Description: The Nordic Seas are a key area at the hemispheric scale since they constitute a transitional basin between the North Atlantic and Arctic Oceans (Figure 1a). The warm and salty North Atlantic waters contribute to the poleward heat transport via the Norwegian (NwAC-W, NwAC-E) and West Spitsbergen (WSC) currents, whereas the Arctic waters carry cool and fresh waters into the Nordic Seas via the East Greenland current (EGC). The interaction between these two surface currents determines the extent of the Polar and Arctic Fronts and acts on the deep-water formation. Several studies have shown large-amplitude variations in sea-surface conditions of the Nordic Seas, during the Holocene, due to changes in the strength and/or thermal characteristics of the NwAC and EGC. However, variability along the EGC is still poorly documented contrary to the NwAC where records depict warmer conditions than present along the main axis during the early Holocene, when summer insolation was higher. Nevertheless, data are not unequivocal since they show regional differences suggesting changes in atmospheric and oceanic circulation patterns. This study aims at documenting the impact of the last deglaciation on surface water masses in the Nordic Seas by reconstructing hydrographic parameters and sea-ice along the NwAC and EGC as well as to discuss the influence of the Arctic vs. North Atlantic fluxes. Here, we report the results from centennial resolution analyses performed on cores M23323, MSM 5/5-712-2 and JM06-WP-16MC (Figure 1a). Dinocyst assemblages were used as a proxy for the reconstructions of sea-surface conditions. We employed the Modern Analogue Technique (MAT) and the Northern Hemisphere dinocyst database that includes 1429 sites. The reconstructed past sea-surface conditions include the temperatures and salinities in summer, as well as the sea-ice cover duration. Likewise, redundancy analyses were done on dinocyst assemblages and environmental parameters in order to determine the statistical weight of changes observed in the distribution of assemblages. In each core, dinocyst assemblages show a clear transition at about 6.8 cal. kyrs BP. It is characterized by assemblages dominated by Nematosphaeropsis labyrinthus accompanied by Spiniferites elongatus and Spiniferites ramosus then by assemblages almost exclusively dominated by Operculodinium centrocarpum. This transition is also confirmed by the redundancy analyses that illustrate a sign shift. Sea-surface reconstructions indicate similar variations between cores but with different orders of magnitude (Figure 1b). Data of core M23323 depict cool summer temperatures (mean of 8°C) and low salinity (〈34.5) with episodic sea-ice (up to 2 months/yr) until 6.8 cal. kyrs BP. Furthermore, this interval corresponds to relatively high concentrations of pollen grains (up to 4000 grains/cm3), which suggest high fluvial discharge from adjacent watersheds. These palynological data suggest an environment marked by a coastal influence with high terrestrial inputs causing low surface salinity and upper water mass stratification. Data of core MSM5/5-712-2 illustrate a comparable variability except that summer temperatures are colder (mean of 5°) with low summer salinity (mean of 33.5) and high seasonal sea-ice cover up to 6 months/yr. Despite very low sedimentation rate, and consequently low temporal resolution in core JM06-WP-16MC, the reconstructions point out large-amplitude oscillations associated with cool conditions (mean of 6°C in summer), a mean salinity of 34 and a seasonal sea-ice cover between 2 and 3 months/yr. After 6.8 cal. kyrs BP, reconstructions from core M23323 indicate a warming trend (≈1.5°C) and the gradual establishment of modern-like conditions with temperatures reaching 9.5°C in summer and a salinity of ~34.8, as the consequence of predominant NwAC. Reconstructions from core MSM 5/5-712-2 illustrate a more stable environment with a slight cooling trend (≈1°C) that could be due to an increase of the EGC and/or ESC fluxes. This transition is not so visible in core JM06-WP-16MC, which depicts a large cyclicity with cool conditions (mean of 7°C in summer), salinity around 34.8 and a seasonal sea-ice cover reaching up to 4 months/yr. However, a freshwater pulse (≈33.5) associated with a sea-ice cover of 4 months/yr at ca. 5.9 cal. kyrs BP is well recorded in cores MSM5/5-712-2 and JM06-WP-16-MC suggesting a strengthening of the EGC and a southward moving of the Polar-Arctic Fronts. Overall, records from these three cores point out a major reorganization of sea-surface conditions during the early-mid- Holocene transition in the Nordic Seas, and particularly in the eastern part. Also, it seems that the decoupling of the western and eastern branches of the Norwegian current as well as the WSC played a predominant role on the variability of sea-surface conditions during this time. The cool conditions reconstructed from core M23323, for the early Holocene period, contrast with warm ones observed along the main axis of the North Atlantic current as recorded, for instance, by diatoms (e.g., Berner et al., 2010) and alkenones (e.g., Calvo et al., 2002). We hypothesize here that this difference in water mass during the early Holocene was notably due to an enhanced freshwater influence from the northwestern Europe, including the Baltic Sea watershed, which lead to an intensification of coastal currents and an enhancement of the upper water mass stratification along the continental margin. Berner, K.S., Koç, N., Godtliebsen, F., 2010, High frequency climate variability of the Norwegian Atlantic Current during the early Holocene period and a possible connection to the Gleissberg cycle: The Holocene, v. 20, p. 245-255. Calvo, E., Grimalt, J., Jansen, E., 2002, High resolution U37 k sea surface temperature reconstruction in the Norwegian Sea during the Holocene: Quaternary Science Reviews, v. 21, p. 1385-1394. [Figures see online publication]

Description: Marine sediment records of dinoflagellate cyst, foraminiferal and ice rafted debris content and stable isotopes from the Nordic seas and Labrador Sea were used to reconstruct the evolution of the surface circulation in the (sub)arctic North Atlantic during the Last Interglacial. Average global temperatures of this time interval, known as Marine Isotope subStage (MIS) 5e, are believed to have been higher than those of the present Holocene interglacial period. However, the abundance peak of warm dinoflagellate cyst taxa and subpolar planktic foraminifera in the eastern Nordic seas during late MIS 5e would suggest that the development of upper ocean interglacial conditions in that area was delayed with respect to the temperate latitudes, and the marine optimum in the eastern Nordic seas with a surface circulation comparable to the modern one was not reached until late MIS 5e. While the lack of a modern type of surface circulation during much of the early MIS 5e probably prevented the formation of Labrador Sea Water (Hillaire-Marcel et al., 2001), our data furthermore suggest that only with the establishment of this intensified modern-type of northward heat transport, an interglacial surface ocean environment also developed in the northern Nordic seas. Hence, our findings illustrate the importance of a correct (stratigraphic) context placement of those last interglacial records from the high Arctic pointing out overall warmer conditions with respect to the Holocene, as these might represent only specific phases of MIS 5e. Hillaire-Marcel, C., de Vernal, A., Bilodeau, G., Weaver, A.J., 2001. Absence of deep-water formation in the Labrador Sea during the last interglacial period. Nature 410, 1073-1077.

Description: Palynological analyses were performed on 53 surface sediment samples from the North Pacific Ocean, including the Bering and Okhotsk Seas (37–64°N, 144°E–148°W), in order to document the relationships between the dinocyst distribution and sea-surface conditions (temperatures, salinities, primary productivity and sea-ice cover). Samples are characterized by concentrations ranging from 18 to 143 816 cysts/cm3 and the occurrence of 32 species. A canonical correspondence analysis (CCA) was carried out to determine the relationship between environmental variables and the distribution of dinocyst taxa. The first and second axes represent, respectively, 47% and 17.8% of the canonical variance. Axis 1 is positively correlated with all parameters except to the sea-ice and primary productivity in August, which are on the negative side. Results indicate that the composition of dinocyst assemblages is mostly controlled by temperature and that all environmental variables are correlated together. The CCA distinguishes 3 groups of dinocysts: the heterotrophic taxa, the genera Impagidinium and Spiniferites as well as the cyst of Pentapharsodinium dalei and Operculodinium centrocarpum. Five assemblage zones can be distinguished: 1) the Okhotsk Sea zone, which is associated to temperate and eutrophic conditions, seasonal upwellings and Amur River discharges. It is characterized by the dominance of O. centrocarpum, Brigantedinium spp. and Islandinium minutum; 2) the Western Subarctic Gyre zone with subpolar and mesotrophic conditions due to the Kamchatka Current and Alaska Stream inflows. Assemblages are dominated by Nematosphaeropsis labyrinthus, Pyxidinopsis reticulata and Brigantedinium spp.; 3) the Bering Sea zone, depicting a subpolar environment, influenced by seasonal upwellings and inputs from the Anadyr and Yukon Rivers. It is characterized by the dominance of I. minutum and Brigantedinium spp.; 4) the Alaska Gyre zone with temperate conditions and nutrient-enriched surface waters, which is dominated by N. labyrinthus and Brigantedinium spp. and 5) the Kuroshio Extension-North Pacific-Subarctic Current zone characterized by a subtropical and oligotrophic environment, which is dominated by O. centrocarpum, N. labyrinthus and warm taxa of the genus Impagidinium. Transfer functions were tested using the modern analog technique (MAT) on the North Pacific Ocean (= 359 sites) and the entire Northern Hemisphere databases (= 1419 sites). Results confirm that the updated Northern Hemisphere database is suitable for further paleoenvironmental reconstructions, and the best results are obtained for temperatures with an accuracy of ± 1.7 °C.