# Effects of frequent or massive sediment supplies on sediment redox dynamics: a microcosm simulation

Corentin Guilhermic, Aurélia Mouret, Hélène Howa, Edouard Metzger, Geoffroy Couasnet, Maria Pia Nardelli

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Corentin Guilhermic, Aurélia Mouret, Hélène Howa, Edouard Metzger, Geoffroy Couasnet, et al. "Effects of frequent or massive sediment supplies on sediment redox dynamics: a microcosm simulation." Estuarine, Coastal and Shelf Science, 2025, 324, pp.109472. 10.1016/j.ecss.2025.109472 hal-05244250

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# Effects of frequent or massive sediment supplies on sediment redox dynamics: a microcosm simulation

Corentin Guilhermic* ID, Aurélia Mouret ID, Hélène Howa, Edouard Metzger, Geoffroy Couasnet ID, Maria Pia Nardelli

Univ Angers, Nantes Université, Le Mans Université, CNRS, Laboratoire de Planétologie et Géosciences, LPG UMR 6112, 49000, Angers, France

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## ARTICLE INFO

**Keywords:**

Experiment

Early diagenesis

Sediment deposition

Transient state

## ABSTRACT

Marine coastal environments are subject to depositional events generated for instance by turbidites, spring tides and estuarine floods. Biogeochemical stability in sediments was already shown to be disrupted following the deposition of a new sediment layer. The present study aims to understand the successive recovery stages of early diagenetic processes following sediment deposition events of varying intensity and frequency, using microcosms simulations. Over a five-week period with weekly sampling, high spatial resolution methods were applied to characterize the behavior of oxygen, nitrogen, manganese and iron in response to i) a single 2.7 cm sediment deposit ("one-time high volume"; OHV) and ii) a series of weekly 0.4 cm thick deposit ("frequent low volume"; FLV), compared to an undisturbed control microcosm. Results indicate that oxygen, ammonium and nitrite concentration in porewaters exhibited similar vertical distributions in FLV and control microcosms. In the OHV microcosm, oxygen penetration depth was returned to control levels after 3 weeks, whereas denitrification fronts within the newly deposited sediments reestablished within one week after disturbance. Regarding manganese and iron, both OHV and FLV treatments showed that the Mn/Fe oxide-enriched surface layer became anoxic after sediment deposition, triggering bacterial reduction of these oxides. Dissolved manganese production occurred one week before dissolved iron production, highlighting preferential organic matter remineralization pathways. These findings suggest that the transient biogeochemical response to sediment deposition can therefore be appreciated through manganese and iron cycling.

## 1. Introduction

Sediment instability on the seabed, driven by abnormally high supplies of detrital material, varies in frequency and intensity depending on the triggering events. Natural seasonal phenomena, such as floods (Deflandre et al., 2002; Ferreira et al., 2024; Mucci et al., 2003; Pastor et al., 2018; Thibault de Chanvalon et al., 2016) or storm-induced seabed reworking (Budillon et al., 2006) can result in the transport and abrupt deposition of large quantities of sediment. Tidal cycles can also affect coastal seabed stability on a daily or biweekly frequency, causing short-term sediment resuspension and deposition (Davies et al., 2023). Additionally, gravity flow events (Bolliet et al., 2014; Haughton et al., 2009; Mulder and Cochonat, 1996; Postma, 1986; Rodrigues et al., 2022) and glacier melting in polar regions (D'Angelo et al., 2018; Hodgkins et al., 2003; Hodson et al., 1998; Łepkowska and Stachnik, 2018; Meslard et al., 2018) can periodically (i.e., from daily to annually)

introduce large amounts of recently eroded sediment to the seabed. In addition to natural phenomena, anthropic activities such as occasional or repeated bottom trawling, dredging, and dumping can further contribute to sedimentary instabilities (Palanques et al., 2024; van de Velde et al., 2018; Wolanski and Gibbs, 1992). Sediment disturbances of this nature are known to impact benthic communities (Alve, 1999; Anschutz et al., 2002; Bolam, 2011; Duros et al., 2017; Fossile et al., 2022; Guilhermic et al., 2023; Mestdagh et al., 2018; Wlodarska-Kowalczuk et al., 2013) and affect diagenetic processes (Anschutz et al., 2002; Deflandre et al., 2002; Mucci et al., 2003; Thibault de Chanvalon et al., 2016; van de Velde et al., 2018).

Many diagenetic reactions are microbially mediated and fueled by organic matter (OM) present in settling particles or directly produced in sediments. These reactions consume a variety of oxidizing components (Berner, 1981; Froelich et al., 1979) and drive changes in both solid phase and porewater composition, creating redox gradients that control

<footnote>* Corresponding author.
E-mail address: cguilhermic@gmail.com (C. Guilhermic).</footnote>
solute and gas fluxes within sediments and across the sediment-water interface. Disruptions of these processes, such as turbidite deposition (Anschutz et al., 2002), landslides (Mucci and Edenborn, 1992), or flood deposits (Barhdadi et al., 2024; Cathalot et al., 2010; Deflandre et al., 2002; Hulot et al., 2023; Pastor et al., 2018; Thibault de Chanvalon et al., 2016), induce physical disturbances that alter diagenetic sequence. Previous studies documented anomalies in the succession of early diagenetic reactions and examined the temporality of recovery following such disturbances (Anschutz et al., 2002; Deflandre et al., 2002; Mucci and Edenborn, 1992). Nevertheless, the temporal resolution of observations after major disturbances is often limited to weeks or months due to scarce samplings, resulting in a lack of detailed descriptions of short-term responses (i.e., within hours to days) after a disturbance event and an inaccurate chronology. A better understanding of geochemical recovery with higher temporal resolution would be of high interest for the development of diagenetic models and the refinement of existing ones (Nmor et al., 2022; Sulpis et al., 2022; van de Velde et al., 2018; Yücel et al., 2010). Experimental studies offer a complementary approach for acquiring high-resolution data on rapid redox dynamics in response to physical and chemical disturbances caused by sediment deposition events, whether erratic (Chaillou et al., 2007) or frequent. They can also help identify the most relevant biogeochemical descriptors of these events.

To provide new insights into the short-term dynamics of benthic environments in response to physical disturbance (i.e., sediment deposition events and subsequent burial), we conducted a combined experiment over 51 days to assess the effects of two types of deposition events on sediment biogeochemical processes (this study) and benthic foraminifera which are unicellular organisms belonging to meiofauna, highly sensitive to sedimentary and biochemical changes (e.g. Murray, 2006). Unlike macrofauna, foraminifera respond to a larger spectrum of environmental gradients. The results concerning foraminiferal communities were previously published in Guilhermic et al. (2023). For this coupled experiment, three microcosms were established: i) A control microcosm with no induced disturbance, ii) a “one-time high volume” (OHV) microcosm simulating a single substantial sedimentation event, and iii) a “frequent low volume” (FLV) microcosm simulating small, weekly sediment deposition events.

This present study specifically examines biogeochemical recovery responses to burial by investigating sediment compaction, reactive metal oxides (Mn and Fe), and dissolved chemical species in porewaters (oxygen, inorganic nitrogen, Mn and Fe). Dissolved compound analyses were performed at submillimeter to millimeter spatial resolution using micro-electrodes, 1D DET (Diffusive Equilibrium in Thin films) and 2D DET. By documenting the physico-chemical response of sediment to unique or frequent sediment deposition events, this study aims to characterize transient biogeochemical processes and identify potential biogeochemical indicators of depositional events in benthic environments. The results will contribute to a better insight into biogeochemical recovery and how such disturbances influence biogeochemical processes, helping to improve diagenetic models implementing transient processes.

## 2. Material and methods

### 2.1. Experimental design

The experiment was performed in three glass aquaria (50 x 15 x 26 cm; 750 cm² surface area), each simulating different sediment disturbance modalities: i) a control microcosm with no induced disturbance throughout the whole experiment, ii) a “one-time high volume” (OHV) microcosm in which only a single sediment load was realized resulting in a thick deposit (2.7 cm after compaction), compared with iii) a “frequent low volume” (FLV) microcosm where small sediment additions were made weekly over 4 weeks, resulting in a total deposit thickness of 1.3 cm after compaction. Each sediment addition occurred 18h before

sampling. This experimental set-up was designed to assess both the biogeochemical recovery following physical disturbances and the response of benthic foraminiferal (i.e. active transport) to burial events. To ensure that the observed effects were solely due to one forcing which is sediment deposition, potential interferences, such as macrofaunal activity were excluded. Indeed, macrofauna may have enhanced physical and biogeochemical recovery or prolonged transient state and may have also induced passive transport of foraminifera. The experiment was performed at a stable temperature of 14 °C and in the dark to limit oxygen production by microphytobenthos. The sediment was collected from the Couplasse mudflat (Bourgneuf Bay, 47°0'57" N, 2°1'29" W) and natural seawater were used to fill the aquaria. The seawater was previously filtered (0.45 µm mesh) and continuously aerated using a bubbling system. A closed water circuit equipped with a pump was installed to renew the aquarium water from a reservoir. On the 35th day, following a breakdown of the pumping system, it was decided to manually replace the aquarium water. From that point onward, the water was completely renewed at each sampling and partially replaced twice a week, with approximately two-thirds of the volume replenished using water from the reservoir. The sediment used for the whole experiment (initial base and sediment additions) was frozen and subsequently unfrozen to kill present meiofauna or macrofauna that might be living in this sediment and might interfere with further treatments. To allow microbial communities to recover from alteration caused by freezing, an initial stabilization period of 21 days was observed before starting the experiment. To seed microcosms in a controlled manner, living foraminifera were collected on the Couplasse mudflat (Guilhermic et al., 2023). Surface sediment was sieved *in situ* to recover the 125–500 µm size fraction. This size fraction included foraminifera and possibly meiofauna or juveniles of macrofauna as well as some organic matter detritus such as phytodetritus and dead meiofauna individuals. It was added in the aquaria on sediment surface on day 7.

The entire experiment lasted 51 days with five main sampling times (T0 = 22 days; T1 = 29 days; T2 = 36 days; T3 = 43 days and T4 = 51 days). Sediment addition was initiated on day 28, with the unique sediment deposit in the OHV microcosm and the first disturbance of the FLV microcosm. Subsequent deposits for the FLV microcosm were carried out on days 35, 42 and 50. At each sampling time, a section of each aquarium was closed and isolated from the rest of the microcosm to prevent water and sediment mixing caused by resuspension during or after sampling. For each induced disturbance, the sediment was homogenized, diluted with the same seawater, stirred and aerated for 1 h before introduction.

### 2.2. Biogeochemical data acquisition

Sediment compaction after disturbance was investigated using daily side-view pictures of the “One-time high volume” and “Frequent low volume” aquaria, captured with a fixed Nikon D3400 camera (n = 60 per microcosm). Image analysis was performed with FIJI® software and EXCEL® (Microsoft) following the detailed protocol (Supplementary material S1). Compaction profiles were compiled with Grapher® (Golden software).

Dissolved oxygen (DO) concentration in porewaters was measured using Clark-type microelectrodes (50 µm tip diameter, Unisense™) attached to an automated micromanipulator, allowing point measurements with a 50 µm vertical resolution (Revsbech, 1989). Measurements were performed in an unsampled section of the aquaria, the other sections being progressively sealed after each sediment sampling. DO measurements were realized one day after each sampling event (i.e., two days after sedimentary disturbance). Diffusive oxygen fluxes at the sediment-water interface (SWI) were quantified using the PROFILE numerical model (Berg et al., 1998).

Sediment porosity and reactive Mn and Fe oxides were investigated by coring the three microcosms at each sampling time (n = 15) using modified syringes as miniature piston corers (2.1 cm diameter).
Sediment cores were sliced every 0.2 cm down to 4 cm depth, then every 0.5 cm from 4 to 6 cm depth. Sediment samples were sealed in pre-weighed vials and immediately frozen. Solid-phase analyses were carried out on freeze-dried samples, corrected for sea salt content using measured salinity. The weight lost during freeze-drying was used to calculate porosity. Relative error on porosity is ±6 %. Freeze-dried samples were homogenized and ground with an agate mortar. Reactive Fe(III) phases and Mn(III, IV) oxides and oxyhydroxides (Fe<sub>asc</sub> and Mn<sub>asc</sub>) were extracted using an ascorbate reagent (Anschutz et al., 1998, 2005; Kostka and Luther, 1994). 0.1 g of dry and crushed sediment was exposed to 10 mL of sodium bicarbonate (0.6M), sodium citrate (0.17M) and ascorbic acid (0.11M) (pH = 8). After centrifugation, 1 mL of supernatant was diluted with 9 mL of 1 % nitric acid for Fe<sub>asc</sub> and Mn<sub>asc</sub> analysis. Concentrations were determined with an ICP-AES (Thermo Scientific iCAP 6300 Radial). Relative error on Fe<sub>asc</sub> and Mn<sub>asc</sub> is ±7 %.

For porewater analysis, two DET (Diffusive Equilibrium in Thin film) gel techniques were used at each sampling time. The principle is to insert a hydrogel probe into sediment, in which a diffusive equilibrium with the solutes present in the porewater is established in approximatively 5h. The concentration distributions obtained therefore cannot fully reflect the very short solute dynamics (<1h). The use of these two DET techniques and the use of several probes were necessary because the dissolved species studied (ammonium, nitrite, dissolved Mn and Fe) required different analytical treatments (colorimetry for ammonium and nitrite and ICP-AES for Mn and Fe) and thus cannot be measured with a single probe. Nevertheless, these methods provided high spatial resolution measurements of the distribution of dissolved species in both the overlaying water and sediments compared to conventional methods such as core sampling.

A 1D DET probe, developed by Davison and Zhang (1994) and Krom et al. (1994) and adapted by Metzger et al. (2013), was used. It consists of a Perspex support with 75 compartments (1 mm depth and 2 mm apart) filled with an ultra-pure agarose-MilliQ water solution. A PVDF hydrophilic membrane, attached with white PVC tape, allows only the dissolved species to be equilibrated with the gel-filled compartments. Prior to deployment, probes were placed into plastic bags partially filled with MilliQ water bubbled with N₂, for at least 6 h to remove dissolved oxygen from the hydrogel, preventing the oxidation of reduced pore-water compounds. 1D DET probes were inserted into sediment microcosms and allowed to equilibrate for 12h. After equilibration, each probe was retrieved, and its membrane was removed. Each gel piece was eluted individually with 500 µL of 5 % HNO₃ (SupraPur Merck), then diluted in 0.8 % HNO₃ (final 200-fold dilution). Dissolved Mn and Fe (Fe₉ and Mn₉) concentrations were determined using the same ICP-AES as for the solid-phase analysis. Relative errors on Fe₉ and Mn₉ are ±16 % and ±18 %, respectively.

2D DET probes were used to measure dissolved inorganic nitrogen concentrations in porewaters. The probe consisted of a 3-mm-thick Plexiglas plate with two 190 mm*95 mm*1 mm depressions on each side of the plate for gels. One side held a 1 mm-thick 2D DET polyacrylamide hydrogel prepared as described by Jezequel et al. (2007) for nitrite analysis (Metzger et al., 2016). The other side contained an ultra-pure agarose-MilliQ hydrogel for ammonium analysis (Metzger et al., 2019). Both gels were covered with a PVDF membrane (0.2 µm, Durapore®) and fixed with white PVC tape. Similar to the 1D DET, probes were preconditioned in degassed MilliQ water for 6 h before being deployed into sediment microcosms for 12h. After retrieval, 2D gels were immediately processed for colorimetric reactions by being combined with one or two reagent gels (see below for details on the reagent gels used). After completion of the colorimetric reaction, images were acquired using a flatbed scanner.

For nitrite mapping, the gel probe was placed in contact with a reagent gel equilibrated with the Griess reagent (Griess, 1879), as detailed by Metzger et al. (2016). Nitrite concentrations were retrieved using the green channel of the scanned images. For ammonium concentration determination, the colorimetric reaction required two reagent gels

simultaneously in contact with the gel probe (Metzger et al., 2019). Ammonium concentrations were analyzed using the red channel of scanned images.

For a better comparison between sampling times and microcosms, laterally averaged 1D profiles were used (2D concentration images are available in Supplementary material S2 and S3). Average concentrations and associated standard deviations were calculated for each pixel line of ammonium and nitrite distribution images.

## 2.3. Production rates and oxide stock calculations

Porewater Mn₉ and Fe₉ profiles were modelled using the Savitzky Golay filter (Savitzky and Golay, 1964; Thibault de Chanvalon et al., 2016), which induces a polynomial interpolation of the data. Each data point was replaced by a polynomial function of degree p that best fits its surrounding n neighboring points. Increasing n enhanced profile smoothing, effectively reducing noise. The polynomial function parameters and smoothing settings were adjusted to obtain the best compromise (here p = 4 and n = 5 for all modelled profiles). Then, production rates were calculated using the first and second derivatives at each point, porosity profiles (Φ fitted with an exponential function), temperature, salinity, and pressure (the last three parameters being assumed constant as a function of depth). The entire procedure was performed using R studio software. Apparent Production Rates (APR) are presented as the sum of all positive production rates along the profiles. They represent the total reduction of reactive oxides (ascorbate-extracted) in the sediment column. The uncertainties associated with production rates were calculated assuming ±1 n for smoothing.

Excess reactive Fe and Mn oxide stocks were calculated after Anschutz et al. (2002) and Thibault de Chanvalon et al. (2016) in mmol m⁻² as follows:

$$ \text{Excess Mn}_{\text{ASC}} = \sum_{l \min}^{l \max} ([Mn_{\text{ASC}}]_l - [\overline{Mn}_{\text{ASC}}]_{\text{deep}}) (1 - \phi_l) \rho t_l $$

Where $[Mn_{\text{ASC}}]_l$ is the concentration of ascorbate-extracted Mn oxides from the layer l; $[\overline{Mn}_{\text{ASC}}]_{\text{deep}}$ is the mean of the minimal constant values of ascorbate-extracted Mn oxides in the deep layers; $\phi_l$ is the porosity of the layer l; $\rho$ is the density of dry sediment (2.65); $t_l$ is the thickness of the sediment layer l. An error on stock values was estimated using error propagation calculations, considering porosity error, on Mn<sub>ASC</sub> or Fe<sub>ASC</sub> errors and the error on slice thickness.

# 3. Results

## 3.1. Sediment compaction and porosity

Pictures taken throughout the experiment were used to observe changes in sediment compaction, thickness and color (i.e., in relation to redox reactions). The top of the initial sediment corresponds to the light-yellow layer above a discontinuous dark layer, in both “One-time high volume” (OHV) and “Frequent low volume” (FLV) microcosms (Fig. 1A). Above, the new deposit appears homogeneous and light brown. In the OHV microcosm, the initial deposit thickness of 43 mm decreased to 29 mm 15h later and then 27 mm within 6 days, followed by a more limited gradual compaction over time (Fig. 1B). In the FLV microcosm, successive sediment additions (between 2- and 6-mm thick layers after compaction) resulted in a final total deposit thickness of 13 mm at the end of the experiment.

Porosity profiles showed a differing response to sediment deposition. In the control microcosm, surface porosity remained stable (0.85–0.90 in the topmost 0.2 cm), with an exponential decrease until 2 cm depth (Fig. 2). Around this depth, porosity stabilized at around 0.77. In contrast, both disturbed microcosms showed increased porosity in the newly deposited layers. In OHV microcosm, porosity increased to 0.94
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Compaction of the newly deposited layer over time in the “One-time high volume” microcosm (blue frame and blue dots) and in the “Frequent low volume” microcosm (green frame and green dots) from the first sediment addition. The top panel (A) is a compilation of pictures taken several hours (0h, 15h, 47h and 64h) after the slurry input in the microcosms. The second panel (B) displays the thickness of the sediment layers added throughout the experiment for the two disturbed modalities, with each point corresponding to one picture. The timeline here is established from the first sediment addition, 28 days after the start of the experiment. The vertical gray area represents the time covered by the images in the upper panel A.</figcaption></figure>

18h after sediment addition and stabilized later at slightly higher value (~0.92) than the control (Fig. 2). Moreover, disruptions in the porosity profiles were observed at previous sediment-water interfaces. FLV microcosm showed consistently high surface porosity, sustained by repeated sediment additions. In all profiles, the uppermost 3 measurements, corresponding to a 0.6 cm thick layer, were characterized by unsteady porosity values.

## 3.2. Oxygen penetration depth and sediment-water interface fluxes

Oxygen Penetration Depths (OPD) were presented in Guilhermic et al. (2023). In the OHV microcosm, OPD showed a significant deepening after the sediment addition and returned to control levels after 3 weeks, while in FLV, OPD did not significantly change throughout the experiment (Supplementary material S4). Modelled dissolved oxygen
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Porosity profiles for each sampling time and each microcosm. The error associated with each porosity value is indicated by the gray coloring around the points. Deposition events are symbolized by dotted lines in the “One-time high volume” and “Frequent low volume” microcosms and are reported on the upper timescale as vertical blue and green lines respectively.</figcaption></figure>

uptakes (DOU) ranged from 35.3 ± 7.9 mmol m⁻² d⁻¹ and 49.3 ± 5.2 mmol m⁻² d⁻¹ throughout the experiment for all microcosms (Table 1). Low DOU variability was observed in the control microcosm over time (39.6 ± 8.1 mmol m⁻² d⁻¹, Table 1). Similarly, in the OHV microcosm, DOU remained stable at around 39.1 ± 8.8 mmol m⁻² d⁻¹ after T0. In the FLV microcosm, T1 and T2 fluxes were slightly higher, but T4 flux was similar to T0 flux.

### 3.3. Inorganic nitrogen species

At T0 in the OHV microcosm (no ammonium data available for the control and FLV microcosms), ammonium concentrations remained below 50 µmol L⁻¹ in the water column, reaching 800 µmol L⁻¹ in porewaters at 8 cm depth (Fig. 3). Ammonium increased with depth similarly across all aquaria and times. The only exception was OHV T1,

**Table 1**

Average of the modelled dissolved oxygen fluxes at the sediment-water interface (mmol m⁻² d⁻¹) and associated standard deviation, based on the number of profiles in the table at the different sampling times for each microcosm. No modelling was performed at T3 due to technical problems.

<table><thead><tr><th rowspan="2">—</th><th colspan="3">Control</th><th colspan="3">OHV</th><th colspan="3">FLV</th></tr><tr><th>mean</th><th>sd</th><th>number of profiles</th><th>mean</th><th>sd</th><th>number of profiles</th><th>mean</th><th>sd</th><th>number of profiles</th></tr></thead><tbody><tr><td>T0</td><td>39.7</td><td>±5.3</td><td>5</td><td>41.7</td><td>±11.6</td><td>5</td><td>35.3</td><td>±7.9</td><td>5</td></tr><tr><td>T1</td><td>40.2</td><td>±5.4</td><td>8</td><td>39.1</td><td>±9.6</td><td>4</td><td>49.3</td><td>±5.2</td><td>5</td></tr><tr><td>T2</td><td>39.5</td><td>±9.8</td><td>7</td><td>39.7</td><td>±6.3</td><td>9</td><td>47.7</td><td>±6.4</td><td>6</td></tr><tr><td>T4</td><td>38.9</td><td>±8.1</td><td>6</td><td>37.9</td><td>±5.7</td><td>5</td><td>36.8</td><td>±9.5</td><td>6</td></tr></tbody></table>
18h after the disturbance, where concentrations were ~600 µmol L⁻¹ in both the water column and newly deposited sediment. Below the former sediment-water interface (SWI), at approximatively 2.9 cm depth, the distribution and concentrations were similar to those observed at the other sampling times. The following week (OHV T2), ammonium returned to control conditions.

Nitrite concentrations were generally higher in the water column than in porewaters, except for FLV T3 and T4) (Fig. 3). Most profiles showed a decrease within the first centimeter of sediment, reaching a constant value at deeper layers. However, differences between microcosms were observed over time. In the control at T0, nitrite was 25 µmol L⁻¹ in the water column, rapidly decreasing to 15 µmol L⁻¹ below 1 cm depth in the sediment. At T1, concentrations more than doubled, creating a steeper gradient. After this sampling time, concentrations in the water column gradually decreased to 4 µmol L⁻¹ at T4, with near-zero concentrations below 1 cm depth and a peak of 5 µmol L⁻¹ within the uppermost centimeter.

In the OHV microcosm at T1, nitrite was relatively constant (3 µmol L⁻¹) in the lower water column and sediment but reached 17 µmol L⁻¹ at 2.5 cm above the sediment (Fig. 3). At T2, bottom water concentrations dropped to 10–17 µmol L⁻¹, stabilizing at 6 µmol L⁻¹ in the newly deposited sediment and below. At T3, a decrease in the overall concentration was observed to reach 0 µmol L⁻¹ in the sediment. At T4 the last sampling time, the water column concentration reached 30 µmol L⁻¹ and around 8 µmol L⁻¹ below 1.5 cm of sediment. In the uppermost sediment, a peak similar to that observed in the control microcosm reached 35 µmol L⁻¹.

In the FLV microcosm, nitrite at T1 and T2 decreased below the newly deposited 0.5 cm sediment layer, from 28 µmol L⁻¹ to 15 µmol L⁻¹ and from 15 to 5 µmol L⁻¹, respectively (Fig. 3). In the successive sampling times, concentrations stabilized at less than 5 µmol L⁻¹.

### 3.4. Manganese and iron reactive oxides

At T0 (before disturbance), the control treatment showed maximum Mnₐsc concentrations between 3 and 8 µmol g⁻¹ and maximum Feₐsc concentrations between 35 and 60 µmol g⁻¹ in the uppermost half centimeter (Fig. 4). While these maximum concentrations remained stable in the first half centimeter in subsequent sampling times, absolute values increased from T1 (10–16 µmol g⁻¹ for Mnₐsc and 60–95 µmol g⁻¹ for Feₐsc). In the OHV microcosm, profiles at T0 were similar to those of the control. However, at T1, Mnₐsc and Feₐsc profiles displayed no enrichment in the first half centimeter and showed constant concentrations at all depths (~2.5 µmol g⁻¹ and 25 µmol g⁻¹ for Mnₐsc and Feₐsc, respectively). Nonetheless, Mnₐsc and Feₐsc enrichments were observed within a 1 cm layer around the former SWI (4 µmol g⁻¹ for Mnₐsc and 30 µmol g⁻¹ for Feₐsc). In subsequent sampling times (T2-T4), Mnₐsc and Feₐsc maximum concentrations were again located at the top of the sediment core, below the new SWI, gradually returning to T0 levels (7.5 µmol g⁻¹ and 65 µmol g⁻¹ for Mnₐsc and Feₐsc, respectively).

In the FLV microcosm, after the first disturbance (T1), the topmost 2 mm showed concentrations similar to those at >1 cm depth (~2.5 µmol g⁻¹ for Mnₐsc and 25 µmol g⁻¹ for Feₐsc) (Fig. 4). However, slightly below (0.2–0.6 cm depth), concentrations increased (5 µmol g⁻¹ for Mnₐsc and 40 µmol g⁻¹ for Feₐsc), corresponding to the former SWI. Mnₐsc enrichment at the newly buried SWI persisted in later sampling times, aligning with each induced disturbance ~18h earlier. The enriched layer consistently corresponded to the former surface prior to each disturbance. At T3 and T4, Mnₐsc concentrations showed a slight decrease in sediment layers deposited after the first two first disturbances (T1 and T2). For Feₐsc, the enrichment observed at the interface between initial sediment and the T1-deposited layer persisted at T2, but no further enrichment was detected at the level of the newly buried SWI. At T3 and T4, Feₐsc enrichment had shifted upwards, aligning with the SWI buried a week earlier.

### 3.5. Dissolved manganese

At T0, Mn₉ in all aquaria increased within the first centimetre of sediment and stabilized at 70–80 µmol L⁻¹ below 2–3 cm depth (Fig. 5). In the control, Mn₉ reached 90–110 µmol L⁻¹ at 2–3 cm depth from T1 to T3.

At T1, 18h after disturbance, OHV Mn₉ profile exhibited a sharp 110 µmol L⁻¹ peak at the former SWI (Fig. 5). Below this peak, the profile had a similar shape to that observed in the control. Between 0 and 2 cm depth, porewater concentrations were constant and similar to those in the water column, at around 60 µmol L⁻¹. At T2, 8 days after disturbance, the Mn₉ peak expanded around the former SWI with a concentration of up to 150 µmol L⁻¹ at 2.9 cm depth. At T3, 15 days after disturbance, it decreased at 120 µmol L⁻¹, disappearing at T4. A similar post-disturbance pattern was observed in the FLV microcosm, where a 100 µmol L⁻¹ peak at 0.5 cm depth, corresponding to the previous SWI, disappeared in later samples (Fig. 5). Below this depth, the profile had a similar shape to that observed in the control.

### 3.6. Dissolved iron

At T0 in the control microcosm, Fe₉ increased within the first 1–2 cm depth before stabilizing at ~150 µmol L⁻¹ below 2 cm depth (Fig. 6). At T1, steep gradients led to a 400 µmol L⁻¹ peak at 2 cm depth. Below this peak, a gradual decrease with depth was observed down to 200 µmol L⁻¹ at the bottom. At T2 and T3, 8 and 15 days after disturbance, the peak persisted with maximum concentrations of 300 µmol L⁻¹ and 400 µmol L⁻¹ respectively, shifting slightly deeper (2 cm and 2.5 cm depth, respectively).

In OHV T1, unlike manganese, no Fe₉ enrichment was observed at the level of the newly buried SWI; instead, a slight decrease in concentrations was rather visible (Fig. 6). A peak of 400 µmol L⁻¹ was observed below the previous SWI at 4.5 cm depth, corresponding to the depth of the maximum observed in the control. Fe₉ concentration was similar within the newly deposited sediment layer and in the water column with a value of 100 µmol L⁻¹. At T2 and T3, the peak rose to the previous SWI (700 µmol L⁻¹), slightly widening at T3. At T4, peak concentration decreased to 320 µmol L⁻¹ at ~3 cm depth.

In FLV microcosm, Fe₉ peaked at T1 and T2 at 2.5 cm and 3.5 cm respectively, corresponding to the maximum concentration values observed for the control (Fig. 6). In the following weeks (T3 and T4), an increase in Fe₉ was visible just below the sediment deposits and in the first deposit (0.6–1.5 cm depth and 1–2 cm depth at T3 and T4 respectively). Deeper, Fe₉ was relatively constant with depth.

## 4. Discussion

### 4.1. Experimental redox setting without sediment disturbance

Previous experimental studies (Chaillou et al., 2007; Hansen and Blackburn, 1991, 1992; Porter et al., 2006) reported that redox gradients in sediments equilibrate within 2–3 weeks, which corresponds to the time observed before the start of our experiment. However, on day 7, the introduction of a high density of sieve-concentrated foraminifera (Guilhermic et al., 2023) led to the formation of a dark gray layer, visible in photos (Fig. 1), suggesting anaerobic degradation of OM concentrated through sieving (e.g., fecal pellets). Despite this, no significant impact of OM addition was observed on O₂ consumption fluxes, which remained stable throughout the experiment, suggesting that oxygen had already reached equilibrium at the time of the first sampling time T0 (Table 1).

Ammonium produced by OM remineralization showed relatively similar profiles across all T0 treatments and the control microcosm throughout the experiment, with the usual vertical distribution characterized by increasing concentrations with depth, as OM is mineralized by suboxic and anoxic pathways (Fig. 3). The strong concentration gradient below ~0.5 cm depth probably favored ammonium efflux
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** 1D plots extracted from 2D DET images of NO₂ concentration (pink curve and standard deviation) and NH₄⁺ concentration (green curve and standard deviation). 1D profiles were calculated using the average value of each pixel line of the nitrite and ammonium distribution images. Blue and green lines in the upper timeline correspond to the deposition events in the “One-time high volume” microcosm and the “Frequent low volume” microcosm, respectively. Note the lack of data for ammonium in the control T0, T4 and FLV T0, T1.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Manganese (Mnasc) and iron (Feasc) extracted by ascorbate in each aquarium at each sampling time. The associated error was 7% for both Mnasc and Feasc and is not represented here being smaller than the size of the points.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Dissolved Mn concentrations in the sediment porewaters and in the water column (empty circles) and associated error (gray band around the profile). Green profiles correspond to production rates (positive when colored green) calculated from the modelled concentration profile (solid line). New sediment layers are symbolized by dashed lines. Each plot is aligned with the original SWI. No data are available for T4 control due to a problem with DET 1D at the time of sampling.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Dissolved Fe concentrations in sediment porewaters and in the water column (empty circles) and associated error (gray band around the profile). Red profiles correspond to production rates (positive when colored red) calculated from the modelled concentration profile (solid line). New sediment layers are symbolized by dashed lines. Each plot is aligned with the original SWI. No data are available for T4 control due to a problem with DET 1D at the time of sampling.</figcaption></figure>

towards the overlaying water, leading to N accumulation in the water column despite regular water renewal. Lower ammonium concentrations in the oxic water column and in the oxic part of the sediment coincided with high nitrite concentrations, suggesting nitrification (Rysgaard et al., 1994), where ammonium is first converted into nitrite

and then nitrate. Nitrite concentration profiles presented a similar distribution pattern across all T0 aquaria and the control microcosm at subsequent sampling times (T1 to T3) (Fig. 3). The sharp decrease in nitrite around 1 cm depth, below the oxic layer, likely marks a denitrification zone, as this process generally occurs where oxygen is scarce (i.
e., <5 µmol L⁻¹; Devol, 2008). Nitrite consumption in suboxic sediments could also be induced by anammox (Brandes et al., 2007). While the distribution of nitrite was comparable across the different sampling times (T0-T4), concentration levels varied. The increase in nitrite concentration from T0 to T1, followed by a decrease between T2 and T4 (Fig. 3), likely resulted from nitrite production in the water column and oxic sediments, which decreased over time.

Below the oxic zone, although denitrification lowered nitrite concentration in the first centimeter, relatively high concentrations persisted from T0 to T3. At T4 (51 days after aquaria establishment), water oxygenation and OPD remained stable in the control microcosm, but nitrite concentrations in the water column had decreased, with only a weak production zone in the oxic sediments. Below this zone, nitrite was absent. Ammonium profiles and gradients were relatively stable between T0 and T4 in the control microcosm, and thus cannot explain the variations in nitrite concentrations. Another possible explanation could be that the regular water renewal was not sufficient to prevent the nitrite buildup at the beginning of the experiment between T0 and T1 (Guilhermic et al., 2023). The sustained remineralization of OM at the sediment-water interface likely enhanced ammonium production, which was subsequently oxidized into nitrite and nitrate.

At T0, porewater Mn₉ and Fe₉ vertical distributions were similar across the three aquaria, whereas they evolved differently depending on the applied disturbance (Figs. 5 and 6). Modelling of Mn₉ and Fe₉ production at T0 (Figs. 5 and 6) did not show well-defined production zones in the sediment, unlike in the control microcosm between T1 and T3. During this period, Mn₉ and Fe₉ were produced by the dissolution of reactive Mn and Fe oxides and diffused upwards, where they were re-oxidized (negative production), leading to an accumulation of reactive oxides in the surface sediment, as expected in the diagenetic sequence described by Froelich et al. (1979) and Burdige (2007). However, at T0, this surface enrichment in Mn and Fe oxides was not observed (Fig. 4). In contrast, this enrichment became visible from T1 to T4 in the control microcosm. Since porosity profiles remained stable throughout the whole experiment in the control microcosm (Fig. 2), no incidence of physical alteration of the sediment column can be considered to explain these changes. Instead, Mn and Fe redox equilibration in sediments appears to have been established from T1, perhaps influenced by the addition of OM and foraminifera on day 7.

At T0, all redox fronts showed similar distributions across microcosms but were geochemically unsteady, probably due to the delayed addition of organic matter and meiofauna, which postponed the establishment of a stable state, as discussed earlier. Subsequently, data from T1 to T4 in the control microcosm suggest a more stabilized system, which can be compared to the disturbed conditions in the OHV et FLV microcosms.

## 4.2. Re-equilibration dynamics after sediment deposit event(s)

Overall, biogeochemical parameters showed different recovery times according to the sediment deposition modality. As described by Froelich et al. (1979), in the absence of structural and geochemical disturbances (e.g., bioturbation, bioirrigation, microenvironments, cable bacteria activity ...), OM remineralization follows a vertical sequence based on energy yields from microbial reactions. Once equilibrium is disrupted, each redox gradient re-establishes accordingly. Our results, consistent with previous studies (Chaillou et al., 2007; Nmor et al., 2022; Pastor et al., 2018), support the idea that recovery times vary depending on several parameters such as biological communities (macrofauna, meiofauna and microorganisms), biological activity (bioturbation and bioirrigation), diffusion of dissolved elements in the sediment, OM content and sediment deposit thickness and compaction.

### 4.2.1. Sediment compaction and porosity evolution

In the OHV treatment, 82 % of final sediment compaction occurred within the first 47h (Fig. 1). Clemo et al. (2022) observed similar

dynamics for the compaction of muddy sediments after reworking, suspension and redeposition. They reported a 34h period before a resuspended 5 cm-thick sediment layer reached its initial thickness. Porosity profiles displayed an exponential decrease with depth across all microcosms at each sampling time (Fig. 2). Nevertheless, for all microcosms at T0 and for the control microcosm throughout the experiment, porosity strongly decreased within the first centimeter due to compaction, then stabilized at depth. In OHV at T1 (18h after disturbance), porosity in the first centimeter was higher than at T0, and then gradually decreased through the thickness of the deposited layer. For subsequent sampling times, porosity profiles remained stable compared to T1. The disturbance thus permanently increased sediment porosity in the deposited layer for the rest of the experiment (23 days). This indicates that a stable state was reached within 60h after deposition for the 2.7 cm-thick deposit. A comparable observation was reported by Chaillou et al. (2007) for a 7-cm thick muddy deposit, where porosity remained unchanged between day 1 and day 295 after the disturbance. Clemo et al. (2022) pointed out the importance of biological community (macrofauna) in sediment compaction. The addition of meiofauna in our experiment may therefore have generated a different physical state of the sediment compared to that observed after the addition of defaunated sediment. This could explain the complete compaction of the sediment, but with different porosity conditions. In FLV, regular sediment deposits also steadily increased surface porosity in the same way than in OHV. Therefore, the repeated weekly sediment additions reactivated the compaction of previously deposited layers, maintaining a transient state for porosity.

### 4.2.2. Oxygen depletion and recovery

Following disturbance, oxygen flux from the water column towards the sediment was interrupted and the oxygen initially present in the sediment was consumed. O₂ profiling 48h after the depositional event confirmed oxygen depletion at the original SWI. Moreover, despite brief oxygenation of the slurry before its addition to the microcosms, all DET probes attested of high concentration of reduced species in the OHV water column (ammonium, Mn₉ and Fe₉) and in the freshly deposited sediment layer (Figs. 3, 5 and 6). Based on modelled DOU in the control microcosm at T1 (40.2 ± 5.4 mmol m⁻² d⁻¹, Table 1) and the O₂ content calculated using O₂ profiles and porosity, oxygen depletion at the previous oxic zone was estimated to occur within 143 ± 40 s. In contrast, Chaillou et al. (2007) reported a 35-min depletion period for a similar event, probably due to a lower O₂ demand (5.5 mmol m⁻² d⁻¹) and deeper oxygen penetration (7 mm). Moreover, two days after sediment deposition, DOU returned to pre-disturbance levels (Table 1), with no significant trends related to disturbance frequency and/or intensity. Meanwhile, OPD variations were correlated with foraminifera migration and bacterial respiration re-establishment (Guilhermic et al., 2023). Indeed, sediment addition triggered an upward migration of foraminifera towards their preferential microhabitats in the uppermost oxygenated layers. A similar OPD deepening was observed in Rhône River flood deposits, where DOU decreased by 20–30 % due to lower OM mineralization (Cathalot et al., 2010; Pastor et al., 2018). In our eutrophic conditions (), stable DOU likely resulted from sustained OM availability.

### 4.2.3. Nitrogen recovery

In OHV, ammonium and nitrite profiles indicated a transient state 18h after the disturbance (T1), with high and equivalent concentrations in both the water column and the newly deposited sediment, and with a nitrification/denitrification zone observed in the water column. During Rhône flood events, Pastor et al. (2018) documented a disruption of ammonium gradient at the original SWI, with a recovery observed 26 days after the deposition of a 10 cm-thick sediment layer (Table 2). However, in that study, no intermediate sampling allowed to better constrain the recovery time. In an experimental set-up with turbidite sediments, Chaillou et al. (2007) hypothesized ammonium gradient
<table><caption>**Table 2**
Recovery time of biogeochemical gradients and components from previous studies.</caption><thead><tr><th>References and type of study</th><th>Thickness deposit (cm)</th><th>OPD</th><th>O<sub>2</sub> fluxes</th><th>NH<sub>4</sub><sup>+</sup></th><th>Mn<br>Dissolved</th><th>Mn<br>Oxides</th><th>Fe<br>Dissolved</th><th>Fe<br>Oxides</th></tr></thead><tbody><tr><td>This study</td><td>2.7</td><td>15-23d</td><td>&lt;2d</td><td>&lt;8d</td><td>15-23d</td><td>15-23d</td><td>&gt;23d</td><td>&gt;23d</td></tr><tr><td>Experimental</td><td>0.2-0.6 per week</td><td>&lt;2d</td><td>&lt;2d</td><td>&lt;1d</td><td>&gt;23d</td><td>&gt;23d</td><td>&gt;23d</td><td>&gt;23d</td></tr><tr><td>Chaillou et al. (2007)</td><td>8-9</td><td>5d</td><td>-</td><td>&gt;295d</td><td>33-75d</td><td>33-75d</td><td>&gt;295d</td><td>&gt;295d</td></tr><tr><td>Experimental</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Anschutz et al. (2002)</td><td>18</td><td>&lt;120d</td><td>-</td><td>&lt;120d</td><td>&gt;120d</td><td>&gt;120d</td><td>&gt;120d</td><td>&gt;120d</td></tr><tr><td>Turbidite in Capbreton canyon</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Deflandre et al. (2002); Mucci et al. (2003)</td><td>10-25</td><td>&lt;21d</td><td>-</td><td>&lt;286d</td><td>&gt;1490d</td><td>&gt;1490d</td><td>&gt;1490d</td><td>&gt;1490d</td></tr><tr><td>Flood deposit in the Saguenay fjord</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cathalot et al. (2010); Pastor et al., (2018)</td><td>10</td><td>&lt;26d</td><td>&lt;26d</td><td>&lt;26d</td><td>&lt;26d</td><td>&lt;26d</td><td>&lt;26d</td><td>&lt;26d</td></tr><tr><td>Flood deposits in Rhône prodelta</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Thibault de Chanvalon et al. (2016)</td><td>8</td><td>-</td><td>-</td><td>-</td><td>75-135d</td><td>75-135d</td><td>75-135d</td><td>75-135d</td></tr><tr><td>Flood deposit in a Loire estuarine mudflat</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Barhdadi et al. (2024)</td><td>30</td><td>-</td><td>-</td><td>-</td><td>&gt;60d</td><td>&gt;60d</td><td>&gt;60d</td><td>&gt;60d</td></tr><tr><td>Deposit in the maximum turbidity zone of the Loire estuary</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

re-establishment after 295 days for an 8–9 cm thick sediment deposit. The faster recovery in our study likely results from higher OM content and thinner sediment deposits. In FLV, the lower quantities of sediment added per event enabled faster complete deposition and a recovery of ammonium and nitrite profiles within a day after each event.

### 4.2.4. Recovery of Mn and Fe redox cycling

18h after the disturbance (T1), reactive Mn and Fe oxides (Mnasc and Feasc) enriched at the previous SWI in OHV were buried (Fig. 4). Mnasc dissolution was indicated by an intense, localized Mn<sub>d</sub> production zone, separated from the previously established production zone in the pre-disturbance (initial) sediment. At T1, no Mnasc or Feasc enrichment was observed at the new SWI (T1 OHV, Fig. 4). Over the following two weeks (T2 and T3), strong dissolution of buried Feasc generated a sharp Fe<sub>d</sub> peak centered at the previous SWI (T2 and T3 OHV, Fig. 6). Another new Fe<sub>d</sub> production zone was observed at around 1 cm depth in the deposited sediment layer, while the Mn<sub>d</sub> production zone extended into this layer (Fig. 5). The surface oxic layer enabled the oxidation of Mn<sub>d</sub> and Fe<sub>d</sub> diffusing upward from the underlying production zones, leading to the re-establishment of Mnasc and Feasc enrichment at the surface, without additional oxide deposition from the water column, as observed in natural environments (Table 2). Furthermore, three weeks after sediment deposition (T4), Mn<sub>d</sub> production at the former SWI was no longer detected, indicating complete dissolution of buried Mnasc, while Fe<sub>d</sub> production persisted (T4 OHV, Fig. 6). Recovery times in natural environments are generally longer (Table 2), likely due to larger reactive Mn and Fe stocks (~100 and ~60 times lower in our microcosms than in Anschutz et al., 2002), along with differences in OM content and quality, and sediment deposit thickness. In FLV, both metal oxides evolved differently. At the end of the experiment, no recovery of reactive oxides or dissolved gradients was observed. Ongoing reduction of trapped oxides observed at one of the last buried interfaces for Mn (layer deposited at T2), and at the first buried SWI for Fe (layer deposited at T1) (Figs. 5 and 6) will be discussed later in relation with the frequency of sediment deposition effects.

### 4.2.5. Influence of deposition frequency on recovery

Based on these observations, the weekly repeated sediment deposits in FLV slowed overall redox re-equilibration compared to the single OHV event. Our results indicate that oxic respiration and denitrification recovered faster than the 7-day deposition interval in FLV, while Mn and Fe remobilization displayed slower dynamics, prolonging their recovery times compared to the single OHV disturbance (Table 2). To our knowledge, the effect of high-frequency sediment deposition on biogeochemical recovery has not been previously documented in experimental or environmental studies and can be considered as an important factor influencing redox re-equilibration in dynamic sedimentary environments.

## 4.3. Effects of sediment disturbances on biogeochemical processes

### 4.3.1. Impact of sediment disturbance on denitrification/nitrification dynamics

During the experiment, sediment deposition events disrupted pre-established nitrite and ammonium concentration gradients, contrasting with the evolution of concentration profiles observed in the control (Fig. 3). In OHV, 18h after sediment addition, a probable denitrification front was present in the water column, between 3.5 and 2.5 cm above SWI, as indicated by the very low nitrite concentrations below this zone (Fig. 3). This likely resulted from turbidity-induced oxygen depletion, as evidenced by the accumulation of reduced species (Figs. 3, 5 and 6). Previous studies showed that high turbidity can enhance denitrification (Abril et al., 2000, 2010; Liu et al., 2013; Sebilo et al., 2006; Xia et al., 2017). However, as suspended particles settled, turbidity decreased and the water column gradually reoxygenated, promoting nitrification and restoring nitrite concentration in the water column, as observed in the upper part of the water column in T1 OHV (Fig. 3). The denitrification-nitrification boundary likely migrated downwards as sediments finished settling. One week later (T2), ammonium and nitrite distributions were re-established.

In FLV, the lower sediment inputs led to faster settling, and the water column was already clear at each sampling time (T1-T4, 18h after sediment additions). This probably explains why transient denitrification-nitrification dynamics were not observed in the FLV water column. However, a constant decrease in nitrite concentrations was noted throughout the experiment, more pronounced than in the control (Fig. 3).

In OHV, a single large sediment deposition event initially enhanced denitrification due to turbidity-induced hypoxia during particle settling, followed by nitrite recovery in the water column thereafter. Our results suggest that, within a short time after the disturbances, sediment addition leading to turbid promotes denitrification, while nitrification prevails in the other treatment, which tends more quickly to an undis turbed state after transient state. Nevertheless, the impact on these processes also depends on the nature of the physical disturbance. For instance, contrary to our findings, Ferguson et al. (2020) showed that bottom trawling reduces denitrification. However, trawling induces resuspension and homogenization of surface sediments, removing sediment structure that supports benthic denitrification.

Additionally, Mn and Fe reactive oxides and their remobilization in response to sediment disturbances may induce anoxic nitrification and denitrification (Burgin et al., 2011; Hulth et al., 1999; Luther et al., 1997; Moncelon et al., 2021; Mortimer et al., 2004; Weber et al., 2006). Therefore, nitrite production or consumption may occur at depths where Mn and/or Fe oxides or dissolved Mn or Fe were present. However, our data do not provide clear evidence of such interactions.
### 4.3.2. Enhanced manganese-iron decoupling due to repeated sedimentation events

As mentioned earlier, in OHV, Fe remobilization was delayed compared to Mn, as Mn oxides trapped at the buried SWI were preferentially reduced. This suggests that microbial metabolism initially favors manganese oxide reduction (Barhdadi et al., 2024; Burdige, 2007; Froelich et al., 1979). Moreover, this decoupling in reduction processes was evident from the apparent production rates of Mn<sub>d</sub> and Fe<sub>d</sub> integrated over the sediment column (APR; Fig. 7). Indeed, a clear shift between Mn<sub>d</sub> and Fe<sub>d</sub> production was observed, with a major increase in dissolved Fe production at T2 and T3, reaching 3 mmol m<sup>-2</sup> d<sup>-1</sup>.

A similar decoupling occurred in FLV, where repeated sediment additions amplified these dynamics. As in OHV, at T1, Mn and Fe oxides slightly accumulated at the base of the freshly deposited sediment layer (T1 FLV, Fig. 4), with strong Mn<sub>d</sub> production occurring around the former SWI (Fig. 5). With subsequent weekly disturbances (T2-T4), Mn oxide enrichment was consistently observed at the base of the most recent deposit, but at lower concentrations than in the control (Fig. 4). Additionally, Mn oxide distribution spread across the successive deposit layers (Fig. 4, T4 FLV). One week after the first deposit (T2), Mn<sub>d</sub> production around the initial SWI had significantly declined, indicating a slowdown in Mn remobilization from trapped oxides (Fig. 5). During the last sampled week (T4), Mn remobilization had shifted between the first and second deposit layers, indicating a greater delay compared to OHV. Meanwhile, Fe oxide enrichment was consistently located below Mn, in older depositional layers, implying an even slower Fe remobilization process (Fig. 4). Indeed, the reduction of initially trapped Fe reactive oxides was only observed from T3 FLV (two weeks after the first burial event), confirming that Mn dynamics, driven by repeated deposition events, caused the delay in Fe remobilization (Fig. 6). The clear temporal succession of Mn and Fe APR observed in OHV was more difficult to apprehend in FLV (Fig. 7). While strong Mn<sub>d</sub> production was observed at

T1, subsequent disturbances appeared to reduce Mn and Fe oxide remobilization rates (Fig. 7). However, increasing Fe APR at T3 and T4 supports the observation of the onset of remobilization previously described using the profiles.

Beyond the sequential remobilization of Mn and Fe, some profiles suggest localized redox interactions between these two elements. At T1, in both OHV and FLV, Fe<sub>d</sub> consumption coincided with the intense Mn<sub>d</sub> production at the former SWI, possibly reflecting Fe<sub>d</sub> oxidation by the residual Mn oxides still present (Figs. 4–6) (Ehlert et al., 2014; Postma, 1986; Schaefer et al., 2017). A similar coupling was observed at T4 in FLV, just above the Fe<sub>d</sub> release zone. These patterns, although less evident at T2 and T3 FLV, may have been masked by delayed Fe remobilization due to successive sediment deposits. These possible localized interactions between dissolved and solid-phase Mn and Fe species further highlight the complexity of post-depositional processes, particularly in zones where oxic–anoxic boundaries shift with each disturbance. However, even if these couplings occur, they do not compensate for the cumulative impact of repeated deposits. While Fe fully recovered in OHV, the weekly sedimentation in FLV continuously interrupted this re-establishment, preventing complete recovery between deposition events. Therefore, repeated sediment deposition not only prolongs biogeochemical transient states, particularly for Mn and Fe, but also intensifies their decoupling. By continuing the experiment and the weekly deposition, Fe oxides might never precipitate at the sediment surface, remaining localized deeper below Mn enrichment zone (Fig. 8). While OM content and deposit thickness are main drivers of biogeochemical recovery (Ferreira et al., 2024; Nmor et al., 2022), our study highlights that deposition frequency, even with low thicknesses, is an additional factor influencing recovery dynamics.

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Temporal representation (T0 to T4) of the calculated Mn<sub>asc</sub> and Fe<sub>asc</sub> stocks and the modelled Apparent Production Rates (APR) of dissolved Mn and Fe.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. Schematic representation of manganese and iron relaxation following an intense sedimentation event and the frequent deposition of multiple thin sediment layers.</figcaption></figure>

4.4. Mn-Fe dynamics as a proxy for recent sediment deposition

The concomitant presence of reactive oxide-enriched layers at anoxic
depths, along with ongoing Mn<sub>d</sub> and Fe<sub>d</sub> production at the same depths,
reveals a transient state following sediment deposition without erosion
(Fig. 8). While the distribution of dissolved oxygen and nitrogen species
was shown to recover quickly after disturbances (<1 week), the distri-
butions of both solid and dissolved Mn and Fe phases within the sedi-
ment column can serve as indicators of former SWI buried by sediment
deposits.

In natural environments, these parameters could be applied to
identify recent burial events without erosion, particularly in areas
experiencing intense particle settling and limited macrofaunal activity
in the sediment column. Potential applications include sediment
discharge zones, such as the fronts of melting tidewater glaciers (Herbert
et al., 2022), turbiditic structures (Anschutz et al., 2002), or hydrody-
namic environments like estuaries (Barhdadi et al., 2024). Moreover, as
suggested by Mouret et al. (2009), different manganese phases have
already been demonstrated to be effective in assessing sedimentation
rates, with accuracy comparable to conventional techniques such as
²¹⁰Pbₓs datation. However, the relaxation rate of Mn and Fe, and
consequently their preservation in sediments as records of past in-
stabilities, depend on multiple factors, including OM content, temper-
ature (which affects bacterial activity and reaction kinetics), porosity,
grain size, metal oxide content in the sediment column and deposit
thickness (Ferreira et al., 2024; Nmor et al., 2022), which require to be

further constrained.

Following the transient state, the re-establishment of redox fronts
within newly deposited layer results in a diagenetic enrichment zone of
reactive manganese and iron oxides at the new sediment surface, asso-
ciated with an underlying Mn<sub>d</sub> and Fe<sub>d</sub> production zone. Frequent
sediment deposits can be identified by the presence of Mn and Fe
reactive oxide enrichments at multiple depths within the sediment col-
umn (Fig. 8), rather than exclusively at the surface, as well as by po-
tential disruptions in porosity profiles. Repeated deposition events
progressively bury previous SWIs, and as the frequency of these events
increases, the decoupling between Mn and Fe becomes more pro-
nounced over time, coinciding with successive accumulated deposits.
The observation in natural environments of multiple sediment layers
containing excess reactive Mn and Fe oxides, along with ongoing
reduction at the same depths, would indicate multiple recent burial
events and sustained transient states. Such patterns are expected in
environments submitted to frequent pulses of sediment discharge.

5. Conclusion

This experimental work allowed to isolate the short-term temporal
dynamics of some redox chemical species in sediments subjected to
unique or frequent sediment deposition events, with sediment thick-
nesses ranging from a few millimeters to a few centimeters.

Frequent small-scale deposition events had no impact on dissolved O₂ penetration depth measured two days after deposition. In contrast, a
single thick (2.7 cm) sediment deposition event led to an OPD deepening of 0.8 mm, persisting for up to 15 days. However, regardless of the new sediment layer thickness, dissolved oxygen uptake recovered within two days after each deposition event.

Nitrite distribution showed that during particle settling in the water column, denitrification appeared to be enhanced, resulting in very low nitrite concentrations in the microcosm subjected to recurrent disturbances. Ammonium gradients re-established within a few days after the intense and unique sedimentation event.

Reactive Mn oxides, enriched below the previous sediment-water interface, were rapidly reduced in both tested conditions once the sediment became anoxic, typically within 1 week after sediment deposition. In contrast, reactive Fe oxides took up to one additional week to undergo reduction in the unique deposition event modality. Under recurrent sediment disturbances, this decoupling was enhanced, leading to a broader distribution of reactive oxides at depth, and a progressive shift in depth between Mn and Fe peaks. The presence of layers enriched in reactive oxides at anoxic depths, associated with the production of dissolved Mn and Fe, can help identifying previous SWIs buried by sedimentary deposits, and can therefore be used as an indicator of frequent sedimentary instabilities.

The experimentally observed model of geochemical transient states involving Mn and Fe could be applied to natural environments to identify ecosystems impacted by high sedimentation events. In coastal environments, further numerical modelling using environmental parameters (e.g., OM content, temperature ...) could help refine quantitative estimates and constrain the temporality of sediment deposition. Finally, this could be an alternative to radiochronology analyses, which can be challenging due to the decay timescales temporality of the radioelements considered, or in areas of high sediment accumulation rate where radioelement decay can be more difficult to quantify.

### CRediT authorship contribution statement

**Coretin Guilhermic:** Writing – original draft, Validation, Methodology, Investigation, Data curation, Conceptualization. **Aurélia Mouret:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Investigation, Data curation, Conceptualization. **Hélène Howa:** Writing – review & editing, Visualization, Validation. **Edouard Metzger:** Writing – review & editing, Investigation. **Geoffroy Couasnet:** Investigation, Data curation. **Maria Pia Nardelli:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

### Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the authors used DeePl in order to improve the readability and language of the manuscript. After using this service, the authors reviewed and edited the content as needed and take full responsibility for the content of the published article.

### Financial support

The CNRS INSU LEFE CYBER program (Fluid Envelopes and Environment) supported our research by funding the BEGIN project (PI: Maria Pia Nardelli). This research is part of a PhD project (first author) funded by the French Ministry of Scientific Research and Innovation and by the University of Angers.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

We thank the Miroiterie Nogentaise Inc. for providing custom-made aquaria following all our requirements. We also thank Dr. Aubin Thibault de Chanvalon for providing R scripts for Mn and Fe production calculations. The first author is grateful to Damien Le Moigne and Sophie Sanchez for their invaluable help during the experiment procedures, and to the bachelor students whose efficiency speeded up the particulate sample analysis. In addition, we thank Eric Bénéteau for ICP-AES data acquisition and Antonio Pusceddu, Santina Saru and Hiba Abyaba who performed the analyses for the biochemical composition of the sedimentary organic matter. Lastly, the authors would like to thank David Burdige and the associate editor for editing the manuscript and the two anonymous reviewers for providing useful comments that helped improving the article.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ecss.2025.109472.

### Data availability

All data presented in this study will be made available in an online repository upon manuscript publication.

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