# Middle Eocene hyperthermal seasonality from Paris Basin marine mollusks

Loïc Marlot, Damien Huyghe, Justine Briais, Mathieu Daëron, Christine Flehoc, Laurent Emmanuel, Didier Merle, Olivier Aguerre-Chariol, Franck Lartaud

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Loïc Marlot, Damien Huyghe, Justine Briais, Mathieu Daëron, Christine Flehoc, et al.. Middle Eocene hyperthermal seasonality from Paris Basin marine mollusks. Palaeogeography, Palaeoclimatology, Palaeoecology, 2026, 685, pp.113547. 10.1016/j.palaeo.2026.113547. hal-05447893

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# Middle Eocene hyperthermal seasonality from Paris Basin marine mollusks

Loïc Marlot<sup>a,b,c</sup>, Damien Huyghe<sup>a,*</sup>, Justine Briais<sup>b</sup>, Mathieu Daëron<sup>d</sup>, Christine Flehoc<sup>b</sup>, Laurent Emmanuel<sup>e</sup>, Didier Merle<sup>f</sup>, Olivier Aguerre-Chariol<sup>g</sup>, Franck Lartaud<sup>h</sup>

## ARTICLE INFO

Editor: Bing Shen

Keywords:

Climate change

Mollusk

Middle Eocene climatic optimum

Stable isotopes

Clumped isotopes

## ABSTRACT

The Earth has experienced hyperthermal events in the past, characterized by maximum durations of hundreds thousand years, significant magnitude, global extent, and drivers associated with increases in greenhouse gas concentrations, therefore making them potential analogues for current climate change. The Middle Eocene Climatic Optimum (MECO) that occurred 40 Ma ago, is marked by a CO₂-driven global warming of +4 to +6 °C, affecting global temperatures. Here, we present a detailed reconstruction of seasonal fluctuations in seawater temperatures during this warming event in littoral environment, based on geochemical analyses (δ¹⁸O and Δ47) of shallow-marine mollusks from the Paris Basin. Our data show a stability in mean winter temperatures compared to pre-MECO conditions, but a marked warming of +10 °C in maximum estuarine water temperatures, with a seasonal temperature range increasing from 12 °C before the MECO to 22 °C at the climax of the event. We demonstrate that at mid-latitudes, annual maximum shallow-water temperatures increased from 30 ± 2 °C before the event to a maximum of 41 ± 4 °C at the warming peak. This pattern is associated with a seasonal regime characterized by dry summers and wet winters, implying that the Paris Basin experienced a super-hot summer Mediterranean climate during the MECO.

## 1. Introduction

Modern anthropogenic greenhouse gases emissions are driving a rapid increase in global temperatures, unprecedented in the historical record. The 2023 Intergovernmental Panel on Climate Change (IPCC) report proposed different scenarios of climate evolution, leading to a return to paleoclimatic conditions observed during the Pliocene, the Early Eocene or even the Late Cretaceous under the most pessimistic scenario (Burke et al., 2018; Tierney et al., 2020). Although no exact analogue exists for the ongoing climate change, studying past episodes of global warming driven by increasing atmospheric greenhouse gas concentrations, can provide valuable insights for better understanding and characterizing future climate conditions (Lear et al., 2021; Tierney

et al., 2020).

After the Early Eocene Climatic Optimum, the Earth entered a long-term cooling trend punctuated by several short-term warming events (Westerhold et al., 2020; Zachos et al., 2001). The Middle Eocene Climatic Optimum (MECO), occurring between 40.5 and 40 million years ago (Bohaty et al., 2009; Westerhold et al., 2020), is marked by a rise in both deep-sea and sea-surface temperatures (SST) of 4–6 °C in ~500 ky (Bohaty and Zachos, 2003; Bohaty et al., 2009; Cramwinckel et al., 2018). In contrast to other Early Paleogene hyperthermals that are commonly linked to CH₄ emissions, such as the Paleocene-Eocene Thermal Maximum (PETM), the MECO is interpreted as the only global warming event primarily driven by elevated atmospheric CO₂ levels (Bohaty and Zachos, 2003; Henehan et al., 2020; Sluijs et al.,

<footnote><sup>a</sup> PSL University/MINES Paris, Centre de Géosciences, 35 rue St Honoré, F-77305 Fontainebleau, France</footnote>

<footnote><sup>b</sup> BRGM (French Geological Survey), F-45060 Orléans Cedex 2, France</footnote>

<footnote><sup>c</sup> Now at IMT Nord Europe, Institut Mines-Télécom, Lille University, Centre for Energy and Environment, Lille F-59000, France</footnote>

<footnote><sup>d</sup> Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Bât 714, CEA-Orme des merisiers, RD 128 F-91191</footnote>

<footnote>GIF-SUR-YVETTE CEDEX, France</footnote>

<footnote><sup>e</sup> Sorbonne Université, UMR CNRS 7193, Institut des Sciences de la Terre de Paris, ISTE, 4 place Jussieu, F-75005 Paris, France</footnote>

<footnote><sup>f</sup> Muséum National d'histoire Naturelle, Département Origines et Évolution (CR2P - MNHN, CNRS, UPMC, Sorbonne Université), 8 rue Buffon, F-75005 Paris, France</footnote>

<footnote><sup>g</sup> Independant, 14 rue Saint-Hubert, 60560 Orry-la-Ville</footnote>

<footnote><sup>h</sup> Sorbonne Université, CNRS, Laboratoire d'Écogéochimie des Environnements Benthiques (LECOB), Observatoire Océanologique de Banyuls, F-66650, Banyuls-sur-Mer, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: damien.huyghe@minesparis.psl.eu (D. Huyghe).</footnote>
2013). The short duration, the magnitude and global impact of the warming, the associated sea level rise (Bohaty et al., 2009; Brachert et al., 2023; Henehan et al., 2020) and resulting biodiversity changes (Bijl et al., 2011; Boscolo Galazzo et al., 2015; Edgar et al., 2013), makes the MECO one of the most compelling paleoclimate analogues for the ongoing global climatic change (Sluijs et al., 2013).

The climatic history of this warming is now well-documented in the oceanic domain, mostly through geochemical analyses of deep-sea benthic and planktonic foraminifera (Bijl et al., 2010; Bohaty et al., 2009; Bohaty and Zachos, 2003; Cramwinckel et al., 2018; Westerhold et al., 2020). However, these proxies provide reconstructions of mean annual temperatures, leaving key climatic components, such as seasonal temperature variations, poorly understood. Characterizing paleo- temperature reconstructions at the seasonal scale is however essential to capture short-term climatic dynamics and to better constrain the mechanisms driving environmental variability (De Winter et al., 2024; He et al., 2025; Ivany and Judd, 2022). Such fine-scale reconstructions provide valuable information about seasonality shifts, extreme events, and their ecological impacts factors often obscured in lower-resolution records. Advancing this level of characterization therefore enhances the accuracy of climate models and improves our ability to interpret the sensitivity of past and future climate systems.

In this study, we aim to identify and characterize seasonal temper-
ature fluctuations, before, during and after the MECO. To achieve this,
we target on shallow-marine environments that hosted mollusk shells,
which allow seasonal-scale temperatures reconstructions through high-
resolution stable isotope analyses of their growth bands (Andreasson
and Schmitz, 1996; Huyghe et al., 2022; Kniest et al., 2024; De Winter
et al., 2020).

## 2. Geological setting

This work focuses on the Paris Basin, which provides a unique record of middle Eocene sediments, with the preservation of coastal deposits that hosted a biodiversity hotspot for shallow-marine ecosystems during the Middle Eocene (Huyghe et al., 2012a; Merle, 2008), allowing for reliable high-resolution paleoclimatic reconstructions.

The Paris Basin is a well-known intracratonic sedimentary basin that underwent long-term subsidence during the Mesozoic, with a period of negligible subsidence during the Paleogene, followed by regional uplift during the Neogene (Cavelier and Pomerol, 1979; Guillocheau et al., 2000; Robin et al., 2003). During the Eocene, the basin extended northward to include the Hampshire and Brussels basins (Pomerol, 1973) and was connected to both the North Sea and the Atlantic Ocean via the English Channel (Fig. 1; Gély and Lorenz, 1991).

Until the end of the Middle Lutetian, marine inflow was primarily from the North Sea. Following the uplift of the Artois and Bray anticlines, inflow became exclusively of North Atlantic origin (Pomerol, 1973; Gély and Lorenz, 1991). Low paleotopographic relief and negligible subsidence during the Paleogene meant sediment distribution was predominantly controlled by eustatic sea-level fluctuations (Mégnien, 1980; Briais, 2015), modulated by tectonic reactivation of anticlines, resulting in significant lateral facies variations complicating basin-wide stratigraphic correlations (Briais, 2015; Gély and Lorenz, 1991; Moreau et al., 2024).

During the Middle Eocene, most shallow-water sedimentary deposits were located in the northern part of the Paris basin. In the Middle Lutetian, sedimentation was dominated by carbonate platform deposits, particularly the Calcaire Grossier Formation. From the Late Lutetian to the Early Bartonian, sedimentation shifted toward lagoonal facies with intense evaporation, leading to the deposition of the Marnes et Caillasses Formation (Briais, 2015; Moreau et al., 2024).

The Bartonian stratigraphy comprises two regional stages: the Auversian, beginning with the Marnes et Caillasses Formation, and followed by siliciclastic shallow-marine deposits, including the Sables d'Auvers and Sables de Beauchamp Formations (Pomerol, 1965). Then the Marinesian local stage, corresponding to the Late Bartonian, is characterized by lagoonal to lagoon-lacustrine calcareous sedimentation (Briais, 2015; Morellet and Morellet, 1948).

This succession of shallow-marine environments enables detailed climatic reconstructions in littoral settings throughout the Middle Eocene, due to exceptionally well-preserved and diverse faunal assemblages in the basin during this period (> 800 species in the Early Bartonian, and >1400 species during the peak of marine biodiversity at the Middle Lutetian; Merle, 2008).

3. Materials and methods

3.1. *Field Sampling*

Fossils analyzed in this study were collected north of Paris, from sites
near Chavençon, Chars, and the protected geological site of Le Guépelle,
recently described by Aguerre-Chariol (2023) (Fig. 1). Sampling was
conducted in sandy layers where fossil shells were well-preserved and
easily extractable, allowing access to whole specimens.

To minimize uncertainties related to species-specific isotopic fractionation, we sampled the most abundant mollusk families present at each site. Thus, we sampled gastropod shells of the species *Torquesia sulcifera* (Deshayes, 1832) (Turritellidae) from the Horizon de Mont Saint-Martin (HMSM) in Chavençon (Turritellidae), and the bivalves

<figure><img src="image_4.png" /><figcaption>Fig. 1. A: Geological map of the Paris Basin and location of the sampling sites. For the Lutetian sites, data are from Andreasson and Schmitz (1996); Huyghe et al. (2012b) and De Winter et al. (2020). B: Paleogeographic map of the Paris Basin during the Lutetian (Merle, 2008) and C: during the Bartonian (Briais, 2015).</figcaption></figure>
*Bicorbula gallica* (Lamarck, 1801) (Corbulidae) from the site of Le Guépelle and *Orthocardium porulosum* (Solander, 1766) (Cardiidae) from the Sables de Cresnes Fm in Chars (Fig. 2). Two or three specimens were sampled per stratigraphic level, except at the uppermost level (Sables de Cresnes Fm.), where only one large sample was available.

## 3.2. Studied taxa

The Turritellidae analyzed in this study include the Bartonian species *Torquesia sulcifera* (Deshayes, 1832). These large gastropods (~10 cm) possess aragonitic shells and are morphologically comparable to extant members of the genus *Turritella*. Turritellidae are commonly used in paleoclimatic reconstructions due to their capacity to record multi-annual seasonal climate variations within their shell growth, and their typically large size allows for high-resolution sampling (Andreasson and Schmitz, 2000; Andreasson and Schmitz, 1996; Huyghe et al., 2012a; Marchegiano and John, 2022). Additionally, modern representatives of this family exhibit a low tolerance to salinity fluctuations (Allmon, 1988), making them reliable indicators of stable marine conditions in paleoenvironmental studies.

The Corbulidae are represented by *Bicorbula gallica*, collected from Le Guépelle section (Fig. 4). Members of the Corbulidae family are typically filter feeders and surface deposit scrapers that inhabit intertidal to shallow subtidal environments (Holmes and Miller, 2006; Mirzaei and Hosseini, 2017). Modern corbulids are widespread in shallow marine waters, ranging from the Norwegian Sea to the western coast of Africa (Fuksi et al., 2018; Holmes and Miller, 2006). They are well adapted to warm temperatures and can tolerate hypoxic conditions as well as significant salinity fluctuations (Fuksi et al., 2018; Holmes and Miller, 2006).

The third species correspond to the bivalve *Orthocardium porulosum*, with only one specimen from the Sables de Cresnes formation at Chars. The Cardiidae family comprises shallow-marine endobenthic species.

## 3.3. Preservation screening

To ensure that the stable isotope composition of biogenic carbonates reliably reflects paleoenvironmental conditions, it is essential that the

mollusk shells used in this study are free from diagenetic alteration. Preservation was assessed using cathodoluminescence microscopy to test the good preservation of growth increments (greenish luminescence indicating preserved aragonite; Barbin, 2013; de Rafélis et al., 2000) and X-ray diffraction (XRD) analyses was conducted on powdered shell material, confirming exclusive aragonite preservation with no detectable secondary phases (Supplementary Material 1). Bivalves of the species *Bicorbulla gallica* show very low but homogenous luminescence. Gastropods are on the contrary completely non-luminescent (totally dark) under CL attesting non recrystallisation of secondary calcite and preservation of their primary aragonitic mineralogy.

## 3.4. Sample preparation and micromilling

After collection, sediment samples were sieved and sorted to isolate complete shells suitable for isotopic analyses. Each shell underwent a standardized cleaning protocol following the method described by (Lartaud et al., 2010). Initially, the shells were rinsed with distilled water, then immersed in a 6 % H₂O₂ solution for six hours to eliminate residual organic matter. This was followed by a 20-min bath in 0.15 N HNO₃ to remove any external recrystallized calcite.

Isotopic sampling was performed using conventional micromilling techniques, with sampling along the growth axis using a 0.1 mm drill bit (Huyghe et al., 2015; De Winter et al., 2020). Multiple samples were extracted from each shell to capture seasonal variations in isotopic composition (δ¹⁸O and Δ₄七年).

## 3.5. Stable Isotope analyses

Each shell was analyzed for δ¹⁸O composition. All stable isotope values are presented in Supplementary Material 2. After micromilling, δ¹⁸O analyses were performed on 50 to 100 µg carbonate powder samples, reacted with 100 % phosphoric acid (H₃PO₄) at 90 °C under vacuum. The resulting CO₂ was ionized and measured using an Isoprime 100 dual-inlet mass spectrometer. Results are reported in delta (δ) notation relative to the Vienna Pee Dee Belemnite standard (‰ VPDB) with ±0.2 ‰ precision (1σ), using CO-1 (for oxygen), NBS-19 (for combined carbon-oxygen calibration), and an internal Carrara marble (calibrated

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Mollusk species analyzed in this work: *Torquesia sulcifera*, *Bicorbula gallica* and *Orthocardium porulosum*. The area of the shell sampled for both δ¹⁸O and Δ₄七年 analyses is illustrated on each shell. The corresponding isotopic signal and the identification of each summer (S) and winter period (W) on a given shell is reported for each shell.</figcaption></figure>
against CO-1) as standards.

Clumped isotope (Δ47) analyses were performed on two *T. sulcifera* shells from the HMSM and two *B. gallica* shells from the base and top of the Sables du Guépelle Formation, corresponding to the Modiolaria zone, the Ermenonville Formation, and the Sables de Beauchamp Formations. These analyses were conducted at the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) using protocols and instrumentation described in (Daëron et al., 2019; Peral et al., 2018) and in the attached analytical report (Supplementary Material 3).

For each analysis, 4 mg of sample powder were dissolved in 100 % H3PO acid at 90 °C for 15 min. The resulting CO2 was then quantitatively recollected by cryogenic trapping and transferred by gas expansion into an Isoprime 100 dual-inlet mass spectrometer equipped with six Faraday collectors (m/z 44 to 49). (δ 45 to δ 49) were converted to δ13C, δ18O, and “raw” Δ47 values as described by (Daëron et al., 2016) using the IUPAC oxygen- 17 correction parameters (Brand et al., 2010). The isotopic composition (δ13C, δ18O) of our working reference gas was computed based on the nominal isotopic composition of all ETH carbonate standards (Bernasconi et al., 2018) and an oxygen-18 acid fractionation factor of 1.00813. Raw Δ47 values were then converted to the I-CDES (InterCarb-Carbon Dioxide Equilibrium Scale) reference frame (Bernasconi et al., 2021) using a pooled regression approach (Daëron, 2021) as implemented by the D47crunch Python library. Full analytical errors are derived from the external reproducibility of unknowns and standards (Nf = 29) and conservatively account for the uncertainties in raw Δ47 measurements as well as those associated with the conversion to the I-CDES reference frame (Daëron, 2021). Seawater δ18O<sub>w</sub> was calculated using the formula of Grossman and Ku (1986). The details of the protocol and results for clumped isotopes analyses are provided in Supplementary Material 3.

### 3.6. Paleotemperature calculation

The primary objective of this study was to estimate paleotemperatures in littoral environments based on δ18O measurements from mollusk shells. Mollusks are assumed to precipitate their shells in near isotopic equilibrium with ambient seawater (Wefer and Berger, 1991; Grossman and Ku, 1986; Quizon et al., 2025; Schlidt et al., 2025). However, cases of isotopic disequilibrium have been identified for some species, such as deep-sea oysters of the genus *Pycnodonte* (Wisshak et al., 2009) or the juvenile portion of the oysters *Maganalla gigas* (Huyghe et al., 2022). Among the fossil used in this study, specimens of the genus *Turritella* have often been employed for (paleo)climatic reconstructions (Andreasson and Schmitz, 1996; Huyghe et al., 2012b; Jones and Allmon, 1995; Scholz et al., 2020). Studies on modern specimens have shown that these gastropods mineralize their shells in equilibrium with seawater, making it possible to reconstruct seasonal temperature gradients when δ18O<sub>w</sub> is well constrained (Scholz et al., 2024). Concerning the bivalves *B. gallica* and *O. porulosum*, these species have not yet been used for paleoclimatic reconstructions. Moreover, for many mollusks, significant variations in growth rate have been documented, especially during winter periods (De Winter et al., 2020; Ivany and Judd, 2022). This requires very high-resolution sampling to ensure that true seasonal extrema are captured in stable isotopes analyses.

These environmental and biological considerations support the combined use of δ18O and Δ47 measurements to reliably estimate δ18O<sub>w</sub> and thus the true temperature extrema. Thus, temperature can be calculated using the aragonite-specific paleotemperature equation from (Grossman and Ku, 1986) (Eq. (1)):

$$T(^{\circ}\text{C}) = 20.6 - 4.34 (\delta^{18}\text{O}_{\text{aragonite}} - \delta^{18}\text{O}_{\text{w}}) \quad (1)$$

where T is the temperature of shell mineralization, δ18O<sub>aragonite</sub> is the stable oxygen isotope composition of the shell, and δ18O<sub>w</sub> is that of ambient water.

The main source of uncertainty in this calculation lies in determining

δ18O<sub>w</sub>. To address this, clumped isotope (Δ47) analyses were conducted on the same mollusk shells, as this proxy is solely temperature-dependent (Eiler, 2011; Quizon et al., 2025; Schlidt et al., 2025). To convert Δ47 values into calcification temperatures, we used the composite OGLS23 calibration which combines >100 biogenic, inorganic and synthetic samples analyzed in 5 different laboratories and reproduced in the I-CDES reference frame (Daëron and Vermeesch, 2024):

$$\Delta_{47}(^{\circ}\text{C}) = 0.1744 - \frac{18.14}{T} + \frac{42.66 \times 10^3}{T^2} \quad (2)$$

Although this Δ47-temperature calibration is largely based on calcitic foraminifera, whereas the samples analyzed in this study are aragonitic, experimental and empirical studies indicate that equilibrium Δ47 values do not exhibit a resolvable dependence on carbonate mineralogy, in contrast to δ18O measurements, which are mineral-specific (e.g., Bonifacie et al., 2017; De Winter et al., 2022). To date, no statistically significant offset in equilibrium Δ47 has been demonstrated between calcite and aragonite. Consequently, Δ47-based paleotemperature reconstructions from aragonitic archives, including mollusk shells, commonly rely on unified equilibrium calibrations rather than mineral-specific equations. Within the I-CDES framework, this approach can be considered appropriate for reconstructing temperatures from the aragonitic mollusk shells analyzed in this study.

Additional parameters can also influence the isotopic composition of biogenic carbonates and the reconstructed paleotemperatures. First, Δ47 reordering can significantly alter temperature estimates under conditions of deep burial or prolonged heating (e.g., Passey and Henkes, 2012; Stolper and Eiler, 2015). We consider such effects to be negligible for our material. As Cenozoic deposits in the Paris Basin are exceptionally thin, with a maximum burial depth of less than 100 m (Briais, 2015), the thermal regime experienced by the aragonitic shells remained far below thresholds known to induce measurable Δ47 reordering. Furthermore, the preservation of aragonite in our samples and the absence of diagenetic indicators support minimal post-depositional alteration. Consequently, we are confident that solid-state reordering did not influence our reconstructed temperatures.

Moreover, evaporation can affect the isotopic composition of shallow-water carbonates. However, Δ47 thermometry is insensitive to variations in water δ18O associated with evaporative processes. In the studied outcrops, sedimentological evidence indicates that the fossil-bearing intervals analyzed here were not deposited under strongly evaporative conditions, limiting the potential impact of evaporation on the reconstructed temperatures (Aguerre-Chariol, 2023).

Then, the temperature obtained from clumped isotope analysis is inserted into Eq. (1) to solve for δ18O<sub>w</sub> for each Formation. Once δ18O<sub>w</sub> is constrained, high-resolution temperature reconstructions can be derived from the δ18O values measured along the growth axis of each shell, using Eq. (1) from (Grossman and Ku, 1986). In addition to temperature, seawater characteristics such as paleo-salinity were estimated using the equation developed by (Pierre, 1999) for the modern Mediterranean Sea, which was considered a reasonable analogue for the Bartonian marine environment of the Paris Basin (Pomerol, 1965):

$$S = (\delta^{18}\text{O}_w + 8.2)/0.25 \quad (3)$$

where S is salinity in parts per thousand (‰), and δ18Ow is expressed in ‰ relative to VSMOW.

### 4. Results

The evolution of the stable isotopic profiles of the shells are presented on Fig. 3. The stable isotopic profiles of the three mollusk species reveal periodic fluctuations, which can be interpreted as reflecting seasonal variations in environmental parameters, including temperature (Fig. 2).

The δ18O values ranged from -1.2 ‰ to -4.5 ‰ for the *Turritelles* of
<figure><img src="image_6.png" /><figcaption>Fig. 3. Composite sedimentary succession of the Bartonian deposits in the Paris Basin, from the studied sites of Chavençon, Le Guépelle and Chars, with the local formations encountered and their associated biozones (Aubry, 1986; Briais, 2015). From left to right are represented the sedimentary deposit profile, the δ¹⁸O values of *T. sulcifera*, *B. gallica* and *O. porulosum*, the Δ₄₇, δ¹⁸Oₗ and salinity calculated from Δ₄₇ and the calibrated temperatures. The white dots correspond to high resolution δ¹⁸O analyses and the purple diamonds correspond to the δ¹⁸O values obtained during clumped isotope analysis. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

the Horizon de Mont Saint Martin (HMSM) at Chavençon, from -1.2 ‰ to -8.9 ‰ for the *B. gallica* across the section of le Guépelle and from -2.5 ‰ to -4.8 ‰ for the *O. porulosum* of the Sables de Cresnes Fm (Fig. 3). The highest δ¹⁸O values for all shells from the Sables du Guépelle Fm shells remained consistently around -2.5 ‰, then increased to -1.5 ‰ in the Agrégat coquillier Fm and stabilized between -1.9 ‰ and -1.2 ‰ until the Sables de Beauchamp Fm. The most negative values of the *B. gallica* first strongly increased from -8.9 ‰ in the base of the Sables du Guépelle Fm to -5.8 ‰ in the rest of this

formation. The δ¹⁸O then increased to -5.2 ‰ in the Agrégat coquillier Fm and -5.0 ‰ in the Modiolaria zone. Finally, these values decreased to reach -6.2 ‰ in the Ermenonville Fm and -5.5 ‰ in the Sables de Beauchamp Fm.

Clumped isotope analyses have been performed on the Bartonian species *T. sulcifera* and *B. gallica*. Specimens of the Sables de Cresnes Fm were too small to perform Δ₄₇ measurements. Δ₄₇ values were converted to temperature values using Eq. (1) from Peral et al. (2018) (Fig. 3).

Seawater δ¹⁸Oₗ values were calculated using estimated

<figure><img src="image_7.png" /><figcaption>Fig. 4. Evolution of the Paris Basin littoral temperatures obtained from this study and previous works for the Lutetian from (Andreasson and Schmitz, 1996; Huyghe et al., 2012a; De Winter et al., 2020); and the seasonal gradient of temperature calculated from the difference between the extremum temperatures of the mollusks analyzed for over the Middle Eocene. Age and calcareous nannofossils zonation are from Gradstein et al. (2012).</figcaption></figure>
temperatures from clumped isotopes analysis and the equation of Grossman and Ku (1986) (see methods). These values ranged from -0.9 ‰ ± 1.3 ‰ to -3.1 ‰ ± 1.2 ‰ along the Le Guépelle section except for one sample at the bottom of Le Guépelle section, which exhibited the most negative δ¹⁸O<sub>w</sub> values of -5 ‰ and -5.3 ‰ ± 1.3 ‰ (Fig. 3). Here, because the δ¹⁸O<sub>w</sub> has been constrained by clumped isotope analyses, the Eq. (3) (Pierre, 1999) is used to calculate local salinity values. Salinity ranged from 20.4 ± 4.8 ‰ to 31.2 ± 4.8 ‰ for the HMSM and from 11.6 ± 3.6 ‰ to 29.2 ± 5.2 ‰ for Le Guépelle section (Fig. 4). Despite this broad range, the salinity at Le Guépelle remains quite stable around 25 ‰, except for one shell at the bottom of the section with a low salinity of 12 ‰, which could be explained by transient strong freshwater discharge or a transport of this specimen from a lower salinity region of the estuary.

The reconstructed temperatures fluctuate throughout the Middle
Eocene in the Paris Basin (Fig. 3). Maximum annual temperatures in-
crease from 32 ± 2 °C in the HMSM to maximum of 41 ± 5 °C at the top
of the Sables du Guépelle and Agrégat Coquiller Fms. Minimum annual
temperatures increased from 12 ± 3 °C to 19 ± 2 °C over the same in-
terval before decreasing toward the top of the section.

5. Discussion

5.1. Stratigraphic position of the MECO in the Paris Basin

During the early Bartonian, the evolution of Earth temperature was characterized by a global transient warming lasting 500 kyrs (from ~40.5 to ~40 My) and corresponding to the Middle Eocene Climatic Optimum (MECO) (Fig. 3) (Bohaty et al., 2009; Bohaty and Zachos, 2003). In the oceanic record, this warming occurred within magnetochron C18n.2n (Bohaty et al., 2009; Westerhold et al., 2020). In the Paris Basin, magnetostratigraphy is infeasible due to the insufficient consolidation of the sedimentary facies, complicating correlations with oceanic records. The Sables du Guépelle Fm and its lateral equivalent, the Sables d'Auvers (Morellet and Morellet, 1948; Pomerol, 1965) have been attributed to the base of the Bartonian according to biostratigraphic and sequential evidences and more specifically to the top part of the calcareous nannofossils zone NP16 (Fig. 3) (Aubry, 1986; Gély and Lorenz, 1991). The interval with the maximum warming identified within the Bartonian composite section in the Paris Basin is located within Zone NP16 (Fig. 3), which is contemporaneous to magnetochron C18n.2n (Gradstein et al., 2020). Thus, we infer that the temperature increase observed within the Sables du Guépelle Fm corresponds to the MECO. It was not possible to point the onset of the MECO, marked by the so called pre-MECO cooling interval (Bohaty et al., 2009), because of the lack of potentially analysable fossils below the HMSM, in the Marnes et Caillasses Fm. However, the sampled shell from the Modiolaria zone seemed to mark the end of the peak of the MECO as temperatures continuously decreased above this stratigraphic level (Fig. 3).

Previous studies achieved in the Paris Basin have proposed varying
stratigraphic positions for the MECO, ranging between the Sables
d'Auvers Fm (Brachert et al., 2022; Kniest et al., 2024) and the Sables de
Cresnes Fm (Dawber et al., 2011; Huyghe et al., 2015). These differences
could be explained, for the Hampshire basin case, by a quite sparse
biostratigraphic resolution concerning the lower Bartonian and the
difficulty to make reliable correlation within the Paris Basin stratigraphy
and with nearby basins due to important lateral facies variations (Aubry,
1985; Briais, 2015; Morellet and Morellet, 1948; Pomerol, 1965). The
discrepancy with the MECO position reported by Huyghe et al. (2015)
within the Sables de Cresnes Fm (NP 17), may stem from the samples'
origin in historical collections, which lack precise stratigraphic control.
Most likely, this study relied solely on δ¹⁸O interpretations, with high
uncertainty on the δ¹⁸O<sub>w</sub> constraint, potentially leading to errors in
reconstructed temperature estimates and subsequent correlations with
the oceanic record. Based on our results, we propose a more precise
positioning of the MECO peak within the Sables du Guépelle, Sables

d'Auvers, and Modiolaria zone (NP16 zone).

This refinement provides a robust chemo-stratigraphic framework across the entire climatic event, enhancing the stratigraphic resolution for the Paris Basin with a precise stratigraphic location of the MECO, compared to previous studies that investigated this event in this area (Brachert et al., 2022; Dawber et al., 2011; Huyghe et al., 2015; Kniest et al., 2024).

5.2. Expression of the MECO in the Paris Basin

The reconstructed temperatures fluctuate throughout the Middle
Eocene in the Paris Basin (Fig. 4). Maximum annual temperatures first
increase from 38 ± 2 °C in the HMSM to maximum of 41 ± 5 °C at the
top of the Sables du Guépelle and Agrégat coquillier Fms, while mini-
mum annual temperatures rise from 15 ± 3 °C to 19 ± 2 °C over the
same interval. Summer temperatures then decrease to the top of the
Guépelle section (34 ± 3 °C) whereas winter temperatures remained
stable around 18 ± 3 °C.

Our results provide new insights into the expression of the MECO in
shallow-water environments. Fig. 4 reports the seasonal variations in
littoral seawater temperatures during the Middle Eocene in the Paris
Basin. Lutetian littoral temperatures are derived from the analyses of
turritellid gastropods (Haustator imbricatarius) and the giant gastropod
Campanile giganteum, as reported by Andreasson and Schmitz (1996),
Huyghe et al. (2012a) and De Winter et al. (2020).

Summer temperatures increased from the Lutetian, peaking at the
base of the HMSM to the Agrégat coquillier Fm before undergoing a
continuous decline until the end of the studied interval. Temperatures
during the colder months did not mirror the summer warming trend and
instead remained relatively stable throughout the Middle Eocene. This
disparity between summer and winter temperature trends resulted in a
significant increase in the seasonal temperature gradient shifting from
11 to 13 °C during the Lutetian to a maximum of 22–23 °C during the
warming peak. The seasonal gradient then decreased gradually to the
Sables de Beauchamp Fm and returning to pre-MECO values in the Sa-
bles de Cresnes Fm but should be treated with caution as no clumped
isotopes measurements were done and only one specimen was analyzed.

Our results diverge from previous studies about the MECO that used
corals (Brachert et al., 2022), and one bivalve (Kniest et al., 2024) from
the Bartonian deposits of the Paris Basin. Both studies reconstructed a
reduced seasonal thermal gradient of 7–8 °C and a wetter summer
climate during the MECO. These two works present very interesting
results but are subject to discussions according to their approach and by
comparison to our results. Indeed, the reason for this discrepancy re-
mains open to discussion but may partly arise from the use of collection-
derived fossil specimens, which lack precise stratigraphic positioning
within the Le Guépelle and Caumont sections, unlike the material
analyzed in our study. Moreover, Brachert et al. (2022), provided a high-
resolution reconstruction of the seasonal temperature gradient using
corals, whereas our study is based on mollusks.

. Their reconstructions relied in several key assumptions, particularly
that the δ¹⁸O<sub>w</sub> remained constant throughout the year. In contrast, our
results indicate that this parameter varied between winter and summer
(Fig. 3), suggesting that previous studies may have underestimated the
seasonal temperature gradient during this interval. Regarding Kniest
et al. (2024), only a single collection-derived fossil was analyzed, which
introduces both stratigraphic uncertainties and neglects potential tem-
perature fluctuations at meteorological timescales.

In this regard, our results, which indicate a pronounced seasonal
temperature gradient and peak precipitation during winter, show better
agreement with climate model simulations for the mid-Eocene (Baatsen
et al., 2020; Li et al., 2022).

5.3. Implication for the climate of the Paris Basin during the MECO

Understanding the impact of an hyperthermal event on the
hydrologic system is also of primary importance (Chen et al., 2018; Peris Cabré et al., 2023). We addressed this question throughout the fluctuations of the δ¹⁸O<sub>w</sub> and seawater salinity in function of temperature, i.e. the season of the year. We report the δ¹⁸O<sub>w</sub> values of seawater in the Paris Basin as a function of temperature derived from Δ<sub>47</sub> values on Fig. 5. In shallow-water environments, salinity values can be inferred directly from δ¹⁸O<sub>w</sub> values and our dataset reveals a strong correlation between these parameters. This relationship highlights distinct seasonal variations in salinity, with elevated values (~30 psu) during the warmest months and significantly lower salinity (~25 to 20 psu) during colder months. Only one sample, that shows very reduced salinities (between 10 and 15 psu) deviates from this correlation (Fig. 5). This sample was possibly exported from an inner part of the estuary or corresponded to a period of very important river discharge.

We present on Fig. 6 a paleoenvironmental reconstruction for winter and summer periods of the Paris Basin during the MECO, integrating the hydrologic system. Under the warm climatic conditions of the Middle Eocene, the seasonal ratio between evaporation and precipitations probably played a critical role in shaping salinity dynamics within the Paris Basin estuary, even if it has been demonstrated on modern estuaries of Florida that precipitations play a major impact than evaporation on this parameter (Summer and Belaineh, 2005). Thus, during summer, increased evaporation likely increased salinity. Conversely, the low salinity observed during colder periods may particularly reflect a seasonal influx of freshwater, possibly driven by enhanced precipitation and runoff during winter months, as observed in tropical modern estuaries (Summer and Belaineh, 2005). The interplay between these processes is further contextualized by the paleogeography of the Paris Basin during the Middle Eocene (Figs. 1 and 6). These findings agree with modelling results arguing for strongly monsoonal climate at this period (Baatsen et al., 2020).

Our results provide a detailed characterization of the climate in the Paris Basin during the MECO. Based on the reconstructed seasonal temperature gradient (~20 °C) and evidence of warm summers coupled with a rainy winter season, the climate aligns with the characteristics of a hot summer Mediterranean climate, according to Köppen's classification. However, contemporary Mediterranean climates typically exhibit maximum mean summer temperatures of ~30 °C. In contrast, our

reconstructions suggest that seawater temperatures in the Paris Basin during the MECO may have reached extreme values of up to ~40 °C during the warmest months, consistent with higher pCO₂ levels (~1000 ppm) during the Middle Eocene (Hönisch et al., 2023).

This reconstructed thermal regime significantly exceeds the thresholds observed in present-day Mediterranean climate type, leading us to propose that the Paris Basin experienced a "super-hot summer Mediterranean climate" during the MECO. Such a climatic state would have been marked by unprecedented summer heat, likely resulting from amplified greenhouse forcing and elevated atmospheric CO₂ concentrations characteristic of hyperthermal events. This climatic configuration underscores the regional amplification of global warming signals during hyperthermal events, particularly in mid-latitude basins like the Paris Basin.

The asymmetrical pattern of freshwater input and rainfall identified from our results agrees with numerical models that also demonstrate higher precipitations during winter in western Europe, reflecting a monsoonal trend (Baatsen et al., 2020; Li et al., 2022).

### 5.4. Expression of the MECO and implications for the future

Fig. 7 presents a comparison in the climatic evolution between the Paris Basin and the oceanic domain. In the oceanic realm, the MECO led to a 4–6 °C rise in deep and sea-surface temperatures (Bijl et al., 2010; Bohaty et al., 2009; Bohaty and Zachos, 2003; Cramwinckel et al., 2018; Sluijs et al., 2013), with peak temperatures reaching 33 °C in the eastern equatorial Atlantic Ocean (Cramwinckel et al., 2018) and 28 °C in the eastern Tasmanian Plateau (Bijl et al., 2010; Bijl et al., 2009). In contrast, shallow-water temperatures in the Paris Basin peaked at 41 °C. This can appear extreme, but this apparent intensification might likely be accentuated by the reduced water column thickness at the Agrégat coquillier Fm., where extremely shallow conditions promote enhanced surface water warming (Aguerre-Chariol, 2023). Furthermore, the pattern of global oceanic temperature increase mirrors that of mean annual and summer temperatures in the Paris Basin, lacking the climatic pattern during cold months. Our results emphasize the importance of integrating shallow-marine records with global datasets to better understand the spatial and temporal complexity of these hyperthermal events.

Concerning the continental domain, Δ₄₇ analyses of lacustrine and paleosol sediments of North-West America indicates a mean annual warming of 9 °C during the MECO (Methner et al., 2016). The climate went from hot and semi / sub-humid during the MECO to dry and cold during the post MECO cool in North-West America and the seasonal gradient seem stronger during the MECO than after (Mulch et al., 2015), suggesting similar trends compared to the Paris Basin.

The evolution of future seasonality in the context of global warming remains uncertain and depends on the climate model used. Most studies agree on a significant rise in summer temperature, but projections for winter temperatures vary. Recent works suggest that European winter temperatures will increase during the 21st century (Carvalho et al., 2021). In contrast, other models indicate that tropical winter SST may decrease slightly due to stronger winter tropical wind (Sobel and Camargo, 2011). Our results show that during the MECO hyperthermal event in mid-latitudes, where tropical climate prevailed, only warm-season temperatures increased significantly, while colder-month temperatures remained stable, confirming a stronger summer response to global warming and relatively stable winter temperatures in these areas.

## 6. Conclusions

This study presents a high-resolution reconstruction of seasonal seawater temperatures in the Paris Basin during the Middle Eocene Climatic Optimum (MECO), using δ¹⁸O and Δ₄₇ analyses of shallow-marine mollusks. Our results reveal a substantial amplification of seasonal temperature variability, with summer temperatures rising by up to

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Correlation graph between the temperatures calculated directly from Δ₄₇ measurements with δ¹⁸O<sub>w</sub> values also calculated from Δ₄₇ measurements. Salinity is calculated from δ¹⁸O<sub>w</sub> values following the equation of Pierre (1999) for the modern Mediterranean Sea.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Simplified 3D diagrams illustrating the hydrological system of the Paris Basin during the MECO. During the summer period, the temperature is high, rainfall is reduced, and evaporation is elevated, implying normal salinity in the estuary. In winter, temperature and evaporation decrease, while rainfall and runoff increase, thus decreasing salinity in the coastal environments of the basin.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Compilation of oceanic deep-sea and sea surface temperatures obtained by various proxies over the Middle Eocene (Cramwinckel et al., 2018) and Paris Basin littoral temperatures obtained from this study and previous works for the Lutetian from (Andreasson and Schmitz, 1996; Huyghe et al., 2012b; De Winter et al., 2020) over the Middle Eocene. Age and calcareous nannofossils zonation are from Gradstein et al. (2012).</figcaption></figure>

10 °C, while winter temperatures remained relatively stable. This seasonal asymmetry led to a pronounced rise in thermal seasonality, reaching a maximum of ~22 °C.

Peak summer temperatures approached 41 °C, suggesting the Paris Basin experienced an extreme form of Mediterranean climate, consistent with contemporaneous elevated atmospheric pCO₂ concentrations. Additionally, reconstructed seawater δ¹⁸O and salinity values highlight a seasonal hydrological cycle dominated by evaporation during dry summers and enhanced freshwater influx during wetter winters, consistent with a monsoonal-like climate regime.

These findings improve the stratigraphic resolution of the middle Eocene within the Paris Basin from chemostratigraphic evidence. It also underscores the importance of incorporating shallow-marine records into global paleoclimatic reconstructions. The reconstructed climate offers an informative analogue for future climate scenarios under continued anthropogenic warming, particularly for regions currently experiencing Mediterranean-like climates.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.palaeo.2026.113547.

### CRediT authorship contribution statement

**Loïc Marlot:** Writing – original draft, Formal analysis, Data curation, Conceptualization. **Damien Huyghe:** Writing – original draft, Validation, Supervision, Project administration, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Justine Briais:** Writing – original draft, Validation, Supervision, Project administration, Funding acquisition, Data curation, Conceptualization. **Mathieu Daëron:** Writing – original draft, Supervision, Methodology, Formal analysis.

**Christine Flehoc:** Writing – original draft, Validation, Supervision, Methodology, Conceptualization. **Laurent Emmanuel:** Writing – original draft, Validation, Methodology. **Didier Merle:** Writing – original draft, Validation, Methodology, Data curation. **Olivier Aguerre-Chariol:** Writing – original draft, Validation, Formal analysis, Data curation.

**Franck Lartaud:** Writing – original draft, Validation, Supervision, Funding acquisition, Data curation.

### Declaration of competing interest

The authors declare no conflict of interest.
## Acknowledgements

The authors would like to thank the RGF (Référentiel Géologique de la France) and the CNRS for their support through the MITI interdisciplinary program *Le Temps* (ARCHIVE), which funded this study. This research was further supported by a PhD grant from the French Ministry of Industry and the BRGM (Bureau de Recherches Géologiques et Minières) awarded to Loïc Marlot. We are also grateful to the Conseil Départemental du Val-d'Oise for granting us access to the protected site of Le Guepelle. Finally, we would like to express our thanks to Nicolas Seigneur and Léo Szewczyk for their valuable proofreading of this article. The authors also thank Editor Bing Shen and two anonymous reviewers for their constructive comments, which helped improve the manuscript.

## Data availability

The authors confirm that all data necessary for supporting the scientific findings of this paper have been provided.

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