Seasonal Subsurface Thaw Dynamics of an Aufeis Feature Inferred From Geophysical Methods

Abstract

Aufeis are sheets of ice unique to cold regions that originate from repeated flooding and freezing events during the winter. They have hydrological importance associated with summer flows and winter insulation, but little is known about the seasonal dynamics of the unfrozen sediment layer beneath them. This layer may support perennial groundwater flow in regions with otherwise continuous permafrost. For this study, ground penetrating radar (GPR) were collected in September 2016 (maximum thaw) and April 2017 (maximum frozen) at the Kuparuk aufeis field on the North Slope of Alaska. Supporting surface nuclear magnetic resonance data were collected during the maximum frozen campaign. These point‐in‐time geophysical data sets were augmented by continuous subsurface temperature data and periodic Structure‐from‐Motion digital elevation models collected seasonally. GPR and difference digital elevation model data showed up to 6 m of ice over the sediment surface. Below the ice, GPR and nuclear magnetic resonance identified regions of permafrost and regions of seasonally frozen sediment (i.e., the active layer) underlain by a substantial lateral talik that reached >13‐m thickness. The seasonally frozen cobble layer above the talik was typically 3‐ to 5‐m thick, with freezing apparently enabled by relatively high thermal diffusivity of the overlying ice and rock cobbles. The large talik suggests that year‐round groundwater flow and coupled heat transport occurs beneath much of the feature. Highly permeable alluvial material and discrete zones of apparent groundwater upwelling indicated by geophysical and ground temperature data allows direct connection between the aufeis and the talik below.

Geophysical Observations of Taliks Below Drained Lake Basins on the Arctic Coastal Plain of Alaska

“…Terry et al. (2020) used surface NMR to detect a talik with VWC ∼0.10 m 3 m −3 beneath the Kuparuk River aufeis on the North Slope of Alaska, similar to our observed values for the remnant taliks below Derksen and Three Creatures basins, and they also observed VWC < 0.03 m 3 m −3 for permafrost, which is similar to our primary surface sites and Blustery Basin results.…”

“…The GMR software does not provide the data residual for the inversion model results. However, we have high confidence that our results are consistent because previous works have used the same surface NMR equipment on the North Slope of Alaska (Creighton et al., 2018; Parsekian et al., 2019; Terry et al., 2020) and the same processing software (Terry et al., 2020).…”

Aufeis fields as novel groundwater‐dependent ecosystems in the arctic cryosphere

“…Hydraulic conductivity ranged from 0.1 to 252.5 m d −1 (Table 2), with 79% of wells showing rates > 10.0 m d −1 , indicating "moderately high (24%)" to "high (55%)" permeability (Bureau of Reclamation 1995). The magnitude of hydraulic conductivity measured in the wells was comparable to that estimated at the field site during winter (i.e., 10s of m d −1 ) using NMR that enabled estimates at sediment depths up to 10+ m (Terry et al 2020). Measurements of SpC from surface waters did not differ significantly from wells and showed a similar spatial pattern, with SpC measured from the three southernmost well transects being higher than SpC measured from the northernmost transects (Table 3; ANOVA-GLM, Tukey's LSD, p < 0.05).…”

“…To our knowledge, this is the first demonstration of a groundwater (e.g., hyporheic/ parafluvial/groundwater) invertebrate fauna in a region of continuous permafrost. We initially predicted that an unfrozen, water-saturated layer of sediments would exist directly beneath the surficial ice layer of the aufeis during winter (Terry et al 2020). Instead, we found an unfrozen layer $ 3 to 5 m below the sediment-ice interface (Terry et al 2020).…”

“…1S). This level of richness is presumably enabled by the relatively high porosity of the subsurface sediments combined with a high level of connectivity between pore spaces (Strayer et al 1997), as indicated by high hydraulic conductivity magnitudes measured both directly within the wells ($ 1 m deep) using NaCl as a tracer and indirectly at greater depths (> 10 m deep) using NMR (Terry et al 2020). To our knowledge, this is the first demonstration of a groundwater (e.g., hyporheic/ parafluvial/groundwater) invertebrate fauna in a region of continuous permafrost.…”