Method for determining stable isotope ratios of dissolved organic carbon in interstitial and other natural marine waters

Abstract

A procedure is described for the analysis of the stable carbon isotopic composition of dissolved organic carbon (DOC) in natural waters from marine and higher-salinity environments. Rapid (less than 5 min) and complete oxidation of DOC is achieved using a modification of previous photochemical oxidation techniques. The CO 2 evolved from DOC oxidation can be collected in less than 10 min for isotopic analysis. The procedure is at present suitable for oxidation and collection of 1–5 μmol of carbon and has an associated blank of 0.1–0.2 μmol of carbon. Complete photochemical oxidation of DOC standards was demonstrated by quantitative recovery of CO 2 as measured manometrically. Isotopic analyses of standards by photochemical and high-temperature sealed-tube combustion methods agreed to within 0.3ℵ.. Photochemical oxidation of DOC in a representative sediment pore-water sample was also quantitative, as shown by the excellent agreement between the photochemical and sealed-tube methods. The δ 13 C values obtained for pore-water DOC using the two methods of oxidation were identical, suggesting that the modified photochemical method is adequate for the isotopically non-fractionated oxidation of pore-water DOC. The procedure was evaluated through an analysis of DOC in pond and pore waters from a hypersaline microbial mat environment. Concentrations of DOC in the water column over the mat displayed a diel pattern, but the isotopic composition of this DOC remained relatively constant (average δ 13C = −12.4ℵ. ). Pore-water DOC exhibited a distinct concentration maximum in the mat surface layer, and δ 13 C of pore-water DOC was nearly 8ℵ. lighter at 1.5–2.0-cm depth than in the mat surface layer (0–0.5-cm depth). These results demonstrate the effectiveness of the method in elucidating differences in DOC concentration and δ 13 C over biogeochemically relevant spatial and temporal scales. Carbon isotopic analysis of DOC in natural waters, especially pore waters, should be a useful probe of biogeochemical processes in recent environments.