Calibration of an early diagenesis model for high nitrate, low reactive sediments in a temperate latitude estuary (Great Ouse, UK)

Abstract

The description and calibration of a reaction-diffusion model of early diagenesis are presented. Unlike previous models it has been developed for a temperate latitude estuary (Gt Ouse, UK) impacted by high nitrate concentrations (annual mean 700 PM). Five variables. 02, No3-, NH4', Sod2-and S'-, are modelled from the steady state distributions of bulk total organic carbon (TOC) (i.e. a l -G model) Three methods for deriving the first order rate constant, k, for TOC mineralisation are tested: (1) data calculated k values [ i e. (depth integrated total mineralisation rate) + (depth integrated TOC inventory)], (2) an exponential formulation, k, = k, e-a" ( k o = k at sedunent surface, a = react~vity coefficient of decrease, z = depth); and (3) use of separate k values for individual mineralisation pathways. Method 1 underestimates observed fluxes of solutes across the sediment-water interface (SWI) by up to an order of magnitude. This is due to an inappropriate use of the calculated kin the model. The calculation of k yields an overall net value which implicitly accounts for all factors acting on rnineralisation. Such factors (e.g. oxidant limitation of organic decay) are explicitly modelled. Consequently, k is significantly reduced by factors applied to it in the model which have previously been accounted for in the calculation. In Method 2, measured NO3-fluxes are overestimated by up to a factor of 7 . To reproduce measured benthic oxygen demands and sulphate reduction rates, a cannot be simultaneously fitted to the NO3-fluxes. The high overlying NO3-concentrations result in model denitrification that cannot reproduce the degree of limitation that actually occurs. Method 3 reproduces the data (i.e. both stoichiometrically derived mineralisation rates and measured solute fluxes at the SWI) to a high degree (r > 0.99, p < 0.001), but at the expense of increasing the degrees of freedom in the model and conceptual simplicity. These results cast doubt over the universal applicability of diagenetic models for estuarine systems exposed to high No3-concentrations. It is concluded that the use of commonly calculated first order rate constants (Method 1) and the frequently used exponential function (Method 2) in diagenetic models cannot be relied upon to reproduce observations in high NO3-estuaries. Previous stoichiometric calculations suggested that all of the measured ammonium fl.uxes across the SW1 in the Gt Ouse could be accounted for with oxygen, nitrate and sulphate reduction alone. With these latter processes the model (Method 3) underestimates the observed ammonium fluxes by up to 44 % at 3 out of 4 sites. This suggests that other mineralisation pathways (e.g. nitrate ammonification) are active in the Great Ouse sediments.

Control of the diffusive boundary layer on benthic fluxes: a model study

“…Organic carbon reactivity is determined by a first order rate constant (k) which is used to simulate all pathways of organic carbon degradation. The use of one k for all mineralisation pathways combined with the decreasing organic carbon concentrations with depth is equivalent to the formulation k = k 0 e -az , where k 0 is the value of the rate constant at the SWI, α is the coefficient of decrease and z is depth as tested in Kelly-Gerreyn et al (1999). Kelly-Gerreyn et al (1999) found that this parameterisation over-predicted rates of denitrification in high nitrate environments (e.g.…”

“…These values are similar to those used by Soetaert et al (1996) and typical for many sediments. All other details (assumptions, equations and parameters) can be found in Kelly-Gerreyn et al (1999). Only details of the implementation of the DBL and 2 examples of the model equations are described hereafter.…”

“…The model is a 1-G type model (Berner 1980) which assumes a simple first order dependence of organic degradation on the concentration of TOC and has been shown to represent observations well (Rabouille & Gaillard 1991, Kelly-Gerreyn et al 1999. Organic carbon mineralisation is mediated via oxygen, nitrate and sulphate, with each pathway producing ammonium (and dinitrogen in the case of nitrate reduction and free sulphide in the case of sulphate reduction).…”

“…Hence, mass continuity requires that the concentration gradient through the sublayer is equal to that across the SWI yielding a boundary condition satisfying: (Boudreau 1997) (2) where C w is the concentration of the solute in the overlying turbulent flow region, C 0 is the concentration at the SWI, φ is the porosity, z is depth in the sediment and δ D is the diffusive sublayer thickness. The numerical scheme adopted in Kelly-Gerreyn et al (1999) is used in the following description of the implementation of the DBL. All model layers (grid cells), each of equal length Δz, are defined by an i-index located in the middle of a cell (Fig.…”

“…The aim here is to use a simple model of early diagenesis (Kelly-Gerreyn et al 1999) to examine the effect of different thicknesses of the DBL on benthic fluxes for sediments in which (1) oxygen consumption is dominated by organic carbon degradation, (2) OPDs are low (< 0.5 cm) and (3) dissolved fluxes across the SWI are predominantly diffusive (i.e. non-bioturbated sediments).…”