Paired δ18O signals of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba show thermal stratification structure in Nordic Seas

The vertical density gradients in the Nordic Seas are crucial for the preconditioning of the surface water to thermolialine sinking in winter. These gradients can be reconstructed from paired oxygen isotope data in tests of different species of planktonic foraminifera, the isotopic signatures of which represent different calcification depths in the water column. Comparison of delta(18)O values from foraminiferal tests in plankton hauls, sediment traps, and nearby core top samples with the calculated delta(18)O(calite) profile of the water column revealed species-specific delta(18)O vital effects and the role of bioturbational admixture of subfossil specimens into the surface sediment. On the basis of core top samples obtained along a west-east transect across various hydrographic regions of the Nordic Seas, delta(18)O values of Turborotalita quinqueloba document apparent calcification depths within the pycnocline at 25-75 in water depth. The isotopic signatures of Neogloboquadrina pachyderma (s) reflect water masses near and well below the pycnocline between 70 and 250 in off Norway, where the Atlantic inflow leads to thermal stratification. Here, temperatures in the calcification depth of N. pachyderma (s) differ from sea surface temperature by approximately -2.5degreesC. In contrast, N. pachyderma (s) calcifies very close to the sea surface (20-50 in) in the Arctic domain of the western Nordic Seas. However, further west N. pachyderma (s) prefers somewhat deeper, more saline water at 70-130 m well below the halocline that confines the low saline East Greenland Current. This implies that the delta(18)O values of N. pachyderma (s) do not fully reflect the freshwater proportion in surface water and that any reconstruction of past meltwater plumes based on delta(18)O is too conservative, because it overestimates sea surface salinity. Minimum delta(18)O differences (< 0.2 parts per thousand) between N. pachyderma (s) and T quinqueloba may serve as proxy for sea regions with dominant haline and absent thermal stratification, whereas thermal stratification leads to delta(18)O differences of > 0.4 to > 1.5 parts per thousand. (C) 2002 Elsevier Science B.V. All rights reserved.