FESDIA (v1.0): exploring temporal variations of sediment biogeochemistry under the influence of flood events using numerical modelling

Abstract

Abstract. Episodic events of flood deposit in coastal environments are characterized by deposition of large quantities of sediment containing reactive organic matter within short periods of time. While steady-state modelling is common in sediment biogeochemical modelling, the inclusion of these events in current early diagenesis models has yet to be demonstrated. We adapted an existing model of early diagenetic processes to include the ability to mimic an immediate organic carbon deposition. The new model version (FESDIA) written in Fortran and R programming language was able to reproduce the basic trends from field sediment porewater data affected by the November 2008 flood event in the Rhône River prodelta. Simulation experiments on two end-member scenarios of sediment characteristics dictated by field observation (1–high thickness deposit, with low TOC (total organic carbon) and 2–low thickness, with high TOC), reveal contrasting evolutions of post-depositional profiles. A first-order approximation of the differences between subsequent profiles was used to characterize the timing of recovery (i.e. relaxation time) from this alteration. Our results indicate a longer relaxation time of approximately 4 months for SO42- and 5 months for DIC (dissolved inorganic carbon) in the first scenario, and less than 3 months for the second scenario which agreed with timescale observed in the field. A sensitivity analysis across a spectrum of these end-member cases for the organic carbon content (described as the enrichment factor α) and for sediment thickness indicates that the relaxation time for oxygen, sulfate, and DIC decreases with increasing organic enrichment for a sediment deposition that is less than 5 cm. However, for larger deposits (>14 cm), the relaxation time for oxygen, sulfate, and DIC increases with α. This can be related to the depth-dependent availability of oxidant and the diffusion of species. This study emphasizes the significance of these sediment characteristics in determining the sediment's short-term response in the presence of an episodic event. Furthermore, the model described here provides a useful tool to better understand the magnitude and dynamics of flooding event on biogeochemical reactions on the seafloor.

Biogeochemical implication of massive episodic flood deposition: Model-Data integration

“…In addition, given the unique sedimentary characteristics of the prodelta (high sedimentation up to 40 𝑐𝑚 𝑦𝑟 −1 , high carbon flux up to 650 𝑔𝐶 𝑚 −2 𝑦𝑟 −1 ), the underlying diagenetic sequence of sediment shows remarkable stationarity during spring and summer (high concentration of DIC, dissolved iron and manganese, strong sulfate reduction Rassmann et al (2020)) despite short-term biogeochemical response linked to fall and winter floods (Cathalot et al, 2010). The reason for such long-term stability but short-term response is still unknown (Pastor et al, 2018), but has been linked to a rather short system's relaxation timescale (4-5 months) and the existence of a possible "Biogeochemical attractor" -a hypothetical concept basically associated with the rapid reorganisation of the perturbed porewater profiles to their pre-flood condition despite being affected by such massive depositional event (Nmor et al, 2022). The latter would help explain the maintenance of the same diagenetic characteristic for multi-temporal data in non-steady conditions (Nmor et al, Journal of Geophysical Research: Oceans 2022).…”

“…The model used for this study is the time dependent, one dimensional reactive transport model, FESDIA (Nmor et al, 2022). This model described the transformation of OC within the sediment column with well adapted capabilities for usage in sudden flood depositional scenarios.…”

“…This model described the transformation of OC within the sediment column with well adapted capabilities for usage in sudden flood depositional scenarios. The full description of the model can be found in Nmor et al (2022) and detailed mass balance equations and reaction kinetic is provided in the Appendix. Here, we basically recap the key biogeochemical reactions and pathway necessary to simulate the flood datasets derived from Pastor et al (2018).…”

“…This advance in the event-depositional algorithm provides some realism to how natural dynamic sedimentary systems work, albeit with an extra layer of complexity and parameterization constraint to the model. The numerical procedure for solving the underlying reactive transport equation capturing the processes in Section 2.2 have been previously discussed (Nmor et al, 2022). In summary, the simulation was carried out in the sediment grid layer of 100 cm thickness with intermittent deposition events treated as an abrupt change to the model dynamics at specified time interval (see Fig.…”

“…The enrichment factor (𝛼) used in the simulations as well as the thickness of the deposited layer is provided in Table 3. These enrichment factors are basically parameterization of the carbon content in the newly deposited sediment layer relative to the ancient layer and is dependent on the source region of the flood (Nmor et al, 2022). The relaxation interval is defined as the timescale over which a particular variable subject to the perturbation signal caused by flood deposition becomes indistinguishable from background variation (see Nmor et al (2022) for introduction and Section 2.5.1.2).…”