Abstract
Abstract. Seawater–sediment interaction is a crucial factor in carbon and nutrient cycling on a wide range of spatial and temporal scales. This interaction is mediated not just through geochemistry but also via biology. Infauna vigorously mix sediment particles, enhance porewater–seawater exchange, and consequently, facilitate chemical reactions. In turn, the ecology and activity of benthic fauna are impacted by their environment, amplifying the sensitivity of seawater–sediment interaction to environmental change. However, numerical representation of the bioturbation of sediment has often been treated simply as an enhanced diffusion of solutes and solids. Whilst reasonably successful in representing the mixing of bulk and predominantly oxic marine sediments, the diffusional approach to bioturbation is limited by a lack of environmental sensitivity. To better capture the mechanics and effects of sediment bioturbation, we extend a published bioturbation model (Lattice-Automaton Bioturbation Simulator; LABS) by adopting a novel method to simulate realistic infaunal behavior that drives sediment mixing. In this new model (extended LABS – eLABS), simulated benthic organism action is combined with a deterministic calculation of water flow and oxygen and organic matter concentration fields to better reflect the physicochemical evolution of sediment in response to bioturbation. The predicted burrow geometry and mixing intensity thus attain a dependence on physicochemical sedimentary conditions. This interplay between biology, chemistry, and physics is important to mechanistically explain empirical observations of bioturbation and to account for the impact of environmental changes. As an illustrative example, we show how higher organic rain can drive more intense sediment mixing by “luring” benthic organisms deeper into sediments, while lower ambient dissolved oxygen restricts the oxic habitat depth and hence tends to reduce bulk mixing rates. Our model, with its oxygen and food availability controls, is a new tool to interpret the trace fossil record, e.g., burrows, as well as to explore biological engineering of past marine environments.
Highlights
Porewater–particle reactions occur extensively in the upper few to hundreds of centimeters of marine sediments, and resulting exchange with the overlying bottom water helps to regulate the chemistry of the oceans and, on relatively longer timescales, the concentrations of atmospheric CO2 and oxygen, and climate, e.g., Hülse et al (2017)
Organic matter concentrations are randomly assigned to individual sediment particles in the range of ≤ 1 wt %, with the probability of high concentration decreasing with depth in the default setting
The sediment particles deposited to the seawater–sediment interface are assumed to have 1 wt % of organic matter (e.g., Müller and Suess, 1979), as a default setting
Summary
Porewater–particle reactions occur extensively in the upper few to hundreds of centimeters of marine sediments (early diagenesis), and resulting exchange with the overlying bottom water helps to regulate the chemistry of the oceans and, on relatively longer timescales, the concentrations of atmospheric CO2 and oxygen, and climate, e.g., Hülse et al (2017). The rates of these reactions, and diagenesis overall, is influenced by benthic marine infauna, whose activities mix solid sediments and porewater solutes and modulate the exchange with overlying seawater. Numerical models of the appropriate processes involved represent invaluable tools in this effort
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