Elsevier

Organic Geochemistry

Volume 28, Issue 5, 17 April 1998, Pages 311-323
Organic Geochemistry

Seasonal variability of the long-chain alkenone flux and the effect on the U37k′-index in the Norwegian Sea

https://doi.org/10.1016/S0146-6380(98)00003-5Get rights and content

Abstract

A seasonally-varying sedimentation pattern was observed for the alkenone flux measured with sediment traps in the northern North Atlantic. In the Norwegian Sea (traps were set at 500, 1000 and 3000 m) the alkenone flux varied between 0.1 and 7.1 μg m−2 d−1 and followed the seasonal pattern of the bulk parameters. Maximum fluxes occurred from mid-October until mid-November and were also high in May. A surprising result was that considerably higher particle fluxes were observed at 3000 m. For the alkenone flux, the highest additional input of 250% was observed during the period when sediment resuspension was greatest in summer. At the Barents Sea continental margin (traps at 1840 and 1950 m) the alkenone fluxes follow the sedimentation pattern of the bulk parameters, with a less visible signal of distinct seasonality observed in the 1950 m trap. The sedimentation of total alkenones varied between 0.8 and 144 μg m−2 d−1 at 1840 m and between 0.5 and 31.0 μg m−2 d−1 at 1950 m. Resuspension and lateral advection contributed significantly to measured fluxes in the two near-bottom traps. Alkenone concentrations were determined in faecal pellets of Appendicularia, ostracods and euphausids from selected samples at the Barents Sea site. The alkenone flux in pellets (4% to 24% of total) was 5 to 6 times higher at 1950 m depth than at 1840 m and the major part (77–78%) of the total flux of C37:3 reaching the near-bottom trap at 1950 m was associated with faecal pellets of the meso-zooplankton. Spatial and temporal variations of the U37k′ signals were observed, indicating that the imprint in the alkenone signal depends on the origin and transport pathway of the organic material. Strong deviations occur in areas where nepheloid layers contribute particles of long residence times to the primary flux.

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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