Weathering Processes in Antarctica as an Analog for Planetary Systems

Abstract:

Ongoing weathering in Antarctica and Mars is believed to be driven dominantly by physical processes. Our recent studies in the McMurdo Dry Valleys (MDV) indicate that both physical and chemical processes are actively occurring. Specifically, ground ice has been sublimating and chemical weathering has been happening for several million years in ice-rich permafrost. These findings are in contrast with previous paradigms that argue the MDV have been stagnant since the Miocene.

We have provided updated models of sublimation calibrated for temperature and humidity by a decade of climate data, and extended back for 200 ka based on stable water isotopes in a core from Taylor Glacier. Our modeling reveals that there is a net mass loss of ground ice each year, yet it is slow enough that it has survived several Ma. Ground ice is pertinent because it is a reservoir of water, and it drives periglacial process that alters the landscape. This stability has implications in landscape development as well as planetary water reservoirs.

Chemically, permafrost is generally considered to be inert, and weathering products observed in permafrost (salts, carbonates, phyllosilicates, and other minerals) are thought to be inherited; however, we recently found that silicate minerals weather in ice-rich permafrost. In the top 7-m of a 30-m permafrost core from Beacon Valley, MDV, over 50% of the water-extractable Mg has weathered out of dolerite clasts within the permafrost, based on Mg isotopes. This weathering process occurs as ions concentrate in water films, which lead to freezing point depression. Similarly, biological activity, in the form of biocrusts, thrives in brine-rich waters that have lowered freezing points but maintain ephemeral, habitable water activities. We also have detected phyllosilicates in the Beacon core and other Antarctic soils that appear to have formed in situ.

Collectively, these studies highlight the importance of salts to depress freezing points in ice-rich permafrost and, thereby, provide water in these otherwise arid and frigid environments, which may support life and promote chemical weathering. Salts are pervasive in these extreme environments, and the lessons that we learn from Antarctica are relevant for planetary systems, especially Mars.