subsurface-ice loss and provide insight into the plausible range of conditions under which multi-million year old ice can exist beneath thin supraglacial tills.