δ13C Analysis of Dissolved Organic Carbon in Eastern Canadian Coastal Waters

The application of carbon stable isotope analysis on dissolved organic carbon (δ13C-DOC) from natural seawater samples has been limited owing to the difficulty of such analysis, with order of magnitude differences between interfering ions and analyte concentrations. High temperature catalytic oxidation allows for the separation of interferences from the organic carbon by precipitation on quartz chips upstream from the oxidation catalyst. Unlike wet chemical oxidation, where salts inhibit the oxidation of organic matter to CO2 via side reactions between the salt anions and the persulfate oxidizing agent, high temperature combustion ensures complete organic matter oxidation in a stream of O2. Using a programmable chemical trap to switch carrier gasses from O2 to He, the OI 1030C combustion unit can be coupled to and 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 δ13C-DOC studies. Here we present DOC concentrations and δ13C-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 Gulf of the St. Lawrence, the St. Lawrence Estuary and the Saguenay Fjord. Measured δ13C-DOC signatures ranged from predominantly marine values of -21.3 ± 0.6 ‰ (vs. VPDB) off the coast of Newfoundland to predominantly terrestrial signatures of -25.8 ± 0.1‰ in Lake Melville. Overall, proper blank subtraction using the isotope mass balance equation and four replicate injections are crucial for the collection of meaningful high quality δ13C-DOC signatures on natural abundance, seawater samples.