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|Title:||Disequilibrium effect in oxygen and carbon isotopic compositions of modern foraminifera from Lakshadweep, India|
|Publisher:||GEOLOGICAL SOC INDIA|
|Citation:||JOURNAL OF THE GEOLOGICAL SOCIETY OF INDIA, 68(6), 1003-1007|
|Abstract:||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 delta O-18 and delta C-13 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, hemprichii 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 parts per thousand and Heterostegina depressa, Neorotalia calcar, Amphistegina lessonii and Nummulites venosus are depleted by up to 0.8 parts per thousand in delta O-18. The carbon isotopic composition of miliolid foraminifera are depleted by < 2 parts per thousand and the rotaliid foraminifera are depleted by about 4 parts per thousand and more. The equilibrium to minor disequilibrium of oxygen isotopic compositions and positive linear correlations between delta O-18 and delta C-13 suggest that metabolic fractionation may be having only a minor effect on the oxygen isotopic compositions of the analyzed benthic foraminifera.|
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