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

Abstract

The application of stable oxygen isotopic ratio of surface-dwelling planktic foraminifera Globigerinoides ruber (white variety; ?18Oruber) to reconstruct past hydrological changes requires a precise understanding of the effect of ambient parameters on ?18Oruber.The northern Indian Ocean, with its huge freshwater influx and being a part of the Indo-Pacific Warm Pool, provides a unique setting to understand the effect of both the freshwater-influx-induced salinity and temperature on ?18Oruber.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 freshwater-influx-induced seawater salinity and temperature on ?18Oruber in the northern Indian Ocean.The analysed surface ?18Oruber mimics the expected ?18O calcite estimated from the modern seawater parameters (temperature, salinity, and seawater ?18O) very well.We report a large diagenetic overprinting of ?18Oruber in the surface sediments, with an increase of 0.18?� per kilometre increase in water depth.The freshwater-influx-induced salinity exerts the major control on ?18Oruber (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 ?18Oruber (?18Oruber-?18Osw) in the northern Indian Ocean [T=-0.59?(?18Oruber-?18Osw)+26.40] is different than reported previously, based on the global compilation of plankton tow ?18Oruber data.The revised equations will help create a better palaeoclimatic reconstruction from the northern Indian Ocean by using the stable oxygen isotopic ratio.The entire data set (newly generated and previously published) used in this work is available both as a Supplement to this article and at PANGAEA (https://doi.org/10.1594/PANGAEA.945401; Saraswat et al., 2022).]