ORGANIC-CARBON ISOTOPE RATIOS IN RECENT SEDIMENTS IN THE ST-LAWRENCE ESTUARY AND THE GULF OF ST-LAWRENCE

The distribution of 13C in the organic fraction of surface sediments has been determined in order to examine the relative contributions of terrestrial and marine organic carbon to sediments in the St Lawrence Estuary, Saguenay Fjord, and the Gulf of St Lawrence, Canada. The δ13C results show that the organic matter deposited within the Saguenay Fjord (-25.9 ± 0.4‰) and the upper St Lawrence Estuary (-25.0 ± 0.6‰) is of terrestrial origin. The variability of δ13C values observed in sediments from the lower St Lawrence Estuary (-25.6 to -21.8‰) suggests the presence of organic matter from both terrestrial and marine sources. Remarkably uniform values are observed in the open Gulf of St Lawrence (-22.4 ± 0.2‰). On the basis of δ13C studies, it is concluded that terrestrial organic matter from the St Lawrence drainage basin is deposited within the lower St Lawrence Estuary and that the organic matter in the open Gulf of St Lawrence is of local or other marine origin. The use of carbon isotope ratios to delineate the extent of terrestrial organic carbon in sediments is briefly compared with the use of C N ratios and lignin concentrations. The δ13C values from the study area are compared to values from other areas. © 1979.