Stable isotope analysis of dissolved organic carbon in Canada's eastern coastal waters

The application of carbon stable isotope analysis of dissolved organic carbon (C-13-DOC) from natural seawater has been limited owing to the inherent difficulty of such analysis, with order of magnitude differences in interfering ions and analyte concentrations. High temperature catalytic oxidation allows for the attenuation of these interferences by precipitation of inorganic ions on quartz chips upstream from the oxidation catalyst. Using a chemical trap, the OI 1030C combustion DOC analyzer unit can be coupled to an IRMS, allowing for the analysis of low DOC content saline waters with relatively high throughput. The analytical limitations and large water volumes traditionally required for these types of analyses have prevented any large-scale C-13-DOC studies. Here, we present C-13-DOC signatures for surface and bottom waters obtained along Canada's East Coast. Included in the study are samples from the Esquiman channel (between Newfoundland and Labrador), Lake Melville, the Saglek and Nachvak Fjords, the Hudson Strait and finally covering the salinity gradient across the St. Lawrence Estuary and Gulf. Measured C-13-DOC signatures ranged from predominantly marine values of -19.9 +/- 0.3 parts per thousand (vs. VPDB) off the coast of Newfoundland to predominantly terrestrial signatures of -26.9 +/- 0.1 parts per thousand in Lake Melville. We observed a large spread in C-13-DOC signatures for samples with a salinity of approximate to 35 between -19.9 parts per thousand and -23.3 parts per thousand demonstrating the difficulty associated to selecting a marine end-member to be used in stable isotope mixing models to determine the fate of organic matter along the freshwater-marine continuum.