Radiocarbon analysis of neutral sugars in high-molecular-weight dissolved organic carbon: Implications for organic carbon cycling

We used compound-specific natural-abundance radiocarbon analyses of neutral sugars to study carbon cycling of high-molecular-weight (HMW) dissolved organic carbon (DOC) at two sites in the North Pacific Ocean. Sugars released from HMW DOC by acid hydrolysis were purified by high-pressure liquid chromatography and analyzed for radiocarbon content via accelerator mass spectrometry. The seven most abundant sugars recovered from HMW DOC have similar radiocarbon values, supporting the hypothesis that these sugars are incorporated into a common family of polysaccharides. Neutral sugar D14C values from surface waters collected in 1999 and 2001 are 89 6 13‰ and 57 6 6‰, respectively; these values are much more enriched in radiocarbon than those found in previous studies that used operationally defined carbohydrate fractions. Radiocarbon values for HMW DOC neutral sugars are the same as, or only slightly depleted relative to, dissolved inorganic carbon (DIC), which is consistent with rapid cycling and a short (,3-yr) residence time. In addition, the D14C value of neutral sugars at 600 m is 20‰ enriched relative to DIC D14C, suggesting that a fraction of dissolved neutral sugars at this depth are introduced by dissolution from large, rapidly sinking particles. Marine heterotrophic bacterial production is fueled by the metabolism of dissolved organic carbon (DOC) (Williams 2000). Numerous studies show that bacterial production is carbon limited, even though DOC concentrations are high throughout the water column (Kirchman 1990; Carlson and Ducklow 1995). The large reservoir of DOC that accumulates in seawater is therefore believed to be largely unavailable to meet bacterial carbon demand (Kirchman et al. 1993; Carlson and Ducklow, 1995). Radiocarbon measurements of deep-sea DOC support the view that a large fraction of DOC is recalcitrant (Williams et al. 1969; Williams and Druffel 1987; Druffel et al. 1992). Average radiocarbon values for DOC below 1,000 m at mid-gyre sites in the North Pacific and North Atlantic Oceans are 2525 6 20‰ and 2390 6 10‰ respectively, consistent with a residence time of 4,000– 6,000 yr, or several ocean-mixing cycles (Druffel et al. 1992). However, at depths ,1,000 m DOC concentrations begin to rise, and surface ocean DOC values can be up to 50 mmol L21 higher than deep-sea concentrations (Hansell and Carlson 1998). The elevated concentrations and the shorter average residence time (;2,000 yr) of DOC in surface waters points to a large (30–50 GT C), recently synthesized DOC reservoir in the upper ocean. This reservoir is considered to be semireactive, turning over on timescales of upper-ocean mixing (months to decades). Because of the large size of the reservoir, semireactive DOC has the poten-