A Coral-based Reconstruction of Seawater δ18O at the Lakshadweep Archipelago in the Arabian Sea, Southwest India

The islands of the Lakshadweep Archipelago (Arabian Sea) receive most of their annual rainfall from the southwest (SW) monsoon, which passes via these islands from the Indian Ocean towards India during the months of June through September. Rainfall data show an increase in precipitation from 1981 to 2008 resulting from the SW monsoon, our study period. In spite of the heavy precipitation during the SW monsoon months, we observe an increase in sea surface salinity (SSS) from April to September. In order to reconstruct regional variations in SSS and to check the ability of corals from this region to record the salinity changes, we measured high-resolution δ¹⁸O and Sr/Ca ratios from a core of a massive Porites lutea coral from Kavaratti Island, Lakshadweep Archipelago. Both, the coral Sr/Ca and the coral δ¹⁸O are well correlated with each other (R=0.33, p<0.001; n=325). The δ¹⁸O_sw is calculated using the centering method proposed by Cahyarini et al. (2008). The regression slopes of coral δ¹⁸O vs. SST (ERSSTv4) and coral Sr/Ca vs. SST (ERSSTv4) used in this study for the reconstruction of δ¹⁸O_sw are -0.153‰/ºC and -0.061 mmol/mol/ºC, respectively. The error of the δ¹⁸O_sw reconstruction is +0.102‰ which applies only to relative δ¹⁸O_sw variations. Our reconstructed δ¹⁸O_sw using coral Sr/Ca and coral δ¹⁸O follows similar climatological trend that of gridded instrumental SSS data sets (UKMO EN4 and Levitus), but has a larger seasonal range implying that the corals are faithfully recording the SSS variations. Based on our studies, we recommend the installation of continuous SST and SSS loggers in these islands for a better understanding of the local climatic changes in this monsoon-dominated region.