Abstract
Abstract. Climate change will alter the balance between frozen and thawed conditions in Arctic systems. Increased temperatures will make the extensive northern permafrost carbon stock vulnerable to decomposition and translocation. Production, cycling, and transport of dissolved organic carbon (DOC) are crucial processes for high-latitude ecosystem carbon loss that result in considerable export off the Arctic landscape. To identify where and under what conditions permafrost DOC is mobilized in an Arctic headwater catchment, we measured radiocarbon (14C) of DOC and assessed DOC composition with ultraviolet–visible spectroscopy (UV–Vis) of surface waters and shallow and deep subsurface porewaters from 17 drainages in the Barrow Environmental Observatory in Alaska. Samples were collected in July and September 2013 to assess changes in age and chemistry of DOC over time. DOC age was highly variable ranging from modern (19 ‰ Δ14C) to approximately 7000 BP (−583 ‰ Δ14C). DOC age increased with depth, over the summer as the active layer deepened, and with increasing drainage size. DOC quality indicators reflected a DOC source rich in high molecular-weight and aromatic compounds, characteristics consistent with vegetation-derived organic matter that had undergone little microbial processing, throughout the summer and a weak relationship with DOC age. In deep porewaters, DOC age was also correlated with several biogeochemical indicators (including dissolved methane concentration, δ13C, and the apparent fractionation factor), suggesting a coupling between carbon and redox biogeochemistry influencing methane production. In the drained thawed lake basins included in this study, DOC concentrations and contributions of vegetation-derived organic matter declined with increasing basin age. The weak relationship between DOC age and chemistry and consistency in DOC chemical indicators over the summer suggest a high lability of old DOC released by thawing permafrost.
Highlights
Soils of the northern permafrost region store approximately 1300 Pg of organic carbon (Hugelius et al, 2014)
The dissolved organic carbon (DOC) : dissolved organic nitrogen (DON) ratio did not change with depth but did increase from July to September (p = 0.01, Fig. 2c) – an indication that dissolved organic matter (DOM) quality or source may have shifted over the summer
An increase in microbially derived or highly processed organic matter should have included an increase in DON and a decrease in DOC : DON ratio as microbially derived organic matter has a lower C : N ratios than plant litter and soil organic matter (Cleveland and Liptzin, 2007) and C : N ratios decline with increased microbial processing of litter and soil organic matter (Lavallee et al, 2020; Moore et al, 2011)
Summary
Soils of the northern permafrost region store approximately 1300 Pg of organic carbon (Hugelius et al, 2014). This extensive carbon pool is vulnerable to climate change as warmer temperatures increase thawing, microbial decomposition, fire frequency, and erosion (Schuur et al, 2013, 2008). Loss of this permafrost carbon to the atmosphere poses a considerable potential feedback to climate change (Schaefer et al, 2014; Koven et al, 2011), with an estimated 5 %–15 % of permafrost soil carbon projected to be released to the atmosphere by the end of this century under the current warming trajectory (Schuur et al, 2015). McFarlane et al.: Labile dissolved organic carbon at the permafrost thaw zone
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