Tracing Water Masses Along the subtropical South American Coast with the Stable Isotopic Composition of Benthic Foraminifera

We explore the applicability of stable isotopic ratios of various indicator species of benthic foraminifera ( Uvigerina peregrina, Pseudononion atlanticum, Hoeglundina elegans, Angulogerina angulosa, Buccella peruviana, Cibicides fletcheri, Cassidulina subglobosa, Bulimina marginata and Hanzawaia boueana) to identify bottom water masses. Samples were collected along the continental shelf of the Argentinean-Uruguayan and Brazilian Atlantic Coast during the winter 2003 and summer 2004. Results show that the stable isotopic composition of living and dead foraminifera is associated with environmental variables (latitude, depth, temperature, salinity and nutrients). Specifically, the δ18O data from Uvigerina peregrina, Hoeglundina elegans and Pseudononion atlanticum follow meridional temperature gradients with the presence of relatively warm Subtropical Shelf Water indicated by lower δ 18O values at the northernmore sites (27° S) and colder Subantarctic Shelf Water with higher δ18O values toward the southern sites (to 37° S). Angulogerina angulosa and Pseudononion atlanticum δ18O values correlated better with salinity than temperature. Results also indicate that δ13C values from Uvigerina peregrina, Hoeglundina elegans, Bulimina marginata and Cassidulina subglobosa tend to be low at sites located within the freshwater plume of the La Plata River. These results suggest that it is possible to identify three regional water masses based on the stable isotopic compositon of these various indicator species.