Description: Density changes in the upper water column of the northern North Atlantic may enhance or reduce vertical convection of surface water with profound effects on meridional overturning and climate in the wider region. This study tests the capability of paired delta O-18 values of two planktonic foraminiferal species - Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba - for the reconstruction of near-surface density stratification in high latitudes or the glacial ocean. Foraminiferal data from two sediment cores of crucial areas of the Nordic Seas were compared with insolation-induced thermal stratification changes as obtained by simulations with the general circulation model ECHO-G. The comparison suggests that insolation was the chief mechanism to change thermocline strength during most of the Holocene. Prior to that, stratification depended by and large on the varying amounts of meltwater injected at the sea surface. Similar to the modern central Arctic Ocean, a pronounced and thick halocline prevented surface waters from deep convection in the central Nordic Seas. Parts of the Norwegian Sea, however, were also stratified but more analogous to the modern Greenland Sea, where deep convection can occur in late winter as a result of the density increase upon a combination of cold temperatures and wind stress. Our findings thus support previous results of an active meridional overturning also in a glacial ocean

Description: Density changes in the upper water column of the northern North Atlantic may enhance or reduce vertical convection of surface water with profound effects on meridional overturning and climate in the wider region. This study tests the capability of paired delta O-18 values of two planktonic foraminiferal species - Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba - for the reconstruction of near-surface density stratification in high latitudes or the glacial ocean. Foraminiferal data from two sediment cores of crucial areas of the Nordic Seas were compared with insolation-induced thermal stratification changes as obtained by simulations with the general circulation model ECHO-G. The comparison suggests that insolation was the chief mechanism to change thermocline strength during most of the Holocene. Prior to that, stratification depended by and large on the varying amounts of meltwater injected at the sea surface. Similar to the modern central Arctic Ocean, a pronounced and thick halocline prevented surface waters from deep convection in the central Nordic Seas. Parts of the Norwegian Sea, however, were also stratified but more analogous to the modern Greenland Sea, where deep convection can occur in late winter as a result of the density increase upon a combination of cold temperatures and wind stress. Our findings thus support previous results of an active meridional overturning also in a glacial ocean.

Description: Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.–min.) in the isotope values when measuring several (≥5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of 〉2‰ of δ18O and δ13C at low absolute levels (δ18O: 〈3‰, δ13C: 〈−3‰) near the river estuaries of Ob and Yenisei and low ranges of ∼1‰ at higher absolute levels (δ18O: 2–5.4‰, δ13C: −3‰ to −1.5‰) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8–7 cal. ka BP) of a sediment core north of Yenisei estuary revealed δ18O and δ13C values which on average are lower by 0.6‰ in both, δ18O and δ13C, than in the upper core section (〈5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.–min.) in the single-valve δ18O and δ13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise.

Description: Observation data from the southern Kara Sea demonstrate high interannual variability of bottom water temperature, salinity, and nutrients in summer. Low oxygen isotope events reconstructed from samples taken along the growth direction on shells of living bivalves show variations which reflect salinity changes probably caused by summer river run-off. These records indicate that the freshwater signal is discernible at the sea floor in water depths of at least 30 m, but is not observed in 70 m water depth. There is no evidence for a correlation between nutrients and carbon isotope composition ofthe bivalve shells.

Description: 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 δ18O values from foraminiferal tests in plankton hauls, sediment traps, and nearby core top samples with the calculated δ18Ocalcite profile of the water column revealed species-specific δ18O 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, δ18O 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 δ18O 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 δ18O is too conservative, because it overestimates sea surface salinity. Minimum δ18O differences (〈0.2‰) 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 δ18O differences of 〉0.4 to 〉1.5‰.

Description: Recent ostracod assemblages were investigated from coretop sediment samples collected in the eastern Kara Sea from water depths down to 300 m. A total of 45 species were identified, 27 of them were reported for the Kara Sea for the first time. The Kara Sea data were compared with our results on the distribution of ostracods in the eastern Laptev Sea. The spatial distribution of recent taxa and the ecological groupings demonstrate a clear relation to dominant environmental factors which range from estuarine to full-marine conditions. Four assemblages related to average summer bottom water salinities were established: (1) a freshwater assemblage from the inner estuaries of the Ob' and Yenisei rivers with salinities less than 2 and from thermokarst lagoons of the southern Laptev Sea coast with strong salinization in winter; (2) a brackishwater assemblage of the outer estuaries of the Ob' and Yenisei rivers with salinities up to 26; (3) a mixed euryhaline–marine assemblage dominated by euryhaline species Paracyprideis pseudopunctillata and Heterocyprideis sorbyana from the inner shelf river-affected zone of the Kara and Laptev seas, where salinities range between 26 and 32; (4) a taxonomically diverse marine assemblage dominated by shallow-water marine taxa from the northern parts of the Kara and Laptev shelves and upper continental slope with stable bottom environments and a salinity higher than 32. Abundant euryhaline species found at greater water depths are identified as part of an ice-rafted assemblage. They are possibly entrained into the newly formed fast ice during autumn storms and freeze-up period and then transported to the distal open-sea areas during summer.