Stable isotope composition (δ18O, δ13C) 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 sampled for water column isotopes and living planktic foraminifer species Neogloboquadrina pachyderma (sinistral coiling) (Ehrenberg), 1861 and Turborotalita quinqueloba (Natland),1938. Their shell delta O-18 and delta C-13 values are compared to water oxygen and dissolved inorganic carbon isotope data to determine the environmental influence on the foraminifers' isotopic ratio. Major controls on the oxygen isotope composition of both species are the shallow depth habitat under permanent ice coverage, the low salinity surface layer, and the rate of metabolic activity. None of the specimens precipitated its shell in isotopic equilibrium with the ambient sea water. They are all depleted in C-13 and O-18, attributed to a species-specific vital effect. For nonencrusted N. pachyderma (sin.) in the 125-250 mum size class, this vital effect amounts to 1.3 parts per thousand in delta O-18 and 2.6 parts per thousand in delta C-13. It increases to higher values in waters under 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 oxygen isotope composition of N. pachyderma (sin.) shows a relationship to salinity measurements and indicates that it is a good quantitative proxy for salinity reconstructions, while no relationship exists in this region between N. pachyderma (sin.) oxygen isotopes and water temperature. (C) 2001 Elsevier Science B.V. All rights reserved.