High-resolution carbon and oxygen isotope records from a scleractinian (Porites) coral of Lakshadweep Archipelago

A 17-year-long δ13C and δ18O record from a scleractinian coral (Porites spp.) of Bangaram island (Lakshadweep Archipelago) is based on a nearly-monthly sample interval. This live coral head of Porites spp. was recovered from the lagoon of Bangaram (Lakshadweep) island at ∼20 m water-depth. The skeletal δ13C and δ18O values varied approximately in phase and exhibit a positive relationship with each other. High-density (monsoon) bands are characterized by enriched δ18O values and low-density (non-monsoon) bands by depleted δ18O values. Coral δ13C and δ18O primarily reflects local oceanographic and climatic variability. The high-density bands were formed during the southwest monsoon (June–September) because of increased turbidity and cloud cover, whereas low-density bands were deposited during the non-monsoon months (October–May). Temporal changes in coral-derived sea surface temperature (SST) show a good agreement with instrumental SST record. The δ18O values in monsoon and non-monsoon bands are mainly due to the sea surface temperature (SST) changes, controlled by monsoon-induced upwelling and insolation changes. Coral-derived SST values clearly show warming events during summer 1993 and 1998. Spectral analysis of δ18O data reveals a teleconnection between the local SST and tropical Pacific climate variability. Enriched δ18O values for the monsoon months of 1984, 1985, 1993 and 1997 are probably indicative of cooler surface water due to the stronger upwelling at the studied location.Generally higher δ13C values correlate with enriched δ18O values of the monsoon bands. This increase in δ13C during the southwest monsoon months is attributed to a decrease in endosymbiotic photosynthesis. Alternatively, an increase in biological production, due to monsoon-induced upwelling, may have resulted in the δ13C enrichment of dissolved inorganic carbon (DIC) in surface waters. However, other possibilities, such as higher δ13C values due to coral mass-spawning events during monsoon times cannot be ruled out. A progressive decrease of ∼0.5‰ in δ13C from 1985 to 2001 may be due to the changes in photosynthesis and/or changing food habits from autotrophy to heterotrophy as the coral grew in size.