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

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.
