Boron and chlorine isotopic signatures of seawater in the Central Indian Ridge

Abstract

Isotopic ratios of boron and chlorine were measured in the upper 2000 m water column of the Central Indian Ridge from two tectonically active areas, one at 5°S and other at 10°S which coincided with the spreading regime of the Central Indian Ridge (CIR). Boron and chlorine isotopic measurements were made using positive thermal ionization mass spectrometry (TIMS) of Cs2BO4 + and Cs2Cl+ ions respectively, mainly to understand their isotopic behaviours to elucidate the consistency of boron and chlorine isotopic compositions of seawater in the oceanic regime. The boron isotopic compositions (d11B values) varied from 37.77‰ to 39.54‰ at 5°S and from 37.75‰ to 39.82‰ at 10°S with considerably heavier values below 300 m depth. The relatively lighter d11B values of seawater within the thermocline in the upper 300 m layer gave an average d11B value of 38.14‰ whereas below the thermocline the average d11B value was 39.3‰. The release of boron from remineralized organic matter and other unknown factor appears to influence the d11B values in causing variations in the upper 300 m layer. The chlorine isotopic composition (d37Cl) of seawater lies within ± 1‰ and indicated difference in their vertical variations at 5°S and 10°S. The d37Cl values ranged from – 0.53 to 0.94‰ at 5°S and from 0.10 to 0.59‰ at 10°S with most of the lighter values (av. 0.06‰) below 300 m depth, showing a trend opposite to that of the boron isotopic variation. The observed heavier d37Cl values (av. 0.64‰) in the upper layer have been attributed to an increase in evaporation to precipitation ratio of seawater that alters the isotopic behaviour of chlorine in the ocean.