The Kinetics of Organic Matter Mineralization in Anoxic Marine Sediments

The kinetics of sulfate reduction and inorganic nutrient production (~C02' ammonium, and phosphate) were examined in the sediments at five sites in the southern Chesapeake Bay, using long term (> 200 d) sediment decomposition experiments. Average first order rate constants for these processes (at 25°C) decreased from 8.2 to 3.7 yrin the surface sediments (0-2 em), to 2.1 to 0.2 yrat 12-14 em. The C/N and C/P ratios of the organic matter undergoing decomposition also increased with depth at these sites. Taken together, these results indicate that the reactivity of the organic matter undergoing mineralization decreases with depth in these sediments. A model based on the multiple-G model for organic matter decomposition (hereafter referred to as the mixture model) was developed to examine the observed kinetics of all of these processes. As in the multiple-G model, the mixture model is based on the mineralization of organic matter in sediments occurring from distinct fractions of organic matter with differing reactivities. However here, differences in reactivity are indicated by differences in both the intrinsic rate constants for decomposition as well as the C/N or C/P ratios of the organic matter in the fractions. The mixture model was useful in interpreting the results of these experiments, and provided explanations for differences in the reactivity of organic matter in the surface sediments at these sites. It also appeared to provide information on the nature of the organic matter undergoing remineralization in these sediments, based on the predicted C/N or e/P ratios of these apparent fractions. The data from this study was also examined using the recently presented power model of Middelburg (1989). This analysis indicated the importance of pre-depositional decomposition in affecting the reactivity of sedimentary organic matter. While all of these models provided insights into organic matter remineralization in marine sediments, they also all had mixed successes in describing (in a unified fashion) sulfate reduction and inorganic nutrient production in these southern Chesapeake Bay sediments. This observation indicates that care must be taken in interpreting information (e.g., rate constants, elemental ratios or apparent initial ages of the sedimentary organic matter) on organic matter in sediments, based on model derived parameters such as those that have been presented here. The ability to independently verify these model derived parameters of the sedimentary organic matter undergoing mineralization will likely be important in further refining these models and improving their usefulness in describing (and predicting) the factors controlling organic matter mineralization in marine sediments. 1. Department ofOceanography,OldDominionUniversity,Norfolk,Virginia,23529-0276, U.S.A.