Abstract
Wildfires have produced black carbon (BC) since land plants emerged. Condensed aromatic compounds, a form of BC, have accumulated to become a major component of the soil carbon pool. Condensed aromatics leach from soils into rivers, where they are termed dissolved black carbon (DBC). The transport of DBC by rivers to the sea is a major term in the global carbon and BC cycles. To estimate Arctic river DBC export, 25 samples collected from the six largest Arctic rivers (Kolyma, Lena, Mackenzie, Ob’, Yenisey and Yukon) were analyzed for dissolved organic carbon (DOC), colored dissolved organic matter (CDOM), and DBC. A simple, linear regression between DOC and DBC indicated that DBC accounted for 8.9 ± 0.3% DOC exported by Arctic rivers. To improve upon this estimate, an optical proxy for DBC was developed based upon the linear correlation between DBC concentrations and CDOM light absorption coefficients at 254 nm (a254). Relatively easy to measure a254 values were determined for 410 Arctic river samples between 2004 and 2010. Each of these a254 values was converted to a DBC concentration based upon the linear correlation, providing an extended record of DBC concentration. The extended DBC record was coupled with daily discharge data from the six rivers to estimate riverine DBC loads using the LOADEST modeling program. The six rivers studied cover 53% of the pan-Arctic watershed and exported 1.5 ± 0.1 million tons of DBC per year. Scaling up to the full area of the pan-Arctic watershed, we estimate that Arctic rivers carry 2.8 ± 0.3 million tons of DBC from land to the Arctic Ocean each year. This equates to ~8% of Arctic river DOC export, slightly less than indicated by the simpler DBC vs DOC correlation-based estimate. Riverine discharge is predicted to increase in a warmer Arctic. DBC export was positively correlated with river runoff, suggesting that the export of soil BC to the Arctic Ocean is likely to increase as the Arctic warms.
Highlights
Fire occurs in most terrestrial ecosystems (Bowman et al, 2009) and is on the increase in the Arctic (Higuera et al, 2008; Hu et al, 2010)
Black carbon (BC) refers to thermally altered organic material and it comes in many forms (Forbes et al, 2006), ranging in chemistry from minimally charred biomolecules (Myers-Pigg et al, 2015) to condensed aromatics formed at high temperatures (Dittmar, 2008)
dissolved black carbon (DBC) loads on days when DBC concentrations were measured (i.e., DBC concentration × daily discharge) varied from 0.26 to 194.5 kg s−1 (Table S1)
Summary
Fire occurs in most terrestrial ecosystems (Bowman et al, 2009) and is on the increase in the Arctic (Higuera et al, 2008; Hu et al, 2010). Once formed, condensed aromatics are ultra-refractory within soils, being preferentially preserved for hundreds to thousands of years (Schmidt et al, 2011). This stability, together with the ubiquity of fire, has resulted in condensed aromatics being distributed throughout the world’s soils (Forbes et al, 2006; Guggenberger et al, 2008), where they have accumulated to represent approximately 10% of the global soil carbon store (Mitra et al, 2013). Without a significant loss term, condensed aromatics should have accumulated to represent an even greater pool of soil carbon than currently observed. The mobilization of soil condensed aromatics into solution and subsequent export by rivers to the oceans is the main loss term identified to date (Guggenberger et al, 2008; Dittmar et al, 2012a; Jaffé et al, 2013)
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