1 Millennial-aged organic matter preservation in anoxic and sulfidic mangrove soils: insights from isotopic and molecular analyses

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3 Sarah Louise Robin<sup>a*</sup>, François Baudin<sup>b</sup>, Claude Le Milbeau<sup>c</sup>, Cyril Marchand<sup>a</sup>

a Institut de Sciences Exactes et Appliquées (ISEA EA7484), Université de la Nouvelle-Calédonie,

145 Avenue James Cook, Nouville, BP R4 98851, Nouméa Cedex, New Caledonia

b Institut des Sciences de la Terre - Paris (ISTeP), UMR 7193, Sorbonne Université, CNRS, 75005 Paris, France

Paris, France

c Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, Université d'Orléans, CNRS,

BRGM, OSUC, F-45071 Orléans, France

10 *corresponding author: sarah.robin@unc.nc

11 Abstract

Mangrove forests are known as coastal carbon sinks, but the long-term (millennium-scale) preservation processes of organic matter (OM) in their soils and the role of sulphur in these processes are still not fully understood. These processes are crucial for better estimating the impact of sea-level variations on the carbon dynamics in mangrove forests, which are particularly sensitive to sea-level changes due to their direct hydrological interactions with coastal waters. This study focuses on a soil layer enriched in mangrove-derived OM that accumulated during a stable sea-level period of the Holocene in New Caledonia (South Pacific). Radiocarbon dating situates this enriched layer at approximately 4,000 cal BP. The aim of this study is to characterize the enriched OM layer using bulk (Rock-Eval), isotopic (δ¹³C and δ¹⁵N), and molecular (lignin and neutral carbohydrates) analyses. This OM has undergone diagenetic processes such as dehydrogenation, and the loss of components such as the main neutral carbohydrates: glucose, xylose, and galactose. However, some Rock-Eval parameters, the total lignin content, and carbon and nitrogen isotopic ratios are characteristic of well-preserved OM, suggesting differential decomposition/preservation processes. In addition, SEM observations highlighted the presence of pyrite associated to preserved root material. Along with high S<sub>org</sub>/TOC ratio, these results suggest potential processes of OM sulfurization preserving it from decomposition. Prolonged sea-level stability in addition to anoxic and sulfidic soil conditions enhanced OM accumulation and long-term sequestration in mangrove soils.

Keywords: mangrove, stable isotopes, Rock-Eval, carbon dating, neutral carbohydrates, lignin

30 1. Introduction
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Mangrove forests are efficient carbon sinks due to their productivity, estimated at 11 Mg C ha⁻¹ yr⁻¹, and their ability to effectively sequester carbon in the soil at an average rate of 180 g organic carbon (OC) m⁻² yr⁻¹ (Alongi, 2020). The presence of anoxic and sulfidic soil conditions, especially at depth, significantly limits OC mineralization, thus facilitating its long-term storage (Kristensen et al., 2008). The combination of high net primary productivity, rapid sedimentation rates, and soil anoxia makes mangrove soil an ideal and favourable reservoir for long-term sequestration of OC (Kristensen et al., 2008). Nevertheless, mangrove forest migration in response to sea-level changes presents challenges in accurately characterizing the carbon stock of the present mangrove forests (Marchand, 2017).

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During the Holocene period, sea-level variations have impacted mangrove distribution (Woodroffe et al., 2016). For about 3,700 years, sea-level on the West Coast of New Caledonia was stable 1.1 m higher than today, before falling to the present mean sea-level at 2,800 cal BP (Yamano et al., 2014). The fall of the sea-level resulted in mangrove seaward migration. However, the presence of the former mangrove forest was recorded in soils of the present-day upper intertidal zone as an organic matter (OM)-rich layer characterized by the presence of preserved red tissues typical of *Rhizophora* trees (Jacotot et al., 2018).

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The aim of the present study was to gain a deeper understanding of the processes governing the long-term preservation of millennial-aged OM in anoxic and sulfidic mangrove soils. By employing isotopic, molecular, and Rock-Eval analyses, we sought to chemically characterize the OM-rich layer and therefore uncover the mechanisms behind OM decomposition and preservation.

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## 2. Methods

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The study site is a semi-arid mangrove forest typical of the West Coast of New Caledonia with *Rhizophora stylosa* present on the seaside of the forest and *Avicennia marina* on the land side. The fall of the sea-level from the late Holocene period infers the presence of the OM-rich layer in depth beneath the *A. marina* stand, where an old *R. stylosa* forest stood prior to migration. This layer is identified at the current study site by the observation of red wood tissues belonging to the *Rhizophora* genus at depth beneath the *A. marina* stand. Previous studies on mangrove forests from

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the same coast identified this layer in areas without vegetation, such as salt-flats, down to depths of 260 cm, further supporting its ancient origin. Furthermore, at the current study site, red wood debris were observed down to 80 cm beneath *A. marina* and over a depth of more than a meter in forests along the same coast, confirming the stable burial of OM (Jacotot et al., 2018). Triplicate fresh *R. stylosa* coarse roots were cut with a saw, and green leaves were collected from the trees to help determine the sources of the OM-rich layer. Soil samples in this specific layer, situated between 30 and 40 cm depth beneath the *A. marina* stand, were collected with an Eijkelkamp gouge auger in May 2021. *A. marina* trees have shallow and widespread root systems (Pi et al., 2009), which limits their influence on the OM pool in this 30-40 cm depth layer. All samples were freeze-dried and ground. Rock-Eval pyrolysis was conducted on a Rock-Eval 7S (Vinci Technologies) and gave multiple parameters on the soil samples including total organic carbon (TOC), total and organic sulphur (S<sub>tot</sub> and S<sub>org</sub>), hydrogen index (HI), oxygen index (OI), and the temperature of the oven at the maximum yield of S<sub>2</sub> peak (TpS<sub>2</sub>), corresponding to the products from OM thermal cracking (Baudin et al., 2015). From these parameters, the S<sub>org</sub>/TOC ratio can be calculated, which help classify the soil in terms of sulphur content (Orr, 1986). Stable isotopic compositions (δ<sup>13</sup>C and δ<sup>15</sup>N) and C/N ratios were obtained on an isotope ratio mass spectrometer Sercon Integra 2. For quality control, duplicate samples were passed through the spectrometer and differences between duplicates were all below 5%. The molecular analysis focused on neutral carbohydrates and lignin-derived phenols. Neutral carbohydrates were extracted through hydrolysis and analysed via GC-MS after derivatization with BSTFA in pyridine (Lallier-Vergès et al., 2008). The GC is a Trace1300 (Thermo Scientific) coupled to a MS ISQ7000 (Thermo Scientific). The GC is equipped with a Restek Rxi-5Sil MS column (60 m x 0.25 mm, 0.25 µm film thickness) (Robin et al., 2024). Quality control consisted of duplicate technical samples (≤11% standard deviation) and measurements of the internal standard (Supplementary Table 1). 11 lignin-derived phenols were quantified after hydrolysis and extraction (Marchand et al., 2005). Chromatographic separation was achieved using a Vanquish Flex system (Thermo Scientific). Samples were separated on a Hypersil GOLD column (1.9 µm particle size), 50 x 2.1 mm (Thermo Scientific). The mobile phase was a mixture of ultrapure water and methanol. MS detection was performed using an Orbitrap Exploris120 (Thermo Scientific) (Robin et al., 2024). A quality control solution was passed every 10 samples and values obtained
were all below 15% difference with expected values. Soil samples were also observed on a scanning electron microscope (SEM) using a JEOL JSM-IT300 LV apparatus coupled with an energy dispersive spectrometer (EDX) Oxford xmax80T device. Radiocarbon dating of the soil samples was performed after hot HCl washing at the radiocarbon dating laboratory of the University of Waikato.

## 3. Results and discussion

<figure><img src="image_1.png" /></figure>

**Figure 1.** (A) Vertical soil core with distribution of percentage total organic carbon content (TOC) and total sulphur content (TS) with depth and (B) SEM photo of preserved root from the organic matter-enriched layer (30-40 cm) with associated EDX spectra of the root and of the coating. The ¹⁴C dating for the soil and the preserved roots are given in cal BP.

The enriched soil layer was identified where preserved roots, potentially belonging to the *R. stylosa* former forest, were observed. The radiocarbon dating (¹⁴C-datation) indicated that the soil layer dated back to approximately 4,000 cal BP (Fig. 1), aligning well with the stable high sea-level period (Yamano et al., 2014). In this layer, TOC reached more than 10% (Fig. 1A) while median TOC content in mangrove soils is 2.2% (Kristensen et al., 2008). Furthermore, the high S<sub>tot</sub> content (5.6%) (Fig. 1A) and S<sub>org</sub>/TOC ratio (0.08) values of the soil layer were indicative of a very high sulphur type of soil (Orr, 1986). The accumulation of roots and other OM debris during this timeframe can be attributed to the prolonged period of sea-level stability and the limited capacity of microbial communities to decompose mangrove tissues in waterlogged, anoxic conditions (McKee et al., 2007). Similar occurrences of mangrove peats with identifiable preserved roots from the Holocene have been reported in the Caribbeans (McKee et al., 2007). In Belize, a comparable scenario to the one described in this study was identified, where an enriched OM layer was found beneath a current
109 *Avicennia germinans* forest, originating from an ancient *Rhizophora mangle* forest (McKee and Faulkner, 2000). However, the authors attributed the formation of peat as an adaptation mechanism to rising sea-levels, rather than indicating a period of sea-level stability (McKee et al., 2007).

112 SEM-EDX analysis revealed the association between the root material and pyrite (FeS₂) (Fig. 1B). We suggest that the intricate interaction between OM and pyrite, probably through adsorption, contributed to an enhanced preservation of OM within the waterlogged and anoxic soil conditions (Shawar et al., 2018; Tessin et al., 2016). The association of OM with minerals slows decomposition processes, and in environments characterized by high sedimentation rates, facilitates the burial of OM (Wakeham and Canuel, 2006). Also, the mineral-OM interface evolves with changes in the chemistry of aqueous phases, OM, and mineralogy (Kleber et al., 2021), but the stable anoxic environment provided by the mangrove soil favours stable OM adsorption onto minerals. We hypothesize that the OM-rich soil layer has remained anoxic due to the inherent anoxic nature of the substrate under *R. stylosa* (Alongi et al., 2000; Lacerda et al., 1995; Leopold et al., 2013). Additionally, during the sea level drop, the migration of the mangrove led to soil coverage. A total of 122 30 cm of mineral particles were deposited over this layer, restricting exchange between the studied layer and the atmosphere, thus maintaining anoxic conditions. Furthermore, the sulphur parameters 123 indicated a high sulphur-OM type (Orr, 1986). We suggest that it is not only the organo-mineral 124 associations that promote preservation, but also the natural sulfurization of OM (process of 125 incorporating sulphur in OM) in sulfidic soils (Raven et al., 2019). In fact, in anoxic environments as 126 in this study, sulphur can form covalent bonds with a carbon atom from the aromatic rings of lignin, 127 enhancing lignin preservation, therefore providing OM resistance to decay (Crémière et al., 2017; 128 Karunarathna et al., 2020). However, sulphur does not tend to form covalent bonds with molecules 129 such as neutral carbohydrates in anoxic environments. This type of bonding is favoured in the 130 presence of oxygen (Lin et al., 2013; Paulsen, 1966). The preservation of root materials can also be 131 attributed to the long period of sea-level stability, which associated with the high primary production 132 of *R. stylosa* trees, enhanced OM accumulation by limiting sediment transport through erosion or 133 disturbances. 134
<figure><img src="image_2.png" /><figcaption>Figure 2. (A) Stable isotope ratios, (B) hydrogen and oxygen indexes, and (C) C/N ratios and TpS2 values of the soil organic matter-enriched layer and the fresh leaf and roots of *R. stylosa*.</figcaption></figure>

The carbon and nitrogen isotopic compositions of the layer (-26.4‰ and 1.26‰, respectively) were close to those of the fresh *R. stylosa* roots (-26.5‰ and 1.22‰, respectively) (Fig. 2A). This result indicates that the preserved roots present in this layer may contribute significantly to the OM composition as their isotopic values must be similar to that of the modern fresh *R. stylosa* tissues. However, the oxygen and hydrogen indexes (OI and HI) obtained after Rock-Eval pyrolysis showed that the OM from the soil layer, if derived from *R. stylosa* tissues, has undergone diagenetic processes, including dehydrogenation, as indicated by the lower HI value (139 mg HC g⁻¹ TOC) compared to the fresh tissues (252 and 410 mg HC g⁻¹ TOC) (Fig. 2B). The higher TpS2 values of the soil layer (452 °C) compared to that of the fresh tissues (347 and 351 °C) (Fig. 2C) also demonstrated more thermally stable OM in the enriched layer, indicative of a mature OM. Still, the OI value of the soil layer was low (81 mg CO₂ g⁻¹ TOC) and close to that of the fresh tissues (Fig. 2B), indicating no oxygenation of the OM. Therefore, soil anoxia may help OM preservation in this layer. The OM in the soil layer exhibited lower C/N ratios (37) than fresh tissues (100 and 54) (Fig.
2C), highlighting either a preference for the degradation of C-rich compounds over N-rich compounds, or the enrichment of N in soil from accumulation of microorganisms biomass, which can obtain N from N₂ fixation (Gotto and Taylor, 1976).

<table><caption>**Table 1.** Neutral carbohydrates and lignin-derived phenols parameters in the soil organic matter-enriched layer and in the fresh roots and leaves of *R. stylosa*.</caption><thead><tr><th>Variables</th><th>OM-enriched layer</th><th><i>R. stylosa</i> fresh roots</th><th><i>R. stylosa</i> fresh leaves</th></tr></thead><tbody><tr><td>Total neutral carbohydrates (mg.g<sup>-1</sup> TOC)</td><td>323</td><td>1,030</td><td>616</td></tr><tr><td>Hex/Pen</td><td>0.05</td><td>0.28</td><td>0.34</td></tr><tr><td>Total lignin-derived phenols (µg.g<sup>-1</sup> TOC)</td><td>4,257</td><td>4,054</td><td>3,581</td></tr><tr><td>C/V</td><td>0.03</td><td>0.06</td><td>0.61</td></tr><tr><td>S/V</td><td>0.15</td><td>0.45</td><td>0.39</td></tr><tr><td>P/(V+S)</td><td>0.18</td><td>0.35</td><td>0.27</td></tr><tr><td>PON/P (Ad/Al)<sub>p</sub></td><td>0.17</td><td>0.03</td><td>0.08</td></tr><tr><td>(Ad/Al)<sub>v</sub></td><td>0.75</td><td>0.34</td><td>1.00</td></tr><tr><td>(Ad/Al)<sub>s</sub></td><td>0.13</td><td>1.10</td><td>4.73</td></tr><tr><td></td><td>2.66</td><td>3.20</td><td>1.88</td></tr></tbody></table>

Hex = Hexoses (mannose + galactose), Pen = Pentoses (xylose + arabinose), P = p-hydroxyl, V = Vanillyl, S = Syringyl, C = Cinnamyl, PON = p-hydroxyacetophenone, and (Ad/Al) = acid over aldehyde ratio.

At the molecular level, the overall neutral carbohydrate (lacking N atoms) content measured in the soil (323 mg g⁻¹ TOC) was lower compared to the fresh roots (1,030 mg g⁻¹ TOC), while the lignin content was similar (4,257 and 4,054 µg g⁻¹ TOC, respectively) (Table 1). In fact, in anoxic mangrove soils, neutral carbohydrates are usually more labile than bulk OC, while lignin is more refractory (Lallier-Vergès et al., 2008; Marchand et al., 2005). The Hex/Pen ratio within the soil's enriched layer (0.05) was nearly an order of magnitude lower than that of the fresh roots (0.28) and leaves (0.34) (Table 1). Polysaccharides originating from plants are notably rich in pentose content, whereas those arising from microbes primarily consist of hexoses (Kögel-Knabner, 2002). Consequently, the reduced Hex/Pen ratio underscores the substantial contribution of plant-derived OM to this layer, in addition to the selective loss of hexose during OM transformation processes. Thus, hexose was removed as CO₂ while pentose stayed in the soil. The smaller ratio was linked to a significant relative increase in arabinose from the fresh roots (19%) to the buried layer (81%) (Fig. 3A), indicating the more refractory behaviour of arabinose compared to bulk OC, alongside the loss of galactose (Fig. 3A). Previous research has highlighted arabinose's refractory behaviour (Marchand et al., 2005), given its presence in hemicellulose, but also pectin, which exhibits higher
174 resistance to decomposition compared to cellulose and hemicellulose (Benner et al., 1990).
175 Additionally, glucose and xylose were measured in relative lower concentrations within the enriched
176 layer (Fig. 3A). Among the fresh tissues, xylose, galactose, and glucose were the most abundant
177 sugars. We suggest that, as neutral carbohydrates serve as a significant energy source for soil
178 microbial communities, the major molecules underwent degradation upon OM burial (Opsahl and
179 Benner, 1999). This degradation is particularly pronounced due to the labile nature of glucose and
180 xylose, which are the least stable carbohydrates (Marchand et al., 2005). Conversely, mannose,
181 ribose, and the deoxy sugars (fucose and rhamnose) remained relatively preserved (Fig. 3A).

<figure><img src="image_3.png" /><figcaption>182
183 Figure 3. (A) Proportions of the 8 measured neutral carbohydrates and (B) proportions of the 4 lignin-
184 derived phenol groups of the soil organic matter-enriched layer and the fresh roots of R. stylosa.</figcaption></figure>

185 Lignin-derived phenols are exclusively associated with vascular plants. Total lignin contents
186 in the studied layer and in the fresh roots were of the same magnitude (Table 1). Still, across all four
187 phenol groups, only the cinnamyl phenols remained constant, while the vanillyl phenols relatively
188 increased in proportions and the syringyl and p-hydroxyl relatively decreased (Fig. 3B).
189 Consequently, the C/V ratio in the soil was halved, while the S/V ratio was reduced to one-third of
190 that found in the fresh roots (Table 1). Among phenol groups, vanillyl predominates in mangrove
191 debris (Benner et al., 1990) and is similarly prevalent in mangrove soils (Lallier-Vergès et al., 2008).
192 Preservation tendencies favour vanillyl and p-hydroxyl phenols over syringyl and cinnamyl groups
193 due to association of some groups to more labile molecules or looser bonds with lignin (Hartley,
194 1973; Kirk, 1980; Lallier-Vergès et al., 2008). Depending on the microbial activity associated to soil
195 conditions, different lignin degradation mechanisms favour certain phenol groups over others (Chen
and Chang, 1985). While the acid/aldehyde ratios were comparable for p-hydroxyl and syringyl groups, they were substantially lower in the soil than in the tissues for the vanillyl group (Table 1). This variance was primarily attributed to a significant enrichment in vanillin (Supplementary Table 2). Low Ad/Al ratios are commonly encountered in anoxic environments due to the absence of side-chain oxidation processes (Lallier-Vergès et al., 2008). In this context, however, vanillin preservation emerges as a key factor. Additionally, notable stability in p-hydroxyacetophenone was evident (Supplementary Table 2), leading to a considerably higher PON/P ratio in the soil compared to the tissues. This may result from the fact that p-hydroxyacetophenone exclusively originates from lignin oxidation, whereas p-hydroxybenzoic acid and p-hydroxybenzaldehyde may also result from the oxidation of non-lignin precursors within the fresh tissues (Benner et al., 1990). The elevated PON/P ratio thus suggests the predominance of lignin within the buried layer.

## 4. Conclusion

The results of this study provide critical insights into the mechanisms behind the long-term preservation of OM in mangrove soils. Isotopic, Rock-Eval, and molecular parameters, including stable isotope ratios and lignin content, indicate that OM preservation is likely enhanced by the soil's stable anoxic conditions, as evidenced by the oxygen index, over thousands of years. Sulfurization processes are also thought to play a role in OM preservation, as indicated by the soil sulphur content and the observed association between pyrite and OM. Nevertheless, the well-preserved OM has undergone weak diagenetic processes, as suggested by the neutral carbohydrate content, hydrogen index, and TpS2 values.

The study provides valuable context for interpreting preserved mangrove peats globally and underscores the role of mangroves in mitigating climate change through effective long-term carbon storage. This study also highlighted the value of using a combination techniques to gain comprehensive insights into OM preservation.

Looking ahead, this research underscores the need for further studies on coastal carbon dynamics, particularly in the face of ongoing climate change and sea-level variations. Understanding
222 these preservation mechanisms is crucial for predicting the future of OC in mangrove ecosystems
223 and for developing strategies to protect and enhance these vital carbon sinks.

224 Acknowledgements

225 The authors acknowledge Kapeliele Gututauava and Pierre Sanlis for the help with the field work.

226 The authors acknowledge Fiona Petchey from the University of Waikato for 14C-dating. The authors
227 acknowledge Ines Le Mao from NC Bioressources for UPLC-MS optimization and analyses and
228 Anne Desnues from IRD for the isotopic analyses.

229 Funding sources

230 This work was supported by the French National program EC2CO (Ecosphère Continentale et
231 Côtière) and le Fonds Pacifique.

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