A stable sulfur and oxygen isotopic investigation of sulfur cycling in an anoxic marine basin, Framvaren Fjord, Norway

In 1993 we measured the δ34S values of total dissolved sulfide (δ34S∑H2S) and sulfate (δ34SSO4−) and the δ18O of sulfate (δ18OSO4−) from water samples collected across the oxic–anoxic interface and in the deep permanently anoxic waters of the stratified Framvaren fjord in southern Norway. Near the chemocline, variations in the δ34SSO4− and δ18OSO4− values were generally less than 1‰ from ambient seawater values. However, a minimum δ34SSO4− value of +19.7‰ was detected at 20 m depth, which coincided with the depth that sulfide first appeared and may reflect sulfide oxidation. Small increases in δ34SSO4− and δ18OSO4− values 3 m below this depth are consistent with a zone of sulfur disproportionation. The δ34S∑H2S value near the interface at 22 m was −19.8‰, which is 41.2‰ depleted in 34S relative to the sulfate collected at that depth. In close agreement with earlier measurements made at Framvaren in 1982, the δ34SSO4− values collected from the deeper anoxic waters showed a marked 34S enrichment with depth, which corresponded with a decrease in the sulfate concentration. These results are interpreted to be the result of active dissimilatory sulfate reduction. A Rayleigh plot for the sulfate data measured in 1993 provides estimates for the sulfur and oxygen isotope enrichment factors (εs and εo, respectively) for sulfate reduction of −41.5‰ and −9.8‰, respectively, with the former value matching closely the observed difference in δ34S between the dissolved sulfide and sulfate near the interface. Our results from 1993, however, contrast with δ34SSO4− and δ34S∑H2S data in the water column made in 1983 by Anderson et al. [Mar. Chem. 23 (1988) 283). We conclude that the results of 1983 may be anomalous, and as a result this may offer additional interpretations than what was previously provided for the origin of reduced inorganic sulfur in the sediments of Framvaren based on their measured δ34S values. We hypothesize that the lower δ34Strs values in the sediments relative to δ34S∑H2S values in the water column could also result from different rates of sulfate reduction, or in shallower sediments just beneath the chemocline, also from disproportionation of S∘, S2O3−, or SO3−. We hypothesize that the observed ratio of 4.4:1 for the measured changes in δ34SSO4− versus δ18OSO4− within the anoxic waters approximates the 4:1 atom ratio of oxygen to sulfur in the residual sulfate as a result of dissimilatory sulfate reduction and reflects little oxygen isotope exchange between intermediates of sulfur metabolism and water either during bacterial sulfate reduction or from sulfide reoxidation processes. Based on comparisons with other studies, we further propose that this lack of isotopic exchange with water, and the subsequent ∼4:1 ratio of δ34SSO4− versus δ18OSO4−, occurs under conditions that promote a unidirectional biochemical reaction for sulfate reduction during which kinetic isotope effects are fully expressed and are consequently reflected in the δ34SSO4− and δ18OSO4− values.