Benthic organic carbon flux and oxygen penetration depth in the Southern Ocean

For the investigation of organic carbon fluxes reaching the seafloor, oxygen microprofiles were measured at 145 sites in different sub-regions of the Southern Ocean. At eleven sites, an in situ oxygen microprofiler was deployed for the measurement of oxygen profiles and the calculation of organic carbon fluxes. At four sites, both in situ and ex situ data were determined for high latitudes. Based on this dataset as well as on previous published data, a relationship was established for the estimation of fluxes derived by ex situ measured O2 profiles. The fluxes of labile organic matter range from 0.5 to 37.1 mgC m**2/day. The high values determined by in situ measurements were observed in the Polar Front region (water depth of more than 4290 m) and are comparable to organic matter fluxes observed for high-productivity, upwelling areas like off West Africa. The oxygen penetration depth, which reflects the long-term organic matter flux to the sediment, was correlated with assemblages of key diatom species. In the Scotia Sea (~3000 m water depth), oxygen penetration depths of less than 15 cm were observed, indicating high benthic organic carbon fluxes. In contrast, the oxic zone extends down to several decimeters in abyssal sediments of the Weddell Sea and the southeastern South Atlantic. The regional pattern of organic carbon fluxes derived from micro-sensor data suggest that episodic and seasonal sedimentation pulses are important for the carbon supply to the seafloor of the deep Southern Ocean.

Organic carbon flux and remineralization in surface sediments

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. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C/m**2/d (mean 3.7 mg C/m**2/d), whereas fluxes on the East Greenland shelf are considerably higher, 9.1-22.5 mg C/m**2/d. On the Norwegian continental slope Corg fluxes of 3.3-13.9 mg C/m**2/d (mean 6.5 mg C/m**2/d) 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 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. 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.