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.
River aufeis (ow 0 fīse) are widespread features of the arctic cryosphere. They form when river channels become locally restricted by ice, resulting in cycles of water overflow and freezing and the accumulation of ice, with some aufeis attaining areas of $ 25 + km 2 and thicknesses of 6+ m. During winter, unfrozen sediments beneath the insulating ice layer provide perennial groundwater-habitat that is otherwise restricted in regions of continuous permafrost. Our goal was to assess whether aufeis facilitate the occurrence of groundwater invertebrate communities in the Arctic. We focused on a single aufeis ecosystem ($ 5 km 2 by late winter) along the Kuparuk River in arctic Alaska. Subsurface invertebrates were sampled during June and August 2017 from 50 3.5-cm diameter PVC wells arranged in a 5 × 10 array covering $ 40 ha. Surface invertebrates were sampled using a quadrat approach. We documented a rich assemblage of groundwater invertebrates (49 [43-54] taxa, X [95% confidence limits]) that was distributed below the sediment surface to a mean depth of $ 69 ± 2 cm (X ± 1 SE) throughout the entire well array. Although community structure differed significantly between groundwater and surface habitats, the taxa richness from wells and surface sediments (43 [35-48] taxa) did not differ significantly, which was surprising given lower richness in subsurface habitats of large, riverine gravel-aquifer systems shown elsewhere. This is the first demonstration of a rich and spatially extensive groundwater fauna in a region of continuous permafrost. Given the geographic extent of aufeis fields, localized groundwater-dependent ecosystems may be widespread in the Arctic.
Concentration‐discharge (C‐Q) relationships are often used to quantify source water contributions and biogeochemical processes occurring within catchments, especially during discrete hydrological events. Yet, the interpretation of C‐Q hysteresis is often confounded by complexity of the critical zone, such as numerous source waters and hydrochemical nonstationarity. Consequently, researchers must often ignore important runoff pathways and geochemical sources/sinks, especially the hyporheic zone because it lacks a distinct hydrochemical signature. Such simplifications limit efforts to identify processes responsible for the transience of C‐Q hysteresis over time. To address these limitations, we leverage the hydrologic simplicity and long‐term, high‐frequency Q and electrical conductivity (EC) data from streams in the McMurdo Dry Valleys, Antarctica. In this two end‐member system, EC can serve as a proxy for the concentration of solutes derived from the hyporheic zone. We utilize a novel approach to decompose loops into subhysteretic EC‐Q dynamics to identify individual mechanisms governing hysteresis across a wide range of timescales. We find that hydrologic and hydraulic processes govern EC response to diel and seasonal Q variability and that the effects of hyporheic mixing processes on C‐Q transience differ in short and long streams. We also observe that variable hyporheic turnover rates govern EC‐Q patterns at daily to interannual timescales. Last, subhysteretic analysis reveals a period of interannual freshening of glacial meltwater streams related to the effects of unsteady flow on hyporheic exchange. The subhysteretic analysis framework we introduce may be applied more broadly to constrain the processes controlling C‐Q transience and advance understanding of catchment evolution.
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