Dynamic Modeling of Early Diagenesis and Nutrient Cycling. a Case Study in an Arctic Marine Sediment

In this paper, we present a dynamic diagenetic model of the oxygen consumption pathways and cycling of carbon, nitrogen, manganese, iron, and sulfur in marine sediments. Sixteen dissolved or solid species are included in our model and the vertical transport processes accounted for are molecular diffusion, bioturbation, irrigation, and burial. Adsorption of dissolved species onto solid sediment is assumed to be reversible and to follow a linear equilibrium isotherm. Interactions between species are formulated in 21 biogeochemical reactions that are regulated through simplified Michaelis-Menten kinetics. The key driving input parameter to the model is the transient flux of organic matter supplied to the sediment surface, below which mineralization is described through the well-known 3-G model that includes a fast, a slow, and a non-degradable pool of organic matter. The model was applied to the Arctic sediment of Young Sound in Northeast Greenland using extensive measurements covering a full annual cycle. The model parameterization was performed in a stepwise process, first focusing on parameters describing transport in the sediment and then on parameters related to biogeochemical reactions. The model successfully simulated the measured concentration-depth profiles of O2, CO2, NH4 , NO3 , Mn , Fe , adsorbed Fe , SO4 , H2S, FeS, FeS2, MnO2, FeOOH, and organic matter; sediment-water fluxes of CO2, O2, NH4 , and NO3 ; depth-integrated process rates of denitrification and sulfate reduction; and depth profiles of iron-and sulfate reduction rates. The model application to the Young Sound sediment provided an excellent means to examine whether our perceptions of the most significant transport processes and biogeochemical reactions were correct. The successful simulation of measured data supported their internal consistency and confirmed previously reported interpretations and conclusions. The imposed supply of organic matter to the sediment surface that reproduced the measured data was found to be almost three times higher for the month of July than the average for the rest of the year. The peak in organic matter supply in mid-July coincided with the disappearance of sea ice. A sensitivity analysis performed for the model showed that the rate constants for the two degradable pools of organic matter were among those input variables that most affected the simulated results. As a result, we are confident of the accuracy of these rate constants of 76 y 1 and 0.095 y . In comparison with results from temperate sediments, these constants showed no significant correlation to the sub-zero Arctic temperatures. The reproduction of measured data for the sulfur cycle could be obtained only when sulfur disproportionation was included in the model as a sink for S, indicating that this process plays an important role in the Young Sound sediment.