Mineralization and burial of organic carbon in sediments of the southern Weddell Sea (Antarctica)

Abstract

Benthic fluxes of oxygen, alkalinity (AT), total carbonate (CT or ΣCO 2) and dissolved organic carbon (DOC) were measured during sediment-water incubations at 16 stations in the southern Weddell Sea (Antarctica) with water depths between 280 and 2514 m. The total sediment oxygen consumption rates (TSOC) were in general low (1.74-3.61 mmol m −2 day −1) and more comparable to measurements in slope and deep-sea sediments at a few thousand meters water depth. The decrease of TSOC with water depth was lower than that observed in many other seas. The mean carbon to nitrogen ratio (C/N) in the solid phase of surficial sediment was 8.3. Measured benthic fluxes of alkalinity, corrected for contributions from nitrification and denitrification, were quantitatively used to correct the fluxes of total carbonate for dissolution of solid phase carbonates. The ΣCO 2 fluxes, originating from carbonate dissolution (0.1661–1.77 mmol m −2 day −1 were 2.6–71 % of the ΣC0 2 fluxes (0.984–3.73 mmol m −2 day −1) resulting from organic carbon oxidation. Measured benthic fluxes of oxygen, ΣC0 2 and nitrate were, together with estimated denitrification rates and sediment C/N ratios, used to model respiration quotients (RQ) for organic carbon oxidation and estimate composition of the organic matter undergoing degradation. Modelled RQ varied roughly between 2/3 and 1 (mean 0.87). Measured fluxes of ΣC0 2 were 1.6–3.2 times higher than integrated organic C mineralization rates (measured during closed incubations of sieved, homogenized sediment), indicating macrofaunal (plus possibly meiofaunal) respiration to be important. However, low abundances of bioirrigating benthic macrofauna and small differences in benthic fluxes of oxygen, ΣCO 2 and alkalinity found between replicate sediment cores, suggested that macrofaunal respiration was quantitatively unimportant in these sediments. The higher measured fluxes of ΣC0 2 compared to the integrated mineralization rates, were therefore most likely caused by a large fraction of the respiration occurring directly on the sediment surface. This degradation of newly deposited organic matter was not reflected in the integrated organic C mineralization rates. Also, there was no obvious effect of this surficial degradation process on the pore water distributions of ΣCO 2. Benthic mass balances of carbon revealed that benthic fluxes of DOC were 3–147% of the corrected fluxes of ΣCO 2, and the recycling efficiencies (E) were up to 35% higher if the DOC fluxes were included in the calculations of E, rather than the inorganic ΣCO 2 flux alone. The recycling efficiencies, including the benthic flux of DOC, ranged between 57 and 88% (mean 78%). Measured rates of inorganic C accumulation (for most stations <0.3 mmol C m −2 day −1 were a factor of 6–7 lower than organic C accumulation rates (0.457–1.94 mmol C m −2 day −1).