A CONCEPTUAL MODEL FOR SEASONAL THAW DYNAMICS OF AN AUFEIS FEATURE INFERRED FROM SURFACE GEOPHYSICAL METHODS

Aufeis are layered accumulations of ice, up to several meters thick, formed from groundwater discharge during freezing conditions. In winter, these features are thought to act as ecological oases by supplying water and heat in an otherwise frozen environment. In summer, the melting ice provides a stable supply of water downstream.

The Kuparuk Aufeis (Northslope, AK) is a large (~ 5 km²) aufeis feature that forms in the cobble bed of the Kuparuk river. Ground penetrating radar (GPR) data were collected along transects corresponding to fifty piezometer locations within the Kuparuk Aufeis during a summer (little surface ice) and late winter (maximum predicted surface ice thickness) campaign to gain insight into the freeze-thaw dynamics of below the aufeis feature. Surface nuclear magnetic resonance (NMR) data were also collected in late winter at select piezometer locations to constrain interpretations.

Results provide evidence for a revised conceptual model for aufeis hydrology that assumes a “wet base” of the surface ice pack. Our data indicate a several meter thick seasonally frozen cobble layer forms between the surface ice and the flowing groundwater below, except in areas of focused discharge to regions below the surface ice (providing a ‘wet base’ to the ice in these areas) and to the surface. These interpretations are drawn from the presence of two strong subhorizontal reflectors observed in the GPR data that are punctuated by sudden rises in the lower reflector, and are supported by NMR profiles of unfrozen water content. The NMR and GPR data further suggest that regions of groundwater upwelling have a deep permafrost boundary (possibly in excess of 20 m in winter) in relatively close horizontal proximity to completely frozen areas that show a permafrost boundary on the order of a few meters during summer time.