Investigation of planktic foraminifera from the Norwegian-Greenland Sea

The vertical density gradients in the Nordic Seas are crucial for the preconditioning of the surface water to thermohaline 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 d18O values from foraminiferal tests in plankton hauls, sediment traps, and nearby core top samples with the calculated d18Ocalcite profile of the water column revealed species-specific d18O 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, d18O values of Turborotalita quinqueloba document apparent calcification depths within the pycnocline at 25-75 m water depth. The isotopic signatures of Neogloboquadrina pachyderma (s) reflect water masses near and well below the pycnocline between 70 and 250 m 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 -2.5°C. In contrast, N. pachyderma (s) calcifies very close to the sea surface (20-50 m) 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 d18O 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 d18O is too conservative, because it overestimates sea surface salinity. Minimum d18O differences (<0.2per mil) 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 d18O differences of >0.4 to >1.5per mil.

Core correlation data and revised composite depth scale of Exp302 (ACEX) cores

Expedition 302 of the Integrated Ocean Drilling Program (IODP), also known as the Arctic Coring Expedition (ACEX), successfully penetrated a sequence of Cenozoic sediments draping the crest of the Lomonosov Ridge in the central Arctic Ocean. The cumulative sedimentary record spans the last 57 m.y. and was recovered from three sites located within 15 km of each other. Merging the recovered cores onto a common depth scale that accurately reflects their stratigraphic placement below the seafloor is a fundamental step toward interpreting this unique sedimentary record. However, the lack of overlapping recovery in adjacent holes and intervals of high core disturbance complicated traditional methods of stratigraphic correlation. Here we present a revised composite depth scale for the ACEX sediments, generated in part by performing a regional stratigraphic correlation with sediments recovered from previous expeditions to the Lomonosov Ridge. The revised depth scale also reassesses the offsets for cores in the upper 55 meters below seafloor, where no overlapping recovery was acquired, and proposes modifications to these depths.