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The largest flux of terrigenous organic carbon into the ocean occurs in dissolved form by way of rivers. The fate of this material is enigmatic; there are numerous reports of conservative behavior over continental shelves, but the only knowledge we have about removal is that it occurs on long unknown time scales in the deep ocean. To investigate the removal process, we evaluated terrigenous dissolved organic carbon concentration gradients in the Beaufort Gyre of the western Arctic Ocean, which allowed us to observe the carbon's slow degradation. Using isotopic tracers of water-mass age, we determined that terrigenous dissolved organic carbon is mineralized with a half-life of 7.1 ± 3.0 years, thus allowing only 21 to 32% of it to be exported to the North Atlantic Ocean.
Terrigenous dissolved organic carbon (tDOC) enters the global ocean by way of rivers at a rate of ~0.25 Pg C yr^sup -1^ (1), constituting the largest transfer of reduced carbon from the continents to the open ocean. The fate of this material once it is delivered to the ocean remains uncertain. Studies that use salinity-DOC relationships across the world's estuaries and ocean margins show conservative tDOC behavior (1, 2), suggesting a long-lived material. In contrast, low concentrations of tDOC tracers in the open ocean (e.g., lignin and stable isotopic compositions) (3, 4) indicate active removal processes, but the time and space scales of the processes remain unknown.
On a per volume basis, the Arctic Ocean receives the greatest loads of terrestrial fresh water and organic matter of any ocean. Rivers draining into the Arctic Ocean comprise ~10% of the global river flux and have high DOC content (5), typically >600 µM C. This material is long lived, such that up to ~25% of the DOC in the surface central Arctic consists of terrigenous components (6, 7). Previous studies that have used DOC-salinity relationships in the eastern Arctic Ocean (e.g., the Eurasian Basin and its bordering seas) have indicated that tDOC in fluvial water mixes conservatively with marine water over the continental margins (8, 9), yet <50% of the tDOC entering the Arctic Ocean survives for export to the North Atlantic (6, 10).
The gyre circulation of the western Arctic Ocean (i.e., the Canada Basin and adjacent continental shelf...