Abstract
Abstract. We introduce a time-dependent, one-dimensional model of early diagenesis that we term RADI, an acronym accounting for the main processes included in the model: chemical reactions, advection, molecular and bio-diffusion, and bio-irrigation. RADI is targeted for study of deep-sea sediments, in particular those containing calcium carbonates (CaCO3). RADI combines CaCO3 dissolution driven by organic matter degradation with a diffusive boundary layer and integrates state-of-the-art parameterizations of CaCO3 dissolution kinetics in seawater, thus serving as a link between mechanistic surface reaction modeling and global-scale biogeochemical models. RADI also includes CaCO3 precipitation, providing a continuum between CaCO3 dissolution and precipitation. RADI integrates components rather than individual chemical species for accessibility and is straightforward to compare against measurements. RADI is the first diagenetic model implemented in Julia, a high-performance programming language that is free and open source, and it is also available in MATLAB/GNU Octave. Here, we first describe the scientific background behind RADI and its implementations. Following this, we evaluate its performance in three selected locations and explore other potential applications, such as the influence of tides and seasonality on early diagenesis in the deep ocean. RADI is a powerful tool to study the time-transient and steady-state response of the sedimentary system to environmental perturbation, such as deep-sea mining, deoxygenation, or acidification events.
Highlights
The deep ocean is being perturbed globally by climate change, including seawater acidification caused by the uptake of ∼ 10 × 109 t of anthropogenic carbon dioxide (CO2) into the ocean each year (Perez et al, 2018; Gruber et al, 2019), roughly a quarter of our total annual emissions (Friedlingstein et al, 2020)
The low NO3 flux could be attributed, for example, to the presence of organic matter with a stoichiometry different than the Redfield ratio used in the current version of RADI or to errors in the nitrification parameters
We note that the choice of calcite dissolution kinetics implemented in RADI does not seem to have a large impact on TAlk and CO2 porewater profiles or on the predicted calcite concentrations
Summary
The seafloor, which covers ∼ 70 % of the surface of the planet and modulates the transfer of materials and energy from the biosphere to the geosphere, remains for the vast majority unexplored. The rate-limiting step for many biogeochemical reactions is solute transport via molecular diffusion through the sediment porewaters or through the diffusive boundary layer (DBL). Most existing models are limited to a steady state and are unable to predict the transient sediment response to time-dependent phenomena such as tides, seasonal change, ocean deoxygenation, or acidification Most of these models do not take the presence of a DBL into account, even though diffusion through the DBL may control the overall rate of many biogeochemical reactions. We describe a new sediment porewater model built upon earlier work termed RADI, an acronym accounting for the main processes included in the model that control the vertical distribution of solutes and solids: chemical reactions, advection, molecular and bio-diffusion, and bio-irrigation. We evaluate its performance in three selected locations and explore other potential applications, such as the influence of tides and seasonality on early diagenesis in the deep ocean
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