Spatio-temporal patterns in carbon distribution in supra-permafrost groundwater at a small-scale site in North-East Siberia

Climate warming can influence a variety of landscape processes, including the transformation and transfer  of water, carbon and nutrients. In the Northern Hemisphere, permafrost underlays large parts of the land surface and represents a large reservoir of  organic carbon that is extremely vulnerable to changing climate conditions. Accelerated thaw can decompose permafrost carbon, and lead to modified exchange processes with the atmosphere (vertical pathway) and hydrosphere (lateral pathways). These carbon export rates are highly dependent on soil water conditions, suprapermafrost groundwater table location, and vegetation community. Depending on depth of thaw and dry or wet soil conditions, changes in the production and availability patterns of dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC), the three main carbon components in water, are expected. Shifts in lateral carbon export become more relevant for quantifying the total local carbon budget with predicted future permafrost degradation due to climate warming and resulting drier soil conditions. 

This study focuses on carbon distribution patterns of the three main carbon components (DOC, POC, DIC) within a floodplain tundra site near Chersky, Northeast Siberia. We compared a wet control site with a dry site affected by a drainage ring built in 2004. A network of piezometers was established to continuously monitor water table trends during the summer season (July to September) in 2017. On several key locations within that network, water was sampled to determine carbon concentrations (DOC, POC, DIC) and carbon isotopes (∆14C-DOC, δ13C-DOC, δ13C-DIC) in 2017. Here, we analyze and discuss the spatio-temporal carbon distribution on both sites with linkages to hydrological conditions (e.g. saturated zone) and carbon isotopic observations. 

The highest concentrations throughout both sites were found for DOC, followed by DIC and POC. DIC is relatively higher at wet sites compared to dry sites. Reversely, the organic carbon components, DOC and POC, were higher at dry sites. ∆14C-DOC can be associated with fresh material and decreased at all measurement sites with time of the season. Within that range, ∆14C-DOC decreased more at dry sites, when thaw depths were deepest within that site and where water tables were lower compared to wet sites, indicating the release of older carbon. Our results show that the distribution of carbon and the respective carbon isotopes are directly related to hydrological flow patterns. Understanding the carbon redistribution processes in these ecosystems is of relevance for assessing the carbon budget in disturbed permafrost areas. These findings will therefore be used to compare climate warming induced permafrost degradation at the dry (drained) site with the wet (control) site.