Landscape connectivity and dissolved organic matter in a degrading permafrost polygonal landscape

Abstract:

In Arctic landscapes dominated by ice-wedge polygons, the degradation of these ice wedges leads to dramatic, interconnected changes in both the physical environment and biogeochemical cycling. As ice wedges thaw, poorly drained, low-centered polygons transform into well-drained high-centered polygons, surrounded by connected water-filled troughs that develop above degrading ice wedges. Thermokarst resulting from ice-wedge degradation allows for substantial re-mineralization of preserved organic matter, but can also cause changes in the lateral transport of dissolved organic carbon (DOC), both in terms of quantity and composition. As yet, it is poorly understood how the connectivity of ice-wedge polygon landscapes contributes to in situ losses of dissolved organic matter (DOM) and the lateral movement of DOM through a watershed. We sampled surface water along a field-mapped flowpath at a study site with actively degrading ice wedges near Prudhoe Bay, Alaska, in July 2019. Our goal is to understand how DOM is mobilized and transformed, from a mid-point in the sub-watershed to an outlet into a drained lake basin. The flowpath ran through a series of troughs and ponds that formed in thermokarst depressions surrounding high-centered polygons, differentiated by water depth, presence of submerged and emergent vegetation, and width between polygon rims. We measured DOC, chromophoric dissolved organic matter (CDOM), dissolved nitrogen, and temperature at each site. Repeat aerial imagery, high-resolution GPS data, and soil cores from this site show clear signs of ice-wedge degradation and increased water connectivity over the past fifty years, including the thawing of previously stable ice wedges to form deep ponds within a six year time period. DOC concentrations were lowest at the outlet, and increased going upstream along the flowpath. There was no difference in either DOC concentration or DOM composition between troughs and ponds, indicating the importance of connectivity. The linkages between hydrology, permafrost thaw, and landform change will be key to understanding the altering carbon and nitrogen cycles in a warming Arctic.