# Lithium isotopes in Palaeozoic stem-tetrapod bioapatite: Preservation, controls, ecology and oceanographic insights

Vincent Luccisano, Jeremy Martin, Romain Amiot, Philippe Telouk, Fanny

Thibon, Sébastien Olive, Olivier Matton, Christophe Lécuyer

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Vincent Luccisano, Jeremy Martin, Romain Amiot, Philippe Telouk, Fanny Thibon, et al.. Lithium isotopes in Palaeozoic stem-tetrapod bioapatite: Preservation, controls, ecology and oceanographic insights. Chemical Geology, 2025, 695, pp.123024. 10.1016/j.chemgeo.2025.123024. hal-05371671

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Research Paper

# Lithium isotopes in Palaeozoic stem-tetrapod bioapatite: Preservation, controls, ecology and oceanographic insights

Vincent Luccisano<sup>a,b,*</sup>, Jeremy E. Martin<sup>b</sup>, Romain Amiot<sup>b</sup>, Philippe Telouk<sup>b</sup>, Fanny Thibon<sup>b</sup>, Sébastien Olive<sup>b,c,d</sup>, Olivier Matton<sup>e</sup>, Christophe Lécuyer<sup>b,f</sup>

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## ARTICLE INFO

Editor: Dr. Vasileios Mavromatis

Keywords:

δ<sup>7</sup>Li

Tetrapodomorphs

Late Devonian

Miguasha

Palaeoenvironment

Seawater

## ABSTRACT

Major ecological changes in vertebrate evolution through geological times include transitions between aquatic and terrestrial environments. However, temporality and modalities of these transitions are poorly known, relying on fossil discoveries and their sedimentary context. Recently, the stable isotopic composition of lithium (δ<sup>7</sup>Li) in mineralised tissues of modern vertebrates has been proposed to finely discriminate aquatic environments. This new proxy has strong potential applications in the fossil record to better understand key ecological changes such as vertebrate terrestrialisation during the Late Devonian. However, the impact of diagenetic processes on the preservation of the original vertebrate δ<sup>7</sup>Li has never been studied to assess the usefulness of this isotopic system. Here we report the first in-depth analysis of δ<sup>7</sup>Li values of mineralised fossil vertebrate tissues, as well as of major and minor elements. The specimens come from the Late Devonian outcrop of Miguasha, known to have preserved stem-tetrapods corresponding to the first steps of the initial vertebrates terrestrialisation. We highlight the good preservation of the majority of the δ<sup>7</sup>Li values with the identification of a diagenetically altered endmember which constitutes a new reference framework to assess the preservation of δ<sup>7</sup>Li in the vertebrate fossil record. δ<sup>7</sup>Li variations of the well-preserved specimens are not controlled by biological factors, but by the environment setting as highlighted by strong correlations between contribution of detrital inputs on the chemical content of fossil bioapatites and their δ<sup>7</sup>Li values. Consequently, lithium isotope composition of vertebrate bioapatite can be used to finely reconstruct past aquatic environments and vertebrate palaeoecology. The δ<sup>7</sup>Li value range of 20 ‰ to 23 ‰ in tetrapodomorphs indicates that they preferentially lived in the most distal and marine-influenced part of the estuary, confirming their euryhalinity and the complex environmental context of the vertebrate terrestrialisation. Tetrapodomorphs and placoderms have δ<sup>7</sup>Li comprised between 20 ‰ and 23 ‰, which are considered potentially valuable proxies for the δ<sup>7</sup>Li of the contemporaneous oceans.

## 1. Introduction

Major steps of vertebrate evolution are closely linked to ecological changes between aquatic and terrestrial environments (e.g. Uhen, 2007; Pyenson et al., 2014). For example, the terrestrialisation of stem- and early tetrapods has traditionally been viewed as a transition from freshwater to land by initially marine organisms (e.g. Barrell, 1916;

Cressler III et al., 2010). However, this view has been questioned, and some evidence argues for a lesser role of freshwater environments with a terrestrialisation process that could originate directly from the marine environment (e.g. Blieck et al., 2007; Goedert et al., 2018). Deciphering the modalities of such a dramatic adaptation requires to carefully reconstruct past aquatic environments, in order to be able to discriminate between fresh, brackish and marine environments.

<footnote><sup>a</sup> Géosciences Environnement Toulouse (GET), Observatoire Midi Pyrénées (OMP), 14 avenue Edouard Belin, Toulouse, France</footnote>

<footnote><sup>b</sup> Université Claude Bernard Lyon1, LGL-TPE, UMR 5276, CNRS, ENSL, UJM, F-69622 Villeurbanne, France</footnote>

<footnote><sup>c</sup> Royal Belgian Institute of Natural Sciences, Directorate Earth and History of Life, Palaeobiosphere Evolution, Rue Vautier 29, 1000 Brussels, Belgium</footnote>

<footnote><sup>d</sup> Université de Liège, Freshwater and Oceanic Science Unit of Research Laboratory of Ecology and Conservation of Amphibians, Quai Van Beneden, 22, 4020 Liège, Belgium</footnote>

<footnote><sup>e</sup> Parc national de Miguasha, 231 route Miguasha ouest, CP 183, Nouvelle, Québec GOC 2E0, Canada</footnote>

<footnote><sup>f</sup> Institut Universitaire de France, France</footnote>

<footnote>* Corresponding author at: Géosciences Environnement Toulouse (GET), Observatoire Midi Pyrénées (OMP), 14 avenue Edouard Belin, Toulouse, France.
*E-mail addresses:* vincent.luccisano@gmail.com (V. Luccisano), jeremy.martin@cnrs.fr (J.E. Martin), romain.amiot@univ-lyon1.fr (R. Amiot), philippe.telouk@ens-lyon.fr (P. Telouk), solive@naturalsciences.be (S. Olive), matton.olivier@sepaq.com (O. Matton), christophe.lecuyer@univ-lyon1.fr (C. Lécuyer).

https://doi.org/10.1016/j.chemgeo.2025.123024

Received 14 June 2025; Received in revised form 24 August 2025; Accepted 27 August 2025

Available online 29 August 2025

0009-2541/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Geological context of this study. **A.** Geological map of the Miguasha area. The red star specifies the location of the Miguasha group (modified from Cloutier et al., 2011). **B.** Stratigraphy of the Escuminac Formation. Red stars pinpoint the investigated beds. The Escuminac and Fleurant formations are encompassed within the Miguasha Group. **C.** Palaeogeographic world map during the Late Devonian. Red star indicates the location of Miguasha (Palaeomap modified from Day and Witzke, 2012). **Abbreviations:** Bon. Fm., Bonaventure Formation; Esc. Fm., Escuminac Formation; Fle. Fm., Fleurant Formation (modified from Cloutier et al., 2011). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

In this context, palaeoenvironmental reconstructions were carried
out through the analysis of stable and radiogenic isotope compositions
in vertebrate mineralised tissues. Several stable (δ¹⁸O, δ³⁴S and δ¹³C) as
well as radiogenic (⁸⁷Sr/⁸⁶Sr) isotope systems have been shown to be
linked to the salinity of aquatic environments and allow the discrimi-
nation of the marine environment from the freshwater one (e.g. Clem-
entz et al., 2006; Fischer et al., 2013; Goedert et al., 2020; Luccisano
et al., 2023). However, these isotopic systems have limitations in
achieving such discriminations especially in transitional environments.
Biological processes in which C and O are involved can affect their
isotopic values independently of the environmental context (Fry, 2006).
C₄ plants for instance have δ¹³C values similar to marine plants and can
lead to the misidentification of terrestrial organisms as marine ones
(Clementz et al., 2006). At low latitude, continental freshwaters can
have δ¹⁸O values close or similar to marine waters (Dansgaard, 1964)

and thus oxygen isotopes alone cannot be used to discriminate envi-
ronments. Sulfur is also limited as an environmental tracer because
continental δ³⁴S values can be quickly masked by marine ones even
under weak marine influences due to the 100–1000 times higher con-
centration of sulfur in seawater relative to freshwaters (Fry and Chum-
chal, 2011). Finally, ⁸⁷Sr/⁸⁶Sr ratios in vertebrate tissues depend on the
local foraging bedrock which can produce a marine isotope signature in
some continental contexts (Holt et al., 2021).

To overcome these limitations and carry out detailed environmental
reconstructions, combinations of several isotopic systems have been
performed, but the establishment of new environmental proxies is also
possible. Recently, lithium stable isotopes (δ⁷Li) have been proposed for
this purpose (Thibon et al., 2022). Li dissolved in the ocean has a resi-
dence time (~1.8 Ma) greater than the mixing time of the ocean (~1600
years) (Lécuyer, 2016). Consequently, Li is homogeneously distributed
in the water column (Misra and Froelich, 2012). Water $\delta^7$Li values are significantly different between present-day seawater and freshwater (Huh et al., 1998; Millot et al., 2004), and such a difference is assumed to have occurred in the fossil record. Recent analyses of $\delta^7$Li values from vertebrate mineralised tissues in a broad ecological and phylogenetic context (Thibon et al., 2022) have successfully distinguished fresh, brackish and seawater environments of several taxonomic groups by their statistically different value range.

However, the usefulness of this modern framework is still uncertain in the fossil record. In particular, the impacts of diagenetic processes on the Li content of vertebrate mineralised tissues are unknown. Bioapatite, in which lithium is incorporated as a substitution of calcium (Supová, 2015), is generally affected by pore fluids known to affect the isotope composition of traditional (e.g. see Zazzo et al., 2004 for oxygen and carbon) and non-traditional (see Weber et al., 2021; Dodat et al., 2023; Guiserix et al., 2024 for calcium, zinc, strontium and magnesium) stable isotope systems. As lithium has no nutritional role in vertebrate organisms, its isotope composition could be influenced by inorganic factors and therefore be buffered by fluids (Banner and Hanson, 1990; Wang and Cerling, 1994) as well as by sediments such as in marine biogenic carbonates (Dellinger et al., 2020; Murphy et al., 2022). The trophic effect has only been briefly investigated by Thibon et al. (2023) in the soft tissues of modern invertebrates and bony fishes. While no significant correlation was established, no study has investigated a possible trophic effect recorded in the vertebrate hard tissues. In these cases, the sediments are mainly affected by marine diagenetic fluids which increase the $\delta^7$Li values. However, their impacts on stable lithium isotope compositions of fossil vertebrates, as well as on their Li concentration, are unknown and may follow another trend.

It is therefore necessary to test the impact of diagenesis on the Li content of fossil vertebrates. This is particularly important because preliminary analyses of a few samples of fossil bone and enamel (Thibon et al., 2022) have highlighted potential differential responses between Li isotopes and concentrations against diagenesis. Although the $\delta^7$Li values of fossil bone and enamel appear consistent with those of modern samples, the Li concentration is generally higher. As marine $\delta^7$Li has changed over geological time (Misra and Froelich, 2012; Wang et al., 2023), higher fossil Li concentration than modern samples could result from alteration by diagenetic processes. Another hypothesis could be that fluctuations in seawater Li concentrations over time could make a sample appear diagenetic during a period when the concentration of Li in seawater was higher than it is today (see Delaney et al., 1989, for a comparison between Cenozoic and Palaeozoic Li/Ca ratios in the ocean).

This study aims to evaluate the preservation of $\delta^7$Li values recorded in fossil vertebrate bioapatite by investigating the potential impacts of diagenesis on these values, but also to reconstruct the $\delta^7$Li values of Late Devonian seawater. We provide the first comprehensive dataset of $\delta^7$Li values from Late Devonian vertebrate fossils, specifically from the Escuminac Formation in the *parc national de Miguasha*, Quebec, Canada. This site provides an ideal setting for the proposed investigations due to its weak to moderate diagenetic history, characterised by maximum temperatures of 430 °C and burial depths up to 1500 m (Hesse and Sawh, 1992; El Albani et al., 2002). The Late Devonian also encompasses the Frasnian-Famennian extinction, an event potentially linked to significant changes in marine chemistry (Racki, 2020). Moreover, the Escuminac Formation contains a diverse assemblage of vertebrate fossils, including tetrapodomorphs (Cloutier, 2013), which provides a unique opportunity to examine the environmental context of vertebrate terrestrialisation. By reporting $\delta^7$Li values in these fossil stem-tetrapod remains, we aim (1) to improve our understanding of the salinity conditions in which early tetrapodomorphs evolved and (2) to explore whether this proxy is valuable to reconstruct seawater chemistry during a crucial period of vertebrate evolution.

# 2. Material and methods

## 2.1. Geological setting

The Escuminac Formation is located on the southern coast of the Gaspé Peninsula, along the Restigouche River estuary, in Quebec, Canada (Fig. 1A; see El Albani et al., 2002; Hesse and Sawh, 1992; Cloutier et al., 2011). Its 119 m of fossiliferous terrigenous Middle Frasnian sediments are enclosed between the Fleurant Formation (Devonian conglomerate) at its base and the Bonaventure Formation (Carboniferous conglomerate) at its top. The Fleurant Formation and Escuminac Formation constitute the Miguasha Group. The sedimentology (Fig. 1B) has been widely studied (see El Albani et al., 2002 and references therein) and is mainly composed of shales, sandstones and siltstones, with occurrences of tidal rhythmites at the base of the formation. Facies are more dominated by shales at the top, while sandstone beds are interstratified throughout the Formation. Leduc and Belles-Isles (1989) divided the formation into 394 beds, numbered from 1 at the base to 394 at the top.

The sediments of the Escuminac Formation were deposited in the inter-tropical zone during the Late Devonian, on the southern margin of the Laurussia-Euramerica continent (red star, Fig. 1C). The environmental setting has been debated, and several hypotheses have been proposed (see Hesse and Sawh, 1992). The most recent consensus (e.g. El Albani et al., 2002; Cloutier et al., 2011) postulates that the Escuminac Formation represents a brackish-marine environment which could correspond to an estuary.

## 2.2. Sampling

All samples come from specimens housed in the collections of the musée d'histoire naturelle du parc national de Miguasha. Different taxonomic groups representing most of the vertebrate diversity of the Escuminac Formation (placoderms, acanthodians, actinopterygians, lungfishes and tetrapodomorphs) were collected and within each group, various mineralised tissues (teeth, bones, spines and scales) were sampled when available (see Tables S1 and S2). The specimens analysed come from the following six separated beds within the Escuminac Formation: 8, 12, 35, 242, 351 and 390 (Fig. 1B).

Biogenic apatite powders were sampled by the curators of the *parc national de Miguasha*, Quebec by using a spherical diamond tip drill bit or by crushing small fragments in an agate mill. Potential traces of sediments or diagenetic minerals were removed using thin needles under binocular microscope. No chemical treatment was performed prior to the elemental and isotope analyses, since such treatment could affect the Li isotope and elemental compositions in an unknown manner. This is because only a few analyses have been conducted on the Li content of fossil bioapatites (Thibon et al., 2022).

## 2.3. Analytical methods

### 2.3.1. Wet chemistry

The following protocol follows that of Thibon et al. (2022). Analytical preparations of vertebrate mineralised tissues were performed in a pressure-cleaned laboratory under a fume-hood to minimize procedural blanks, using distilled reagents and pre-cleaned containers. 5–10 mg of sample powder was first dissolved with a mixture of concentrated (1 N) HNO₃ and H₂O₂. After 48 h, concentrated (1 N) HCl was added to continue digestion in inverted aqua regia and heated for 24 h before evaporation. The extraction and purification of lithium were carried out according to a routine procedure (James and Palmer, 2000; Vigier et al., 2009), recently adapted to biological materials and tissues (Thibon et al., 2021a, 2021b, 2022). Evaporated samples were diluted in 0.5 mL of 1 N HCl and loaded onto AG 50-X12 resin (200–400 mesh) in 8.5 cm high Teflon columns. Lithium was eluted with 1 N HCl. A second purification step, following the same procedure as the first Li extraction,
<caption>**Table 1**</caption>

Bioapatite δ<sup>7</sup>Li, Li concentration and Ca/P ratio values of the mineralised tissues of the Miguasha vertebrates (see the Table S1 for the detailed dataset). **Abbreviation:** n, number of samples; min, minimum; max, maximum; sd, standard error.

<table><thead><tr><th rowspan="2">Bed</th><th rowspan="2"></th><th colspan="5">δ<sup>7</sup>Li (%<sub>o</sub> vs L-SVEC)</th><th colspan="5">Li (µg/g)</th><th colspan="5">Ca/P</th></tr><tr><th>n</th><th>min</th><th>mean</th><th>max</th><th>sd</th><th>n</th><th>min</th><th>mean</th><th>max</th><th>sd</th><th>n</th><th>min</th><th>mean</th><th>max</th><th>sd</th></tr></thead><tbody><tr><td rowspan="6">All data</td><td>390</td><td>4</td><td>10,31</td><td>12,37</td><td>14,43</td><td>2,0</td><td>4</td><td>0,72</td><td>0,77</td><td>0,82</td><td>0,1</td><td>4</td><td>0,57</td><td>1,10</td><td>1,52</td><td>0,4</td></tr><tr><td>351</td><td>7</td><td>0,61</td><td>7,88</td><td>19,17</td><td>7,8</td><td>8</td><td>0,83</td><td>3,48</td><td>11,55</td><td>3,5</td><td>8</td><td>0,87</td><td>1,93</td><td>4,28</td><td>1,1</td></tr><tr><td>242</td><td>19</td><td>5,02</td><td>13,73</td><td>21,70</td><td>6,3</td><td>19</td><td>0,87</td><td>2,17</td><td>3,83</td><td>1,0</td><td>19</td><td>0,78</td><td>1,66</td><td>3,38</td><td>0,9</td></tr><tr><td>35</td><td>5</td><td>3,50</td><td>7,07</td><td>15,75</td><td>4,9</td><td>5</td><td>1,29</td><td>2,49</td><td>3,62</td><td>0,8</td><td>5</td><td>0,95</td><td>1,81</td><td>2,77</td><td>0,9</td></tr><tr><td>12</td><td>11</td><td>0,04</td><td>9,48</td><td>20,41</td><td>7,0</td><td>11</td><td>1,68</td><td>5,96</td><td>18,75</td><td>5,4</td><td>11</td><td>2,35</td><td>4,27</td><td>9,99</td><td>2,3</td></tr><tr><td>8</td><td>11</td><td>-0,50</td><td>7,40</td><td>23,00</td><td>6,4</td><td>12</td><td>1,44</td><td>3,62</td><td>8,90</td><td>2,5</td><td>12</td><td>1,36</td><td>3,67</td><td>7,73</td><td>2,0</td></tr><tr><td rowspan="3">Unaltered</td><td>351</td><td>4</td><td>5,67</td><td>12,53</td><td>19,17</td><td>7,45</td><td>5</td><td>0,83</td><td>1,84</td><td>3,4</td><td>1,04</td><td>5</td><td>0,87</td><td>1,21</td><td>1,44</td><td>0,22</td></tr><tr><td>12</td><td>7</td><td>7,26</td><td>13,70</td><td>20,41</td><td>4,89</td><td>7</td><td>1,68</td><td>2,85</td><td>3,80</td><td>0,90</td><td>7</td><td>2,35</td><td>2,97</td><td>3,54</td><td>0,45</td></tr><tr><td>8</td><td>8</td><td>3,37</td><td>9,48</td><td>23,00</td><td>6,24</td><td>8</td><td>1,44</td><td>2,31</td><td>2,86</td><td>0,46</td><td>8</td><td>1,36</td><td>2,57</td><td>4,13</td><td>s</td></tr><tr><td rowspan="3">Altered</td><td>351</td><td>3</td><td>0,61</td><td>1,77</td><td>3,23</td><td>1,34</td><td>3</td><td>2,74</td><td>6,21</td><td>11,55</td><td>4,69</td><td>3</td><td>2,47</td><td>3,15</td><td>4,28</td><td>0,99</td></tr><tr><td>12</td><td>4</td><td>-0,03</td><td>1,66</td><td>3,80</td><td>1,71</td><td>4</td><td>4,91</td><td>11,54</td><td>18,75</td><td>4,99</td><td>4</td><td>4,48</td><td>6,55</td><td>9,99</td><td>2,59</td></tr><tr><td>8</td><td>3</td><td>-0,50</td><td>1,84</td><td>4,57</td><td>2,56</td><td>4</td><td>2,41</td><td>6,23</td><td>8,90</td><td>2,86</td><td>4</td><td>4,01</td><td>5,89</td><td>7,73</td><td>1,54</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** δ<sup>7</sup>Li values plotted against the Li concentration (A) and the Ca/P ratio (B) as well as the Li concentration vs the Ca/P ratio (C) of the hard tissues of the Miguasha aquatic vertebrates. Linear regressions correspond to the diagenetically altered samples marked with black circles. The threshold ratio for separating diagenetic from well-preserved samples is 4 for Ca/P ratios and 4 µg/g for Li concentrations.</figcaption></figure>

ensured complete separation of Li. Four procedure blanks in total and three replicates of the cow bone meal reference material SRM-1486 were included. The feasibility of the procedure was verified and published in Thibon et al. (2021a, 2021b) and applied to modern and fossil apatite samples in Thibon et al. (2022).

### 2.3.2. Elemental analyses

Concentrations of major and trace elements (see Table S1 for the complete list) were measured with an inductively coupled plasma atomic emission spectrometer (ICP-AES, iCAP Pro Series, Thermo Scientific) and using an inductively coupled plasma mass spectrometer (ICPMS iCap-Q, Thermo Scientific), respectively. Indium (In) and scandium (Sc) have been used as internal standards for minor and major elements, respectively. Blank samples (pure 0.5 N HNO<sub>3</sub>) were also introduced.

### 2.3.3. δ<sup>7</sup>Li measurements

For the <sup>7</sup>Li/<sup>6</sup>Li delta notation, the reference was NIST L-SVEC-1 SRM 8545 and the 'δ<sup>7</sup>Li' notation means delta notation versus L-SVEC for mineralised tissues. Li isotope analyses were carried out using a Thermo-Fisher Neptune Plus multi-collector inductively coupled plasma mass spectrometer following the procedure developed for low concentrations of Li in biological materials (Balter and Vigier, 2014; Bastian et al., 2018; Thibon et al., 2021a, 2021b, 2022). Samples and standards were analysed under dry plasma conditions with an Aridus 2 (Cetac) desolvator system in low resolution mode. The standard bracketing method (using L-SVEC reference material with a Li concentration of 4 µg/g) was used to perform instrumental mass bias correction. The four total Li procedural blanks, which included chemical and analytical blanks, ranged between 10 and 20 pg. For 10 mg of material with a Li concentration of 1 µg/g, the maximum blank contribution was negligible (0.4 %). On average, internal reproducibility was ±0.04 %. Precision (external reproducibility) was estimated over 1 to 3 repeated measurements and ranged from 0.1 to 2.5 ‰ (2SD). Bone meal reference material (SRM-1486) was measured at -5.8 ± 0.6 ‰ (2SD, n = 3), which constitute the base for future comparison. Li concentrations were also estimated using voltage measurements during isotopic measurements (using normalization to a calibrated L-SVEC solution).

## 3. Results

Isotopic and elemental data are presented in the Table 1 and Figs. 2-7 (see the Table S1 for the whole and detailed dataset that includes diagenetically altered and well-preserved samples). δ<sup>7</sup>Li values range from -0.5 ‰ to 23.0 ‰. The δ<sup>7</sup>Li values are heterogeneous within each bed, with 2SD from 2.0 to 7.8 ‰ (Table 1).

For the whole dataset, while average δ<sup>7</sup>Li values (Figs. 2A-B, Table 1) are similar in beds 8 (7.4 ± 6.40 ‰), 12 (9.5 ± 7.0 ‰), 35 (7.1 ± 4.9 ‰)
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Rare Earth Elements content within the Miguasha fossil bioapatites. **A.** NASC normalized REE (µg·g⁻¹/NASC) profiles for the samples of Bed 12 (see Fig. 1). **B.** *La*/Gd$_N$ plotted against *La/Yb$_N$* values (normalized to NASC; Haskin and Frey, 1966; Gromet et al., 1984) for various fossil and extant bioapatite samples, as well as modern waters (Reynard and Balter, 2014, and references therein). Mid-Cretaceous faunas are from Hassler et al. (2018), Tiupampa fauna from Pochat-Cotilloux et al. (2023), Devonian conodonts from Grandjean and Albarède (1989) and Grandjean-Lécuyer et al. (1993), Triassis and Eocene fishes from Grandjean and Albarède (1989), Quaternary fishes from Elderfield and Pagett (1986), Quaternary oceanic waters from Zhang and Nozaki (1996), and modern estuarine and coastal waters from Elderfield et al. (1990). δ⁷Li values (C) and Li concentrations (D) for this study vs. sum of REEs.</figcaption></figure>

and 351 (7.9 ± 7.8 ‰), they are higher in bed 242 (13.7 ± 6.3 ‰) and 390 (12.4 ± 2.0 ‰). Li concentrations (Fig. 2A, C, Table 1) range from 0.7 µg/g to 18.8 µg/g. If beds 8 (3.6 ± 2.5 µg/g), 35 (2.5 ± 0.8 µg/g), 242 (2.2 ± 1.0 µg/g) and 351 (3.5 ± 3.5 µg/g) show similar average values, the mean Li concentrations measured in bed 12 are higher (6.0 ± 5.4 µg/g) while lower in bed 390 (0.8 ± 0.1 µg/g). It must also be noted that for the whole dataset (before screening for diagenetically altered samples), apparent differences between beds and taxa in terms of Li isotopes and concentrations are not statistically supported (Supplementary material Figs. S2–3).

Ca/P ratio (Figs. 2B-C, Table 1) for the entire dataset is 2.6 ± 1.9 (Table S1). Between beds, the beds 242, 35, and 351 have similar average Ca/P ratios of 1.7 ± 0.9, 1.81 ± 0.9, and 1.9 ± 1.1, respectively. In contrast, beds 12 and 8 have higher average Ca/P ratios, of 4.3 ± 2.3 and 3.7 ± 2.0, respectively, and bed 390 shows a lower average ratio of 1.1 ± 0.4. It must be noted that for the whole dataset (before screening for diagenetically altered samples), apparent differences between beds and taxa are mostly not statistically supported except for beds 8 and 12 (Supplementary material Fig. S2–3).

Following the nomenclature of NASC normalized Rare Earth Element (REE) profile shapes by Reynard et al. (1999) and Reynard and Balter (2014), Miguasha specimens show a significant positive anomaly of middle rare-earth elements (MREEs) relative to light and heavy ones (LREEs and HREEs) (Fig. 3A and Supplementary material Fig. S4), but not as high as in bell-shaped profiles (e.g. Reynard et al., 1999). Most specimens, regardless of their position in the stratigraphy, have a similar REE profile shape (see Supplementary Material Fig. S4). However, specimens from bed 12 (Fig. 3A) show a distinct profile with a lower MREE than the other beds. When plotted against data spanning a wide range of time periods and taxa, the *La/Yb*<sub>N</sub> vs *La/Gd*<sub>N</sub> ratios (normalized to NASC; Haskin and Frey, 1966; Gromet et al., 1984) of Miguasha are

aligned with Devonian conodonts (Fig. 3B) (Reynard and Balter, 2014).

Globally, there is no relationship between the Li isotopic and elemental content and the sum of REE (Figs. 3C-D) and some elemental concentrations (Supplemental material Fig. S6). When plotted against the Li concentration and the Ca/P ratio (Fig. 2 and Supplemental material Fig. S7), the δ⁷Li values have no correlation as concerns Li concentration inferior to 4 µg/g and Ca/P ratio inferior to 4 (Supplemental material Table S4). Conversely, when these two parameters exceed these values, correlations with the δ⁷Li values appear (Supplemental material Table S5). When plotted against some elemental ratios (Fig. 4), there is no correlation with the Li concentration, whereas moderate correlation (Pearson coefficient *r* around 0.7, *p*-value<0.01) is found with the Li/Ca, Al/Ca, Rb/Sr and Rb/Ca ratios of the whole data set (Fig. 5). There is no correlation between the δ⁷Li values and the following elemental ratios: Mn/Ca, Fe/Ca, Mn/Sr, Sr/Ca, Mn/Ca (Fig. 6). On the contrary, moderate correlations between the δ⁷Li values and other elemental ratios (Li/Ca, Rb/Ca, Al/Ca and Rb/Sr in Fig. 7) are found (−0.6 < Pearson coefficient *r* < −0.5; *p*-value <1.816*10⁻⁵).

Mann-Whitney-Wilcoxon test was performed on the δ⁷Li values of the whole dataset (before screening for diagenetically altered samples) in order to highlight potential differences between taxa of different size (Fig. 10), and between different analysed tissues within the same taxa (Fig. 11). There is no statistical difference between taxa or animal size and between tissues within the same taxa.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Diagenetic control on the Li concentration of the hard tissues of the Miguasha aquatic vertebrates. Li concentration plotted against the Mg/Ca (A), Fe/Ca (B), Mn/Sr (C), Sr/Ca (D) and Mn/Ca (E) ratios. Linear regressions are calculated for the entire dataset.</figcaption></figure>

# 4. Discussion

## 4.1. Diagenetic control on Li content and isotopic composition of Devonian bioapatite

### 4.1.1. Early diagenesis impacted the Li content of the Miguasha vertebrates

Miguasha specimens reveal a wide range in REE concentrations, indicating variable degrees of diagenetic uptake following burial (Fig. 3 and Supplementary material Fig. S4-S6) (Reynard et al., 1999; Kocsis et al., 2009; Hinz and Kohn, 2010; Suarez and Kohn, 2020). The comparable pattern of the REE profiles between specimens from Beds 35, 351, 242, and 390 and several specimens from Bed 8 suggest that they

are autochthonous and underwent similar diagenetic processes (Trueman, 2013). Conversely, those from Bed 12 (Fig. 3A) and some from Bed 8 may depict different REE profiles, which means they have undergone distinct burial and diagenetic processes. This discrepancy may be attributed to the non-contemporaneity of the different beds. All REE patterns from Miguasha specimens are distinct from the bell-shaped profile (high MREE enrichment) illustrating bioapatite recrystallisation (Reynard et al., 1999). The moderate enrichment of MREEs provides evidence of adsorption mechanisms from sediments deposited in estuarine or coastal environments (Lécuyer et al., 2003). Regarding the La/Yb<sub>N</sub> and La/Gd<sub>N</sub> ratios (Fig. 3B), the Miguasha sample values are aligned with those of the Devonian conodonts which experienced low-grade
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Terrigenous inputs control on the Li concentration of the hard tissues of the Miguasha aquatic vertebrates. Li concentration plotted against the Li/Ca (A), Rb/Ca (B), Al/Ca (C) and Rb/Sr (D) ratios. Linear regressions are calculated for the entire dataset.</figcaption></figure>

metamorphism (Reynard and Balter, 2014). Yet, their REE profiles being different from the bell-shaped ones of Devonian conodonts (Fig. 3B) (Reynard et al., 1999), low-grade metamorphism processes may have less affected the Miguasha samples. However, following other works (Reynard et al., 1999; Trueman et al., 2003), extensive diagenesis is characterised by La/Yb<sub>N</sub> ratios of less than 1. This case only concerns a few Miguasha specimens (7/59; Fig. 3B). This suggests that potential diagenetic alteration could have occurred but at a low degree, such as during early diagenesis.

When plotted against several elemental concentrations and ratios (Fig. 4) traditionally used as diagenesis proxies within sediments (e.g. Adiatma et al., 2024), the Li concentration of the Miguasha samples exhibit recurrent patterns. Firstly, there is no correlation between the Li concentration and other elemental concentrations and their ratios for most of the dataset (Supplementary material S4–5). This indicates that the early diagenesis has impacted a few portions of the analysed samples. However, some points, which are always the same independently of the considered proxy and coming from the beds 8, 12 and 351 (marked with a black circle in the Fig. 4), show that the increase in Li concentration occurs with the increase of some diagenetic proxies (e.g. Mg/Ca, Fe/Ca, Mn/Ca, Sr/Ca, Mn/Sr, Fe, Pb, S, Sum of REE; Figs. 4, S4, S6). For example, Li concentration values of these specimens appear to correlate with Sr/Ca and Mn/Ca ratios, Fe and Pb concentrations (Figs. 4, S6), but the small number of samples precludes statistical testing for most of the proxies (Supplementary material Table S4–5). Iron and lead are preferentially incorporated after the death of an

organism. Such assumed correlations therefore suggest that the high Li concentrations result from secondary incorporation subsequent to the death of the organisms. The stratigraphic sequence (Hesse and Sawh, 1992; El Albani et al., 2002; Cloutier et al., 2011) indicates the presence of fibrous calcite (Fig. 1C) associated with pyrite (FeS<sub>2</sub>) in beds 8, 12 and 351. Hence, the presence of pyrite in these beds may account for the observed correlation between Li concentrations and Fe and Pb concentrations, which can be attributed to Fe substitution. It is also possible that the secondary incorporation of lithium within these samples originates from the same fluids from which the carbonate concretions precipitated in the Escuminac Formation. These correlations, which cannot be statistically tested due to the small number of data, support that the Li content of these particular samples come from a diagenetic reservoir which is different from biological signatures.

The Li concentrations measured in modern vertebrates from a diverse phylogenetic context, i.e. marine, freshwater and terrestrial, (Thibon et al., 2022), are lower than 4 µg/g. The majority of Devonian vertebrates from Miguasha exhibits similar concentrations. However, samples from beds 8, 12 and 351, which are characterised by an enrichment in diagenetic elements (Fig. 4) have high Li concentrations, up to 18.8 µg/g. Assuming no difference in seawater Li concentration between the upper Devonian and the present day, this confirms that their Li content derives from a diagenetic origin.

Previous studies have evaluated the preservation of the elemental composition of fossil bioapatites from different geological periods by using the Ca/P ratios (e.g. Holmden et al., 1996; Matton et al., 2012;
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Diagenetic control on the δ<sup>7</sup>Li values of the hard tissues of the Miguasha aquatic vertebrates. Li concentration plotted against the Mg/Ca (A), Fe/Ca (B), Mn/Sr (C), Sr/Ca (D) and Mn/Ca (E) ratios.</figcaption></figure>

Hassler et al., 2018; Martin et al., 2022). In modern vertebrates, the Ca/P ratio follows a stoichiometric relationship, with typical biological value around 2 (see Weatherell and Robinson, 1973 for human bone). It is interesting to observe that, when plotted against biological Li concentrations, the Ca/P ratio is congruent with biological values too: the Ca/P ratios of specimens from beds 35, 242 and 351, 1.8 ± 0.9, 1.66 ± 0.9 and 1.9 ± 1.1 respectively, (Fig. 2B, Table 1) are similar to those observed in the enamel and dentine of Miguasha fish (2.00 and 2.22, respectively; Matton et al., 2012), Palaeozoic conodonts (2.12 to 2.37; Holmden et al., 1996), and modern human enamel and dentine (2.05 and 2.17, respectively; Weatherell and Robinson, 1973). Conversely, when the Li concentration exceeds 4 µg/g, the Ca/P ratio increases out of the values recorded in modern vertebrates (Pearson coefficient r = 0.828, p-value <0.005; Fig. 2B, Table S4). The high Ca/P ratio mean

values observed in beds 8 (12, 3.7 ± 2.0) and 12 (4.3 ± 2.3) are distinct from those observed in modern hard phosphatic tissues (Fig. 2B, Table 1). In the latter case, it is possible to postulate that the corresponding samples, the same ones from the beds 8, 12 and 351 in which there is an enrichment in heavy elements, have been affected by diagenesis, modifying the relative abundance between Ca and P. These findings indicate poor preservation of bioapatite, suggesting that the high concentrations may result from secondary incorporation of lithium via diagenetic fluids, e.g. hydrothermal ones, or the addition of a secondary carbonate-rich phase. The rise in the Li concentration in diagenetic fluids at low and high temperatures, as well in vent and pore fluids housed in deep hydrothermal systems, supports this hypothesis (Scholz et al., 2010).

To summarise, the use of several proxies to trace the diagenetic
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Terrigenous input control on the δ<sup>7</sup>Li values of the hard tissues of the Miguasha aquatic vertebrates. Li concentration plotted against the Li/Ca (A), Rb/Ca (B), Al/Ca (C) and Rb/Sr (D) ratios. Pearson coefficient *r* is calculated for the entire dataset.</figcaption></figure>

influence on the Li provenance within the Miguasha bioapatite
 elemental concentration and ratios of heavy elements, Ca/P ratio, REE)
allows us to conclude that Li in most of the samples from beds 8, 12 and
351 comes from a diagenetic fluid. Therefore, the influence of this
diagenetic Li on the δ<sup>7</sup>Li values, i.e. the assessment of the diagenetic
fractionation, must be evaluated in order to select the samples that could
be used for palaeoecological and palaeoenvironmental interpretations.

4.1.2. Determination of diagenetic versus biogenic endmember on fossil vertebrate δ<sup>7</sup>Li values

A stable isotope mixing model could be used to estimate the contri-
bution percentages of different sources to the stable isotope ratio within
a sample. Here, the identified sources that could contribute to the iso-
topic signature of Miguasha samples are seawater, freshwater and
diagenetic fluids. Seawater and freshwater are used as the two sources
for the estuary because continental-derived waters contribute signifi-
cantly to the isotope budget of estuarine waters (e.g. Berezovski et al.,
2025). Diagenetic fluids are characterised by a δ<sup>7</sup>Li value of almost 0 ‰
(Scholz et al., 2010). The maximum estimate of the Late Devonian
seawater δ<sup>7</sup>Li value is 25 ‰ (20 ‰ ± 5 ‰ following Kalderon-Asael
et al., 2021) and this value could be considered homogeneous on a large
scale due to the long residence time of dissolved Li in the water column
(~1.8 Ma) relative to the ocean mixing time of 1600 years (Lécuyer,
2016). In the long term, the seawater δ<sup>7</sup>Li is mainly derived from the
freshwater inputs in the oceans whose Li content comes from the clay
formation and the weathering regime (e.g. Dellinger et al., 2015).

Miguasha represents a geological record, from 59.5 ka to 2.5 Ma (Cloutier et al., 2011), thus suggesting that the seawater δ<sup>7</sup>Li composition could be considered constant or could has changed over this period. Adiatma et al. (2024) reconstructed seawater δ<sup>7</sup>Li values from the Early to the Late Ordovician and showed variations of 4 ‰. However, the evolution in the short term, i.e. 1 Ma or less, showed little variation (<2 ‰) of the marine δ<sup>7</sup>Li value. The same tendency in the short-term variation of seawater δ<sup>7</sup>Li is observed during the Carboniferous (Wang et al., 2021). The Miguasha sedimentary deposits are thought to reflect an estuary throughout their history (Cloutier et al., 2011). The beds, which represent palaeoenvironments in which freshwater and marine vertebrates were prevalent, exhibit similar mean δ<sup>7</sup>Li values and variability before and after the removal of diagenetic data (Table 1). As the environment was permanently an estuary, the similar mean δ<sup>7</sup>Li values for each bed suggest that both continental and marine sources within the estuary exhibited relatively consistent δ<sup>7</sup>Li values. Consequently, despite its long duration, it can postulate that the seawater δ<sup>7</sup>Li value was constant during the depositional interval represented in Miguasha. As the Miguasha environment is considered to be an estuary (e.g. El Albani et al., 2002; Cloutier et al., 2011), freshwater was an important contributor. Today the mean freshwater δ<sup>7</sup>Li value is 23 ‰ (Huh et al., 1998) for a seawater value of 31.0 ± 0.5 ‰ (Millot et al., 2004), i.e. an offset of 8 ‰, but the freshwater value could vary from 1 ‰ to 44 ‰ (Huh et al., 1998). During the Permian, the seawater δ<sup>7</sup>Li value could vary between 5 ‰ and 20 ‰ with a postulated freshwater δ<sup>7</sup>Li value ranging between 3 ‰ and 15 ‰ (Cao et al., 2022). This suggests a
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Sources contributions on the δ⁷Li values of the hard tissues of the Miguasha aquatic vertebrates for diagenetically altered (top, *n* = 12) and unaltered (bottom, *n* = 49) samples.</figcaption></figure>

seawater-freshwater offset between 2 ‰ and 12 ‰, encompassing the present-day offset of 8 ‰. The geodynamic and climatic contexts between the Late Devonian and the Permian were different, so we can assume that the mean Late Devonian and Permian freshwater and seawater δ⁷Li values were different too. As there is no consensus on the seawater δ⁷Li values for the Late Devonian period (ranging from 11.8 to 25.0 ‰, as reported by Kalderon-Asael et al., 2021 and Wang et al., 2023), and in the absence of a dedicated study, it is more parsimonious to hypothesise a similar offset to that observed today during the Late Devonian period in the current model. Another argument in favour of an assumed modern-like seawater-freshwater offset in the Late Devonian is the similar climate setting, i.e. an icehouse Earth, contrary to the Permian period during which the Earth transitioned to a greenhouse model (e.g. Griffis et al., 2023). Consequently, our analysis postulates a mean freshwater δ⁷Li value of 17 ‰ estimated from a seawater δ7Li value of 25 ‰.

Consequently, in order to quantify the diagenetic Li isotopic contamination of some fossil bioapatites, a stable isotope mixing model was performed within a Bayesian framework using the R package “simmr” (Govan et al., 2023). The selected sources are diagenetic fluids (δ⁷Li = 0 ‰, [Li] = 0.347 µg/g; Scholz et al., 2010), freshwater (δ⁷Li = 17 ‰, [Li] = 0.01105 µg/g; Huh et al., 1998) and seawater (δ⁷Li = 25 ‰, [Li] = 0.183 µg/g; Millot et al., 2004; Kalderon-Asael et al., 2021). For the diagenetically altered samples coming from the beds, 8, 12 and 351, the mixing model indicates that putative diagenetic fluids contribute to 65.9 ± 15.3 % of the Li stable isotopes, whereas seawater and freshwater contribute to only 8.2 ± 3.9 % and 25.3 ± 17.2 % respectively (Fig. 8A). This confirms that the diagenetically-derived Li in these samples is also characterised by an isotopic fractionation of diagenetic origin. Therefore, these specimens cannot be interpreted in terms of palaeoecological or palaeoenvironmental aspects.

On the contrary, the same analysis performed on the samples not identified as diagenetically altered by the elemental proxies, shows that the Li stable isotope compositions mostly (Fig. 8B) originate from a mixture of seawater and freshwater, i.e. 38.9 % ± 15.2 % and 37.9 % ± 23.5 % respectively. As the Miguasha sediments are expected to have undergone moderate diagenesis (Hesse and Sawh, 1992; El Albani et al., 2002), it is expected that a small proportion of the fossil bioapatite samples could be influenced by diagenetic fluids, i.e. a contribution of 23.2 ± 8.7 % (Fig. 8). However, the diagenetic contribution is clearly different between the two sub-samples established by diagenetic proxies. Consequently, isotopic fractionation could occur but to a lesser extent in the unaltered samples than in the diagenetically-altered ones. Comparisons between the δ⁷Li values and elemental concentrations and ratios could help to assess whether fractionation is significant in the assumed unaltered sub-samples.

Scholz et al. (2010) presented a compilation of δ⁷Li values for low- and high-temperature diagenetic hydrothermal fluids, as well as vent and pore fluids hosted in hydrothermal systems. All are characterised by an initial surface δ⁷Li value close to 0 ‰. This study also modelled the evolution of Li concentration and isotope ratios with increasing fluid burial. Irrespective of the model employed (transport-reaction modeling), the fluids depict positive δ⁷Li values in the superficial sediments, subsequently exhibiting a decline to around 0 ‰ at depths below 1000 m. In parallel, the Li concentration in the fluids rises. Consequently, diagenesis is supposed to alter the original Li stable isotope compositions by decreasing the values to 0 ‰.

When plotted against several diagenetic proxies (Fig. 6), the total Miguasha δ⁷Li values do not correlate with them. This indicates that the majority of the δ⁷Li values are not of diagenetic origin. More specifically, calculation of the Pearson coefficient r shows no correlation between the δ⁷Li values and the Mg/Ca, Mn/Ca, Mn/Sr, Sr/Ca ratios for the samples interpreted as unaltered (−0.215 < r < 0.214 and 0.14 < p-value < 0.95; Supplementary Material Table S4). These specimens exhibit a wide range of δ⁷Li from 3.4 ‰ to 23.0 ‰ which is consistent with the observed range in modern aquatic vertebrates. On the contrary, there are some more or less statistically supported correlations with the Mg/Ca (r = −0.59, p-value = 0.053) and Fe/Ca (r = −0.40, p-value = 0.22 ratios (Supplementary Material Table S5) for the diagenetically-altered specimens. The latter are identified as the same specimens coming from beds 8, 12 and 351, and exhibit low δ⁷Li values from −0.5 ‰ to 4.6 ‰, with a tendency to average 0 ‰ regardless of the correlation considered. Thus, when the Li is of diagenetic origin, the δ⁷Li values are close to those of diagenetic fluids, implying significant fractionation during interactions between bioapatite and aqueous fluids of crustal origin.

Regarding the Ca/P ratio, the Miguasha samples can be divided into two categories when their δ⁷Li values are reported against their Li concentrations and their Ca/P ratios (Fig. 2 and Supplementary material Fig. S7). The first group gathers specimens from the beds 8, 12 and 351 with Li concentration values that are above those documented for modern aquatic vertebrates (Thibon et al., 2022), with the Fe and Pb concentrations (Fig. S6) and Ca/P ratios negatively correlated with δ⁷Li
values (Fig. 2C). Ca/P ratios exceeding 4 indicate a significant alteration of the phosphatic tissues, characterised by a loss of phosphorus due to apatite dissolution. This process, which could be the observed filling of bioapatite cavities by diagenetic calcites in some Miguasha specimens, is likely accompanied by the addition of a Ca- and Li-rich phase, namely calcium carbonate (e.g. Marriott et al. (2004a, 2004b) for Li/Ca ratio in organic calcium carbonate), which could also explain the high Li concentration of the concerned samples (Fig. 2C). Therefore, the low δ⁷Li values of these Li- and Ca-concentrated vertebrate specimens may reflect a combination of the original bioapatite signature with post-depositional diagenetic fluids. In this case, the diagenetically-derived Li content of these samples also implies a diagenetically-derived isotopic signature, different from the biological one assumed for aquatic vertebrates (Thibon et al., 2022).

The second group gathers the majority of Miguasha vertebrate samples. It includes samples with low Li concentration values (< 4 µg/g) and Ca/P ratios (< 4) similar to those observed in modern vertebrates (Fig. 2) (Thibon et al., 2022). Focusing on these samples shows that there is no clear correlation between the δ⁷Li values and the Ca/P ratio (Supplementary material S7). This reflects the preserved stoichiometry of the hard tissues of these aquatic vertebrates. Therefore, we conclude that the considerable range of δ⁷Li values (+3.4 ‰ to +23.0 ‰) is indicative of diverse mixtures between a high δ⁷Li seawater endmember and a low δ⁷Li freshwater (river input) endmember. Our understanding of the lithium geochemical cycle suggests that the continental crust is ⁷Li-depleted in comparison to the oceanic one (e.g. Hoefs, 2009).

In accordance with the maximum estimates of Hesse and Sawh (1992) and El Albani et al. (2002), the sediments of the Escuminac Formation underwent diagenesis at an average temperature of 430 °C and were buried to a depth of approximately 1500 m. Therefore, the reported evolution of δ⁷Li values and Li concentration of diagenetic fluids from hydrothermal systems are consistent with the δ⁷Li-Li concentration correlation observed in the Miguasha specimens (Pearson coefficient r = -0.687, p-value<0.03; Table S4). The minimum measured δ⁷Li values (-0.5 ‰) are thus consistent with the interaction between bioapatite and crustal-derived aqueous fluids (Supplementary material in Kalderon-Asael et al., 2021). Consequently, samples with low δ⁷Li values, high Li concentrations and Ca/P ratios and enrichment in heavy elements correspond to diagenetically altered samples (highlighted with black circles in Figs. 2, 6). They are thus unsuitable to reconstruct the ecology of Devonian vertebrates or the lithium isotope composition of contemporaneous seawater. This represents 12 samples in this study, which have been eventually excluded from our palaeoenvironmental interpretations. The preservation of stable isotope compositions in fossil bioapatites is assessed through the correlation of isotopic and elemental diagenetic proxies (e.g., Pochat-Cotilloux et al., 2023; Guiserix et al., 2024; Cowen et al., 2025). However, despite the theoretical impact of diagenetic fluids on the δ⁷Li values of samples identified as unaltered (nearly 25 ‰, Fig. 8), the absence of correlation between δ⁷Li values and Li concentration (Fig. 2A), as well as between δ⁷Li values and many elemental ratios (Figs. 2B, 3C, 6), indicates that this impact indicates no significantly altered isotopic compositions.

## 4.2. Controlling factors on the δ⁷Li values in fossil bioapatite

### 4.2.1. Biological control

Investigations of δ⁷Li values in invertebrates and vertebrates have shown that isotopic fractionation could be caused by biological controls in addition to environmental ones. Analyses of several inner organs in aquatic invertebrates and vertebrates (Thibon et al., 2023) and terrestrial mammals (Balter and Vigier, 2014) show that their δ⁷Li values are not similar between organs of the same individual. Isotopic fractionation is thus tissue-dependent in these cases; however, no study has evaluated this process on vertebrate hard tissues. Among the Miguasha fauna, several tissues (tooth, bone, scale, spine) from the same taxa were analysed. Although the sample size is small, there is no statistical

difference between the δ⁷Li values from different tissues within the same taxon (Fig. 11). Therefore, it appears that there is no significant isotopic fractionation between hard tissues, in contrast to that observed in soft tissues.

Thibon et al. (2022) highlighted a positive relationship between ‘fish’ δ⁷Li values and the body mass, which is believed to correlate with gill surface area thus filtering important volumes of water, resulting in a high lithium uptake and a low degree of isotopic fractionation. δ⁷Li values of large marine ‘fish’ is therefore close to that of seawater (Thibon et al., 2022). At Miguasha, the range of fossil vertebrate δ⁷Li values does not appear to be fully explained by metabolic factors. Most sampled taxa, including placoderms, acanthodians, actinopterygians, and dipnoans, exhibit an adult body length range of 30 to 86 cm (Cloutier, 2013; Cloutier et al., 2009; Chevrinais et al., 2017), and δ⁷Li values from +0.6 ‰ to +21.7 ‰. Moreover, the larger tetrapodomorphs Eusthenopteron and Elpistostege have been documented to reach lengths of up to 1.5 m (Cloutier et al., 2009, 2020), with δ⁷Li values from +3.5 ‰ to +23.0 ‰. Despite its smaller size of ~60 cm long, the placoderm Bothriolepis exhibits comparable δ⁷Li values to those observed in tetrapodomorphs. The absence of statistical difference between body size and isotopic values (Fig. 10; Supplementary material Fig. S4) suggest that the observed variations in δ⁷Li values from Miguasha fossils are more likely the result of differences in palaeoenvironmental and palaeoecological conditions than to purely physiological factors.

### 4.2.2. Environmental control

When plotted against several elemental ratios (Fig. 5) used as detrital input proxies (Adiatma et al., 2024), the Li concentration of well-preserved Miguasha vertebrate hard tissues does not show any well-supported statistical correlation (0.26 < r < 0.42, 0.002 < p-value <0.06; Table S4). However, correlations are observed when diagenetic samples are included (Fig. 5), highlighting their direct contribution to these correlations. This means that there is no significant environmental control such as terrigenous input on the amount of Li within the vertebrate hard tissues. The Miguasha palaeo-estuary could be considered as a mixture of marine and freshwater, with the chemical composition of the latter coming directly from the runoff of continental waters. The palaeo-estuary is thus the destination of these continental freshwaters, where the Li concentration deriving from these two reservoirs is homogenised.

Statistically supported correlations are observed between the δ⁷Li values and the detrital input proxies (Supplementary Material Table S4–5). This implies that the magnitude of changes in Li isotope compositions is driven by the amount of terrigenous input. This is consistent with previous analyses on vertebrate hard tissues from a wide ecological and phylogenetic context, where the environment is the key factor in understanding the δ⁷Li variations (Thibon et al., 2022). As Miguasha sedimentary successions were interpreted as reflecting an estuary environment, the elemental composition of the environmental water, which is the primary source of non-nutrient elements in bioapatites, can be assumed to be a mixing between seawater and terrigenous-derived freshwater in terms of geochemical budget (e.g. Goedert et al., 2018). As the aluminium (Al) concentration has been proved to be linked to terrigenous contamination in sediments (e.g. Abbasi et al., 2021; Sharif et al., 2024) and because this element is not biologically incorporated (e.g. Sivakumar et al., 2012), the proportions of marine and terrigenous inputs in the elemental composition of fossil bioapatites can be approximated by the equations as follows:

$$F_{\text{seawater}}(\%) = \frac{\frac{[\text{Al}]}{[\text{Ca}]} \text{apatite} - \frac{[\text{Al}]}{[\text{Ca}]} \text{freshwater}}{\text{seawater} - \frac{[\text{Al}]}{[\text{Ca}]} \text{freshwater}} \times 100 \quad (1)$$

$$\text{Detrital input}(\%) \approx F_{\text{freshwater}}(\%) = 100 - F_{\text{seawater}} \quad (2)$$

In which F is the fraction (%) of the Al/Ca ratio incorporated from seawater ($F_{\text{seawater}}$) and freshwater ($F_{\text{freshwater}}$) in the fossil bioapatites. The
<caption>**Table 2**</caption>

Percentage of the freshwater contribution ($F_{\text{freshwater}}$) reported for each bed and investigated taxon (see the Table S2 for the detailed dataset). **Abbreviation**: n, number of samples; min, minimum; max, maximum; sd, standard error.

<table><thead><tr><th rowspan="2"></th><th colspan="5">$F_{\text{freshwater}}$ (%)</th></tr><tr><th>n</th><th>min</th><th>mean</th><th>max</th><th>sd</th></tr></thead><tbody><tr><td>Bed 390</td><td>4</td><td>10.47</td><td>25.24</td><td>47.91</td><td>16.23</td></tr><tr><td>Bed 351</td><td>4</td><td>4.96</td><td>26.97</td><td>55.33</td><td>24.52</td></tr><tr><td>Bed 242</td><td>19</td><td>1.48</td><td>26.82</td><td>92.86</td><td>28.59</td></tr><tr><td>Bed 35</td><td>5</td><td>5.54</td><td>36.22</td><td>73.70</td><td>24.84</td></tr><tr><td>Bed 12</td><td>7</td><td>1.69</td><td>9.95</td><td>30.39</td><td>10.38</td></tr><tr><td>Bed 8</td><td>8</td><td>1.72</td><td>40.57</td><td>85.91</td><td>27.77</td></tr><tr><td>Actinopteryrii</td><td>5</td><td>28.40</td><td>43.36</td><td>85.91</td><td>24.03</td></tr><tr><td>Tetrapodomorpha</td><td>11</td><td>1.48</td><td>18.28</td><td>55.33</td><td>19.46</td></tr><tr><td>Placodermi</td><td>19</td><td>2.38</td><td>23.78</td><td>92.86</td><td>25.24</td></tr><tr><td>Acanthodii</td><td>7</td><td>3.50</td><td>22.54</td><td>67.02</td><td>21.70</td></tr><tr><td>Dipnoi</td><td>5</td><td>10.36</td><td>53.25</td><td>83.76</td><td>29.28</td></tr></tbody></table>

latter could be approximated to the proportion of *Detrital input* (%) in the elemental composition of fossil bioapatites. $[\text{Al}/\text{Ca}]_{\text{freshwater}}$ is 0.03 and $[\text{Al}/\text{Ca}]_{\text{seawater}}$ is $3.13*10^{-6}$ (see Closset et al. (2021) and Weyhenmeyer et al. (2019) for freshwater Al and Ca concentrations respectively; and

Angel et al. (2015) and Krumgalz and Holzer (1980) for seawater Al and Ca concentrations respectively). The calculation of the detrital input, approximated from the Al/Ca ratios, is on average 27.5 % for the whole dataset (Table 2) and the following logarithmic equation (Fig. 9) linked the $\delta^7\text{Li}$ to the detrital input (DT):

$$ \delta^7\text{Li} = -4.265\ln(DT) + 23.94 \quad (3) $$

The correlation between DT and $\delta^7\text{Li}$ values (Pearson coefficient $r$ = 0.739, p-value = $3.04*10^{-9}$) supports the environmental control of the $\delta^7\text{Li}$ value within fossil bioapatite, more specifically by the terrigenous inputs in the case of an estuary. The wide range of detrital inputs may indicate that the Miguasha palaeo-estuary was not a homogeneous mixing of marine and freshwaters and that the proportions between them could vary depending on a distal-proximal transect in which the vertebrates lived preferentially. This is illustrated by the wide range of both the $\delta^7\text{Li}$ values and the detrital inputs per bed investigated (Fig. 9, Table 2). By interpreting these parameters as a function of taxa, it is consequently possible to reconstruct their preferred palaeoecology within the estuary.

<figure><img src="image_11.png" /></figure>

**Fig. 9.** Boxplots showing the well-preserved $\delta^7\text{Li}$ values of the hard tissues of the Miguasha aquatic vertebrates according to the tissues type and per taxa. The red stars correspond to *Elpistostege*. Mann-Whitney-Wilcoxon tests performed on all taxa of all beds individually indicate no significant difference (p-value>0.05). Mixed tissues correspond to the melting of two different tissues among spine, bone and scale. The absence of significant difference (ns) between pairs of taxa are highlighted by a Mann-Whitney-Wilcoxon test (p-value>0.05). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

<figure><img src="image_12.png" /></figure>

**Fig. 10.** Boxplots showing the well-preserved $\delta^7\text{Li}$ values of the hard tissues of the Miguasha aquatic vertebrates according to the taxonomy and per beds. The absence of significant difference (ns) between pairs of taxa are highlighted by a Mann-Whitney-Wilcoxon test (p-value>0.05).
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Assessment of the detrital inputs within the chemical composition of the hard tissues of the Miguasha aquatic vertebrates. Data are expressed as a function of the investigated beds (A) and taxa (B). Grey area corresponds to the uncertainties of the non-linear regression. Red bar and number correspond to theoretical detrital endmember with uncertainties. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

### 4.3. Reconstruction of the palaeoecology of the Miguasha aquatic vertebrates

The preferred habitat is where the organisms have lived most of their lives. However, it is also possible that these organisms had broader ecological habits. In this case, it is possible that marine organisms sporadically entered the estuary and vice versa. The large variations in δ<sup>7</sup>Li values and detrital inputs could suggest that some specimens were estuary visitors, originating from either freshwater (high detrital inputs and low δ<sup>7</sup>Li values) or seawater (low detrital inputs and high δ<sup>7</sup>Li values). As the diversity of these ecological habits cannot be fully reconstructed, the following palaeoecological reconstructions only concern the organisms' preferred environment, as inferred from their Li isotopic composition.

Li stable isotope fractionation between environmental water and bioapatite is not yet well understood. Previous studies (Thibon et al., 2022) have identified a negative shift between the δ<sup>7</sup>Li values of seawater and marine vertebrate hard tissues, a pattern that was also observed between the δ<sup>7</sup>Li values of seawater and invertebrate soft tissues (Thibon et al., 2023). While the quantification of this isotopic fractionation remains unknown, modern data suggest that the bioapatite δ<sup>7</sup>Li values decrease from terrestrial to freshwater and marine vertebrates, enabling palaeoecological inferences to be made.

The estimate of the terrigenous contribution within the chemical

content of Miguasha vertebrates varies from 1.5 % to 92.9 %. These percentages are independent of the taxa considered, since high and low percentages are recovered for all of them (Table 2). The non-linear relationship between the detrital inputs and the δ<sup>7</sup>Li values (Fig. 9) indicates that detrital inputs lower than 5 % correspond to δ<sup>7</sup>Li values higher than 20 ‰. The latter could be considered as reflecting a potential marine endmember with organisms preferentially living in the most distal part of the estuary, i.e. with the highest marine influences. It is therefore possible to discuss the preferred living environment of each taxon recovered from the Miguasha sedimentary deposits.

The δ<sup>7</sup>Li values of extant actinopterygians have been used to differentiate their living environments between freshwater, brackish, and marine environment (Thibon et al., 2022). δ<sup>7</sup>Li values are a strong tool to distinguish subtle ecological differences within Miguasha fauna (Fig. 12). For instance, the dipnoan *Scaumenacia* displays low δ<sup>7</sup>Li values associated with significant detrital inputs (~5 ‰ and from 10 % to 80 % in two different beds respectively; Figs. 9–12, Supplementary material S2), which suggest a prevailing influence of freshwater. Therefore, this vertebrate may have inhabited a proximal region of the estuary. This corroborates hypotheses regarding *Scaumenacia* ecology, as this post-Middle Devonian dipnoan holds specific morphological features such as cranial ribs. This adaptation reveals air-breathing ecology and suggests an analogous lifestyle to that of modern freshwater dipnoans, characterised by a reliance on freshwater habitats and occasional terrestrial excursions (Long, 1983).

The actinopterygian *Cheirolepis* and the acanthodian *Homalacanthus* exhibit δ<sup>7</sup>Li values that are intermediate between those of *Scaumenacia* and the highest recorded δ<sup>7</sup>Li values at Miguasha (Fig. 12). More precisely, the values range from 4.9 ‰ to 12.4 ‰. These medium-sized taxa, typically a few tens of centimetres long, exhibits different ecology: *Homalacanthus* was a secondary consumer feeding on conchostracans, e.g. *Asmusia membranacea*, while *Cheirolepis* was a predator hunting conchostracans, *Homalacanthus* and other *Cheirolepis* specimens (Chevrinais et al., 2017). The observed trophic interactions suggest that these organisms preferentially inhabited the estuarine zone, where their prey were present, and did not venture into the more distal, marine-dominated regions of the estuary that were preferred by larger predators. This is consistent with detrital inputs exceeding 25 % for *Cheirolepis* and 17 % for *Homalacanthus*, with the exception of two samples from the bed 12 (Fig. 9, Table 2).

The placoderm *Bothriolepis* exhibits a wide range of δ<sup>7</sup>Li values, spanning from 5.1 ‰ to 21.7 ‰ in bed 242 (Fig. 12), corresponding to detrital inputs between 3.9 % and 92.9 % (Fig. 9, Table 2). This variability indicates that *Bothriolepis* had a flexible ecology, inhabiting both marine environments and more proximal areas of the estuary. It confirms the ecological euryhalinity of this genus already demonstrated by Goedert et al. (2025). The presence of the species in beds 35 and 390 (Fig. 12), which are characterised by a wide range of δ<sup>7</sup>Li values (4.6 ‰ to 15.8 ‰ in bed 35, and 10.3 ‰ to 14.4 ‰ in bed 390) linked to continental inputs, serves to support its adaptability to various environments. Furthermore, palaeontological records indicate the presence of *Bothriolepis* in exclusively marine environments (e.g., Young, 1984; Long and Trinajstic, 2010), as well as in freshwater (Dupret et al., 2023), thus providing additional evidence for its ecological plasticity. The lower δ<sup>7</sup>Li values observed at Miguasha point to the capacity of *Bothriolepis canadensis* to flourish in habitats influenced by freshwater, such as coastal or estuarine ecosystems.

The δ<sup>7</sup>Li values, up to 23.0 ‰, observed in tetrapodomorphs (*Eusthenopteron* and *Elpistostege*) are the highest measured in the Miguasha fauna. Among modern aquatic vertebrates (Thibon et al., 2022), fully marine species, such as “fish” or cetaceans, exhibit some of the highest δ<sup>7</sup>Li values, while freshwater species have δ<sup>7</sup>Li values close to 0 ‰ and negative terrestrial values. Consequently, these latter taxa are likely indicative of marine-dominated regions of the palaeoestuary as indicated by low detrital inputs (<5 %) for δ<sup>7</sup>Li values higher than 20 ‰ (Figs. 9, 12). Such an ecology in Devonian tetrapodomorphs is
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** δ<sup>7</sup>Li values (boxplots and raw values) of Miguasha vertebrate samples according to their stratigraphic position. Published values for modern seawater (MS; Millot et al., 2004) and Late Devonian seawater (LDS1 and LDS2; Kalderon-Asael et al., 2021 and Wang et al., 2023, respectively) as well as for modern vertebrates are added for comparison (Thibon et al., 2022). Blue arrow represents the postulated seawater-river gradient in the Miguasha palaeoestuary according to the modern vertebrate values as well as the δ<sup>7</sup>Li value range of analysed samples. Standard errors are smaller than the point size. Silhouettes are not to scale. **Abbreviations:** LDS, Late Devonian seawater; MFA, modern freshwater actinopterygians; MFAm, modern freshwater amphibians; MFC, modern freshwater chondrichthyans; MS, modern seawater; MSA, modern seawater actinopterygians; MSC, modern seawater chondrichthyans. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

congruent with the euryhaline ecology postulated in some Palaeozoic
tetrapods (Blieck et al., 2007; Goedert et al., 2018) and suggests that the
tetrapod terrestrialisation could have occurred in various aquatic
environment.

4.4. Future application of δ<sup>7</sup>Li of the fossil bioapatite in palaeoceanographic research

The seawater chemistry during the Late Palaeozoic period has been
extensively studied (e.g. Zeebe, 2012; Lenton and Daines, 2017 and the
references within) given the pivotal role it played in global climate
change (for instance, the Late Palaeozoic Ice Age, as outlined by
Sutherland, 1870) and biodiversity crisis (such as the Frasnian-
Famennian and Permian-Triassic mass extinctions). The Li isotopic
composition of past seawater has been reconstructed using a variety of
proxies, including foraminifera, brachiopods, and marine carbonates (e.
g., Hathorne and James, 2006; Misra and Froelich, 2012; Washington
et al., 2020). Published estimates of Devonian seawater δ<sup>7</sup>Li values
range from 11.8 to 20 ± 5 ‰ (Kalderon-Asael et al., 2021; Wang et al.,
2023), which are lower than the value of 31 ± 0.5 ‰ measured in
present-day seawater (Millot et al., 2004) (Fig. 3). Seawater δ<sup>7</sup>Li values
are homogeneously distributed (Misra and Froelich, 2012) due to the

longer residence time of dissolved Li in the water column (~1.8 Ma)
compared to the ocean mixing time (Lécuyer, 2016). The Escuminac
Formation sediments represent a geological record spanning between
59.5 ka (minimum estimate) to 2.5 Ma (maximal estimate) (Cloutier
et al., 2011). It can thus be assumed that seawater δ<sup>7</sup>Li values did not
undergo significant changes during this time interval.

It is interesting to note that the Miguasha δ<sup>7</sup>Li values that mainly derived from seawater in tetrapodomorphs and placoderms are comprised between 20 ‰ and 23 ‰. Most extant marine vertebrates, even those of modest size, exhibit δ<sup>7</sup>Li values with an offset of a few ‰ relative to ambient seawater (Thibon et al., 2022). Consequently, the Late Devonian seawater δ<sup>7</sup>Li could be higher than at least 23 ‰, which is consistent with the 20 ± 5 ‰ value proposed by Kalderon-Asael et al. (2021) but not with Wang et al.'s (2023) estimate between 11.8 ‰ and 13.8 ‰ (Fig. 12). Therefore, the identification of marine-derived δ<sup>7</sup>Li value within Miguasha vertebrates highlights future implications of these data for reconstructing the seawater δ<sup>7</sup>Li during the Phanerozoic. However, further studies focusing on quantifying the isotopic fractionation between the ambient water and the vertebrate hard tissues must be conducted before any in-depth discussion.

Recently, Brand et al. (2025) proposed a set of equations relating
ocean temperature to the Li/Ca ratio of brachiopods shells. The Li/Ca
ratio of the Miguasha fauna is interpreted here as a terrigenous contamination proxy according to Adiatma et al. (2024). The assumed heterothermy of all the investigated taxa and the correlation with the δ⁷Li values go in this direction. However, the Miguasha taxa lived in a brackish environment where detrital inputs controlled the hard tissues δ⁷Li values and the Li/Ca ratio. Further analyses on exclusively marine taxa could identify a temperature-dependent variation of the Li/Ca ratio in vertebrate hard tissues which could be applied to palaeotemperature reconstructions.

## 5. Conclusions

We have discussed the preservation of the original δ⁷Li values of vertebrate mineralised tissues against diagenetic processes based on Li concentration and isotopic ratio in vertebrates from the Late Devonian deposit of Miguasha. The relationships between δ⁷Li and major and minor element concentrations in fossils highlight the conditions under which the isotopic signal is preserved with Li concentrations and Ca/P ratios similar to modern vertebrates, no MREE enrichment, no correlation between Li and heavy element concentrations, and those under which it is not. Conversely, when preservation criteria are not met using these proxies, vertebrate δ⁷Li values are close to the 0 ‰, which is close to the composition of diagenetic fluids. The identification of these two endmembers, i.e. preserved and altered, constitutes a solid reference framework for evaluating the preservation of δ⁷Li values within fossil bioapatites.

At the first order, the Li isotope compositions of the well-preserved fossil bioapatite from Miguasha are not controlled by biological factors such as taxa, size or tissues. On the contrary, it seems that the main factor influencing the δ⁷Li values is the environment, and in the case of the Miguasha palaeo-estuary, the amount of terrigenous input into the water where the vertebrates lived. Consequently, the preserved δ⁷Li values allow us to discuss the palaeoecology of the Miguasha vertebrates and to reconstruct their preferred living environment. In particular, this confirms the euryhalinity of the Late Devonian tetrapodomorphs and calls into question the environment that allowed vertebrate terrestrialisation.

The δ⁷Li values from fossil vertebrate hard tissues can be used in palaeoecological and palaeoenvironmental reconstructions. The identification of a marine endmember at 23 ‰ is consistent with some reconstructions of the seawater δ⁷Li values. Further work focusing on the environmental water-bioapatite isotopic fractionation will help to assess whether fossil bioapatite can be used as a new palaeoceanographic proxy.

### CRediT authorship contribution statement

**Vincent Luccisano:** Investigation, Visualization, Writing – original draft, Writing – review & editing. **Jeremy E. Martin:** Conceptualization, Methodology, Investigation, Writing – review & editing. **Romain Amiot:** Funding acquisition, conceptualization, Investigation, Writing – review & editing. **Philippe Telouk:** Conceptualization, Formal analysis, Methodology, Investigation. **Fanny Thibon:** Conceptualization, Formal analysis, Methodology, Investigation, Writing – review & editing. **Sebastien Olive:** Investigation, Writing – review and editing. **Olivier Matton:** Sampling, Writing – review and editing. **Christophe Lécuyer:** Sampling, Conceptualization, Investigation, Resources, Writing – review & editing.

### 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.

### Acknowledgements

This work was funded by the Labex LIO postdoctoral fellowship 2022 (Université de Lyon) and was made possible by the INSU/CNRS MC-ICP-MS national facility at ENS-Lyon. Sébastien Olive received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 101032456-TNT. Li concentration and isotope analyses were performed at the geochemical facility of the Laboratoire de Géologie de Lyon, Terre, Planètes, environnement. This work was supported by the LGL-TPE internal call in 2024. The authors thank Richard Cloutier (Université du Québec à Rimouski, Canada) for discussions about the fauna and the environment of the Miguasha palaeoestuary. C.L. is grateful to the staff of the Parc national de Miguasha for the creation of a photomicrograph database, their assistance in sampling vertebrate fauna, and their packing for transport overseas.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemgeo.2025.123024.

### Data availability

Data will be made available on request.

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**Update**

Chemical Geology

Volume 695, Issue , 5 November 2025, Page

DOI: https://doi.org/10.1016/j.chemgeo.2025.123054
Fig. 9 Boxplots showing the well-preserved δ<sup>7</sup>Li values of the hard tissues of the Miguasha aquatic vertebrates according to the tissues type and per taxa. The red stars correspond to *Elpistostege*. Mann-Whitney-Wilcoxon tests performed on all taxa of all beds individually indicate no significant difference (p-value>0.05). Mixed tissues correspond to the melting of two different tissues among spine, bone and scale. The absence of significant difference (ns) between pairs of taxa are highlighted by a Mann-Whitney-Wilcoxon test (p-value>0.05). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11 Assessment of the detrital inputs within the chemical composition of the hard tissues of the Miguasha aquatic vertebrates. Data are expressed as a function of the investigated beds (A) and taxa (B). Grey area corresponds to the uncertainties of the non-linear regression. Red bar and number correspond to theoretical detrital end-member with uncertainties. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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