Dynamics of sigmaCO2 in a surficial sandy marine sediment: the role of chemoautotrophy

Abstract

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 (ZCOz) 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 XCO, production rates (4910 to 5570 nmol cm-3 d-l). The second zone (8 to 9 mm) below the suboxic layer showed a net CO2 uptake (161 to 191 nmol d-l) which coincided with the zone of maximum C-labeled bicarbonate fixation (R114Cp.0C). This implies that anoxic CO2 fixation is associated with anoxic processes probably involving sulfur species, since both NO3and metal oxides are absent. The CO, 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 ZCO, (55 to 97 nmol cm-3 d-l). The role of SzO3,in CO2 fixation was examined in completely anoxic and closed sediment incubations (jars). The presence of 0.5 mM S203' did not induce higher CO2 fixation rates than S203,--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 fixatlon may be higher in marine sediment than previously thought.