Oxygen isotope composition of seawater and salinity in the western Indian Ocean: Implications for water mass mixing

Abstract

In this study, the quasi-meridional distribution of oxygen isotopes in seawater (δ18Osw) and salinity were investigated to understand water mass mixing in the western Indian Ocean along the two transects 60°E and 67°E, and covering latitudinal ranges from 3°S to 12°S and 3°S to 25°S, respectively. We identified the water mass mixing using the relationship between δ18Osw and salinity newly presented herein. Results show three linear mixing lines divided into three water layers on the δ18Osw salinity diagram. A positive correlation associated with evaporation and precipitation was detected in the surface layer (<100 m), while in the intermediate (100–1000 m) and deep (>1000 m) layers, distinctive vertical mixing lines depending on the latitude were observed. Vertical mixing trends were discerned in the northern (Arabian Sea Surface Water [ASSW], Indian Deep Water [IDW], and Circumpolar Deep Water [CDW]), and southern (South Indian Subtropical Underwater [STUW], Antarctic Intermediate Water [AAIW], IDW, and CDW) areas of the study region. Based on a simple mixing model of conservative behaviors for δ18Osw and salinity, we estimated the contribution of each of the five water masses in the western Indian Ocean. ASSW and STUW overlying the intermediate layer showed the highest contributions in the northern and southern areas of study region, respectively. In the intermediate layer, AAIW had the highest fraction in the southern area while IDW generally occupied the intermediate layer overlying CDW from north to south. Our estimations were similar to the results of published studies obtained in the western Indian Ocean, and the water mass fractions obtained in this study well reflected the presence of major water masses depending on latitude and depth.