Oxygen isotopic ratio of Globigerinoides ruber (white variety) in the surface sediments of the northern Indian Ocean

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. For δ¹⁸O analysis, 10-15 well preserved shells of Globigerinoides ruber white variety, were picked from 250-355 μm size range. The stable oxygen isotopic ratio was measured by using Finnigan MAT 253 isotope ratio mass spectrometer, coupled with Kiel IV automated carbonate preparation device. The precision of oxygen isotope measurements was better than 0.08‰. 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.