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.

Results from ikaite investigations in the Kara Sea

The authigenic carbonate mineral ikaite is specific of low-temperature high latitude environments. The depletion of ikaite carbon in 13C isotopes in most cases implies a causal relation of ikaite generation with methane geochemistry. In this paper we present new data on ikaite minerals in Holocene sediments sampled along the Yenisei channel at the southern (74°N) and northern (77°N) ends. Stable carbon isotopes of the ikaite crystals were studied in conjunction with the hydrochemistry and isotope geochemistry of the sediments. Pore water and natural gas samples were separated from sediments to describe the methane carbon isotope distribution pattern throughout two sedimentary sequences embedding the ikaite crystals of different isotope composition (-24 per mil and -42 per mil). The biogenic nature of the methane is indicated by 51 C values being as low as -104.4 per mil. In the case of the moderately depleted sample (-24 per mil) from the southern location the small-scale ikaite formation fits best into the concept of a 'closed» sediment system, with a limited diagenetic carbon dioxide source being present. In the second case, formation of highly abundant and isotopically depleted ikaite crystals (-42 per mil) were caused by upwards flux of biogenic methane from below. Contribution of two main carbon sources to the ikaite crystals was estimated by using a isotope-mass balance equation. Organic-derived CO2 constitutes the principal source in both samples, amounting to 50 % of the total carbon of the strongly depleted ikaite crystals (-42 per mil) sampled at the northern end and 83 % for the moderately (-24 per mil) depleted crystals from the southern end. Methane-derived CO2 comes to 42 % for the isotopically light ikaite crystals and to 9% for the isotopically heavy crystals. The importance of sediment lithology and diffusive transport for ikaite formation is emphazied.