Morphometric data for Globigerinoides sacculifer and Globorotalia tumida (size and Fourier coefficients) and temperature proxy reconstructions (UK37' and Tex86H) from samples from ODP sites 165-999/165-1000 and 202-1241 spanning the last 10Ma

Sediment samples from both Site 165-999/165-1000 (Atlantic) and Site 202-1241 (Pacific) were chosen at 1Ma intervals over the period 0.3-9.3Ma. Samples were washed and sieved <150µm. Splits of the sediment fraction were picked completely to obtain, where possible, at least 30 specimens each of planktic foraminifer species Globigerinoides sacculifer and Globorotalia tumida, on which outline analysis (Fourier) was performed. Sea surface and thermocline temperatures were reconstructed from palaeoenvironmental proxies (UK37' and Tex86H respectively).

TEX86H proxy and BIT index of sediment core GeoB9528-3 and reconstructed temperature data from NW Africa surface sediments

Changes in the strength of Atlantic meridional overturning circulation (AMOC) are known to have profound impacts on global climate. Coupled modelling studies have suggested that, on annual to multi-decadal time scales, a slowdown of AMOC causes a deepening of the thermocline in the tropical Atlantic. However, this process has been poorly constrained by sedimentary geochemical records. Here, we reconstruct surface (UK'37 Index) and thermocline (TEX86H) water temperatures from the Guinea Plateau Margin (Eastern tropical Atlantic) over the last two glacial-interglacial cycles (~ 192 kyr). These paleotemperature records show that periods of reduced AMOC, as indicated by the d13 C benthic foraminiferal record from the same core, coincide with a reduction in the near-surface vertical temperature gradient, demonstrating for the first time that AMOC-induced tropical Atlantic thermocline adjustment exists on longer, millennial time scales. Modelling results support the interpretation of the geochemical records and show that thermocline adjustment is particularly pronounced in the eastern tropical Atlantic. Thus, variations in AMOC strength appear to be an important driver of the thermocline structure in the tropical Atlantic from annual to multi-millennial time scales.

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.