Comparison of organic matter oxidation approaches in sediment diagenesis models

Sediment diagenesis models have been developed over the past few decades using various algorithms and notation for similar processes. Organic matter oxidation in particular has been parameterised in a number of different ways. The reactions involved are the sequence of decreasing energy-yield oxidation pathways (aerobic mineralisation, denitrification, Mn(IV) reduction, Fe(III) reduction, SO4 2- reduction and methanogenesis). In this paper we examine the differences between three common approaches that have emerged in the literature and quantify whether the differences have a significant effect on organic matter oxidation rates. Firstly, the equations of three approaches were expressed using a common notation, highlighting the similarities and differences between the rate expressions. All approaches have an oxidation rate that is the product of the rate constant kOM, organic matter concentration OM, a limitation term ΘOx, and an inhibition term ΘIn. The ΘOx and ΘIn of Approach 1 apply Monod kinetics. The ΘOx and ΘIn of Approach 2 apply Blackman kinetics, which has previously been described as not significantly different to a Monod formulation. Approach 3 uses Monod terms while mixing the limitation and inhibition functions differently to Approach 1. While Approaches 1 and 2 usually use six oxidation pathways, Approach 3 uses three pathways, combining manganese, iron and sulfate reduction and methanogenesis into one anoxic pathway. Methanogenesis in Approach 1 is a product of the previous five inhibition terms, whereas in Approach 2 it is the difference between the total rate and the sum of the five higher-energy rates. The algorithms of the three approaches were compared with a simple calculation of organic matter oxidation on a homogenous unit of sediment over 3 years, in marine and freshwater conditions. The differences between the approaches were illustrated in terms of the points in time when the oxidants are exhausted. In all three approaches, O2 was exhausted within 1.7 to 1.8 days. The NO3 - was exhausted after 1.7 to 1.8 days in Approaches 2 and 3, but after 2.8 days in Approach 1. In Approach 1 Mn(IV) was exhausted at day 798 and Fe(III) at day 857, whereas these components were not exhausted in Approach 2 in the three year simulation, and not simulated in Approach 3. In the freshwater experiment, SO4 2- was depleted by days 425 and 215, respectively. While the time to depletion differed considerably, it is of note that both approaches allowed depletion of SO4 2- prior to the more thermodynamically favourable oxidants. Approaches 1 and 3 allow for more overlap of the rates of different oxidation pathways whereas Approach 2 only allows for overlap when the oxidant concentration is below its limiting concentration. This represents a different conceptualisation of the microbial processes in the sediment with bacteria either partially or completely inhibited. Implications of this include that at the time when the corresponding reduction pathway began in Approach 2, in Approach 1 25% of NO3 - , 17% of Fe(III) and 1% (marine experiment) or 38% (freshwater experiment) of SO4 2- had already been consumed. Approaches 1 and 2 exhibited a steady decline in total organic matter oxidation rate as organic matter concentration decreases, whereas in Approach 3 there was an increase in the total rate when denitrification and anoxic processes overlapped due to the lack of inhibition of anoxic processes. The differences between the approaches are significant enough to suggest that care must be taken in choosing which best represents the modeller's conceptualisation of microbial processes. The effects of secondary reactions and transport processes between sediment layers on the chemical profiles will be tested in future work using a full diagenesis model.