AME 12:165-176 (1997)  -  doi:10.3354/ame012165

Net consumption and production of CO₂ in the surface layers of a sandy marine sediment were examined with a depth resolution of 1 mm. A transient state diagenetic model fitted to measured porewater profiles of total inorganic carbon (SigmaCO₂) in open incubated sediment plugs revealed 3 distinct zones. The first was an upper oxic/suboxic zone of 5 to 8 mm depth with high net SigmaCO₂ production rates (4910 to 5570 nmol cm^-3 d^-1). The second zone (8 to 9 mm) below the suboxic layer showed a net CO₂ uptake (161 to 191 nmol cm^-3 d^-1) which coincided with the zone of maximum ¹⁴C-labeled bicarbonate fixation (R_[14C]TOC). This implies that anoxic CO₂ fixation is associated with anoxic processes probably involving sulfur species, since both NO₃^- and metal oxides are absent. The CO₂ fixation processes were, however, dependent on supply of oxidation equivalent from above, since they were completely inhibited under anoxic conditions in the overlying water. A third zone, situated below 16 mm in the deepest reduced sediment, had low net production rates of SigmaCO₂ (55 to 97 nmol cm^-3 d^-1). The role of S₂O₃^2- in CO₂ fixation was examined in completely anoxic and closed sediment incubations (jars). The presence of 0.5 mM S₂O₃^2- did not induce higher CO₂ fixation rates than S₂O₃^2--free controls. When thiosulfate was increased to 2 mM, a stimulation of CO₂ fixation occurred, indicating chemoautotrophy by e.g. disproportionation. The fact that significant CO₂ fixation also occurred initially in thiosulfate-free, anoxic control sediment indicated that hetero-/chemolithotrophic CO₂ fixation may be higher in marine sediment than previously thought.

Mineralization · Carbon fixation · Diagenetic modelling · Sulfate reduction · Thiosulfate · Marine sediment