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

In 1993 we measured the delta(34)S values of total dissolved sulfide (delta(34)S(SigmaH2S)) and sulfate (delta(SO4)(34)(-)) and the delta(18)O of sulfate (delta(18)OSO(4)(-)) 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 delta(34)S(SO4)(-) and delta(18)O(SO4)(-) values were generally less than 1 parts per thousand from ambient seawater values. However, a minimum delta(34)S(SO4)(-) value of + 19.7 parts per thousand was detected at 20 in depth, which coincided with the depth that sulfide first appeared and may reflect sulfide oxidation. Small increases in delta(34) S-SO4(-) and delta(18)O(SO4)(-) values 3 m below this depth are consistent with a zone of sulfur disproportionation. The delta(34) S-SigmaH2S value near the interface at 22 in was - 19.8 parts per thousand, which is 41.2 parts per thousand depleted in S-34 relative to the sulfate collected at that depth. In close agreement with earlier measurements made at Framvaren in 1982, the delta(34)S(SO4)(-),_ values collected from the deeper anoxic waters showed a marked 34 S 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 (epsilon(s) and epsilon(o) respectively) for sulfate reduction of - 41.5 parts per thousand and - 9.8 parts per thousand, respectively, with the former value matching closely the observed difference in delta(34)S between the dissolved sulfide and sulfate near the interface. Our results from 1993, however, contrast with delta(34)S(SO4)(-) and delta(34)S(SigmaH2S) 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 delta(34)S values. We hypothesize that the lower delta(34)S(trs) values in the sediments relative to delta(34)S(SigmaH2S) 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-n, S2O3-, or SO3-. We hypothesize that the observed ratio of 4.4:1 for the measured changes in delta(34)S(SO4)(-) versus delta(18)O(SO4)(-) 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 similar to 4:1 ratio of delta(34)S(SO4)(-) versus delta(18)O(SO4)(-), 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 delta(34)S(SO4)(-) and delta(18)O(SO4)(-) values. (C) 2003 Elsevier Science B.V. All rights reserved.