The Ephemeral Signature of Permafrost Carbon in an Arctic Fluvial Network

Abstract

Arctic fluvial networks process, outgas, and transport significant quantities of terrestrial organic carbon (C), particularly dissolved organic carbon (DOC). The proportion of permafrost C in these fluxes, however, is poorly constrained. A primary obstacle to the quantification of permafrost‐derived DOC is that it is rapidly respired without leaving a unique tracer of its presence. In this study, we investigated the production of bacterial respiratory carbon dioxide (CO2; measured as dissolved inorganic carbon; DIC) during maximum late‐summer thaw in sites spanning a fluvial network (Kolyma Basin, Siberia) to assess whether the biodegradation of permafrost DOC could be detected by the presence of a persistent aged (14C‐depleted) signature on the DIC pool. Using Keeling plot interpretation of DIC produced in bioincubations of river water, we show that bacteria respire varying sources of DOC moving downstream through the fluvial network. Respiration of permafrost (production of aged CO2) was only detected in heavily permafrost thaw influenced sites. In nonpermafrost thaw impacted sites, ambient DIC was modern (14C‐enriched), but rather than precluding the respiration of permafrost OC upstream, we suggest that 14C‐depleted DIC is overwhelmed by modern DIC. Investigation of dissolved organic matter composition via Fourier transform ion cyclotron resonance mass spectrometry highlighted that elevated levels of aliphatic and nitrogen‐containing compounds were associated with the production of aged DIC, providing molecular‐level insight as to why permafrost‐derived dissolved organic matter is rapidly respired. Overall, results from this study demonstrate the difficulty of tracing inputs of a highly reactive substrate to systems with diverse organic matter sources.

Increasing Organic Carbon Biolability With Depth in Yedoma Permafrost: Ramifications for Future Climate Change

“…Studies of DOM measured in yedoma‐dominated watersheds support that the dissolved fraction of the total OM pool is particularly biolabile (Mann et al, ; Spencer et al, ; Vonk et al, ) and, though small, is relevant for estimating GHG production potentials from thawing permafrost. While the DOM measured in these studies partially originates from thawed permafrost OM, it is also influenced by additional DOM sources in the watershed and in situ processing, making it difficult to identify and trace permafrost‐derived DOM in these systems (Drake et al, ).…”

“…The decreasing relative abundances we observed for aliphatic compounds at all depths and incubation temperatures except Y‐12m at 3 °C (Figures e and e) suggest that these compounds are preferentially mineralized into GHG following permafrost thaw. Aliphatic compounds have been previously shown to have high substrate potential for GHG production and be rapidly degraded following permafrost thaw (Drake et al, ; Spencer et al, ; Stapel et al, , ). Thus, increasing abundance of aliphatic compounds in yedoma permafrost with depth (this study; Ewing et al, ; Stapel et al, , ) suggests increasing OC quality and, by extension, increasing anaerobic C mineralization potentials per unit OC with depth.…”

Thermokarst Effects on Carbon Dioxide and Methane Fluxes in Streams on the Peel Plateau (NWT, Canada)

“…In fluvial networks, CO 2 concentrations are typically most pronounced in headwaters (Butman & Raymond, 2011, Crawford et al, 2013Hutchins et al, 2019), owing to relatively strong hydrologic connectivity between streams and soils (Hope et al, 2004;Hotchkiss et al, 2015) and to CO 2 production within streams from processing of organic and mineral substrates (Drake et al, 2015;Zolkos et al, 2018). In regions with organicrich yedoma permafrost, for instance, dissolved organic carbon (DOC) is relatively biolabile and readily oxidized by microbes following thaw, resulting in rapid aquatic CO 2 production (Drake et al, 2015(Drake et al, , 2018. In regions with more sediment-and mineral-rich permafrost, such as in ice-rich glaciated terrain, the chemical weathering of minerals exposed by thaw can consume or produce CO 2 in streams (Tank et al, 2012;Zolkos et al, 2018).…”

Spatiotemporal Variability and Sources of DIC in Permafrost Catchments of the Yangtze River Source Region: Insights From Stable Carbon Isotope and Water Chemistry

“…Temperature, precipitation, and hydrologic partitioning are also important drivers for mineral weathering (Li et al, 2016;Raymond, 2017). CO 2 evasion from stream water surfaces to the atmosphere can enrich the residual DIC pool in 13 C due to isotopic fractionation (Doctor et al, 2008;Drake et al, 2018;van Geldern et al, 2015). In some cases, CO 2 outgassing can lead to enrichments of δ 13 C-DIC by 2.4‰ per log unit decrease of excess CO 2 partial pressure in downstream transects (Doctor et al, 2008).…”