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 pore waters 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 y 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 throughout the summer and a weak relationship with DOC age. In deep porewaters, DOC age was also correlated with several biogeochemical indicators, 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 high lability of old DOC released by thawing permafrost.
Highlights
Soils of the northern permafrost region store nearly 1700 Pg of organic carbon (Tarnocai et al, 2009; Schuur et al., 2008)
dissolved organic carbon (DOC) Δ14C values ranged from 19 ‰ to -583 ‰, or from an average conventional radiocarbon age of modern to 7000 175 y BP, and decreased from July to September (p < 0.01, Fig. 2a), consistent with a decline in DOC 14C values from spring through late summer and fall reported for Arctic rivers and their tributaries (Neff et al, 2006; Wild et al, 2019)
DOC: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
Summary
Soils of the northern permafrost region store nearly 1700 Pg of organic carbon (Tarnocai et al, 2009; Schuur et al., 2008). This extensive carbon pool is vulnerable to climate change as warmer temperatures increase thawing, microbial decomposition, fire frequency, and erosion (Schuur et al, 2013; Schuur et al, 2008). Lateral transport of dissolved carbon is a crucial mechanism for terrestrial carbon loss 40 in the Arctic and results in an export off the landscape of up to 25% of net ecosystem productivity (calculated from Mcguire et al, 2009), exceeds net ecosystem exchange of carbon in some northern ecosystems (Aurela et al, 2002; Billett et al, 2004; Christensen et al, 2007; Roulet et al, 2007), and at the global scale is comparable in magnitude to the land and ocean CO2 sinks (Drake et al, 2018a; Tank et al, 2018)
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