Vapor diffusion through sublimation till: Implications for preservation of ancient glacier ice in the McMurdo Dry Valleys, Antarctica.

摘要

Recent studies in the McMurdo Dry Valleys of Antarctica (MDV) purport long-term preservation of buried-ice deposits, some as old as ∼8.1 Ma. If these studies are correct, then the ice deposits likely contain the only pre-Quaternary records for atmospheric evolution on the planet. However, skepticism related to uncertainty over the physical processes that could facilitate long-term ice preservation endures.;To address these concerns, I conducted field-based and numerical-modeling studies to: (1) determine the origin of buried-ice deposits; (2) describe processes that produce and modify overlying debris; (3) numerically model the rate of modern ice loss for typical debris-covered glaciers; and (4) calculate environmental conditions most conducive to ice preservation.;Field results show that debris-covered ice typically forms from rock fall onto former glacier accumulation zones. Rock-fall deposits may travel englacially for hundreds of meters before returning to the ground surface (forming sublimation till) as overlying ice sublimes. Results from both one- and two-dimensional models for Fickian diffusion highlight till thickness, porosity, and climate as key variables governing the rate of subsurface-ice sublimation. In addition, stochastic changes in till texture and surface roughness related to the growth and decay of contraction-crack polygons also play major roles in modulating ice sublimation. Overall, the results show that water vapor flows into and out of sublimation tills at rates dependent on the non-linear variation of soil temperature with depth. Although current climate conditions favor a net loss of buried ice of -0.059 mm a-1 for the region, ice loss beneath typical sublimation tills (∼30% porosity and thermal diffusivity ∼3x10-7 m2s-1) is reduced to zero with any one of the following changes in mean annual conditions: till temperatures decrease by -3.7°C (from -24.3°C to -28.0°C); relative humidity increases from ∼52% to 82%; or recharge from snowmelt equals ∼2 x 10-3 mm per day. All of the above could be achieved with a modest increase in cloud cover, consistent with proxy records for climate change over the last ∼1 Ma. The results indicate that future research may extract multi-million-year-old records of atmospheric change from buried ice deposits in the MDV.