Disequilibrium Effect in Oxygen and Carbon Isotopic Compositions of Modern Foraminifera from Lakshadweep, India

Oxygen isotope palaeothermometry can be applied to biogenic carbonates only if they are precipitated in isotopic equilibrium with the ambient water. The selection of species satisfying this condition is therefore vital to the integrity of palaeoclimatic interpretation. This paper analyzes δ¹⁸O and δ¹³C of seven coexisting species of benthic foraminifera from the lagoon waters of Lakshadweep, India. It aims to (I) select the species secreting their tests in isotopic equilibrium and (II) assess the amount of disequilibrium for species not precipitating their tests in isotopic equilibrium. It is found that Marginopora vertebralis and Amphisorus hemprichu precipitate their tests in oxygen isotopic equilibrium and, therefore, these species are suitable for calibration in isotopic palaeotemperature reconstructions. Sorites is enriched by 0 3%0 and Heterostegina depressa, Neorotalta calcar, Amphistegina lessonit and Nummulites venosus are depleted by up to 0 8%0 in δ¹⁸O. The carbon isotopic composition of miliolid foraminifera are depleted by <2%0 and the rotaliid foraminifera are depleted by about 4%0 and more. The equilibrium to minor disequilibrium of oxygen isotopic compositions and positive linear correlations between δ¹⁸O and δ¹³C suggest that metabolic fractionation may be having only a minor effect on the oxygen isotopic compositions of the analyzed benthic foraminifera.