Excess ground ice of condensation-diffusion origin in University Valley, Dry Valleys of Antarctica: Evidence from isotope geochemistry and numerical modeling

摘要

This study investigates the origin and age of ground ice in the uppermost 1. m of permafrost in University Valley, one of the upper valleys in the McMurdo Dry Valleys of Antarctica. In contrast to other regions in the MDV, mean daily air and soil temperatures at the coring sites are always below 0. °C, which allows for unique cryogenic processes to occur. In the two cores that were analyzed, excess ground ice was measured throughout, ranging between 23% and 85%. Isotope geochemical trends in the ice-rich permafrost indicate that the ground in Core 5 (65. cm long) and the uppermost 52. cm of Core 7 originated from condensation-diffusion of water vapor; whereas the ground ice between 57-90. cm in Core 7 originated from freezing of liquid water. Using numerical modeling, we show that the excess ground ice of condensation-diffusion origin formed by the long-term thermal contraction-expansion of the cryotic sediments, which allowed for the ice content to exceed pore-filling capacity. Absolute age estimates of the sandy-loam sediments based on Optically Stimulated Luminescence dating indicate that soils have been accreting at the site for at least the last 170. ±. 36. ka. years, and this places an upper limit to the age of the ground ice. Absolute soil ages allowed us to link the change in ground ice origin in Core 7, which took place around 152. ±. 12. ka. years, with shifts in climate conditions since marine isotope stage 5e interglacial period. Our findings offer a new process of ground ice emplacement in sediments in cold-dry environments and allow an alternative explanation regarding the enigmatic origin of excess ground ice identified by Mars Odyssey and Phoenix in the northern martian plain, which is that overfilled pore ice can form by vapor deposition and repeated thermal cycling without the presence of melt water.