DYNAMICS OF SIGMA-CO2 IN A SURFICIAL SANDY MARINE SEDIMENT - THE ROLEOF CHEMOAUTOTROPHY

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 tran sient state diagenetic model fitted to measured porewater profiles of total inorganic carbon (Sigma CO2) in open incubated sediment plugs re vealed 3 distinct zones. The first was an upper oxic/suboxic zone of 5 to 8 mm depth with high net Sigma CO2 production rates (4910 to 5570 nmol cm(-3) d(-1)), The second zone (8 to 9 ma) below the suboxic laye r showed a net CO2 uptake (161 to 191 nmol cm(-3) d(-1)) which coincid ed with the zone of maximum C-14-labeled bicarbonate fixation (R([)14( C]TOC)). This implies that anoxic CO2 fixation is associated with anox ic processes probably involving sulfur species, since both NO3- End me tal oxides are absent. The CO2 fixation processes were, however, depen dent on supply of oxidation equivalent from above, since they were com pletely inhibited under anoxic conditions in the overlying water. A th ird zone, situated below 16 mm in the deepest reduced sediment, had lo w net production rates of Sigma CO2 (55 to 97 nmol cm(-3) d(-1)). The role of S2O32- in CO2 fixation was examined in completely anoxic and c losed sediment incubations (jars). The presence of 0.5 mM S2O32- did n ot induce higher CO2 fixation rates than S2O32--free controls. When th iosulfate was increased to 2 mM, a stimulation of CO2 fixation occurre d, indicating chemoautotrophy by e.g, disproportionation. The fact tha t 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.