Organic carbon flux and remineralization in surface sediments from the northern North Atlantic derived from pore-water oxygen microprofiles

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Abstract

Organic carbon fluxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen microprofiles. A wire-operated in situ oxygen bottom profiler was deployed, and oxygen profiles were also measured onboard (ex situ). Diffusive oxygen fluxes, obtained by fitting exponential functions to the oxygen profiles, were translated into organic carbon fluxes and organic carbon degradation rates. The mean Corg input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C m−2 d−1. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C m−2 d−1 (mean 3.7 mg C m−2 d−1), whereas fluxes on the East Greenland shelf are considerably higher, 9.1–22.5 mg C m−2 d−1. On the Norwegian continental slope Corg fluxes of 3.3–13.9 mg C m−2 d−1 (mean 6.5 mg C m−2 d−1) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites off southern Norway in 1997 the temporal variability of diffusive O2 fluxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the seafloor. Degradation rates of 0.004–1.1 mg C cm−3 a−1 at the sediment surface were calculated from the oxygen profiles. First-order degradation constants, obtained from Corg degradation rates and sediment organic carbon content, are in the range 0.03–0.6 a−1. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in Corg flux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.

Introduction

Early diagenetic processes in surface sediments are closely coupled to the flux of organic carbon (Corg) to the seafloor. These processes drive the recycling and burial of Corg and thus affect the sedimentary record. Therefore, the quantification of Corg fluxes is of major interest for early diagenetic modeling and geochemical budgets, which, in turn, are an important aspect of a quantitative understanding of the carbon cycle at present and in the past.

Most of the remineralizable organic carbon arriving at the seafloor in meso- and oligotrophic environments is degraded in the immediate vicinity of the sediment/water interface, consuming dissolved oxygen as a primary electron acceptor. Furthermore, oxygen also functions as the final oxidant for anaerobic pathways. Therefore, the measurement of pore-water oxygen microprofiles provides a suitable tool for the determination of Corg fluxes through the sediment/water interface and of Corg remineralization rates.

In addition to the determination of benthic oxygen uptake by in situ chamber or laboratory core incubation, O2 microelectrodes have been established as an appropriate method for the determination of diffusive oxygen fluxes by high-resolution O2 profiles (Revsbech et al., 1980; Revsbech and Jørgensen, 1986; Reimers et al., 1986). Reimers (1987) and Glud et al. (1994) have emphasized that it is important to measure O2 profiles in situ in order to avoid sampling and pressure artifacts during core retrieval.

Numerous flux measurements have been performed in deep-sea sediments of the low and temperate latitude oceans (e.g. Reimers et al., 1984; Archer et al., 1989; Jahnke et al., 1989; Hales et al., 1994; Cai and Reimers, 1995; Glud et al., 1994). Little data, mostly obtained ex situ, exist for high latitudes beyond 60° N or S, such as the areas near Svalbard (Hulth et al., 1994), the Arctic Ocean (Boetius and Damm, 1998), the Southern Ocean (Rabouille et al., 1998; De Wit et al., 1997) and the Weddell Sea (Schlüter, 1991). Focusing on the northern high latitudes, Graf et al. (1995) and Rowe et al. (1997) quantified sediment oxygen demand (SOD) on the basis of core and chamber incubation for the European Nordic Seas and the Northeast Water Polynya (NEWP). Glud et al. (1998) investigated benthic mineralization and exchange in coastal Arctic sediments from northern Norway and Svalbard. Nevertheless, in situ O2 data coverage is poor for the open Norwegian and Greenland Seas, which are of great importance for global ocean circulation through deep-water renewal processes (Killworth, 1979) and for earth’s climate system (Rudels et al., 1989). Little is known about the contribution of sedimentary Corg remineralization and fixation in the northern North Atlantic to the global carbon budget.

In situ O2 profiles measured in the Norwegian and Greenland Seas at 568–3627 m water depth as well as ex situ profiles from the East Greenland shelf at 189–794 m water depth are presented here. Apart from the in situ chamber measurements performed by Rowe et al. (1997) in the NEWP, these data are, to our knowledge, the first in situ O2 microprofile measurements obtained from ice-covered areas and from the deep northern North Atlantic. The regional distribution of fluxes and degradation rates over shelf areas and continental margins off Greenland and Norway as well as that of the deep basins is addressed here. The influence of the ice regime on particle flux (Ramseier et al., 1999) and on sedimentary Corg flux will be discussed below.

Furthermore, this study provides an appropriate data set for a basin-wide Corg flux budget of the European Nordic Seas. Global benthic flux budgets (Jahnke, 1996), which exclude the high-latitude ocean due to sparse data coverage, can now be extended to the high-latitude North Atlantic.

In calculating budgets such as these, it is important to address the aspect of spatial and temporal variabilities for assessing uncertainties in projecting single flux measurements up to an annual scale. Due to distinct seasonal variations in light availability at high latitudes, the amount of primary produced Corg also changes considerably during the year (Antoine et al., 1996). Martin and Bender (1988) and Soetaert et al. (1996) investigated the response of surface sediment metabolism to seasonal variations in Corg rain rates by modeling pore-water metabolites and oxidants. These models reveal that the amplitude and phase lag of benthic fluxes to seasonal variations in organic carbon rain rates are closely related to the degradation rate. Some investigations reveal a rapid response of benthic metabolism to increased rain rates (Graf, 1989; Smith et al., 1994); other investigators have observed only a slight seasonal response of O2 fluxes, although particle trap data suggest considerable variability in export flux at this site (Sayles et al., 1994). In order to assess such variabilities in our working area, several specific locations were revisited at different seasons, and multiple measurements performed at one site were compared.

Finally, Corg fluxes were compared to data obtained from other polar regions, such as the NEWP (Rowe et al., 1997), the Arctic Ocean (Boetius and Damm, 1998) and the Svalbard area (Hulth et al., 1994).

Section snippets

Materials and methods

On several cruises with the research vessels “Polarstern”, “Meteor”, “G.O. Sars” and “Johan Hjort” to the Norwegian and Greenland Seas, in situ oxygen profiles were measured at 14 locations (Table 1, Fig. 1). At some of these and at additional sites, ex situ O2 profiles, pore-water manganese, and Corg content were measured. Water depths range from 193 m on the East Greenland shelf to 3627 m in the central Greenland Sea. The sites off Norway lie in the domain of the Norwegian Current (Atlantic

Oxygen profiles and Corg flux

Some exemplary in situ oxygen profiles measured in the Norwegian and Greenland Seas are shown in Figs. 3a–i. Some of the additional ex situ (laboratory) measurements from the East Greenland shelf are depicted in Figs. 3j–l). Fluxes derived from the profiles are listed in Table 2. At the deep-sea stations (Figs. 3a and b) oxygen decreases only slightly with depth, indicating that the sediment is deeply oxygenated, corresponding to low Corg input. At Station 36/246 (3278 m water depth) the lowest O

Discussion

Prior to a discussion of the regional distribution of fluxes and rates, data quality as well as temporal and spatial variabilities are discussed. At the end, Corg flux in the area is compared to that found in other regions.

Conclusions

O2 pore-water profiles measured in this study suggest that deep-sea sediments of the Norwegian and Greenland Sea are deeply oxygenated and that Corg fluxes are comparatively low. At some sites, e.g. on the Vøring Plateau as well as on the shallow East Greenland shelf, extraordinarily high oxygen uptake rates have been found, which were assumed to be caused by particular hydrographic settings and influenced by the NEWP.

Corg fluxes determined on the Norwegian continental slope are generally

Acknowledgements

We thank the officers and crews of the research vessels “Polarstern”, “Meteor”, “G.O. Sars”, and “Johan Hjort” for their patient support. We are grateful for technical and laboratory assistance by A. Lunau and E. Steen. We have appreciated fruitful discussions with W. Ritzrau, R.O. Ramseier and other colleagues within the Sonderforschungsbereich 313 at Kiel University and at GEOMAR. We are very grateful to Tom Noji for his mediation between Kiel and the Institute of Marine Research, Bergen,

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