Stable isotope composition (delta O-18, delta C-13) of living planktic foraminifers in the outer Laptev Sea and the Fram Strait

The upper water column in the Fram Strait and the outer Laptev Sea was samp led for water column isotopes and living planktic foraminifer species Neogl oboquadrina pachyderma (sinistral coiling) (Ehrenberg), 1861 and Turborotal ita quinqueloba (Natland),1938. Their shell delta O-18 and delta C-13 value s are compared to water oxygen and dissolved inorganic carbon isotope data to determine the environmental influence on the foraminifers' isotopic rati o. Major controls on the oxygen isotope composition of both species are the shallow depth habitat under permanent ice coverage, the low salinity surfa ce layer, and the rate of metabolic activity. None of the specimens precipi tated its shell in isotopic equilibrium with the ambient sea water. They ar e all depleted in C-13 and O-18, attributed to a species-specific vital eff ect. For nonencrusted N. pachyderma (sin.) in the 125-250 mum size class, t his vital effect amounts to 1.3 parts per thousand in delta O-18 and 2.6 pa rts per thousand in delta C-13. It increases to higher values in waters und er permanent ice cover. T. quinqueloba reveals a mean vital effect of about 1.3 parts per thousand in delta O-18 and 2.6 parts per thousand in delta C -13. Th, general isotopic trends are similar for N. pachyderma (sin.) and T . quinqueloba. Differences in the species' isotope ratio at the same sites are caused by different calcification depths and metabolic activity. The ox ygen isotope composition of N. pachyderma (sin.) shows a relationship to sa linity measurements and indicates that it is a good quantitative proxy for salinity reconstructions, while no relationship exists in this region betwe en N. pachyderma (sin.) oxygen isotopes and water temperature. (C) 2001 Els evier Science B.V. All rights reserved.