Dissolved organic matter composition of Arctic rivers: Linking permafrost and parent material to riverine carbon

Abstract

Recent climate change in the Arctic is driving permafrost thaw, which has important implications for regional hydrology and global carbon dynamics. Permafrost is an important control on groundwater dynamics and the amount and chemical composition of dissolved organic matter (DOM) transported by high‐latitude rivers. The consequences of permafrost thaw for riverine DOM dynamics will likely vary across space and time, due in part to spatial variation in ecosystem properties in Arctic watersheds. Here we examined watershed controls on DOM composition in 69 streams and rivers draining heterogeneous landscapes across a broad region of Arctic Alaska. We characterized DOM using bulk dissolved organic carbon (DOC) concentration, optical properties, and chemical fractionation and classified watersheds based on permafrost characteristics (mapping of parent material and ground ice content, modeling of thermal state) and ecotypes. Parent material and ground ice content significantly affected the amount and composition of DOM. DOC concentrations were higher in watersheds underlain by fine‐grained loess compared to watersheds underlain by coarse‐grained sand or shallow bedrock. DOC concentration was also higher in rivers draining ice‐rich landscapes compared to rivers draining ice‐poor landscapes. Similarly, specific ultraviolet absorbance (SUVA254, an index of DOM aromaticity) values were highest in watersheds underlain by fine‐grained deposits or ice‐rich permafrost. We also observed differences in hydrophobic organic acids, hydrophilic compounds, and DOM fluorescence across watersheds. Both DOC concentration and SUVA254 were negatively correlated with watershed active layer thickness, as determined by high‐resolution permafrost modeling. Together, these findings highlight how spatial variations in permafrost physical and thermal properties can influence riverine DOM.

Permafrost Boundary Shift in Western Siberia May Not Modify Dissolved Nutrient Concentrations in Rivers

“…Due to the high importance of the Arctic Ocean and permafrost-dominated sub-arctic continental zones in the global carbon cycle and the high vulnerability of circumpolar zones to climate warming [20][21][22], numerous studies have been devoted to the biogeochemistry of organic carbon and nutrients in large rivers of the circumpolar zone [3,[23][24][25][26][27][28][29][30]. The main source of information on long-term fluxes of dissolved and suspended material from Northern Eurasia to the Arctic Ocean is data from the Russian Hydrometeorological Survey (RHS) collected at key gauging stations of almost all major Russian Arctic rivers [31][32][33][34].…”

Groundwater Discharge in the Arctic: A Review of Studies and Implications for Biogeochemistry

“…Not only did Walvoord and Striegl [2] find that the export of DOC decreased in the Yukon River basin and its tributaries, but DOC concentrations have been found to decrease with increasing active layer thickness [68]. As soil type influences not only the rate of permafrost thaw, but also the DOC concentration in the active layer, the relationship between these three factors will evolve as the climate continues to warm [68].…”

Thaw Depth Determines Dissolved Organic Carbon Concentration and Biodegradability on the Northern Qinghai‐Tibetan Plateau

“…The interaction between thaw depth, flow path, and DOC dynamics is observable seasonally, where early-season DOC derives from surface organic soil (Mann et al, 2012;Treat et al, 2014), while late season flow paths leach deeper, preprocessed DOC (Holmes et al, 2008;Pokrovsky et al, 2011;Striegl et al, 2005). The negative correlation between thaw depth and DOC concentration has also been observed spatially, where regions with deeper active layers have lower soil and stream DOC concentration (Kawahigashi et al, 2004;O'Donnell, Aiken, Swanson et al, 2016), with the notable exception of regions with deep peat deposits, where active layer depth is strongly positively associated with DOC export (Frey & Smith, 2005). The link between thaw depth and DOC properties, including biodegradability, is much less consistent, with BDOC from some Arctic rivers appearing to be highest during winter base flow or spring snowmelt (Holmes et al, 2008;Wickland et al, 2012), and others showing extremely low winter BDOC (Mann et al, 2012), potentially due to differences in rate and extent of permafrost degradation (Abbott et al, 2014).…”

“…The long-standing conceptual model of how DOC flux will respond to permafrost degradation is that the permafrost table controls the depth of water flow and consequently, available DOC sources (Kawahigashi Geophysical 10.100210. /2017GL075067 et al, 2004Koch et al, 2013;O'Donnell, Aiken, Swanson et al, 2016;Striegl et al, 2005). The interaction between thaw depth, flow path, and DOC dynamics is observable seasonally, where early-season DOC derives from surface organic soil (Mann et al, 2012;Treat et al, 2014), while late season flow paths leach deeper, preprocessed DOC (Holmes et al, 2008;Pokrovsky et al, 2011;Striegl et al, 2005).…”

“…In contrast with top-down permafrost thaw, where thawed organic matter is gradually exposed to cold and waterlogged conditions at the bottom of the active layer, thermokarst formation delivers thawed SOC and DOC directly to aquatic ecosystems Krieger, 2012;Walter Anthony et al, 2014). While there is growing consensus that thermokarst formation increases DOC biodegradability at the point of thaw (Abbott et al, 2014;Cory et al, 2013;Ewing et al, 2015), there is conflicting evidence about how active layer deepening might affect DOC production and transport in different permafrost landscapes (Abbott, Jones et al, 2016;Carey, 2003;Kawahigashi et al, 2004;Laudon et al, 2013;O'Donnell, Aiken, Swanson et al, 2016;Olefeldt & Roulet, 2014;Tank et al, 2016;Vonk et al, 2015). Warm permafrost regions such as the Qinghai-Tibetan Plateau (QTP) experience large seasonal fluctuations in thaw depth, potentially providing an analogue for future Arctic and Boreal conditions in regard to DOC production and transport.…”