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 Delta C-14 values from surface waters collected in 1999 and 2001 are 89 +/- 13 parts per thousand and 57 +/- 6 parts per thousand, 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 Delta C-14 value of neutral Sugars at 600 m is 20 parts per thousand enriched relative to DIC Delta C-14, suggesting that a fraction of dissolved neutral sugars at this depth are introduced by dissolution from large, rapidly sinking particles.