The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark)1

Abstract

The cyclic transformations of inorganic sulfur compounds in the sediments of a Danish fjord were followed for 2 years. The in situ rate of sulfate reduction measured with a radiotracer technique together with chemical determinations of various sulfur compounds are used to calculate a budget of the complete sulfur cycle. Sulfate reduction rates were high at the sediment surface (25–200 nmol SO 4 2‐ cm −3 d −1 ) and there was still measurable activity at a depth of 1.5 m. Sulfate reduction is also compared with the benthic community metabolism measured as the oxygen uptake rate of the sediment. Sulfate reduction accounted for 53% of the total mineralization of organic matter in the sediment. Only 3% of the sulfide was derived from organic sulfur. Of all the sulfide produced, 10% was precipitated by metal ions within the anoxic sediment while the rest was reoxidized at the surface. The results demonstrate the dynamic nature of the sulfur cycle, with turnover times for sulfate of 4–5 months and for free sulfide of 1–5 days.