N in Marine Organic Matter

An important use of nitrogen stable isotope ratios is the reconstruction of the degree of nitrate utilization in surface waters at the time the organic matter was produced. The applicability of this tool in paleoceanographic studies has been shown repeatedly (e.g. Altabet and Francois 1994 Holmes et al. 1997 Freudenthal et al. 2001). To determine changes in the fraction of unutilized nitrate in surface waters, Altabet and Francois (1994) defined the following equations  

Further decomposition of organic matter occnrs at the sediment/water interface and in the npper sections of the sediment. However, observations 

In the marine environment, snlfnr occnrs most commonly in its oxidized form as dissolved snlfate in seawater or as precipitated snlfate in evaporites and in its rednced form as sedimentary pyrite. The ratio of is a sensitive indicator for the 

Only a small fractionation is associated with the precipitation of snlfates in seawater. Theoretical considerations as well as snlfur-isotope measurements in natural environments revealed an isotopic difference between seawater sulfate and 

By far the largest fractionation is associated with the bacterial sulfate reduction (see a recent review by Canfield 2001). Dne to the activity of snlfate-redncing bacteria, snch as Desulfovibrio desulfur icons, organic matter is oxidized according to the following equation  

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