Oxygen and carbon isotope composition of modern planktic foraminifera and near-surface waters in the Fram Strait (Arctic Ocean) – a case study

Abstract. The upper 500 m of the water column and the sediment surface along an E–W transect in the Fram Strait were sampled for recent planktic foraminifera. The δ¹⁸O and δ¹³C values of the tests are compared to the stable isotope composition of the water samples taken from the same depths, and related to the characteristics of the water column. The polar species Neogloboquadrina pachyderma clearly dominates the species assemblage in the Fram Strait in the early summer, while the subpolar Turborotalita quinqueloba accounts only for 5–23%. In this area the average depth of calcification of N. pachyderma lies between 70–150 m water depth, T. quinqueloba shows a similar range with 50–120 m water depth. The δ¹⁸O values of N. pachyderma show an average vital effect of about −1.5‰ compared to calculated equilibrium calcite values. Except for the upper ∼75 m, the vertical profiles of δ¹³C of the net-sampled shells are nearly parallel to the values measured in the water column with an average offset of −1.6‰ and −3.6‰ for N. pachyderma and T. quinqueloba, respectively. The discrepancy found in the upper ~75 m might indicate an influence of the "carbonate ion effect" on the carbon isotope incorporation in the tests. Oxygen and carbon isotopes from the sediment surface yield higher values than those from the water column for both species. This may be because specimens from the water column reflect a modern snapshot only, while tests from surface sediments record environmental parameters from the past ~1000 years.