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Abstract:

Kelts (1988) was the first to point out that the ionic composition of lacustrine waters could exert an important effect on the preservation of organic matter in the sediments of these systems. In lakes with high sedimentation rates, the sediments become devoid of molecular oxygen at or very near the sediment-water interface. The rates of anoxic (anaerobic) decomposition of organic matter vary with the amount and "type" of organic matter being buried as well as with the electron acceptor being reduced. The amount of energy produced per organic molecule being oxidized is more for microbial sulfate reduction than for methane production. Therefore, where sulfate is abundant, organic carbon should be more readily oxidized than where other anions such as Cl and HCO3 dominate the chemistry.

We have tested this hypothesis by determining in-situ sulfate reduction via radiolabel techniques in sediments from Freefight Lake, a Na+-Mg2+-SO42– saline lake in southwestern Saskatchewan. These data are then compared to sulfate reduction rate measurements from other sedimentary environments. Of particular interest is our previous work on algal flat sediments from Na+-Mg2+-Cl salt lakes and pans in Bonaire, N. A. The maximum sulfate reduction rate for the nearshore, algal flat sediments of Freefight Lake was 400 nmol/ml/d. This is 2 to 2.5 times lower than rates from similar settings in Bonaire, even though the sulfate concentrations were 10–20 times higher at Freefight Lake. In addition, the S:OC ratios of the Freefight Lake sediments were extremely low, indicating the lack of sulfate reduction in these sediments. Although the total reduced sulfur concentration increased with depth at Freefight Lake, an extremely high percentage of this total S2– is in the form of acid-volatile sulfur. This would not have been predicted from the low sulfate reduction rates.

A number of important conclusions can be drawn from this work. They include the following: (1) sulfate reduction rates in salt lake sediments are not solely dependent on SO4 2– concentrations nor on the ionic composition of the lake waters; (2) even though the organic carbon concentrations of these sediments are high, sulfate reduction is not rapid; and (3) the low sedimentation rate (0.03 cm/yr) may lead to the utilization of the most metabolizable of the carbon present in the oxic zone by aerobic bacteria, thereby slowing sulfate reduction rates.

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