Variations in Salinity and Oxygen Isotopes in the Southern Indian Ocean

Stable isotopes of water (deuterium, D, and oxygen, δ18O) have been used as important tracers in the ocean for river run-off and importantly,for interior water masses in the world ocean because these isotopes are conservative once ocean water masses leave the surface. Like salinity, δ18O in the ocean can vary due to evaporation/precipitation, the influx of freshwater from river runoff and the melting of glaciers or sea ice. Paired δ18O and salinity can help to provide information about provenance and mixing of deep water masses. Previous studies have shown that surface waters generally exhibit a linear δ18O - salinity relationship where the slope depends on regional conditions. To better understand processes governing the isotopic signal in the surface ocean and their effect on deeper water mass values data from a range of depths and locations is needed. However, data sets with both surface and deeper waters are extremely sparse and limited, particularly in the Indian and Southern Oceans. In this study, water samples were collected along two transects between 30-42°S and 77-115°E in the southern part of the Indian Ocean crossing from subtropical to subantarctic waters. For all locations water masses from the surface to 20 meters above the seafloor were sampled during November - December 2018. These transects are nominally located north and south of the subtropical front (STF), which separates the sub-antarctic and sub-tropical surface waters, and cross the formation zone for Subantarctic mode water (SAMW). Over 200 samples were collected and analyzed for water isotopes and compared to shipboard CTD measurements of salinity. Surface data show a correlation between δ18O and salinity similar to that reported by Srivastava et al. (2007) for the Southern Indian Ocean.