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

Abstract

A 17-year-long δ 13C and δ 18O 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 δ 13C and δ 18O values varied approximately in phase and exhibit a positive relationship with each other. High-density (monsoon) bands are characterized by enriched δ 18O values and low-density (non-monsoon) bands by depleted δ 18O values. Coral δ 13C and δ 18O 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 δ 18O 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 δ 18O data reveals a teleconnection between the local SST and tropical Pacific climate variability. Enriched δ 18O 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 δ 13C values correlate with enriched δ 18O values of the monsoon bands. This increase in δ 13C 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 δ 13C enrichment of dissolved inorganic carbon (DIC) in surface waters. However, other possibilities, such as higher δ 13C values due to coral mass-spawning events during monsoon times cannot be ruled out. A progressive decrease of ∼0.5‰ in δ 13C 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.