Stable isotope record of planktonic foraminifera in the 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 delta18O and delta13C 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 13C and 18O, attributed to a species-specific vital effect. For nonencrusted N. pachyderma (sin.) in the 125-250 µm size class, this vital effect amounts to 1.3 per mil in delta18O and 2.0 per mil in delta13C. It increases to higher values in waters under permanent ice cover. T. quinqueloba reveals a mean vital effect of about 1.3 per mil in delta18O and 2.6 per mil in delta13C. The 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.