The Organic Matter ENabled SEDiment model (OMEN-SED) is a one-dimensional, analytical reaction–transport model for early diagenesis in marine sediments. It explicitly resolves organic matter (OM) degradation and associated biogeochemical terminal electron acceptor, reduced species and nutrient dynamics in porous media under steady-state conditions. OMEN-SED has been specifically designed for coupling to global Earth system models and the analytical solution of the coupled set of mass conservation equations ensures the computational efficiency required for such a coupling. To find an analytical solution, OMEN-SED expresses all explicitly resolved biogeochemical processes as a function of OM degradation. The original version of OMEN-SED contains a relatively simple description of OM degradation based on two reactive OM classes, a so-called 2G model. However, such a simplified approach does not fully account for the widely observed continuous decrease in organic matter reactivity with burial depth/time. The reactive continuum model that accounts for the continuous distribution of organic compounds over the reactive spectrum represents an alternative and more realistic description but cannot be easily incorporated within the general OMEN-SED framework.
Here, we extend the diagenetic framework of OMEN-SED with a multi-G approximation of the reactive continuum model (RCM) of organic matter degradation by using a finite but large number of OM fractions, each characterized by a distinct reactivity. The RCM and its multi-G approximation are fully constrained by only two free parameters,
The degradation of organic matter (OM) in marine sediments is a key component of the global carbon cycle and climate
Benthic biogeochemical dynamics are driven by a complex and dynamic interplay of transport and reaction processes that operate over different temporal and spatial scales. This underlying complexity compromises our ability to understand, quantify and predict diagenetic dynamics. In this respect, reaction–transport models (RTMs) that account for transport (advection, molecular diffusion, bioturbation and bioirrigation) and reaction (production and consumption) processes are, in combination with observational data, powerful analytic, diagnostic and predictive tools.
Because of its prominent role, the description and parametrization of OM degradation in diagenetic models is critical for their ability to capture and predict sediment–water exchange and burial fluxes.
The rate of OM degradation and thus the intensity of the associated biogeochemical cycling is first and foremost driven by the supply of OM at the sediment–water interface. However, it is not only the quantity of that flux that determines benthic biogeochemical rates but also its quality
Existing OM degradation models can be broadly divided into two different classes according to their description of apparent reactivity of the bulk OM,
Continuum models, on the other hand, assume a continuous distribution of OM compounds over the reactivity spectrum, thus avoiding the need to partition the bulk material into a limited number of discrete compound classes. One can distinguish theoretically derived continuum models
The application of continuum models in the framework of local, one-dimensional diagenetic models is straight forward as these models generally solve the coupled reaction–transport equations numerically. However, computational power rapidly becomes a limiting factor if a coupled set of reaction–transport equations were to be numerically solved on a global scale and over longer timescales. Therefore, global diagenetic models designed for large-scale applications and the coupling to Earth system models are generally either highly simplistic, limited in their scope or rely on the analytical solution of the reaction–transport equation
Continuum models, on the other hand, merely require constraining the two free parameters that define the shape of the initial distribution and the continuous decrease of OM reactivity with time/depth. Therefore, the RCM approach captures a wider range of OM reactivity scenarios over both short and long timescales, while using fewer parameters than multi-G models
In order to extend OMEN-SED with a second OM degradation model, here we developed a multi-G approximation of the RCM and directly integrated it into the mathematical framework of OMEN-SED. We first provide a short summary of OMEN-SED, including a description of the general model approach and the generic algorithm used to match internal boundary conditions and to determine the integration constants for the analytical solutions. We provide a detailed description of the newly integrated RCM approximation for organic matter degradation, followed by an evaluation of the performance of OMEN-SED-RCM by (1) comparing simulated depth profiles of OM, terminal electron acceptors (TEAs) and metabolic byproducts with observations from selected sites, as well as (2) benthic exchange fluxes of TEAs along a global ocean depth transect. In addition to the model–data comparison, OMEN-SED-RCM results are also compared with original OMEN-SED simulation results. We then force OMEN-SED-RCM with global observational data sets of sediment surface OM contents and bottom water concentrations to explore global patterns of
OMEN-SED-RCM is based on the diagenetic model OMEN-SED.
OMEN-SED accounts for both the advective and diffusive transport of dissolved and solid species. Sedimentation is simulated as a constant rate,
Here, we extend the 2G-model description of OM degradation incorporated in the original version of OMEN-SED (v. 1.0) with a multi-G approximation of the RCM. This new version, OMEN-SED-RCM, accounts for a continuous yet dynamic distribution of OM compounds over a range of reactivities and captures the widely observed, continuous decrease in apparent OM reactivity with depth/burial age
Our RCM approximation by a multi-G description of OM degradation (see Fig.
Based on distribution of
The initial fraction
The derived degradation rate constants for OM
Quality of the multi-G approximation (expressed as the sum of the reactive fractions) as a function of the number of reactivity bins,
To illustrate the capabilities of OMEN-SED-RCM in simulating local diagenetic dynamics and compare its performance with the original 2G model description implemented in OMEN-SED, we here simulate the simulations for the same four sediment cores that were used to benchmark OMEN-SED
OMEN-SED-RCM (blue curves), 2G OMEN-SED (magenta curves)
Figure
More specifically, for the two Iberian margin sites (108 and 2213 m), OMEN-SED-RCM fits observations generally well. A slight overestimation of
Overall, a comparison between OMEN-SED and OMEN-SED-RCM results shows that both models reproduce observations equally well. They mainly differ in simulating the deeper parts of the
Finally, inversely fitted OM reactivity parameters
Model boundary conditions and parameters for the simulated sediment profiles in Fig.
To evaluate the performance of OMEN-SED-RCM in capturing OM degradation pathways and the resulting TEA fluxes across different depositional environments we replicate the global ocean transect simulation by
Here, we use this global relationship to estimate parameter
In addition to OM reactivity, the reoxidation of reduced substances (i.e.
Seafloor depth dependency of key model parameters and boundary conditions
Superscripted numbers denote the following references:
Simulated (lines) and observed (points) benthic fluxes of
Figure
Figure
We acknowledge that coastal and shallow environments are highly dynamic in their constantly shifting boundary conditions and rarely reach a steady state, which is a prerequisite to OMEN-SED(-RCM)'s analytical model approach
To evaluate the performance of OMEN-SED-RCM in reproducing observed global patterns of benthic–pelagic exchange fluxes and sediment redox zonation, we force OMEN-SED-RCM with existing global data sets and parameterizations to simulate diffusive
Figure
The mismatch between model results and observations has several reasons. First, it can be partly explained by a bias in the observational data set towards shallower OPD, i.e. exemplified by the upper limit of 1000 mm in the observed OPD, a likely artefact of the data selected, since oxygen is known to penetrate several metres
Furthermore, two simplifications/limitations in the model configuration and in the boundary conditions: first, the applied OM concentration at the SWI
The simulated DOUs and OPDs using the rescaled
Finally, the reoxidation of reduced products can make up a substantial fraction of the oxygen consumption in marine sediments. These secondary reactions consume products originating from anaerobic mineralization (Aller, 1990; Boudreau and Canfield, 1993) and can make up to 56 %
Sulfate is the most important terminal electron acceptor in marine sediments on a global scale
To this end, we set up a one-dimensional OMEN-SED-RCM simulation that is representative of a shallow depositional environment with a high OM input and a high sedimentation rate. OM concentration is set to 1.1 wt %. We assume bottom water concentrations of
Model boundary conditions representative of shallow shelf marine sediments for the SMTZ simulations. Calculation for
Water depth versus OPD for
Depth of the SMTZ as a function of OM reactivity parameters and sedimentation rates (
In agreement with results of
Simulation results also show that OM reactivity exerts the dominant control on the depth of the SMTZ. Changes in sedimentation rate and OM deposition shift (not shown here) but do not change the general pattern. Increasing the deposition flux of OM at the SWI would lead merely to the shallowing of the SMTZ on both ends of the
Here we present OMEN-SED-RCM, an extension of the original analytical diagenetic model OMEN-SED
We show that the new version of OMEN-SED is not only able to reproduce observed pore water profiles across a wide range of depositional environments and capture observed global patterns of TEA-fluxes, oxygen penetration depths and biogeochemical reaction rates but also accounts for the widely observed continuous decrease in OM reactivity with sediment depths/burial time and thus provides a more realistic description of anaerobic degradation pathways. This added functionality offers an alternative to the common but simpler 2G model description implemented in the original model, extending the model's applicability to a wider range of environments and timescales, while requiring fewer parameters to describe a wider spectrum of OM reactivity. These improvements were implemented while maintaining the computational advantages of the original version.
Primary pathways of organic matter degradation, secondary redox reactions and stoichiometries implemented in the reaction network.
Sediment characteristics and transport parameters.
Note: DIC and ALK coefficients are the values of
Values for biogeochemical parameters used in OMEN-SED-RCM. The variables
The commented OMEN-SED-RCM source code (MATLAB) to this article is available for download in the two links provided below or see
Data sets used in this study are available from the studies cited in the text and figures.
PP, DH, and SA conceived and designed the study. PP carried out the model simulations, wrote and validated the new model code, analysed the data and wrote the manuscript. DH provided the nutrient data, helped with the implementation and contributed to the editing of the manuscript. SA contributed the editing of the manuscript.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We would like to thank Bernard Boudreau and one anonymous reviewer for their constructive critiques and insightful comments, which have improved the paper. Sandra Arndt and Philip Pika were supported by funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant (agreement no. 643052) (C-CASCADES).
This research has been supported by Horizon 2020 (grant no. C-CASCADES (643052)). Dominik Hülse was supported by a postdoctoral fellowship from the Simons Foundation (award ID 653829).
This paper was edited by Paul Halloran and reviewed by Bernard Boudreau and one anonymous referee.