Abstract: "The climate record from the last four decades (1981-2020) shows that the North Slope of Alaska has experienced rapid increase in winter air temperatures by up to 3.5oC and snowfall increasing by ~40 mm. These shifts are altering not only surface energy balances but also the thermal and hydrological states of the soils and shallow aquifers above permafrost. These supra-permafrost aquifers (SPAs) have high concentrations of dissolved organic carbon (DOC) leaching rapidly from peat, the main SPA material. The transport of DOC into aquatic water bodies and biogeochemical transformation into CO2 and CH4 depend on thermal and hydrologic conditions. However, the hydro-biogeochemical responses of SPAs to long-term climate are largely unknown.

In this study, we combined high-resolution, high-fidelity modeling using the Advanced Terrestrial Simulator (ATS) to evaluate 40 years of climate impact on the thermal regime, groundwater flow, and DOC transport across a representative hillslope-riparian transect in the Imnavait Creek watershed a typical headwater region on the North Slope. We combined long term and specific year-long meteorological, land-surface and subsurface observations for model calibration, followed by long term ATS simulations using historical meteorological forcings. Rising winter air temperatures and snowfall have led to increased annual outflow from the hillslope-riparian zone to the creek as well as warmer soil temperatures in winter, more extensive zero-curtain zones (subsurface locations where soil temperature hovers near 0oC due to latent heat exchange during freeze/thaw) and longer zero-curtain periods. The model showed that in 2018 the zero curtain extended into the typical coldest day of the year (January 22), when up to 180 days of zero curtain period was observed near the riparian zone. This situation would allow for sustained liquid water saturation with negligible subsurface flow as well as microbial activity deep within the cold season. Our findings provide insight on the mechanisms driving high early spring and late winter greenhouse gas emissions from the SPA. This work shows that there may be more hydrological activity in the Arctic winter at this site that previously thought, which could drive enhanced carbon emissions under future climate."