CARBON AND OXYGEN ISOTOPE RATIOS IN THE SAGUENAY FJORD AND THE ST LAWRENCE ESTUARY AND THEIR IMPLICATIONS FOR PALEOENVIRONMENTAL STUDIES

Surface water samples (1 m depth) from the Saguenay Fjord and the upper St Lawrence Estuary have been analysed for δ13C, ΣCO2, and δ18O. In the Saguenay Fjord, the δ13CPDB and concentration of the total dissolved CO2 vary from -10·9‰ and 3·6 ml/1 near the head to -0·4‰ and 30·3 ml/1 at the mouth. The upper St Lawrence Estuary exhibits a δ13C range of -4·2 to -1·6‰ and a ∑CO2 concentration range of 19·8 to 37·6 ml/1. The measured δ13C, ∑CO2, δ18O and salinity distributions are compared to a conservative mixing model. The excellent agreement between the measured and predicted δ13C values in the Saguenay Fjord indicates that the carbon isotope ratio behaves conservatively. In the upper St Lawrence Estuary, the much poorer agreement suggests that significant in situ processes involving the dissolved CO2 are operative. The suggestion that estuarine paleotemperatures may be obtained by extrapolating δ13C-δ18O shell carbonate curves to marine conditions is open to criticism. The δ13C-δ18O water curves observed in this work illustrate two different situations that could produce errors in paleotemperatures estimated by this method. © 1979.