Large salinity gradient and diagenetic changes in the northern Indian Ocean dominate the stable oxygen isotopic variation in Globigerinoides ruber

Abstract

Abstract. The application of stable oxygen isotopic ratio of surface dwelling Globigerinoides ruber (white variety) (δ18Oruber) to reconstruct past hydrological changes requires precise understanding of the effect of ambient parameters on δ18Oruber. The northern Indian Ocean, with huge freshwater influx and being a part of the Indo-Pacific Warm Pool, provides a unique setting to understand the effect of both the salinity and temperature on δ18Oruber. Here, we use a total of 400 surface samples (252 from this work and 148 from previous studies), covering the entire salinity end member region, to assess the effect of seawater salinity and temperature on δ18Oruber in the northern Indian Ocean. The analyzed surface δ18Oruber very well mimics the expected δ18O calcite estimated from the modern seawater parameters (temperature, salinity and seawater δ18O). We report a large diagenetic overprinting of δ18Oruber in the surface sediments with an increase of 0.18 ‰ per kilometer increase in water depth. The salinity exerts the major control on δ18Oruber (R2 = 0.63) in the northern Indian Ocean, with an increase of 0.29 ‰ per unit increase in salinity. The relationship between temperature and salinity corrected δ18Oruber (δ18Oruber - δ18Osw) in the northern Indian Ocean [T= -0.59*(δ18Oruber - δ18Osw) + 26.40] is different than reported previously based on the global compilation of plankton tow δ18Oruber data. The revised equations will help in better paleoclimatic reconstruction from the northern Indian Ocean.