The role of water and ice beneath the surface: insights into the Cryosphere from Near-Surface Geophysical observations

Abstract:

Much of our planet’s Cryosphere is undergoing unprecedented changes across local and global scales. Yet, understanding the impact of these changes is often constrained by our inability to directly measure properties and processes hidden within the frozen earth. Here, we present results from a novel multidisciplinary field experiment in Interior Alaska that highlights the value of Near-Surface Geophysics to understanding the complex structure and dynamics of the Cryosphere. At a thermokarst site near Fairbanks, AK, an innovative combination of geophysical and biogeochemical measurements are collected along a thaw gradient to better understand the importance of subsurface water and ice dynamics on permafrost thaw, microbial activity, and soil carbon losses. Complementary passive seismic monitoring and borehole nuclear magnetic resonance (NMR) provide high-temporal resolution measurements of near-surface changes in unfrozen water content and ice saturation. Co-located gas probes allow for repeat sampling of greenhouse gas (GHG) concentrations within the permafrost, and a combination of surface chambers and eddy covariance flux towers monitor gas fluxes to the atmosphere. Integration of these data reveal spatial patterns in thaw extent, subsurface unfrozen water content, and permafrost GHG concentrations. Specifically, we found permafrost at the margins of the collapse-scar bogs to be warm (-1 to -0.3 °C), wet (up to 25% liquid water by volume), and to contain significant amounts of methane (up to 30% of the soil gas). This suggests that unfrozen water in permafrost may be an important driver of soil carbon losses prior to collapse. Furthermore, passive seismic monitoring revealed complex spatial and temporal relationships between wintertime changes in near-surface water and ice content and surface carbon dioxide (CO₂) fluxes, providing a better understanding of the mechanisms behind cold-season soil carbon losses. These findings demonstrate the advantage of Near-Surface Geophysics in providing unparalleled observations of subsurface properties, with clear linkages to the fields of Hydrology, Seismology, and Biogeosciences, in addition to the far-reaching opportunities presented within the Cryosphere.