Dissimilatory sulfate reduction and methane production in Gotland Deep sediments (Baltic Sea) during a transition period from oxic to anoxic bottom water (1993-1996)

During a transition period from oxic to anoxic conditions in the bottom water, rates of sulfate reduction and methane production, methane fluxes, as well as concentration profiles of sulfate, sulfide and methane were measured in sediments at a central site of the Gotland Deep (Stn AL 93, 241 m depth), which is regarded as representative for the deepest part of this basin. During this period from 1993 to 1996 oxic conditions in the bottom water prevailed from spring 1994 until summer 1995 with oxygen concentrations decreasing progressively with time. In the sediments methane production occurred primarily in layers below 1 m depth and flux rates of methane to the sediment surface were characterized by a steep concentration gradient from approx. 5 mM at 4 m depth to values close to 30 μM at the surface, determined by diffusion processes and anaerobic oxidation of methane. Both processes were independent of changes at the sediment surface. Differences in the flux rates of methane between the deeper part with a mean value of 259 μmol m-2 d-1 and the upper layers with a mean of 47.7 μmol m-2 d-1 indicate that a considerable proportion of the methane is oxidized within the anoxic horizon of the sediment (71 to 86% in the layer from 40 to 70 cm). Low rates of methane production found within the top 20 cm of the sediment during periods of oxic bottom water increased after depletion of oxygen and resulted in a clear maximum of the methane concentration in the top 2 cm. Sulfate concentrations declined exponentially from values of 11.5 mM in June 1994 and 8.5 mM in October 1995 at the sediment surface to values of 2.5 mM at 20 cm depth and of less than 0.5 mM at 50 to 60 cm depth. High sulfate reduction rates (150 to 250 nmol cm-3 d-1) in the upper part of the sediment (8 to 13 cm) coincided with maxima of sulfide concentrations. During the time period of this investigation an increase of maximum sulfide concentrations in the sediment from 1 to 10 mM was measured together with decreasing oxygen concentrations in the deep water. At the same time sulfate reduction established a small but distinct maximum at the top layer of the sediment (0 to 2 cm). The relative importance of sulfate reduction and methanogenesis in the carbon budget of the Gotland Deep sediments is calculated on the basis of the actual measurements.

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