# Measuring alkalinity fluxes at the water-sediment interface in aquatic ecosystems of the Tunisian littoral: impact on pH and inorganic carbon storage

Walid Oueslati, Haifa Ben Mna, Mohamed Amine Helali, Takwa Mghirbi, Amina Ben Mefteh, Sarah Ben Jeddi, Valérie Mesnage

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Walid Oueslati, Haifa Ben Mna, Mohamed Amine Helali, Takwa Mghirbi, Amina Ben Mefteh, et al.. Measuring alkalinity fluxes at the water-sediment interface in aquatic ecosystems of the Tunisian littoral: impact on pH and inorganic carbon storage. Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, Association des Sédimentologistes français, pp.115-118, 2025, 2-907205-83-8. 10.70665/NRKC6125. hal-05011429

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# MEASURING ALKALINITY FLUXES AT THE WATER-SEDIMENT INTERFACE IN AQUATIC ECOSYSTEMS OF THE TUNISIAN LITTORAL: IMPACT ON PH AND INORGANIC CARBON STORAGE

Walid OUESLATI<fnref n="1" />, Haifa BEN MNA<fnref n="1" />, Mohamed Amine HELALI<fnref n="1" />, Takwa MGHIRBI<fnref n="1" />, Amina BEN MEFTEH<fnref n="1" />, Sarah BEN JEDDI<fnref n="1" /> and Valérie MESNAGE<fnref n="2" />

Corresponding author: walid.oueslati@fst.utm.tn

**Abstract:** In the context of climate change and rising atmospheric CO₂, coastal marine sediments, rich in algae and seagrass beds, are potential blue carbon sinks and thus contribute to climate mitigation. However, ocean acidification due to CO₂ fluxes has been the subject of scientific debate for over 20 years. Indeed, sediments, through the anaerobic reactions of early diagenesis, promote the production of alkalinity in pore water, which can mitigate the effects of open-water acidification. The aim of this research is to model pore water alkalinity concentrations in four different Tunisian coastal ecosystems, in order to quantify the release of benthic alkalinity and the precipitation of authigenic carbonates. The results show that these sediments are an important source of alkalinity, with fluxes ranging from 1.1 to 7.27 mmol m⁻<fnref n="2" /> d⁻<fnref n="1" />. Additionally, the elevated alkalinity can promote the precipitation of carbonates, enabling the sediments to act as a sink for inorganic carbon.

**Key words:** sediment-water interface, alkalinity, blue carbon, inorganic carbon sequestration, Tunisia

## 1 INTRODUCTION

The ocean plays an important role as a natural carbon sink, absorbing nearly, a third of atmospheric CO₂ through ecosystems such as seagrasses, macroalgae, phytoplankton, mangroves and salt marshes (Wang et al., 2023), helping to mitigate the effects of climate change. However, the release of CO₂ into seawater leads to ocean acidification, which affects calcifying organisms such as corals and molluscs. Coastal marine sediments play a buffering role against this acidification, helping to sequester inorganic carbon. Coastal wetlands, which cover about 6% of the earth's surface, are particularly vulnerable to human impacts from agriculture and tourism. Recent studies have analysed the impact of acidification on the dissolution of calcium carbonate in the sediments (Eyre et al., 2014; Nesbit & Mucci, 2020). Traditionally sediments have been considered as zones of carbonate dissolution. However, recent research suggests that coastal sediments can also act as inorganic carbon sinks (Sun & Turchyn, 2014; Oueslati et al., 2019) models show that authigenic carbonate precipitation in marine sediments can play an important role in the global carbon cycle (Schrag et al., 2013). Furthermore Oueslati et al. (2019) have shown that the sediments of the Ghar El Melh lagoon in Tunisia generate alkalinity and therefore contribute to the acidification of the coastal waters in this area. In this study, we wish to examine the role of other coastal wetlands in Tunisia by diversifying the types of ecosystems studied, in particular lagoons (Korba and Lake North of Tunis), a coastal Sabkha (Sabkha of Ariana in Greater Tunis) and an open sea (Monastir Bay in central Tunisia, characterized by an abundance of *Posidonia Oceanica*). The Ghar El Melh lagoon was also reassessed in this study. The methodology adopted is based on modelling alkalinity concentrations in porewaters, using a computer code (PROFILE, Berg et al. 1998) to calculate flows at the sediment-water interface.

## 2 MATERIALS AND METHODS

### 2.1 Study sites

This research work complements the results of Oueslati et al. (2019) on the Ghar El Melh lagoon, by studying how the sedimentary compartment contributes to the generation of alkalinity, likely to diffuse into the water column to mitigate the acidification of overlying waters, while promoting the precipitation of authigenic calcite and the sequestration of inorganic carbon. The sites studied (Fig. 1) are (1) lagoons (Korba lagoon and Tunis northern lake), (2) a coastal sabkha (Ariana sabkha) and (3) open sea (Monastir Bay, rich in *Posidonia Oceanica*, which promotes blue carbon sequestration). We also re-examined the Ghar El Melh lagoon.

<footnote n="1">¹ LR01ES06 Ressources Minérales et Environnement, Université de Tunis El Manar (Tunisie)</footnote>

<footnote n="2">² UMR 6143 M2C, Université Rouen Normandie, Université Caen Normandie, CNRS</footnote>
<figure><img src="image_2.png" /><figcaption>Figure 1: Study sites</figcaption></figure>

## 2.2 Sampling, pore water alkalinity determination and mathematical modelling

At each study site, a sediment core was taken using a 9 cm PVC corer; except for the Monastir Bay site, where two sediment cores were taken. The sediment cores were then cut into sub-samples under a nitrogen flow. The resulting samples were then centrifuged at 2,500 rpm at low temperature to extract the pore water. Total alkalinity (TA) was measured on this pore water using the method of Rodier et al. (1997). Sediment porosity was calculated on the basis of the water content obtained by drying at 110°C. To assess the diffusion flux of total alkalinity at the sediment-water interface, as well as its rate of production or consumption, we modelled total alkalinity (TA) concentrations. Assuming steady-state, compaction and the absence of bioirrigation (Boudreau, 1997), the equilibrium equation for the alkalinity in pore water is expressed as follows:

$$ \left[ \frac{\partial \phi[TA]}{\partial t} \right]_x = \frac{\partial}{\partial x} \left[ \phi(D_s + D_B) \frac{\partial[TA]}{\partial x} \right] + R_{net}^{TA} $$

where $x$ represents the depth (in cm), $\phi$ the porosity and $t$ the time (in s). $R_{net}^{TA}$ is the rate of net production (positive) or consumption (negative) associated with reactions (in nmol cm$^{-3}$.s$^{-1}$) in the entire sediment, including particles and pore water, for the alkalinity TA present in the pore water. This equation is solved numerically for $R_{net}^{TA}$ using PROFILE code (Berg et al., 1998). For input values to the program, we used porosity data as well as a data set of the average pore water alkalinity. DS and DB values required in PROFILE were estimated according to Li and Gregory (1974).

## 3 RESULTS AND DISCUSSION

Figure 2 illustrates alkalinity profiles, with an increase of concentrations from the sediment-water interface to the first 10-15 centimeters, indicating TA production in this layer of sediment. Beyond this depth, with the exception of the northern lake of Tunis, concentrations show a marked decrease. Alkalinity production rates in pore water (Table 1) are significant. They amount to 3.5 mmol m$^{-2}$ d$^{-1}$ in the Ghar El Melh lagoon, 3.71 mmol m$^{-2}$ d$^{-1}$ (on average) in Monastir Bay, 1.5 mmol m$^{-2}$ d$^{-1}$ in Ariana de Sabkha, and 8.66 mmol m$^{-2}$ d$^{-1}$ and 0.47 mmol m$^{-2}$ d$^{-1}$ in Lac Nord de Tunis. Detailed modeling of diagenesis in the Ghar El Melh lagoon (Oueslati et al., 2019) shows that the chemical precipitation of carbonates is facilitated by the production of alkalinity during the processes of anaerobic mineralization and framboidal pyrite formation.

Pyrite oxidation and reoxidation of free $Fe^{2+}$ in pore water are very limited. Assuming that chemical carbonate precipitation is the secondary diagenetic reaction responsible for alkalinity consumption in deep sediment layers (Figure 2), authigenic calcite precipitation rates range from 0 to 1.39 mmol m$^{-2}$ d$^{-1}$ (Tab. 1). These rates are relatively low compared with those measured by Zhu et al. (2002) in Amazon delta sediments (4 mmol m$^{-2}$ d$^{-1}$) and higher than those reported by Reimers et al. (1996) in undisturbed muddy sediments from the Santa Barbara Basin (0.2 mmol m$^{-2}$ d$^{-1}$). Although alkalinity is consumed by the precipitation of carbonates and the reoxidation of reduced chemical elements ($Fe^{2+}$ and $FeS_2$), even at low levels, a significant amount remains present in the sediments and may diffuse into the overlying waters. These diffusion fluxes are in the same range as those measured in North Sea sandy sediments (1.7 - 6.6
mmol m⁻² d⁻¹, Brenner et al., 2016), and in Australian lagoon sediments populated by seagrass beds (0 - 2 mmol m⁻² d⁻¹, Eyre and Ferguson, 2002).

<figure><img src="image_3.png" /><figcaption>Figure 2: Vertical profiles of measured (circle) and calculated (solid red line) alkalinity in Sabkha of Ariana (a), Ghar El Melh lagoon (b), Korba lagoon (c), Monastir bay S1 (d), Monastir bay S2 (e) and Lake north of Tunis (f). Alkalinity production rate is presented in solid green line.</figcaption></figure>

<table><caption>Table 1: Diffusion flux and production/consumption rate of alkalinity (mmol m⁻² d⁻¹)</caption><thead><tr><td></td><td>PRODUCTION RATE</td><td>DIFFUSION FLUX</td><td>CONSUMPTION RATE</td></tr></thead><tbody><tr><td>Monastir bay S1</td><td>303</td><td>-2.82</td><td>-0.2</td></tr><tr><td>Monastir bay S2</td><td>4.39</td><td>-3.69</td><td>-0.7</td></tr><tr><td>Lake North of Tunis</td><td>0.47</td><td>-0.47</td><td>0.0</td></tr><tr><td>Sabkha Ariana</td><td>1.5</td><td>-1.1</td><td>-0.4</td></tr><tr><td>Ghar El Melh lagoon</td><td>3.5</td><td>-2.8</td><td>-0.7</td></tr><tr><td>Korba lagoon</td><td>8.66</td><td>-7.27</td><td>-1.39</td></tr></tbody></table>

## 4 CONCLUSION

In this study, by modeling the alkalinity concentrations in the pore water of different coastal marine environments, we demonstrate that early diagenesis of organic matter, generates significant amounts of alkalinity. The latter plays an important role in the global carbon cycle, notably through the precipitation and subsequent burial of authigenic carbonates. On the other hand, it is a source of alkalinity for overlying water and the open ocean, contributing to pH regulation and CO₂ absorption. These positive results, linked to climate change and the dissolution of marine carbonates, could encourage decision-makers to protect these coastal ecosystems, which undoubtedly provide valuable services.
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