Seasonal variability of the long-chain alkenone flux and the effect on the U37k′-index in the Norwegian Sea
Introduction
Recent studies of the flux of organic matter into the deep ocean have prompted the search for key organic compounds (biomarkers) as tracers for its production, flux and burial into the sediment. Many paleoceanographic studies have concentrated on the long-chain unsaturated methyl ketones (alkenones) with 37 and 38 carbon atoms. It is generally accepted that haptophyte (prymnesiophyte) algae, predominantly the coccolithophorid species Emiliania huxleyi, are the biological source for these alkenones. Laboratory experiments with this species and several field studies have shown that the degree of unsaturation within the series of alkenones, expressed in the U37k′-index, depends on the water temperature during algal production (Brassell et al., 1986; Prahl and Wakeham, 1987; Prahl et al., 1988; reviewed by Brassell, 1993). This relationship can serve as a tool for paleo sea surface temperature estimations from sediment data (e.g. Sikes et al., 1991; Eglinton et al., 1992; Lyle et al., 1992; Rosell-Melé et al., 1995a). A number of authors have confirmed a good relationship between water column and sediment data (e.g. Conte et al., 1992; Prahl et al., 1993; Sikes and Volkman, 1993) and a good concordance with δ18O isotope data (e.g. Brassell et al., 1986; Freeman and Wakeham, 1992; Sikes and Keigwin, 1996). Different formulae have been applied to calibrate these data for the water column (Prahl and Wakeham, 1987; Conte and Eglinton, 1993; Prahl et al., 1993; Sikes and Volkman, 1993) and sediments (Rosell-Melé et al., 1995a).
The Norwegian–Greenland Sea and adjacent areas are of central importance in the renewal of deep ocean waters and for the circulation system of the world ocean. The Norwegian–Greenland Sea (Fig. 1) is characterized by strong east to west hydrographic gradients. Temperate and ice-covered surface waters separated by distinct oceanic gradients occur in close vicinity. Today, warm saline Atlantic water moves northward on the eastern side of the Norwegian Sea in the Norwegian current (NC), a relatively warm (6–10°C) and saline (S>34.9) branch of the North Atlantic Drift (NAD). The Atlantic water masses at the continental slope at 75°N have temperatures above 0°C and extend down to depths between 600–800 m in the West Spitsbergen Current (WSC). The boundary between Atlantic Water and the fresh and cold (S<34.8 and T<0°C) polar water is the Bear Island Front. On the western side of the Sea the East-Greenland (EGC) current carries cold (<0°C), less saline (S=30–34) polar water southward along the East Greenland coast (Swift, 1986). Between these two main current systems Arctic surface water masses are formed by mixing in two large gyres (Jan Mayen Current and East-Iceland Current). The area is characterized by the formation of distinct oceanographic fronts (Johannessen, 1986).
Coccolithophorids represent a major phytoplankton group in the Norwegian–Greenland Sea and several studies have described the occurrence and distribution (e.g. Samtleben et al., 1995a). Coccolithophore assemblages of the North Atlantic group (T>10°C, up to 18 species) occur primarily south of the Iceland–Scotland Ridge and are normally dominated by E. huxleyi. The Norwegian group (T>5°C, generally more than seven species), occurs within the Norwegian current up to 75°N and is characterized by Syracosphaera and Acanthoica species. The Arctic group shows low diversity. While Algirosphaera robusta and Ophiaster hydroideus are primarily observed in areas with stronger influence from Atlantic water, E. huxleyi, Alisphaera unicornis and Calciopappus caudatus are distributed up to the polar front. Only two species are present in the polar domain, mainly Coccolithus pelagicus (Samtleben and Schröder, 1992; Samtleben et al., 1995b). Studies report strong seasonal variations of the floras which lead to large differences in composition and abundance between non-production (autumn to early summer) and production (summer) periods. Emiliania huxleyi, in particular, produces strong blooms in summer within various regional groups (Samtleben and Schröder, 1992; Samtleben et al., 1995a, Samtleben et al., 1995b; Baumann et al., in press). High cell densities of E. huxleyi are observed south of Iceland in May/June, in the region of the Vøring Plateau in July, in the Barents Sea during August and west of Jan Mayen not before September (Samtleben et al., 1995a). There are only rare examples among the trap assemblages which resemble the living communities with regard to relative species abundance. The settling assemblages are always dominated by either E. huxleyi or C. pelagicus (Andruleit, 1997).
Our aim was to determine the seasonality of the alkenone flux at two different sites in the Norwegian Sea and to test the applicability of the alkenones as tracers for source and transport processes of the organic matter before reaching the sediment in this area. The aspect is part of the objectives of the Sonderforschungsbereich 313 of Kiel University to understand the links between pelagic surface processes and particle export to the deep Norwegian–Greenland Sea and provide information for the reconstruction of the paleoenvironment from the sedimentary record (Schäfer et al., 1995).
Section snippets
Sample collection
The particulate material was collected with multisample sediment traps (Kremling et al., 1996) moored in the Norwegian Sea (NB — 69°41.2′N; 0°27.8′E; water depth 3290 m) at 500, 1000 and 3000 m (August 1991–July 1992) and at the Barents Sea continental margin (BI — 75°11.8′N; 12°29.2′E; water depth 2050 m) at 1840 and 1950 m (March–July 1991) (Fig. 1). At the latter station the temporal resolution of the two bottom near sediment traps was high (7 days) to obtain information on the effect of lateral
Norwegian Sea
In general the seasonal particle flux in the Norwegian Sea shows a clear seasonal maximum during late summer/autumn (summarized in von Bodungen et al., 1995). A complex food web with a low ratio of new to regenerated production efficiently retains biogenic elements in the pelagic zone in spring. The phytoplankton growth is controlled and limited strongly by herbivorous copepods. In late summer and during autumn the development of pteropods can play an important part in the pelagic control of
Conclusions
The alkenone sedimentation in the Norwegian Sea and on the Barents Sea continental margin showed a seasonally varying sedimentation pattern. Spatial and temporal variations of the U37k′-index were observed in the sediment trap material. The least modification of the alkenone signal in particles on their way from surface to the near-bottom traps occurs during phases of rapid sedimentation after short residence time in the upper pelagic zone or transport packaged in faecal pellets. In general
Acknowledgements
The authors thank the crews of RV Meteor and Poseidon for their help during the cruises. We acknowledge the support of our colleagues in subproject A1 of the SFB 313, who provided pre-processed and split sediment trap material and made it possible to consider bulk parameter flux data. Additionally, water column filter samples were kindly made available for alkenone measurements by C. Samtleben, SFB 313 Synpal-Working Group. Thanks are due to A. Rosell-Melé for the helpful discussions concerning
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