Tracing the signature of various frontal systems in stable isotopes (oxygen and carbon) of the planktonic foraminiferal species Globigerina bulloides in the Southern Ocean (Indian Sector)

Twenty-five surficial sediment samples, collected on board ORV Sagar Kanya during her 199th and 200th cruises along a north-south transect between latitudes 9.69◦N and 55.01◦S, and longitudes 80◦E and 40◦ E were studied for isotopic variations (values of δ18O and δ13C) of the indicator planktonic species Globigerina bulloides. The results indicate that from latitudes 9.69◦N to 15◦ S both these isotopes (δ18O and δ13C) fluctuated significantly. Between latitudes from around 15◦S to 30–35◦S δ18O values steadily increased, whereas δ13C showed a decreasing trend. However, to the south of latitudes 30–35◦S, both isotope values showed a similar response with a gradual increase up to latitude 50◦S, beyond which δ18O continued to increase while δ13C declined. The characteristic patterns of the values of both isotopes indicates that the signatures of different water masses are associated with various frontal systems and/or water masses across the transect. The signature of the Polar Front at around latitude 50◦S shows the specific response of the isotopic values (δ18O and δ13C) of G. bulloides. Such a response beyond 50◦S latitude is ascribable to the general decrease in the ambient temperature, resulting in a continuous increase in δ18O values, while δ13C values decrease as a result of reduced photosynthesis in regions approaching higher latitudes owing to low light penetration. To further corroborate our results, those of many such transects from geographically distinct regions need to be studied for isotopic variations in the calcareous shells of planktonic foraminiferal species. The results have the potential to be used as a proxy to assess the movement of frontal systems in southern high latitude regions.