Remineralization of organic carbon in eastern Canadian continental margin sediments

• Published in: Deep-sea research. Part II, Topical studies in oceanography Topical studies in oceanography, 2000-01, Vol.47 (3), p.699-731
• Main Authors: Silverberg, Norman, Sundby, Bjørn, Mucci, Alfonso, Zhong, Shaojun, Arakaki, Takeshi, Hall, Per, Landén, Angela, Tengberg, Anders
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/S0967-0645(99)00123-X

Undisturbed sediment samples were collected for chemical analyses at six sites during winter and summer cruises to the eastern Canadian continental margin. Micro-electrode oxygen profiles were obtained in freshly collected multicorer samples, and replicate cores were incubated at in situ temperature for 48 h to monitor changes in the concentrations of dissolved oxygen and nitrate. In addition, box cores were subsampled vertically for porewater chemistry, porosity, and particulate carbon. The data obtained are combined with estimates of sedimentation rate based on sediment trap measurements, 210 Pb dating and historical data to evaluate the role of benthic processes in the carbon cycle on the eastern Canadian continental margin. With one exception, oxygen uptake rates determined from incubations and calculated from micro-profiles were very similar, indicating that exchange of oxygen across the sediment–water interface was dominated by molecular diffusion. On the basis of this observation, transport by diffusion is assumed for the calculation of the flux rates for other solutes from their respective porewater gradients. The fluxes of oxygen into the sediments were low, but generally comparable to other continental margins at comparable depths. They varied from 1.4 to 1.8 mmol/m 2/d in December 1993 and from 2.8 to 4.5 mmol/m 2/d in June 1994. Uptake of nitrate by the sediment occurred at all sites except for the continental slope off Nova Scotia. Both oxygen and nitrate uptake were higher in summer than in winter, indicative of a lingering response to the input of organic matter associated with the early spring bloom. At one of the sampling sites, Miscou Channel, the measured oxygen uptake rate far exceeded the flux calculated from the oxygen gradient. The difference suggests biologically enhanced exchange with the overlying waters at this site, consistent with the greater abundance of benthic organisms. The rate of organic carbon mineralization at the seafloor (1.6 – 4.2 mmol/m 2/d) was estimated as the sum of the sediment oxygen and nitrate uptake rates. The contribution of other electron-acceptors to the mineralization of organic carbon is assumed to be accounted for by the oxidation of most of their reduced by-products by O 2 and NO 3 −. The return fluxes of dissolved inorganic carbon (∑CO 2 efflux corrected for carbonate dissolution) were generally comparable to the carbon oxidation rates. A detailed carbon budget was established at one of the sampled s