Publication Date:
2012-02-23
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]
Type:
Conference or Workshop Item
,
NonPeerReviewed
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