# Short Papers from the 19th French Conference of Sedimentology

Jean-Yves Reynaud, Eric Armynot Du Châtelet

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Jean-Yves Reynaud, Eric Armynot Du Châtelet. Short Papers from the 19th French Conference of Sedimentology. 19e Congrès Français de Sédimentologie, Nov 2024, Villeneuve d'Ascq, France. 84, Association des Sédimentologistes français, 2025. http://hal-05279613

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SHORT PAPERS
FROM THE
19th
FRENCH
CONFERENCE
OF
SEDIMENTOLOGY

SHORT PAPERS

DES
AST
ASSOCIATION
SEDIMENTOLOGISTES
FRANÇAIS
ASF n°84 Publication

Short Papers from the 19th French Congress of
Sedimentology

At the conclusion of the 19th French Sedimentology Congress that was held in Lille in November 2024, the association board and the organizers decided to highlight a set of research papers, oral or poster, in the form of extended abstracts that would match the standards of scientific journal articles but with a faster online publishing.

The selection of contributions was made on a voluntary basis, with the support of the session convenors. Each submitted article was then peer-reviewed, with non-anonymous reviewers, ensuring a high level of quality. The full process lasted 3 months until the last paper was accepted for online publication on March 2025.

Each paper produced under a Credit-By CC licence, has a DOI registrered on CrossRef, and is landed on the HAL repository of French Public Research Institutes and Universities (https://hal.science/). This ensures an improved accessibility and visibility, as well as a perennial Open Access to the publication on internet.

This volume gathers all these short papers. Free for the readers, free for authors, well-born and well-grounded, this is a Diamond Publication that we trust academic authorities will consider as relevant for the evaluation of research in sedimentology and researchers publishing at ASF.

We thus hope that there will be numerous other ASF Short Papers in the future!

The volume editors,

Jean-Yves Reynaud & Eric Armynot du Châtelet

<figure><img src="image_2.png" /></figure>

The French Association of Sedimentologists (ASF) is a non-for-profit society based in Paris, France. It is dedicated to the collection and dissemination of research outcomes and training products related to Sedimentology in a broad sense. It publishes Congress Proceedings, Books and Field Trips. More info on https://www.sedimentologie.fr/.

ISBN: 2-907205-83-8

DOI: 10.70665/IPJT8001

ISSN: 0990-3925

CC-BY © 2025
Contents

Billières, C., Billy, J., Bourillot, R., Féniès, H. Characterization of barchanoid dune systems on the Cap Ferret spit (Aquitaine) 1–4

Bindelli, L., Haspert, F., Re, M., Hernandez, L., Kazmierski, L. A first approach to modelling fine-sediment dynamics in the Santa Cruz River estuary, southern Patagonia 5–8

Bourrier, E., Mathieu, G., Leyrit, H., Le Bivic, R., Ottavi-Pupier, E., Denis, M., Duquennoy, J., Dujanny, A. Distribution of rootless cones and landforms resulting from water-lava interactions at hagi in the Aðaldalur valley (Iceland) 9–12

Boussagol, P., Vennin, E., Duret, M., Bonnotte, A., Noirot, E., Deulvot, C., Bundeleva, I. Biotic processes involved in the mineralization of tufa deposits 13–20

Cazalis-Henry, S., Seuront, L., Delaeter, C., Hennion, C., Deplechin, O., Spilmont, N. Impact of microplastic white tide on the light and dark metabolism of the macroalgae Fucus serratus: petroleum-based vs biosourced polymers 21–26

De Perthuis, A., Gagnaison, C., Potel, S. The Centre-Val de Loire region (France) in relation to the Middle Miocene climatic optimum: a more accurate stratigraphic dating 27–30

Didier, J., Fanget, B., Jaillet, S., Mallet, E., Monvoisin, G., Régnier, E. Retour d'expérience de carottage avec du matériel léger en eau peu profonde à l'aide d'une aiguille vibrante à béton 31–34

Dodinet, P., Kowalewski, I., Kohler, E. Organic matter in the sediments of Guaymas Basin, a study by Rock-Eval 7s® 35–38

Faÿ, O., Kroth, M., Claes, H., Farrant, A.R., Vandycke, S., Woods, M. Chalk marl seams: not quite what they seem.. 39–42

Faÿ, O., Lasseur, E., Paquet, F. Flamborough head's Upper Cretaceous Chalk: insights into a mixed deep-sea sedimentation system 43–47

Frigola, C., Grégoire, G., Murat, A., Poizot, E., Haitz, F., Le Lorec, B., Peuziat, I., Costa, S., Méar, Y. Carbonate sources and dynamics in coastal environments: application off the coast of Saint-Valéry-en-Caux (Seine-Maritime) 49–54

Garcia, C., Brasseur, B., Mathieu, L.-A., Michaelis, D., Bacon, J., Saulnier-Copard, S., Antoine, P. Weichselian lateglacial and Holocene fluvial dynamics variability in the Somme basin: the evolution of an alluvial peatland 55–58

Le Lorec, B., Grégoire, G., Murat, A., Gallon, R., Méar, Y., Poizot, E., Tessier, B., Dézileau, L., Poprawski, Y., Laignel, B. Evolution of sedimentary depositional environments from the iron age to the present day in Cul-de-Loup Bay (Normandy, France): an illustration of intertidal environment vulnerability under significant human impact 59–65

Murat, A., Poizot, E., Grégoire, G., Le Goff, V., Weill, P., Mensens, C., Pancrazzi, L., Frigola, C., Méar, Y. The dynamics of silica sediment at the ECUSED site in the Baie de Seine based on the COVA methodology: (1) glauconite traced by arsenic.... 67–73

Ortiz, A., Baudesson, A., Flehoc, C., Guerrot, C., Lopez, X., Douez, O., Rumbach, G. Hypothesis on the karstification/dolomitization of the Barremian reservoir in Parentis Basin 75–80
Reynaud, J.-Y., Ouchaou, R., Trentesaux, A., Seif Dine, R., Robin, N., Raynal, O., Rohmer, F., Buchwald, T., Antoine, P. MASW-2d seismic as a tool for investigating the architecture of Holocene coastal deposits: examples in the North of France (Boulonnais and Flemish coastal plain) 81–85

Seuront, L. Contamination of Pyrenees mountain streams by agricultural synthetic fibre ropes and a first assessment of their potential effect on aquatic and terrestrial gastropods 87–95

Tessier, B., Lortie, T., Cuitino, J.I., Reynaud, J.-Y., Scasso, R.A., Richiano, S.M., Pancrazzi, L., Salameh, E., Condomines, M., Dezileau, L., Lekander, M. Rhythmites preserved in intertidal flat successions of the hypertidal Santa Cruz – Chico River estuary (southern Patagonia, Argentina) 97–102

Billy, J., Baudouin, V., Garcin, M., Noury, G. Beneath the Vendée dunes: former coastal systems, interface between marsh and foreshore revealed by ground-penetrating radar and core-penetration testing 133–137

Décultot, C., Chevalier, N., Denis, L. Impact of dike removal on carbon balances in estuarine intertidal zones 103–108

Denis, L., Kocyla, L., Duong, G. The impact of ploughing on sediment mineralization rates in the salt marshes of the Somme estuary 109–114

Moreau-Ledegen, M., Martinez, M., Aguirre-Ureta, B., Ben Saïd, E., Rabu, B., Pellenard, P., Pierson-Wickmann, A.-C. Deep-time astronomical forcing on organic carbon isotope values: a tool for global correlation at 10^5-year scale 139–143

Oueslati, W., Ben Mna, H., Helali, M.A., Mghirbi, T., Ben Mefteh, A., Ben Jeddi, S., Mesnage, V. Measuring alkalinity fluxes at the water-sediment interface in aquatic ecosystems of the Tunisian littoral: impact on pH and inorganic carbon storage 115–118

Plègue, K., Spilmont, N., Denis, L. Carbon budgets within temperate coastal ecosystems and comparison with picard estuaries (north of France) 145–150

Poizot, E., Méar, Y., Murat, A., Frigola, C., Grégoire, G., Le Lorec, B. Multi-proxy sediment trend analysis (MSTA): sedimentary dynamics from source to sink 119–124

Uguen, M., Gaudron, S.M., Cozzolino, L., Seuront, L. Influence of the reproductive cycle and sex on microplastic toxicity in mussels 125–132
# CHARACTERIZATION OF BARCHANOID DUNE SYSTEMS ON THE CAP FERRET SPIT (AQUITAINE)

Clélia BILLIÈRES¹, Julie BILLY², Raphaël BOURILLOT³ and Hugues FÉNIÈS⁴

¹ BRGM, Bureau de Recherche Géologique et Minières, F-33600 Pessac, France;
Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France

² BRGM, F-33600 Pessac, France

³ Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, F-33600 Pessac, France

⁴ CV Associated Engineering

Corresponding author: c.billieres@brgm.fr

**Abstract:** The Cap Ferret spit (Atlantic coast, France) is defined by barchanoid dune systems formed by a large input of sand remobilized by aeolian dynamics. GPR investigations have revealed distinct internal architectures within the barchanoid dunes, mainly characterized by eastward-dipping reflectors and complex structures illustrating sand remobilization. These structures highlight a progressive evolution in dune morphology, from west to east, i) low topographic and interconnected dunes without specific morphology (proto-barchans), ii) interconnected dunes with crescent-shaped morphology (barchanoid ridge) and iii) isolated dunes with a well-defined crescent-shaped morphology (isolated barchans). This study investigates these distinct stages of development, linking internal stratigraphy and geomorphological evolution, and provides insights on the dynamics of coastal barchanoid dune formation and migration.

**Keywords:** Barchanoid dunes, Spit, GPR, Internal architecture, Dune migration, France.

## 1 INTRODUCTION

The coastal zone is the interface between marine and terrestrial environments and can extend several kilometers inland. It represents a complex system with interactions between land and marine dynamics processes. In the literature, the beach/dune interface, coastline or geomorphological changes are the most documented topics (e.g. Brossard and Nicolae Lerma., 2020), but fewer studies include the retro-littoral domain. Indeed, holistic study of the coastal zone is crucial, especially in the context of coastal evolution and territorial adaptation to climate change, and requires consideration of the marine part, the land/sea interface, and the inland domain. For example, sedimentary spits and coastal barriers need to be considered on their entire geosystem, i.e., along an ocean/barrier/lagoon continuum.

Barchan dunes are one of the most iconic dune categories in the literature and can be located in desert areas as well as along the coast (e.g. Bristow, 2019; Fu et al., 2019; Gomez-Ortis et al., 2008; Oliveira, et al., 2008). They are mainly defined as crescent-shaped, with the convex side facing into the wind and horns pointing downwind, and literature suggests that they are formed on dry substrates. Unlike desert barchanoids, those on the coast remain understudied, with limited documentation on their internal structures. Studies referring to barchan migration or geomorphological evolution often consider high dome-shaped sand deposits at starting point (e.g., numerical models or theoretical evolutionary models based on distinct wind directions, etc., Hersen, 2004). However, these idealized cases are not representative of the barchan dune initiation, especially in coastal areas.

This study focuses on the inland domain of an emblematic coastal system on the Atlantic coast of Gironde, the Cap Ferret spit, a 23-km-long and 5-km-wide sedimentary spit (Fig. 1-A). This coastal spit has evolved over the last 3000 years through southward progradation (Féniès et al., 2005), shifting the Leyre estuary and shaping the geomorphology of the Arcachon Basin. The spit displays several dune systems and interdune areas, in equal proportion. This spit is particularly well suited for examining the coastal zone in its entirety, nevertheless only the lagoonal part and ocean domains were studied previously (e.g. Allard, et al., 2009; Féniès, et al., 1998; Nahon, 2018). The terrestrial domain (spit itself) has been given little consideration, apart from the coastline and the modern foredune (e.g. Brossard and Nicolae Lerma., 2020). However, the spit configuration provides an excellent opportunity to explore the complexity and interconnection of the coastal environment as well as the study of barchanoid dune systems in coastal environments. The study focuses specifically on the different dune systems present on this spit. Analysis of 2016 LIDAR data (1 m resolution) enabled the characterization of four dune typologies on the Cap Ferret spit (Billières, 2022). From west to east (Fig. 1-B), these include a longitudinal active foredune (modern coastal dune, not studied here) followed by barchanoid dune systems fixed since the mid-19th century (maritime pine forestation), and defined as: (i) proto-barchan (PB) dunes, referred to as undetermined dune type in Froidefond et al. (1985); (ii) barchanoid ridges (BR); and (iii) isolated barchans.
Here, each barchanoid dune displays distinct morphological characteristics, notably in height or shape. What insights can be obtained regarding their internal architecture? In this study, the internal structure of coastal barchans is analyzed using ground-penetrating radar (GPR) profiles, which enable the identification of sedimentary structures.

<figure><img src="image_3.png" /><figcaption>Figure 1. A. Location map of the studied areas; B. Barchanoid dune typologies identified on the Cap Ferret Spit (Atlantic coast, France): Proto-barchan, Barchanoid ridge and isolated barchan.</figcaption></figure>

## 2 MATERIALS AND METHODS

Several GPR surveys have been carried out between 2022 and 2024 to provide insights into the dunes' internal structures and their underlying substrate. GPR is a non-intrusive geophysical method, based on the transmission and reflection of electromagnetic pulses to visualize sedimentary structures. Two instruments were used: a Mala ProEx system with three antennas operating at center frequencies of 100, 250 and 500 MHz and a dual frequency 200-900 MHz IDS-CHASER, which enables deep penetration while maintaining high resolution even with a high frequency. A total of 65 km of profiles were recorded along the spit. REFLEXW V10.5 software was used to process data, applying corrections such as background removal, gain adjustment and topographic correction, etc. The interpretation of these processed profiles was conducted using the DELPH Road Map software (V5), which also allows 3D visualization of the data. A constant radar velocity of 0.14 m/ns was estimated, with CMP method (Billy et al. 2020), for the time to depth conversion. Profile analysis allowed (i) visualization of internal architecture via reflectors, (ii) identification of various dune facies, (iii) calculation of internal structure dip angles, and (iv) dune development phases identification. Proto-barchan and barchanoid ridge systems were the most extensively surveyed due to their accessibility (via dune paths).

## 3 RESULTS AND DISCUSSION

### 3.1 Internal architecture of barchanoid dunes

The internal architecture of the three families of barchanoids dunes analyzed in this study is primarily characterized by eastward-dipping oblique reflectors (Fig. 2-B,C), downlapping onto the top of a basal unit. In their western part, these reflectors are truncated at the top, illustrating the effects of dune migration by aeolian processes. This erosion surface represents the current dune topography and is concordant with the 19th century dune topography (at the period of dune fixation by vegetation). GPR images also reveal distinct sequences within dunes, which correspond to phases of sediment mobilization influenced by wind dynamics. From west to east:

- Smaller PB dunes are defined as low altitude (5-15 m) interconnected dunes (Fig. 1-B.2). They are featured by homogeneous internal structures, with reflectors displaying gentle dip angles, from 5° to 15°, becoming steeper toward the eastern part of the dunes. Some PB dunes reach twice the altitude of smaller PB dunes (15-30 m) suggesting dune overlap. These are characterized by reflectors dipping up to 25° and draping stratification at the dune crest.

- BR dunes are taller than PB (25-40 m) and the crescent-shaped dune is well defined (Fig. 1-B.3) but dunes are still interconnected. GPR images (Fig. 2-B) demonstrate greater complexity of their
internal architecture. Their western parts share similarities with small PB in terms of facies, but with steeper dip angles, ranging from 25° to 30°. Their eastern parts show distinct overlapping units. It is interpreted here as evidence of eastward dune migration, with dunes growing and overlying existing sand accumulations (former dunes or part of nearest dunes).

- Barchans are individual dunes (Fig. 1B.4,5), with a clearly defined crescent shape. Its internal architecture is close to BR dunes in terms of prograding and overlapping facies, the dip angles (10 to 25°) and heights (25-40 m) but barchans can be wider (up to 500 m).

The internal architecture described below is coherent with findings by Bristow (2019) and Oliveira et al. (2008) for desert and coastal barchans, respectively. Indeed, in both studies, key features include oblique and parallel reflectors indicative of sedimentary layering formed by wind and sediment transport. Prograding facies align with sediment migration direction, showing stratification that flattens near the crest of the lee slope. Dip angles range from 24° to 31° (Bristow, 2019) decreasing near the slip face. Differential slope inclinations are also observed, with the windward slope shows a higher dip angle compared to the leeward slope, reflecting variations in sediment deposition and transport processes.

<figure><img src="image_4.png" /><figcaption>Figure 2. A. Location map of the GPR profile studied; B. Example of a GPR profile of a barchanoid ridge; C. Representative 2D model of a barchanoid ridge (Vertical Exaggeration = 9)</figcaption></figure>

## 3.2 Barchanoid development model: initiation to individual barchan

Geomorphological observations complete these internal architecture analyses, revealing a main west to east gradient across the 5-km-wide spit in terms of dune heights and shape. This west to east evolution pattern reflects the gradual transformation of the dunes, which is also identified in their internal architecture (e.g. oblique reflectors, slopes, overlying units). The evolution of dune shapes begins with sand accumulation (embryonic dune) lacking specific forms (PB), progressing to interconnected dunes showing the initiation of crescent shape (BR) and to individual dune (barchans). This pattern highlights distinct phases of barchanoid dune development that are rarely described in the literature.

The progression from PB dunes to BR dunes marks a critical phase in the evolutionary sequence, where dunes evolve from embryonic, poorly organized sand masses (PB) into larger, more structured forms with increased internal complexity (BR). The final stage, represented by mature barchans, reflects the isolation and individualization of dunes, completing the maturity sequence. These individual barchans continued their eastward migration with this mature shape.

# 4 CONCLUSION

By integrating GPR-based internal architecture data with geomorphological observations, the processes underlying the transition from PB dunes to BR dunes, and then to mature barchans have been highlighted. Our study site provides a rare opportunity to observe the earliest phase of this evolutionary sequence, with the observation of PB dunes. These embryonic dunes are scarcely documented in the literature. Additionally, the poorly defined morphology of PB dunes often excludes them from studies in contrary of well-defined
shape such as BR dunes or isolated barchans. Their identification at the Cap Ferret Spit enriches the understanding of coastal sedimentary dynamics. Without the intensive vegetation of the spit in the 19th century, this information, particularly on PB dunes, might have been partly masked by the natural mobility of the dunes.

Moreover, the evolutionary sequence observed in this study offers a broader framework for interpreting coastal barchanoid systems in other regions. The evolutionary pattern in dune height, width, and internal architecture reflects a maturity gradient that is likely influenced by sediment availability, wind regime, and local topographic constraints, need to be further investigated. The ability to document and preserve early dune stages provides insights into the initial conditions that precede more developed forms, contributing to a comprehensive understanding of coastal barchanoid dune dynamics.

## 5 ACKNOWLEDGMENT

The work is part of the DUNES research program, co-funded by the BRGM, The Nouvelle-Aquitaine Region, the ONF, the municipality of Lège Cap Ferret, the Landes Department, Vermilion REP SAS, CCGL, AEAG, and the EC2CO HYBIGE CNRS program. The authors would like to thank our ONF collaborators, Eric Lenain and Aurelien Brossard, and our BRGM colleagues Nicolas Bernon, Pierre Bourbon and Marie Branellec for their field assistance. This paper benefitted from the reviews of Marie-Hélène Ruz and Anne-Lise Montreuil.

## 6 REFERENCES

Allard, J., Chaumillon, E., Féniès, H., 2009. A synthesis of morphological evolutions and Holocene stratigraphy of a wave-dominated estuary: The Arcachon lagoon, SW France. Continental Shelf Research, 29, 957-969.

Billières, C., 2022. Etude du système dunaire de la flèche du Cap Ferret : Synthèse de l'état des connaissances, Analyses morphologique des dunes, SIG, Traitement, analyse et interprétation des données Géoradar. Rapport de stage de fin d'étude. BRGM, p. 20-23.

Billy, J., Baudouin, V., Portal, A., Deparis, J., Bitri, A., Garcin, M., 2020. An Innovative Approach for a Comprehensive Characterization of Coastal Dune Systems through Internal Architecture and the Associated Intrinsic Geophysical Properties. Journal of Coastal Research, 95, 387-391.

Bristow, C., 2019. Bounding surfaces in a barchan dune: annual cycles of deposition? Seasonality or Erosion by Superimposed Bedforms?. Remote sensing, 11, 965.

Brossard, V., Nicolae Lerma, A., 2020. Geomorphologic characteristics and evolution of managed dunes on the South West Coast of France. Geomorphology, 367, 107312.

Féniès, H., Faugères, J.C., 1998. Facies and geometry of tidal channel-fill deposits (Arcachon Lagoon, SW France). Marine Geology, 150, 131-148.

Féniès, H., Lericolais., 2005. Internal architecture of an incised valley-ll on a waveand tide-dominated coast (the Leyre incised valley, Bay of Biscaye, France). Comptes Rendus Geoscience, 337, 12571266.

Froidefond, J.M., Legigan, P., 1985. La grande dune du Pilat et la progression des dunes sur le littoral aquitain. Bull. Inst. Géol. Bassin d'Aquitaine, Bordeaux, n°38, p.69-79.

Fu, T., Wu, Y., Tan, L., Li, D., Wen, Y., 2019. Imaging the structure and reconstructing the development of a barchan dune using ground-penetrating radar. Geomorphology, 341, 192-202.

Gomez-Ortiz, D., Martin-Crespo, T., Rodriguez, I., Sanchez, M.J., Montoya, I., 2008. The internal structure of modern barchan dunes of the Ebro River Delta (Spain) from ground penetrating radar. Journal of Applied Geophysics, 68, 159-170.

Hersen, P., 2004. Morphogenèse et Dynamique des Barchanes. Thèse de l'Université Paris-Diderot - Paris VII. 243 p.

Nahon, A., 2018. Évolution morphologique actuelle d'une flèche littorale holocène : le Cap Ferret, à l'embouchure du Bassin d'Arcachon. Thèse de l'Université de Bordeaux, 248 p.

Oliveria, J., Medeiros, W., Tabosa, W., Vital, H., 2008. From barchan to domic shape: evolution of a coastal sand dune in northeastern Brazil based on GPR survey. Brazilian Journal of Geophysics, 26(1).
# A FIRST APPROACH TO MODELLING FINE-SEDIMENT DYNAMICS IN THE SANTA CRUZ'S RIVER ESTUARY, SOUTHERN PATAGONIA

Lucas BINDELLI¹,², Federico HASPERT¹, Mariano RE¹, Lucas HERNÁNDEZ¹ and Leandro KAZMIERSKI¹

¹Subgerencia Laboratorio de Hidráulica, Instituto Nacional del Agua (INA), Argentina.

²Facultad de Ingeniería, Universidad de Buenos Aires (FIUBA).

Corresponding author: lucasbindelli@gmail.com

**Abstract:** A Hydrodynamic numerical model coupled with a Particle Tracking python Library was generated, with the objective of describing particle plume dynamics. A general comparison of modelled plume dynamics with fine-sediment plume dynamics obtained from SENTINEL-2 satellite images was made. Results show a geostrophic-like movement for particles going outside the estuary into the sea at ebb tide, with most of them re-entering at flood tide, and some leaving the estuary's influence zone towards North-East. The comparison with satellite images shows good agreement, though further details should be put into particles concentration and location for a better representation.

**Keywords:** Numerical modelling, Fine-Sediment plume Dynamics, Hypertidal, Estuaries, Argentina.

## 1 INTRODUCTION

Santa Cruz's River Estuary (SCRE), located in Santa Cruz Province, Argentina, presents an area of 141 km², receiving river discharges of both the Santa Cruz and Chico River, with mean values of 715 m³/s and 25 m³/s respectively (Bindelli, 2018) and containing visible amounts of clay sediment. It also presents a semi-diurnal and macro-tidal regime that can reach a tidal range of up to 12 m. As stated in Dinápoli et al. (2024), it is part of a region of astronomical dominance, with river discharges and wind having minor influence in hydrodynamics. The sediment plume can be observed in satellite images, presenting a clear tidal influence due to the estuary's high tidal range, and presumably some seasonal influence as well, due to the river's seasonal regime. As for the fine sediment characterization, samples were taken from the estuary as part of an Environmental Assessment for engineering works on the area, presenting Kaolinite as the main component, followed by some presence of quartz (Carol et al., 2017).

In this work, a coupled hydrodynamic and particle tracking model is used to describe the macro-tidal dynamics of the SCRE. Then, as a first approach to modelling the fine-sediment dynamics, a comparison of these results to SENTINEL 2 satellite images is made.

## 2 MATERIALS AND METHODS

The modelling strategy was based on a system of nested models (Fig. 1) to achieve greater discretization/precision/accuracy inside the estuary, using module FLOW of Delft3D software (Deltares, The Netherlands).

<figure><img src="image_5.png" /><figcaption>Figure 1 - Nested model strategy and domains. SCRE: Santa Cruz's River Estuary.</figcaption></figure>

For this, first a regional model of the Argentinian Sea (named Rank 0) was created, with an extension able to properly simulate tidal dynamics in the Continental Shelf and forced by 14 tidal components obtained from
the global model FES2014b. Then, a nested model was built (Rank 1), using as open boundary conditions the results generated by the Rank 0 model, increasing grid resolution at a 1:5 ratio to preserve an acceptable precision, without disproportionally increasing computational time. This nesting technique was applied two more times, creating a Rank 2 and finally a Rank 3 model, accurately propagating tidal information from open seas towards the estuary's mouth (Bindelli et al., 2023). Ranks 0 to 2 were modelled in 2D, while for Rank 3 a 3D representation was made, using a grid resolution of 100 m and a sigma model composed of 20 layers with varying distribution to accurately represent bottom friction effects. The bathymetry was generated by combining different sources. GEBCO (General Bathymetric Chart of the Oceans) global data with 30" resolution was used for the deeper parts, and for the shallower parts, bathymetric charts provided by the Hydrographic National Service (SHN), coastal lines obtained from LANDSAT satellite images post-processing and bathymetric surveys of specific parts of the estuary were used.

For calibration, a combination of surveyed water level and velocity data was employed. Water levels were measured near the estuary's mouth for 3 days, and 8 velocity profiles were measured with an ADCP (Acoustic Doppler Current Profiler) at different points in the estuary and in different times of the tidal wave, for a period of 10 to 25 minutes each. Results range between 0.75 to 2 meters per second with constant direction during each measurement. The calibration parameters were Manning's coefficient (0.025) and horizontal eddy viscosity (540 square meters per second) showing good agreement with all the data (Fig. 2).

<figure><img src="image_6.png" /><figcaption>Figure 2 – Comparison of levels and velocities between measured and modelled data.</figcaption></figure>

Once the hydrodynamic model was calibrated, a coupling with a particle tracking module was made, using a python library named OceanParcels. This library is a Lagrangian simulator that takes as input velocity fields generated by the Delft3D model. Originally created for ocean circulation, it can be used for multiple applications, such as active or passive particle tracking of water, plankton, plastic and fish<fnref n="1" />. In this case, it was used for representing clay (Kaolinite) coming from the Santa Cruz River into the estuary and its interaction with the tidal dynamics.

This work being a first approach to modelling fine sediment plume dynamics, some hypotheses were made to simplify the analysis:

- River discharge is minimal compared to tidal flow (see Tab. 1 in Results), therefore only a fixed discharge of 715 m³/s from the Santa Cruz River is included. This implies that particle movement is mainly due to tidal dynamics and that the influence of the river discharge is of second order, disregarding the river's cyclic behavior. The analysis is then performed by comparing stages of low and high tide rather than comparing times of the year.

- For this stage, only depth-averaged velocity fields were used to perform particle tracking, neglecting vertical displacements, since no threshold depth was established on satellite images for the visibility of the fine sediment plume.

- There is no diffusion process considered in the simulations, since the estuary is well mixed (no stratification), which leads to a homogeneous distribution of sediment in the estuary.

- Since the main kind of particles present in the estuary are Kaolinite (Carol et al., 2017), which is an insoluble and stable clay material, the modelled particles are presumed to be inert (non-reactive with the surrounding environment).

<footnote n="1">¹ https://oceanparcels.org/</footnote>
- Since simulations are short in morphological scale, no morphological changes are included, and only fixed amounts of particles are modelled for a better understanding of particle behavior in this time span.

The simulation period was three months, to allow the comparison of satellite images of fine sediment plumes at different tidal levels. Also, different kinds of simulations were undertaken: On the one hand, simulations were made to obtain characteristic parameters of the overall movements, like particle's residence time inside the estuary and the length of the trajectory for neap and spring tides. This was made by placing small groups of particles at the interface between the Santa Cruz River and the estuary. On the other hand, a homogeneous distribution of particles along the entire estuary's surface was laid, to get the equivalent of the sediment's plume dynamics.

Finally, the results obtained were compared to SENTINEL 2 images. At this stage, only schematic comparisons were made, aiming at capturing the fine sediment plume's dynamic and not a detailed characterization of the concentration of sediment in the estuary.

## 3 RESULTS AND DISCUSSION

Regarding Delft3D-FLOW simulations, Table 1 shows the variation of the mean discharge of Santa Cruz River, which denotes a seasonal regime, but if this is compared to the tidal flow rate that enters or exists the estuary at each cycle (Tab. 2), river discharge accounts for 2% to 8% of total flow, with mean value of 4%. Also, the maximum ratios encountered only happen at specific and brief periods of time, which is when river discharge is at its maximum (during March) and astronomical tide is on a neap cycle. This validates the hypothesis that river discharge is of secondary importance to modelling fine-sediment plume dynamics and can be replaced by mean discharge on a first approach. These calculations were made using measured data at Charles Fuhr Hydrometric Station (located in Santa Cruz River, upward from the estuary), and tidal flow was obtained directly from the model by computing flow rates at the mouth of the estuary.

<table><caption>Table 1 - Monthly Mean Discharge of Santa Cruz River.</caption><thead><tr><th>Month</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th><th>9</th><th>10</th><th>11</th><th>12</th></tr></thead><tbody><tr><th>Discharge [m³/s]</th><td>840</td><td>1090</td><td>1277</td><td>1155</td><td>925</td><td>690</td><td>510</td><td>384</td><td>306</td><td>331</td><td>444</td><td>604</td></tr></tbody></table>

<table><caption>Table 2 - Tidal Flow Characteristics of SCRE obtained by Delft3D-FLOW.</caption><thead><tr><th>Maximum Tidal Flow Rate [Qmax]</th><th>Mean Tidal Flow Rate [Qmean]</th><th>Minimum Tidal Flow Rate [Qmax]<sup>min</sup></th><th>Mean River Discharge [Qmod]</th><th>[Qmod] / [Qmax]<sup>min</sup></th></tr></thead><tbody><tr><td>111,090 m³/s</td><td>37,902 m³/s</td><td>16,038 m³/s</td><td>715 m³/s</td><td>4%</td></tr></tbody></table>

Regarding the OceanParcels simulations, for the case of small groups of particles located at the interface between the river mouth and the estuary, particles follow elongated circulations, with, average length of particle trajectory per cycle of 25 kilometers for spring tides and 15 kilometers for neap tides. Also, the average permanence time of particles was between 2 and 3 weeks for non-stranded particles, although a more thorough analysis should be performed, since sample size was not statistically robust.

For the case of homogeneous particle distribution simulations, a circulation pattern can be observed: ebb tides create deflection towards the South-West when particles leave the estuary and flow tides create deflection towards the North-East for these outer particles. Most of these enter back into the estuary, but some of them leave the estuary's hydrodynamic influence zone and travel to the North-East along the coast until they leave the domain (Fig. 3a, b). This behavior is probably due to geostrophic circulation.

Another interesting pattern observed for longer period simulations is that stranded particles (or with very little movement) in the estuary's margins are present all along the coast. This behavior could have a role in the formation of the tidal flats of the estuary (Fig. 3c).
<figure><img src="image_7.png" /><figcaption>Figure 3 – Modelled particle dynamics: a) Ebb tide, b) Flood tide, c) Stranded particles at the end of the simulation.</figcaption></figure>

Finally, comparison of these results to satellite images (Fig. 4) shows similarities between both fine sediment (satellite images) and particle (modelled) plumes, extending outside the estuary and leaning towards the South-West at ebb tides. The modelled plume presents a smaller extension though, possibly due to the original position of particles.

<figure><img src="image_8.png" /><figcaption>Figure 4 – Modelled particle plume (left) and fine sediment plume captured by satellite image (right).</figcaption></figure>

## 4 CONCLUSION

Even though simplifications have been made, results show a good agreement for the overall dynamics, representing the shape and distribution of the particles in a similar way to that observed for fine sediment in the satellite images. On the other hand, the limitations present in the current analysis should be further investigated: The modelling domain should be extended towards the ocean to capture the entirety of the sediment's plume; sensibility analysis should be made for the inclusion of diffusion processes, as well as considering a variable river discharge. Also, some calibration should be made in terms of particle concentration and turbidity threshold in the visible range of satellite images. Possible application of these modelling tools could be to establish a deposition rate of sediments for a better assessment of the sediment accretion process in the tidal flats in the geomorphological short term.

## 5 ACKNOWLEDGMENT

The authors would like to express appreciation for the support of CNRS by means of the International Research Project "Hypertide", enabling the presentation of this work at the 19th French Congress on Sedimentology. They also thank Emmanuel Poizot for his careful review.

## 6 REFERENCES

Bindelli, L., 2018. Aprovechamiento hidrocinético en el estuario del río Santa Cruz. Trabajo Final de Ingeniería Civil. Facultad de Ingeniería, Universidad de Buenos Aires.

Bindelli, L., Re, M., Kazimierski, L.D., 2023. Hydrokinetic Energy Potential in Argentine Estuaries with High Tidal Ranges. IEEE Latin American Transactions, 21, 1049-1055.

Carol, E., Spalletti, L.A., Schwarz, E., 2017. Determinación de mineralogía por difracción de rayos X de sólidos suspendidos en muestras del río Santa Cruz. Centro de Investigaciones Geológicas, UNLP.

Dinápoli, M., Simionato, C.G., 2024. Study of the tidal dynamics in the Southwestern Atlantic Continental Shelf based on data assimilation. Ocean Modelling, 188, 102332.
# DISTRIBUTION OF ROOTLESS CONES AND LANDFORMS RESULTING FROM WATER-LAVA INTERACTIONS AT HAGI IN THE AÐALDALUR VALLEY (ICELAND)

Émeline BOURRIER<fnref n="1" />, Gaëlle MATHIEU<fnref n="1" />, Hervé LEYRIT<fnref n="2" />, Réjanne LE BIVIC<fnref n="2" />, Elsa OTTAVI-PUPIER<fnref n="2" />, Maxime DENIS<fnref n="2" />, Julien DUQUENNOY<fnref n="2" />, Arnaud DUJANNY<fnref n="2" />

Corresponding author: emeline.bourrier@gmail.com

**Abstract:** The region of Lake Mývatn and the Aðaldalur Valley in Iceland is known for its high concentration of rootless cones. To study the processes involved in the formation of those volcanic landforms, a 1.1 km<fnref n="2" /> study area rich in volcanic landforms was delineated near the Hagi guesthouse in the Aðaldalur valley. The rootless cones and other facies were referenced, and a high resolution orthomosaic and 3D model were produced using drone acquisition. A new classification of rootless volcanic cones and mounds was produced, as well as an architecture of the Hagi zone. This area is interpreted as a continuous transition of explosive interactions between lava and water saturated sediments followed by a basaltic flow due to depletion of water. The lava flow mechanically erodes some of the rootless cones.

**Keywords:** Hydrovolcanism, Iceland, Mývatn, Laxa lava, rootless cones

## 1 INTRODUCTION

The region between Lake Mývatn and the Aðaldalur Valley in Iceland is known for its high concentration of rootless cones. These are the result of local explosions at the surface of a lava flow, generated when it interacts with a shallow body of water (Thorarinsson, 1951) or water saturated sediments (Fagents and Thordarson, 2007). They are composed of two main types of volcanic ejecta: lapilli, with a diameter ranging from 2 to 64 mm, and spatter (> 64 mm), soft, plastically deformable bombs, which can aggregate and weld when hot (Boreham et al., 2018). They are not conventional volcanoes, as they are not directly fed by a magma chamber. The Younger Laxá Lava that forms these cones is a basaltic lava with a chaotic "aa" surface, which flowed from the Þrengslaborgir-Lúdentarborgir eruptive fissure towards the prehistoric lake of Mývatn, then followed the Laxá valley to Skjálfandi Bay in northern Iceland, covering a total distance of 63 km with an average thickness of ~ 17 m and a total area of around 220 km<fnref n="2" /> (Höskuldsson et al., 2010). A classification of crater cone morphology was proposed for the proximal sector around Lake Mývatn (Noguchi et al., 2016) and then modified by distinguishing hornitos (metric cones of spatter) and applied to the entire valley (Boreham et al., 2018).

The aim of this study is to identify the primary and secondary processes involved in the formation of volcanic landforms in the Aðaldalur valley (Iceland), a former glacial valley less studied than the area around lake Mývatn, located around 50 km from the eruptive fissure. Primary processes include the fragmentation of lava by interaction with water, as well as the sedimentary transport and deposition of the pyroclastic fall that forms the relief. Secondary processes include syn- to post-depositional remobilization of tephra. For this purpose, a 1.1 km<fnref n="2" /> study area rich in volcanic landforms was delineated near the Hagi guesthouse in the Aðaldalur valley.

## 2 MATERIALS AND METHODS

### 2.1 Field work

A two-week field mission was carried out in July 2023. The morphologies of the Mývatn cones were compared with those of the Hagi zone to establish differences and similarities. Then, the cones in the Hagi zone were identified and referenced according to their pyroclastic products, color and angle of slope.

### 2.2 Drone and photogrammetry

The drone used for the acquisition was the DJI Phantom 4 Pro v2, equipped with an RGB optical imaging sensor. Acquisition was carried out with 75% longitudinal and 60% lateral overlap. 26 targets (or markers), whose coordinates were recorded using GPS, were randomly placed across the acquisition zone. These targets were used as Ground Control Point to achieve a precise georeferencing under 10 cm over the entire area. During data acquisition, the drone's speed was set at 3.5 m/s, and its altitude between 30 and 50 m

<footnote n="1">¹Institut Polytechnique UniLaSalle Beauvais, 19 rue Pierre Waguet 60000 Beauvais, France</footnote>

<footnote n="2">² Institut Polytechnique UniLaSalle, UPJV, B2R (GeNumEr), U2R 7511, 19 Rue Pierre Waguet, BP 30313, 60026 Beauvais, France</footnote>
(AGL). All photographs were taken from the same angle, at nadir, and the resolution was set at 20 megapixels.

The 4,500 photos captured by the drone were processed using Agisoft Metashape software. This software enables photogrammetric processing (SFM) of digital images to obtain a dense point cloud or textured model. A 12 cm resolution orthomosaic and a 3D model were obtained after processing.

# 3 RESULTS AND DISCUSSION

## 3.1 Classification

The classification established by Noguchi et al. (2016) was completed based on information gathered on the field (Fig. 1). Scoriaceous cones were classified into three categories: (i) Lapilli cones (LC), which were divided into two subgroups according to their slope angle: 30° (LC1), or less than 30° (LC2); (ii) lapilli and spatter cones (LSC), whose determining factor is the position of the spatter: on top of the cone (LSC1), or inside (LSC2); (iii) spatter cones (SC), with a slope angle close to 45° and a break of slope at the base of the cone (SC1) or without a break of slope (SC2). The main additions are (i) the creation of a second type of lapilli and spatter cone, and (ii) the definition of mound facies (LM, LSM, SM) describing craterless volcanic ejecta accumulations.

<figure><img src="image_9.png" /><figcaption>Figure 1. Rootless cones classification, based on the morphology and the tephra granulometry</figcaption></figure>

## 3.2 Zone facies referencing

The facies observed in the field were referenced in ArcGIS (Fig. 2A) and their distribution analyzed. A DEM was produced using orthomosaic data (Fig. 2B).

<figure><img src="image_10.png" /><figcaption>Figure 2. (A) Distribution of volcanic facies; (B) DEM of the zone.</figcaption></figure>

The northern zone is the richest in lapilli. This tephra platform is at least 4 m thick (Fig 2B). It is covered with scoriaceous cones (LC2, LSC1) with circular craters (Fig 2A). To the north-west, around ten lapilli cones,
sometimes with incomplete spatter crowns, are aligned in a west-east direction. In the northeastern part, smaller alignments, oriented north-south, have broadly similar granulometries and slope angles of less than 20°. On the eastern and southern edges of the platform, the more frequent facies are craterless LM or LSM (Fig. 2A).

The central zone has a lower topography (Fig. 2B) and is covered by dense vegetation. In this area, the few outcrops are those of pahoehoe lava tubes that trend north-westwards in the central plain and north-eastwards to the east of the tephra platform. The rare pyroclastic facies are spatter cones (SC) and lapilli and spatter cones (LSC) (Fig. 2A).

The southern zone is predominantly a lava sheet flow with a ropy surface and west-east oriented ogives (ridges) over several hundred metres. The pyroclastic cones are rare, isolated and generally replaced by craterless facies. At their contact, the orientation of the lava ogives changes, turn around each side of the cone. In addition, in the south-west area, the cone (LSC2 facies) with an elongated shape is a cone with a horseshoe-shaped crater open to the north-east that releases a flow at its base (Fig. 2A).

## 3.3 Interpretation and discussion

The facies architecture illustrates the gradual depletion of water in a wetland area and a river within the Aðaldalur glacial valley, following the continuous input of a voluminous flow of basaltic lava (Fig. 3).

<figure><img src="image_11.png" /><figcaption>Figure 3. The depletion of interactions between water and large basaltic lava flow</figcaption></figure>

Two periods can be distinguished in the development of the lava flow in the study area. In the first phase, the lava's arrival on the river and wetland triggered intense explosive activity, creating a platform of pyroclastic fall (tephra blanket), from which numerous tephra cones (LC) emerge and tend to cover their neighbours. The gradual reduction in the amount of water during explosions led to a reduction in fragmentation, resulting in the formation of a spatter layer at the top of the lapilli cones (LSC). This zone is well preserved in the northern sector north of the road. In a second phase, following dewatering, the continuous flow of lava partially covered this new relief from the south, before being diverted on either side to continue its flow towards the north of the valley. Thus, in the southern sector, the greater frequency of scoriaceous mounds (LM, LSM) on the edges in contact with the flow results from the deformation of the first cones and their mechanical erosion by the continuous flow of basaltic lava, which explains the disappearance of the craters. According to this interpretation, the tephra relief acts as an obstacle that can be deformed by the flow. The latter also induces greater deformation of the basaltic flow surface, materialized by the fragmentation of the solidified crust into blocks and the formation of metre-scale pressure ripples (ogives) (Fig. 3).

Rather than aa facies (Thorarinsson, 1951), the lava exhibits a ropy pahoehoe facies emplaced as a sheet flow with a low enough to allow the formation of a stable upper crust on the surface of the pahoehoe flow but high enough to deform the crust as the lava progresses (Boreham et al. 2018). The preservation of surface ogives makes it possible to distinguish the path of the various lobes that bypassed isolated cones or groups of lapilli cones. The V-shape of the lava lobes surrounding the cones may be typical of an interaction between the relief and a fluid basaltic lava flow (Fig. 3).
## 4 CONCLUSION

The new classification of rootless volcanic cones and mounds allows us to propose an architecture for the Hagi zone. This is interpreted as a zone of explosive interaction between lava and water saturated sediments. Following the initial strong explosions that produced the lapilli cones (LC) covering the lapilli platform, the water supply to the wetland was progressively reduced, limiting the explosive character to a spatter cover at the top of some cones (LSC) and the formation of spatter cones (SC). The lava flow then flowed around these new lapilli topography, partially covering and deforming it. The results provide new insights into the development of rootless cones and the secondary processes that may have affected them.

In the future, non-circular or asymmetrical morphologies of cones, excluded from this initial classification, could be studied to refine the classification of cones and to complete the distribution of volcanic facies. Other areas in the Aðaldalur valley will have to be studied to complete description and to explain (1) the phenomenon of water-lava interaction, (2) the origine of transverse alignments of cones and (3) the detour of lava by a new obstacle. In addition, the search for Martian analogues could enable us to distinguish a scenario of water or ice source depletion during the setting up of large basaltic flows.

## 5 ACKNOWLEDGMENT

The authors are grateful to La Fondation Dassault Systèmes for the financing, APEX UniLaSalle Beauvais for the support provided for this work, and Rina Noguchi and Armann Höskuldsson for their help and support, for his review.

## 6 REFERENCES

Boreham, F., Cashman, K., Rust, A., Höskuldsson, Á., 2018. Linking lava flow morphology, water availability and rootless cone formation on the Younger Laxá Lava, NE Iceland. Journal of Volcanology and Geothermal Research, 364, 1-19.

Fagents, S., Thordarson, T., 2007. Rootless volcanic cones in Iceland and on Mars. The Geology of Mars: Evidence from Earth-Based Analog. Cambridge University Press, pp. 151-177.

Höskuldsson, Á., Dyrh, C., Dolvik, T., 2010. Grænavatnsbruni og Laxárhraun yngra. Fall Meeting JFI 2010, Ágrip erinda Jaðfræðafélag Íslands, 65 bls.

Noguchi, R., Höskuldsson, Á., Kurita, K., 2016. Detailed topographical, distributional, and material analyses of rootless cones in Myvatn, Iceland. Journal of Volcanology and Geothermal Research, 318, 89-102.

Thorarinsson, S., 1951. Laxárgljúfur and Laxárhraun. A Tephrochronological Study. Geografiska Annaler, 1-2, 89 p.
# BIOTIC PROCESSES INVOLVED IN THE MINERALIZATION OF TUFA DEPOSITS

Pierre BOUSSAGOL<fnref n="1" />, Emmanuelle VENNIN<fnref n="1" />, Madeline DURET<fnref n="2" />, Aline BONNOTTE<fnref n="3" />, Élodie NOIROT<fnref n="3" />, Chrystel DEULVOT<fnref n="3" />, Irina BUNDELEVA<fnref n="1" /> and Pieter VISSCHER<fnref n="4" />

Corresponding author: emmanuelle.vennin@u-bourgogne.fr

**Abstract:** In the Bourgogne-Franche-Comté region, tufa deposits, form in rivers fed by karstic springs, raising questions about their biotic or abiotic origins. These deposits form in calcium carbonate-saturated waters with vegetation and microbial communities. The study aims to understand the bio-physicochemical conditions of tufa formation in rivers, which show diverse carbonate depositional structures. To address this question, a multi-analytical and multi-scale approach are employed, combining microscopy methods (fluorescence, confocal, SEM, and Cryo-SEM), analysis of physico-chemical conditions (pH, conductivity, Ca<fnref n="2" />⁺ concentration, alkalinity), and mineralogical assessments, along with a mapping of the structures to understand the context of their formation. It also assesses the mineralization kinetics revealing a complex interaction between physicochemical and biological processes, always involving microbial activity.

**Keywords:** Rivers, Carbonate mineralization, Microbialite, Exopolymeric substances, Biotic/Abiotic

## 1 INTRODUCTION

Calcium carbonates deposits found in river stream, known as tufa, raise questions about their origins, particularly regarding the role of microbial communities in their formation. Tufa typically forms at ambient temperatures in calcium carbonate-saturated water, often in association with vegetation and photosynthetic microorganisms (Shiraishi et al., 2008; Pedley et al., 2009; Roche et al., 2019). The formation of tufa involves complex interactions between environmental, physicochemical, and biological factors (Ford & Pedley, 1996; Merz-Preiß. & Riding, 1999; Boussagol et al., 2023). Several mechanisms, both biotic and abiotic, have been proposed to explain carbonate precipitation in freshwater environments, with microbial activity, especially cyanobacteria, often playing a key role. However, the debate continues as to whether tufa formation is predominantly biotic or abiotic in origins.

Modern tufa deposits are primarily composed of calcite and influenced by factors such as water flow (depth, velocity, etc.), CO₂ degassing, and climate (precipitation, temperature, humidity, etc.). Tufa often displays lamination characterized by alternating dense and porous layers. Microbial activity, especially from cyanobacteria and biofilms, is closely associated with calcite precipitation, with the degradation of extracellular polymeric substances (EPS) promoting mineral crystal growth. Studies suggest that tufa formation is influenced by seasonal variations, with more efficient production in warmer conditions (Arenas & Jones, 2017). The biogenic origin of carbonates, although less frequently discussed, is sometimes linked to the formation of micrite or small crystals, while sparry crystals are mostly considered abiotic (Pedley et al., 2009). Perri et al. (2012) propose that bacteria may influence carbonate precipitation in rivers through the formation of mineral nanostructures. Three main mechanisms for tufa formation have been identified: abiotic control from CO₂ degassing (Kano et al., 2003), physical control with some biofilm influence (Ford & Pedley, 1996; Merz-Preiß. & Riding, 1999), and biofilm components, like cyanobacteria, contributing to calcite mineralization (Pedley et al., 2009; Dupraz et al., 2009).

The study aims to explore tufa formation in rivers fed by karst springs in Bourgogne-Franche-Comté (France), using multi-analytical approaches, combining microscopy methods (fluorescence, confocal, SEM, and Cryo-SEM), analysis of physico-chemical conditions (pH, conductivity, Ca<fnref n="2" />⁺ concentration, alkalinity), and mineralogical assessments to understand the biotic and abiotic factors involved.

## 2 MATERIALS AND METHODS

This study adopts a multi-scale approach to investigate calcium carbonate production in river systems by integrating geomorphological, sedimentological, and biological factors.

Detailed mapping of tufa deposits was performed using high-resolution LiDAR data (20 cm resolution from the HD LiDAR program, National Institute of Geographic and Forest Information, Baume-Les-Messieurs).

<footnote n="1">¹Biogéosciences, UMR 6282 CNRS, Université de Bourgogne, 6 Boulevard Gabriel, F-21000, Dijon, France.</footnote>

<footnote n="2">²Department of Geosciences, University of Fribourg, Switzerland</footnote>

<footnote n="3">³Plateforme DimaCell, UMR 1347 Agroécologie, INRAE, Institut Agro, Université de Bourgogne, F-21000 Dijon, France.</footnote>

<footnote n="4">⁴Departments of Marine Sciences & Earth Sciences, University of Connecticut, Storrs, CT 06269, USA. E-mail</footnote>
The mapping is supplemented by field observations, including the identification of areas suitable for observing microbial mats and small structures, such as oncoids, measuring less than 20 cm.

These field observations were combined with physico-chemical analyses of river water, including pH, conductivity, and calcium concentration, which were monitored monthly from July 2023 to May 2024 in Baume-les-Messieurs, and several times per year in Marangea from 2021 to 2024, using a portable multi-parameter probe (ProfiLine 3320 WTW). Water samples were collected for calcium concentration and alkalinity analysis (GET Laboratory, Toulouse, France). The saturation index (SI) for calcite is calculated using PHREEQC software (version 3.6.2) along with the corresponding database (Parkhurst & Wissmeier, 2015), taking into account pH, alkalinity, temperature, and calcium concentration at the time of sampling. This method allows for the identification of river sections where the physico-chemical conditions favor carbonate precipitation, as well as the determination of whether specific times of the year are more conducive to this process.

The results of the physico-chemical analyses conducted on river water, from the source to the downstream, are systematically geolocated and mapped to identify any potential physico-chemical variations along the river's course.

The sedimentological analysis examined tufa formations at macro, meso, and micro scales (Shapiro, 2000; Dupraz et al., 2009) using a range of microscopy techniques. 150 thin sections were observed using polarizing and fluorescence microscopes (Zeiss Axioscope 7, Zeiss group, Oberkachen, Baden-Wurttemberg, Germany) coupled with a deltaPix camera (DeltaPix, Smoerum, Denmark) processed by DeltaPix InSight software (v.7.0) and scanning electron microscopy (SEM). Five samples were analyzed using confocal microscopy (Leica SP8 with a 10x dry objective, following Gérard et al., 2013 protocol) Five fresh tufa samples, collected in spring 2024, were observed with cryo-SEM (FE-SEM Hitachi SU 8230 with Quorum PP3010T cryo- preparation; DIMACELL platform, university of Burgundy, Dijon, France

# 3 RESULTS AND DISCUSSION

## 3.1 Geological context

This study focuses on Baume-les-Messieurs and Marangea rivers flowing through the French Jura. The Jura is a crescent-shaped fold-and-thrust belt that developed during the Mio-Pliocene at the foreland of the western Alps (Turrini et al., 2009). It features Cenozoic and Mesozoic sedimentary successions, primarily limestone and marl, shaped by compressional tectonics from the Alpine orogeny. The studied rivers flow along uplifted Mesozoic limestone plateaus marked by fault zones and are deeply incised due to intense erosion and glacial processes (Campy, 1982). This geomorphological evolution has led to significant karstification, creating dolines, sinkholes, and subterranean drainage networks (Campy, 1982; Bichet & Campy, 2008; Chauve et al., 1993). The valleys, known as "Reculée," are formed in Jurassic crinoidal and coral limestones, discharging water through springs at the contact with impermeable marl strata. Currently, the rivers erode Quaternary tufa deposits. The Baume-les-Messieurs River (the Dard) is studied over 1,500 meters, showcasing significant slope variation, with an average slope of about 6% (Fig. 1A-B). It features a 30-meter drop over ten meters at the tufa mound, along with several waterfalls. The river's flow is laminar in the first 150 meters and the last 400 meters, while the intermediate section is characterized by turbulent flow in a straight channel (Fig. 1C). The Marangea river (average slope of 11%) is studied over 950 meters (Fig. 1A-B). This river is marked by a perched tufa-mound, located a few meters from the source and features a torrential flow for about 400 meters before becoming calmer and taking on a slightly meandering morphology.

## 3.2 Geomorphological structures

Tufa carbonate concretions in the studied rivers are mainly found on or around geomorphological structures. A tufa mound, often associated with karst springs, is prominent formations (Fig. 1D). This mound develops in areas with steep slopes (over 30%), reaching sizes of over 100 m² and heights exceeding 10 meters. Tufa deposits also form dams across watercourses, varying from simple to successive or crossed structures depending on slope and river conditions (Fig. 1E). Simple dams appear in moderate slopes (2%–30%), while successive dams occur in steeper areas. These dams range from a few centimeters to several meters in height and can extend for hundreds of meters.

## 3.3 Tufa composition

The tufa deposits in the riverbeds composed of calcite, are categorized into three macro-structures: lobates, planar, and oncoids. These structures can be either laminated, which displays planar or concentric layering often based on organic remnants (Fig. 1G-a and H) or non-laminated (i.e., clotted; Fig. 1G-b and I), characterized by high porosity and disorganized structure. Based on microscope analyses, five main components were identified: Biotic components, peloids, micrite, microspar and sparite (mineral classification from Folk (2005); see classification from Vennin et al. (2019) and Fig. 1J).
<figure><img src="image_12.png" /></figure>

Figure 1. (A) Location of the study area. (B) Geological map of the Bourgogne-Franche-Comté region (from BRGM; https://www.brgm.fr/fr/implantation-regionale/bourgogne-franche-comte). The red lines indicate rivers with tufa deposits, while the colored stars represent the location of the two rivers studied in this research (Baume-les Messieurs and Marangea). (C) Topographic maps (LiDAR) of the Baume-Les-Messieurs Reculée. (D) Illustration of the tufa-mound of Baume-Les-Messieurs. (E) Illustration of a succession of crossed complex dams forming across the entire width of the river of Marangea. (F) Illustration of a succession of complex dams in Baume-Les-Messieurs River. (G) Illustration of the two main layering that form tufa deposits, (a) laminated and (b) non laminated clotted. (H) Alternation of dense laminae, composed of sparite and microspar, and porous laminae made of micrite and microspar. (I) Illustration of bryophyte remains encrusted by a mix of micrite and microspar forming clotted layers. Illustration of dense laminae, composed of sparite and microspar, and porous laminae made of micrite and microspar observed with optical microscope (J) and (K) in fluorescence microscopy (green emission 555 nm) displaying high density of microbial filaments. (L) Chlorophyll autofluorescence detected by emission between 610 and 730 nm using confocal microscope, highlighting the presence of numerous microbial filaments. (M) Calcein emission, with the detection window set between 517 and 530 nm. The detection of calcein indicates the presence of free Ca²⁺ ions in the sample surrounding microbial filaments.
Lobate structures (m- to several meters) in tufa deposits are composed of cyanobacteria (Rivularia, Phormidium, Calothrix, etc.) with bryophytes and filamentous green algae, either embedded or on the surface of carbonate structures. During spring and summer, when rivers flow, these structures are covered with thick layers of algae, microbial mats, and decaying plant fragments. Texturally, they have a clotted core made of organic remains, surrounded by alternating layers of micrite and microspar. Microscopic analysis reveals that the clotted core contains peloids, while fan-shaped microbial filaments cluster in micritic and sparitic layers. Planar structures form discontinuous mm to cm-thick calcitic layers on riverbeds, appearing granular and becoming slippery when coated with microbial mats in spring and summer. Microscopy shows they are laminated, with micritic and sparitic layers trapping organic fragments (bryophytes, and leaf and wood fragments) and microbial filaments. Oncoids, found along riverbanks, are multi-cm ovoid carbonate biogenic structures with a clotted core composed of disorganized peloids, micrite, and microspar. The core has high porosity (over 30%) and is covered by a laminated cortex of alternating micrite and microspar laminae (up to 1 mm). Sparite may develop on their surface. Observations in all the studied samples, from scanning electron microscopy (SEM), both conventional and cryogenic, show that the calcite crystals in the tufas exhibit a rhombohedral morphology, characterized by the accumulation of rhombic crystallites forming regular polygons. These crystals can grow larger than 40 µm and display a pyramidal or chevron-shaped granular surface. Cryo-SEM analysis of fresh samples reveals a significant presence of extracellular polymeric substances (EPS), which often degrade during conventional SEM observations. The laminated layers are rich in EPS, creating a spider web-like network (Dupraz et al., 2004; Boussagol et al., 2024), with calcite nucleation occurring at the nodes of degrading EPS. Micritic agglomerates are encapsulated within the EPS matrix and associated with biological remains. In non-laminated samples (i.e. clotted), there is a high concentration of casts from microbial filaments and bryophyte structures, with the internal structure composed of small micrite crystals that form granular walls, later covered by larger micrite and spar crystals.

Confocal microscopy observations revealed significant natural chlorophyll fluorescence in all examined samples, particularly in the outer parts, indicating better preservation of chlorophyll molecules and organelles. Chlorophyll compounds appeared in both laminated and clotted layers, primarily as microbial filaments (Fig. 1L). In laminated structures, these filaments were concentrated in dense laminae of sparite and microspar, oriented perpendicularly to the lamination, while in porous laminae of micrites, they were more disorganized. In clotted layers, chlorophyll was associated with remnants of bryophytes, often reduced to cell walls and some microbial filaments. Calcein staining detected free Ca²⁺ and Mg²⁺ cations, with fluorescence overlapping areas of chlorophyll fluorescence, suggesting that these cations are primarily located on the walls of microbial filaments and bryophyte remains (Fig. 1M). Extracellular polymeric substances, observed in cryo-SEM, enveloped organic remnants in all meso-fabric structures. The EPS likely played a key role in releasing Ca²⁺ ions during degradation, making them detectable through calcine staining. This highlights EPS as a promising candidate for facilitating calcium release.

## 3.4 Biotic control on carbonate production

Observations of non-laminated clotted samples consistently indicated a high concentration of biological compounds, particularly EPS, which coat microbial filaments and surrounding organic materials, including bryophyte remains. These findings correlate with confocal microscopy results, showing free calcium ions around these organic structures. Considering that EPS captures free Ca²⁺ due to negatively charged acidic groups (Dupraz & Visscher, 2005; Shiraishi et al., 2008) they inhibit calcium carbonate (CaCO₃) precipitation (Dupraz & Visscher, 2005; Decho & Gutierrez, 2017). As EPS degrades, calcium can be released, increasing the local concentration of Ca²⁺ and enhancing the saturation index for calcite (SIcalcite) near microbial filaments, which promotes calcite nucleation (Dupraz & Visscher, 2005; Dupraz et al., 2004, 2009) at the nodes formed by degrading EPS and around organic remains (Boussagol et al., 2024).

Cryo-SEM observations (Fig. 2) revealed that micrite crystals, predominantly rhombohedral, consist of an accumulation of rhombic crystallites, akin to structures found in lacustrine carbonate mud deposits formed from EPS degradation (Boussagol et al., 2024). This carbonate mineralization process is crucial in forming the non-laminated, characterized by organic remains encrusted by peloids, micrites, and microspars. Overall, these observations illustrate the significant role of biological processes and EPS degradation in the mineralization of tufa deposits. The porous laminations of laminated layers resemble the composition of clotted structures, indicating a dual biotic influence on carbonate precipitation. Calcium supply arises from the degradation of EPS, crucial for calcite nucleation. Additionally, these carbonates serve as substrates as we observed a new EPS fixation, mineralization, and micrite growth upon previous lithified layers.

Formation of non-laminated clotted layers and porous laminae is linked to microbial and bryophyte development, mainly during spring, summer, and early autumn when chlorophyll production is high. Locally, the micritic fringes are covered with microspar suggesting a change in precipitation conditions and enhanced mineralization. Cryo-SEM observations reveal microspar crystals, ranging from cubic to rhombohedral shapes, often encapsulated in degrading EPS, highlighting the interplay of biotic and physicochemical processes in carbonate deposits.
<figure><img src="image_13.png" /><figcaption>**Figure 2.** Cryo-SEM illustrations of the relationship between EPS and calcite crystals in a fresh tufa sample. (A) High concentration of EPS forming a veil with diatoms as well as numerous microbial filaments embedded within the microbial matrix; EPS veil takes on a typical spiderweb shape as it degrades (Dupraz et al., 2004). (B) Close-up view of degrading EPS showing the occurrence of numerous calcite nucleation points resulting from the clustering of nano-globules leading to the formation of rhombohedral crystallites. (C) Microspar crystals embedded in the EPS matrix. (D) EPS thread coated with calcite crystals.</figcaption></figure>

In the laminated layers, the occurrence of sparitic laminae in which microbial filaments are observed in an erected and vertical position suggests a progressive precipitation of some of the sparites. This implies that the formation of sparites may be contemporaneous with the development of the filaments, evoking a mixed precipitation process involving biotic factors. This is consistent with sparite crystal morphology, whose walls display trigonal, pyramidal, and chevron growths, which are generally associated with biotic influence (Pedley et al., 2009; Roche et al., 2019). In contrast, sparites lacking filaments might result from more physico-chemical processes, associated with an increase in ionic concentration. However, it cannot be ruled out that the calcium released from the degradation of EPS partially contributes to the formation of sparites in the absence of filaments. Therefore, the presence of larger crystals and microspar in dense laminae implies a more efficient carbonate production process. These layers are preferentially observed in higher hydrodynamic setting of the river (main channel and during increasing drainage), driven by enhanced CO₂ degassing. Seasonal droughts may contribute to increased Ca²⁺ concentrations and evaporation, favoring higher calcite saturation indices.

### 3.5 Biotic vs physicochemical processes

As illustrated in figure 3, tufa deposit formation is a complex process influenced by multiple factors, particularly the calcite saturation index (IScalcite), which is determined by the physico-chemical conditions of water and watershed characteristics (Shiraishi et al., 2008; Roche et al., 2019; Kano et al., 2003). CO₂ degassing, evaporation, water agitation, evaporation, and photosynthesis raise pH and promote calcium carbonate precipitation (Jacobson & Usdowski, 1975; Herman & Lorah, 1988). High concentrations of Ca²⁺, driven by water from karstic aquifer, temperature, and exopolysaccharides (EPS), further enhance this process (Pentecost, 1975; Shiraishi et al., 2008; Arenas et al., 2014; Bouton et al., 2020). The occurrence of EPS has been observed in all samples analyzed by Cryo-SEM (Fig. 2E), showing that the nucleation points of calcite and their growing are associated with EPS degradation (Boussagol et al., 2024). Calcium release is generally observed around microbial and algal filaments, which explains their encrustation by micrites and
microspars, the main components of clotted layers and porous laminae in laminated layers suggesting a more 'biotic' control. The interaction of abiotic and biotic factors, such as the photosynthetic role of microbial filaments and EPS, contributes to different carbonate components, ranging from sparite crystals and clots layers. Despite the distinction between "more abiotic" and "more biotic" poles, both processes are intertwined, with microbial filaments potentially influencing even the most abiotic sparite formations. The formation of tufa appears to lie along a biotic-abiotic gradient, with no clear dominance of one over the other, as both processes can lead to similar precipitation outcomes through varied pathways, such as the rise in pH or Ca²⁺ concentration (Boussagol, 2024). Thus, carbonate deposits reflect a mix of physicochemical and biological influences. As observed in modern lacustrine micrite sediments (Boussagol et al., 2024), the precipitation of micrite associated with ESP decay, may conduce to maintain mineralization during the early diagenesis (Boussagol, 2024).

<figure><img src="image_14.png" /><figcaption>Figure 2. (A) Summary diagram showing the processes involved in the formation of continental carbonate deposits. (B) Microstructure of a speleothem sampled from the Baume-les-Messieurs cave and observed under optical microscopy. (C) Laminated layer sampled from a tufa dam in Marangea. (D) "Clotted" layer sampled from a waterfall, mainly composed of encrusted organic remains (Marangea). (E) Cryo-SEM illustration of a tufa sample showing the presence of calcite nucleation points that form at the nodes of EPS during their degradation (Baume-les-Messieurs).</figcaption></figure>

## 4 CONCLUSION

The study of tufa deposits in karstic aquifer fed rivers highlights several key findings regarding calcite carbonate precipitation and early diagenetic processes: (1) Microbial Role: Microscopic studies show microbial filaments and extracellular polymeric substances (EPS) play a crucial role in calcite nucleation, creating mineralization sites. The degradation of EPS may contribute to local increases in Ca²⁺ concentration, favoring calcite precipitation in the tufa formation. (2) Biotic-Abiotic Gradient: tufa formation occurs along a gradient where both biotic (microbial) and abiotic factors interact.

## 5 ACKNOWLEDGMENT

This work is a contribution from the SEDS team in the Biogéosciences Laboratory (uB, Dijon, France) to the project Carbostock-Envergure/OSU THETA and Stock-car/OSU THETA. The authors sincerely thank Dr. Concepción Arenas Abad for her constructive comments, which helped improving the quality of this manuscript. The authors warmly thank DimaCell Facility Imaging Center (INRAE, UBFC, France) which manages and maintains the analytical equipment used in this study. We also thank Lucie Masson (Bourgogne Franche-Comté University, Dijon) for preparing the thin sections, Ludovic Bruno for the DRX
analyses and Alex Pourprix and Arnaud Grivel for their participation as master student in the field acquisition.
We thank geosciences Environnement laboratory (Toulouse, France) for waters analysis.

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# IMPACT OF MICROPLASTIC WHITE TIDE ON THE LIGHT AND DARK METABOLISM OF THE MACROALGAE *FUCUS SERRATUS*: PETROLEUM-BASED VS BIOSOURCED POLYMERS

Solène CAZALIS-HENRY<fnref n="1" />, Laurent SEURONT<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Camille DELAETER<fnref n="1" />, Camille HENNION<fnref n="1" />, Ombeline DEPLECHIN<fnref n="1" /> and Nicolas SPILMONT<fnref n="1" />

Corresponding author: solene.henry@univ-lille.fr

**Abstract:** In a context of global change, the rising intensity of extreme climatic and pollution events, particularly plastic pollution and accumulation of plastic pellets, threaten intertidal organisms' living conditions and physiology. In response to the need for reduced fossil fuel reliance and the use of potentially toxic chemical compounds, the plastics industry is developing bioplastics marketed as "biosourced" or "biodegradable." However, bioplastics often share the same environmental issues as conventional plastics. To address gaps in understanding how conventional and bioplastics affect brown algae, we studied the impact of four virgin plastic polymers —polypropylene (PP), a biosourced polypropylene with a scallop shell matrix (PPSS), a biodegradable polylactic acid (PLA), and a biodegradable biopolyester (BioP)— on CO₂ fluxes in *Fucus serratus* under massive microbead exposure. Results showed limited effects of PP, PPSS, and BioP on the algae's photosynthesis and respiration, indicating resilience to short-term plastic exposure. In contrast, PLA exposure significantly increased dark respiration rates, suggesting a metabolic stress. However, prolonged high energy demand could be detrimental to algal health and resilience. Our future research will explore the effects of these polymers in a wider ecological context, both under natural seasonal variations and/or combined with an exceptional thermal stress.

**Keywords:** Photosynthesis, respiration, pollution, bioplastic, extreme event

## 1 INTRODUCTION

Brown algae have a global distribution (Coyer et al., 2006; Raffaelli and Hawkins, 1999) and are recognized as ecosystem engineer species that drive significant ecological changes, influencing both habitats and resources available to other organisms (Bertness et al., 1999; Wernberg et al., 2010). These engineer species play a crucial role in enhancing biodiversity, primarily by creating new habitats, offering shelter from predators (Raffaelli and Hawkins, 1999) and enriching the ecosystem with organic matter (Duggins et al., 1989) through their high productivity (Mann, 1973; Birkett et al., 1998; Vadas et al., 2004). Brown algae serve as excellent models for studying how environmental factors influence photosynthesis, particularly in response to light intensity variations (Huppertz et al., 1990; Gevaert et al., 2002; Delebecq et al., 2011) and temperature changes (Davison et al., 1991; Major and Davison, 1998). Macroalgae are highly susceptible to the impacts of climate change and human activities (Pereira et al., 2015); in particular, brown algal communities have experienced significant declines, with notable shifts in their assemblages in recent years (Wernberg et al., 2011; Raybaud et al., 2013; Christie et al., 2019; de la Hoz et al., 2019).

In an era of global change, plastic contamination of the oceans is an increasingly alarming and potentially growing threat to a wide range of marine organisms (Avio et al., 2017). Coastal ecosystems are particularly impacted, ranking among the most plastic-polluted environments (Serra Gonçalves et al., 2019; Wu et al., 2022). This pollution includes macroplastic (> 5 mm), microplastics (< 5 mm) and nanoplastics (< 100 nm) items, either produced directly by industry (primary plastic) or resulting from the fragmentation of larger plastic items (secondary plastic; Cole et al., 2011; Paul-Pont et al., 2018; Delaeter et al., 2022). Nowadays, bioplastics are increasingly considered as sustainable alternatives to conventional petroleum-based plastics (European Bioplastics, 2023). These materials can be classified into three primary categories based on their origin and end-of-life characteristics: (i) petroleum-based biodegradable plastics, e.g. biopolyester (BioP); (ii) bio-based non-biodegradable plastics, e.g. biosourced polypropylene enhanced with a biological matrix, such as scallop shell (PPSS) and (iii) bio-based and biodegradable plastics, e.g. polylactic acid (PLA). While bioplastics are often presented as environmentally-friendly solutions, recent research indicates they may generate more microplastics, i.e. almost 3 times greater to what is generated by fossil fuel plastic (Yang et al., 2022) and pose adverse effects on the physiology of intertidal organisms, sometimes exceeding the impacts caused by conventional plastics (Green et al., 2016; Yokota and Mehlrose, 2020).

<footnote n="1">¹Univ. Lille, CNRS, Univ. Littoral Côte d'Opale, IRD, UMR 8187 LOG, Station Marine de Wimereux, F-59000 Lille, France</footnote>

<footnote n="2">²Department of Marine Resources and Energy, Tokyo University of Marine Science and Technology, Tokyo, Japan</footnote>

<footnote n="3">³Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa</footnote>
Although there is a growing awareness of the harmful physical and chemical effects of plastics on primary photosynthetic microorganisms (e.g. Larue et al., 2021; Sánchez-Fortún et al., 2021), a significant knowledge gaps remain regarding their impact on macroalgae, especially brown algae. To the best of our knowledge, only six studies have investigated the effects of conventional micro- and nanoplastics on macroalgae (Web of Science, accessed on 4 December, 2024). Notably, only one of these studies (Zardi et al., 2024) has opportunistically focused on brown alga, despite their critical ecological role in primary production. In this context, the present study investigates the effect of an intense plastic stress corresponding to a white tide (i.e., massive concentrations of pellet spills; Galgani and Rangel-Buitrago, 2024) of four virgin plastic polymers, including a conventional non-biodegradable petrosourced polymer, polypropylene (PP), and a polymer from each category of bioplastics (i.e. PPSS, PLA and BioP) on the physiology of brown algae *Fucus serratus*.

# 2 MATERIALS AND METHODS

## 2.1 Sampling design

Individuals of *F. serratus* were collected during low tide on an intertidal rocky reef (the Fort de Croy, 50°45'47"N 1°35'59"E) located along the French shores of the eastern English Channel. The algae were subsequently acclimatized 48 hours at the laboratory in a flow-through natural seawater aquarium representative of *in situ* conditions (T = 10°C; S = 33). The organisms collected had an average exchange surface area of 48.63 ± 17.94 cm² (mean ± standard deviation), and were selected based on the absence of both reproductive cells and predation marks.

## 2.2 Experimental design

The experiment was designed to simulate intense plastic stress corresponding to a white tide of plastic pellets experienced by *Fucus serratus* during a typical half tidal cycle at the sampling site (i.e. 6h immersion with a water temperature of 10°C). Organisms were exposed to a high concentration of plastic particles equivalent to an exchange surface —to promote the standardization of the concentrations used (see Delaeter et al., 2022 for a review)— of 17,800 mm² of plastic, realistically corresponding to the white tide event that occurred along the French shores of the eastern English Channel following the accidental spillage of 8-tonne of pellets from a tanker-truck in 2016 (i.e. 20 ml L⁻¹ of polypropylene (PP); Seuront, 2018; Gravier and Haut, 2020; Seuront et al., 2021; Uguen et al., 2022). Accordingly, the concentrations used were 10 g L⁻¹ PP, 11.1 g L⁻¹ PPSS, 7.1 g L⁻¹ PLA and 14.9 g L⁻¹ BioP. For each experimental treatment (control and plastic treatments), 5 algae were used (N = 25). The algae were incubated in a 1.5 L glass jar containing 1 L of continuously oxygenated control seawater, and PP, PPSS, PLA and BIOP solutions, incubated in temperature-controlled cabinets (MIR-154, Panasonic, Japan; temperature resolution ± 0.3°C) at 10°C for 6h.

## 2.3 Fluxes measurements

The metabolism (net photosynthesis, gross photosynthesis and respiration) of *Fucus serratus* was assessed by measuring inorganic carbon fluxes under emersion in a 2-L benthic chamber connected to a closed circuit for CO₂ analysis following Migné et al. (2002). Variations in CO₂ concentration in air were measured in the chamber using a non-dispersive infrared gas analyzer (µmolC mol air⁻¹ min⁻¹; model LI-COR IRGA LI-830) and recorded at 1 s intervals during incubations lasting 5 min. Measurements were carried out at a light intensity of 850 µmol photons m⁻² s⁻¹ using two lamps (NANLITE FS-300)—representing the light intensity conditions of the study site at solar noon (9-year average; Halawi Ghosn et al., 2023)— and in darkness in order to estimate net photosynthesis (NP) and respiration (R), respectively. Between two consecutive measurements, the benthic chamber was opened to renew the ambient air. At the end of the series of experiments, the algae were dried for 48 h at 70°C to assess their dry weight (DW). CO₂ flux (µmol C gDW⁻¹ h⁻¹) was calculated by regression analysis of CO₂ concentration versus time during light and dark incubations. Gross photosynthesis (GP) was determined by using measurements of respiration and net photosynthesis (Eq. [1]).

$$GP=NP + CR \quad [1]$$

## 2.4 Statistical analyses

Graphical representation and data analyses were performed using the R 4.3.1 programming language (R Core Team, 2019). The Kruskal-Wallis (KW) test has been shown to be more robust to use than the F-test (ANOVA), particularly when the distribution assumptions of the datasets are questionable (Nwobi and Akanno, 2021). Insofar as the overall residuals of the comparisons did not follow a Normal distribution (Shapiro-Wilkson test < 0.05), the KW test was used to compare the effect of different plastic polymers on respiration, net and gross photosynthesis in *F. serratus* in multiple comparisons. A post-hoc Dunn test (Dunn, 1961) was performed where appropriate to identify treatments that differed significantly from the control.
## 3 RESULTS AND DISCUSSION

Net and gross photosynthesis (Fig. 1A and 1C) showed no significant differences between the control and various plastic treatments (p > 0.05). This result contrasts with previous studies showing that microplastics can impair the photosynthetic processes of cyanobacteria (Tetu et al., 2019) and the freshwater green microalgae *Chlamydomonas reinhardtii* (Dong et al., 2021). Similarly, prolonged exposure to 1 g L⁻¹ of polypropylene reduced the maximum net photosynthesis rates in the bare leaves of seagrass *Zostera marina* (Molin et al., 2023). Our results may hence suggest that brown intertidal algae exhibit greater resilience to plastic pollution. However, prolonged exposure may nevertheless have adverse effects on their physiological parameters and growth, which warrants the need for further investigation.

Respiration of *F. serratus* was unaffected by exposure to a massive pollution tide containing polypropylene microplastics, polypropene composed of a scallop shell matrix, and biopolyester (p > 0.05) compared to control (Fig. 1B). However, respiration increased significantly following an exposure to 7.1 g L⁻¹ of PLA (p < 0.05) compared to the control (Fig. 1B). This also contrasts with previous studies reporting that microplastic inhibit respiration in microalgae (Dong et al., 2021) and seagrass beds (Malte Molin et al., 2023). Respiration-stimulating effects have also been recorded following heavy metal exposure and were attributed to an increased demand for ATP to promote detoxification and repair processes (Meharg, 1993; Scheidegger et al., 2011; Dao and Beardall, 2016) or to compensate for a reduced supply of ATP from photophosphorylation (Ernst, 1980). Poskuta et al. (1996) also suggested that, when CO₂ fixation is inhibited, the redox equivalents generated during photosynthesis are transferred to the mitochondria where they are oxidized, intensifying respiratory electron transport. Although the mechanisms underlying the observed increase in respiration remain speculative and need to be studied in more detail, the response to exposure to the biosourced and biodegradable polymer (i.e. PLA) indicates the activation of a stress-related mechanism in *Fucus serratus*.

<figure><img src="image_15.png" /><figcaption>Figure 1. Carbon fluxes (mean ± se) measured in the benthic chamber of *Fucus serratus* (in µmol C gDW⁻¹ h⁻¹) in light (A) or in dark (B) and corresponding gross photosynthesis (C) under emersion. Letters (a or b) denote significant differences at α = 0.05.</figcaption></figure>

We observed that polylactic acid (PLA), a polymer notably commonly used in medical applications such as sutures, bone fixation materials and drug delivery devices, exhibited a more pronounced deleterious effect than conventional plastics. Similar results have been reported in studies examining the impacts of plastics on marine organisms, such as altered respiration rates in oysters and disruption of phytoplankton assemblages (Green, 2016; Yokota & Mehlrose, 2020). These results could be explained by the presumed fragility of this polymer (Casalini et al., 2019) and by the ability of bioplastics to leach their additives more easily than conventional ones (Quade et al., 2022). Although further work is needed to identify plastic pellet additives before and after lixiviation (Seuront et al., 2022) and allow us to validate this hypothesis.

## 4 CONCLUSION

Taken together, our results show that conventional microplastic particles (PP) and bioplastics (PPSS and BioP) have limited effects on both photosynthetic and respiratory performances of *Fucus serratus*, demonstrating that their ability to act as a CO₂ source or sink is unaffected and indicating relatively high resilience in the face of massive short-term plastic exposure. However, exposure to PLA significantly affected respiration rates, suggesting an increase of metabolic stress. If the increase of respiration persists over the long term, these high energy requirements can have more serious consequences for the health and resilience of these organisms. Aware of the limitations of studying exposure to a single stress, future research is needed to assess the effects of these four plastic polymers in a broader ecological context, e.g. following a heat stress event and during seasonal monitoring.

Given that PLA is considered as an alternative to one of the most widely used conventional polymers, polyethylene (PE; Ali et al., 2023), and that it is both biosourced and biodegradable, and among the most
produced biopolymers in 2021 (European Bioplastics, 2023), we propose that future plastic pollution management strategies should take into account the environmental challenges it shares with conventional petroleum-based plastics. Despite its sustainable origins, PLA's environmental behavior —in particular its degradation process and impact on ecosystems— highlights the need for further research before promoting more “environmentally-friendly solutions”.

## 5 ACKNOWLEDGMENT

The authors would like to thank Kevin Robache for his help in processing the MAREL Carnot data, and Mathilde Schapira for reviewing the manuscript. This work was financially supported by the ‘Agence Nationale de la Recherche’ (ANR) through the project ECUME [Project Number = ANR-22-CE06-0029]. S. Cazalis-Henry is financially supported by a PhD scholarship from the ANR ECUME and the University of Lille, France. This paper has been reviewed by Mathilde Schapira.

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# THE CENTRE-VAL DE LOIRE REGION (FRANCE) IN RELATION TO THE MIDDLE MIOCENE CLIMATIC OPTIMUM: A MORE ACCURATE STRATIGRAPHIC DATING

Adrien DE PERTHUIS, Cyril GAGNAISON and Sébastien POTEL

Institut Polytechnique UniLaSalle UPJV, B2R (GeNumEr), U2R 7511, F-60026 Beauvais cedex France

Corresponding author: adrien.deperthuis@unilasalle.fr

**Abstract:** This study is focusing on eight paleontological sites of the Miocene formations (Aquitanian to Messinian) of the Centre-Val de Loire Region (France). By a multi-approach (sedimentology, geochemistry, palaeontology), the aim is, firstly, to give a more precise age to those Miocene formations; and, secondly, a better understanding of the paleoenvironment evolution in the Loire sedimentary basin during the Miocene, related to the Middle Miocene Climatic Optimum.

**Keywords:** Middle Miocene Climatic Optimum, Paleoenvironment, Palaeontology, Absolute Dating.

## 1 INTRODUCTION

Between -16.9 and -13.8 Ma a hyperthermal event called the Middle Miocene Climatic Optimum (MMCO) took place (Hansen et al. 2013). During the MMCO, the climate in Western Europe was subtropical to semi-arid (Costeur & Legendre 2008), with temperatures 4 to 5 °C higher than today (Hansen et al., 2013). The Centre-Val de Loire region, located south-west of the Paris Basin, was characterised by various depositional environments (lacustrine, fluvial and shallow-marine; Fig. 1) (Mégnien et al. 1980), recorded in 5 Formations encompassing the Miocene and thus providing geological data for the entire MMCO: 1- The Beauce lacustrine limestone formation (Oligocene-Aquitanian), which covers a large part of Beauce and Sologne (Ginsburg, 2000). 2- The Orléanais and Blésois sand and marl formation (Burdigalien-Langhien), which extends between Orléans and Angers, corresponds to a series of terrigenous sediments of fluvio-lacustrine origin. These sediments are channelized by folded plicative East-West tectonic structures inherited from the Alpine orogeny. 3- The Faluns Formation (Langhien), which is a shallow marine lateral equivalent of the Orléans and Blésois sands and marls. It is exposed from the Loire-Atlantique to Blois. Locally, the marine sedimentation continues until the Tortonian (Gagnaison 2020). 4 & 5- The fluvio-lacustrine formation of the Sologne sands and clays (Middle Miocene-Pliocene), which outcrops in the east of the Loire sedimentary basin and grades westwards to the Redonian marine sands formation. The aim of this study is to refine the dating of these different sedimentary formations to characterize the palaeoenvironmental changes associated to the MMCO.

<figure><img src="image_16.png" /><figcaption>**Figure 1.** Miocene formations of the Loire valley (Gagnaison 2020, modified). Black stars: sites studied. Beige: Beauce lacustrine limestone (Oligocene-Aquitanian). Red: Fluvial sands and clays (Burdigalian-Pliocene). Yellow: faluns (Langhian-Tortonian).</figcaption></figure>
# 2 MATERIALS AND METHODS

## 2.1 Sites studied

Eight outcrops with geological or palaeontological data that are still accessible have been selected (Fig. 1): Selles-sur-Cher in the Beauce lacustrine limestone formation (Aquitanian); Mauvières, La Guimardière, Pont-Boutard, Tavers and Contres in the Orléanais and Blésois sands and marls formation (Burdigalien-Langhien); Doué-la-Fontaine and Saint-Hilaire-de-Clisson in the Faluns formation (Tortonian). A systematic review of the geological and palaeontological data for these sites is ongoing.

## 2.2 Dating

Previous to this study, the age of the site of Mauvières was determined using absolute dating (U/Pb, U/Th; Monchal *et al.* 2025) on calcite nodules from palaeosols. This method will be applied to other sites if possible. Three sites have been dated using strontium isotope stratigraphy on samples of shark tooth enameloid. The Tavers site has been dated by sequence stratigraphy, based on the maximum flooding of the Langhian (Miller *et al.* 2005).

## 2.3 ATR-FTIR spectroscopy

Several enamel and bone samples from the different study sites have been analysed to assess the secondary recrystallization rates of the samples (Aufort *et al.* 2019).

# 3 RESULTS AND DISCUSSION

## 3.1 Dating

Former absolute ages obtained on the Mauvières site, established the base of the formation of the Orléans and Blésois sands at -19.3 Ma (Monchal *et al.* 2025). In addition to absolute dating, sequence stratigraphy has been used to date the Tavers site at -15.5 Ma (Perthuis *et al.* *accepted*), making it the site closest in time to the Miocene Climatic Optimum (-15.6 Ma; Kochhann *et al.* 2017). At this site, molluscs (Neritidae, Pachychilidae and Dreissenidae; de Perthuis *et al.*, *accepted*) showing a marine influence were found at the top of the conglomerate, which correlates with the maximum marine transgression of the Langhien at -15.5 Ma (Miller *et al.* 2005). Strontium isotope stratigraphy yielded an age of -15.3 Ma for the Contres site and of -9.7 Ma and -7.4 Ma respectively for the Doué-la-Fontaine and Saint-Hilaire-de-Clisson sites. All these dates constitute a new dataset for the geological sites of the Loire depression (Fig. 2), the ages of which was previously known only from the biostratigraphy of continental mammals (Ginsburg 2000).

<figure><img src="image_17.png" /><figcaption>Figure 2. Lithostratigraphic column of the Miocene formations in the Loire valley.</figcaption></figure>
## 3.2 ATR-FTIR spectroscopy

Dental enamel samples do not show secondary recrystallization, unlike bone samples, which, depending on the considered site, show recrystallization between 40 and 50%. Future results from isotopic analyses of dental enamel will therefore not be influenced by secondary recrystallization.

## 4 CONCLUSION

Application of combined dating methods, in particular U/Pb dating and strontium isotope stratigraphy, enabled five sites to be precisely dated between the Lower Miocene and Upper Miocene in the Loire valley. At the same time, a review of the sedimentological and palaeontological data from these sites has been conducted using new field and laboratory data (Perthuis de et al., accepted). The clay mineral composition and a taxonomic review of the fossil remains is in progress. As a first result, ATR-FTIR spectroscopy analyses show that dental enamel of the various sites is not affected by recrystallization. Geochemical analyses (δ¹³C and δ¹⁸O) have been launched on the dental enamel of several groups of mammals (Rhinocerotidae, Suidae and Proboscidea). All these new data will enable a more precise reconstruction of the palaeoenvironments of the various sites studied and to trace their evolution on the scale of the sedimentary basin in direct link with the MMCO. This study could then serve as a methodological example for future studies in other European sedimentary basins affected by rapid temperature changes.

## 5 ACKNOWLEDGMENTS

The authors would like to thank Étienne Balan and Julie Aufort (Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590, Sorbonne Université) for their invaluable help for ATR-FTIR spectroscopy analysis; Sidonie Revillon (SEDISOR) for carrying out the strontium isotope stratigraphy analysis; and the Chaire GéoAgro Énergies (UniLaSalle-Beauvais) for the funding. This paper was reviewed by Victorien Paumard and Sébastien Rohais.

## 6 REFERENCES

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Costeur, L., Lengedre, S., 2008. Mammalian communities document a latitudinal environmental gradient during the Miocene Climatic Optimum in Western Europe. Palaios, 23, 280-288.

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Hansen, J., Sato, M., Russell, G., Kharecha, P., 2013. Climate sensitivity, sea level and atmospheric carbon dioxide. Philosophical Transaction of the Royal Society, A 371, 20120294.

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Mégnien, C., Mégnien, F., Debrand-Passard, S., 1980. Synthèse géologique du bassin de Paris, stratigraphie et paléogéographie. Mémoires du BRGM, 101, 466 pp.

Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E., Sugarman, P.J., Cramer, B.S., Christie-Blick, N., Pekar, S.F., 2005. The Phanerozoic record of global sea-level change. Science, 310, 293-298.

Monchal, V., Rateau, R., Drost, K., Gagnaison, C., Mennecart, B., Toullec, R., Chew, D., 2025. U-Pb direct dating on calcite paleosol nodules: first absolute age constraints on the Miocene continental succession of the Paris Basin. EGUsphere 2024.

De Perthuis, A., Mennecart, B., Barrier, P., Chenot, É., Falconnet, J., Gagnaison, J.C., Georgalis, G.L., Gilbert, C., Guevel, B., Langevin, D., de Lapparent, F., Lemierre, A., Maubert, F., Ossó, À., Potel, S., Thivaiou, D., Tissier, J., Toullec, R., Xerri, S., Gagnaison C. (accepted). Révision des données sédimentologiques et biostratigraphiques des gisements à vertébrés des sables de l'Orléanais, à Beaugency, Tavers et Le Bardon (Miocène Moyen; Loiret, France). Geodiversitas.
Zachos, J.C., Pagani, M., Sloan, L., Thomas, E., Billups, K. 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292, 686-693.
# RETOUR D'EXPÉRIENCE DE CAROTTAGE AVEC DU MATÉRIEL LÉGER EN EAU PEU PROFONDE À L'AIDE D'UNE AIGUILLE VIBRANTE À BÉTON

Julien DIDIER<fnref n="1" />, Bernard FANGET<fnref n="2" />, Raphaël GALLET<fnref n="2" />, Stéphane JAILLET<fnref n="2" />, Emmanuel MALLET<fnref n="2" />, Gaël MONVOISIN<fnref n="3" /> and Edouard RÉGNIER<fnref n="4" />

Corresponding author: bernard.fanget@univ-smb.fr

**Abstract:** Le document présente une expérimentation de carottage en eaux peu profondes à l'aide d'une aiguille vibrante à béton. L'étude a été menée dans le lac souterrain des grottes de Choranche, avec une mini-barge spécialement conçue pour le transport du matériel. Différentes configurations ont été testées, révélant que l'utilisation d'un tube en inox de 90 mm avec un poids adapté et une aiguille vibrante fixée proche de la tête était la plus efficace. Cette méthode a permis d'extraire une carotte de 1,8 m en 2 min 15 s. L'expérimentation conclut à l'intérêt de cette approche, bien que des améliorations mécaniques puissent être envisagées.

**Keywords:** Carottage, sédiments, aiguille vibrante.

## 1 INTRODUCTION

Dans le cadre de la première session des journées techniques du réseau de métier des Carotteurs Continental Français CCF (13-16 novembre 2023), nous nous sommes associés avec le réseau de métier Milieux Souterrains et Karst pour tester un nouveau dispositif de carottage dans des conditions complexes. Deux objectifs ont été fixés pour ces journées : (1) l'utilisation de moyens de carottage légers, démontables et transportables permettant l'échantillonnage de sites jusqu'alors inaccessibles ; (2) de tester un mode de carottage par vibration pour des sédiments particulièrement compacts en eaux peu profonde.

Compte tenu de ces objectifs, le site retenu pour réaliser ces essais en tenant compte de nos objectifs a été le lac souterrain des grottes de Choranche dans le Vercors.

## 2 MATERIELS

Le laboratoire EDYTEM a développé une mini barge de voyage servant au carottage lacustre (Fig. 1A). L'ensemble du matériel de carottage peut être transporté dans 6 bagages de 23 kg (conditionnement hors format, Fig. 1B, admis en transport aérien).

<figure><img src="image_18.png" /><figcaption>Figure 1. 1A: Mini barge de carottage en configuration de prélèvement. 1B: Ensemble du dispositif conditionné pour le voyage en 6 bagages acceptés par le transport aérien.</figcaption></figure>

Cette mini-barge est composée de deux canoës gonflables conçus spécialement pour cet usage par la société Mekong (Crest – Drôme) et d'une plate-forme en cadres d'aluminium. Cette plate-forme est fixée sur les bateaux et permet d'accueillir deux coffres utilisés aussi comme sièges, un moteur électrique et un mat de carottage pivotant pour faciliter la mise en œuvre du carottier et son extraction. En complément, des matelas servant de fond sont rajoutés dans les bateaux comme réserve de flottaison dans le cas d'avaries. Cette mini

<footnote n="1">¹Réseau CCF - CHRONO-ENVIRONNEMENT, Univ. Bourgogne Franche Comté</footnote>

<footnote n="2">²Réseau CCF et MSK - EDYTEM, Univ. Savoie Mont Blanc</footnote>

<footnote n="3">³Réseau MSK -GEOPS, Univ. Paris Saclay</footnote>

<footnote n="4">⁴Réseau CCF - LSCE, Univ. Paris Saclay</footnote>
barge (dimensions totale 3,6x2,3 m ; surface de travail 2,3x1,2 m ; réserve d'air totale 1530 kg) peut accueillir deux opérateurs et a une force d'extraction de 500 kg limitée par le treuil. Lors de la conception, l'ensemble des contraintes mécaniques a été optimisé par la méthode des éléments finis associée à un plan d'expériences. Les caractéristiques hydrostatiques et hydrodynamiques ont été calculées aussi bien dans le cas d'un usage normal que dans celui d'avaries (cas d'un canoë percé). Des tests en laboratoire et sur le terrain ont permis de vérifier l'ensemble des calculs et des conditions de sécurités.

Les bagages ont été optimisés pour transporter une quinzaine de tube PVC de 63 mm de diamètre avec une longueur de 2 m ainsi qu'un système de carottier de type Nesje développé par EDYTEM modifié accueillir des tubes de 63 mm comprenant deux moutons de frappe d'environ 10 kg. Cet ensemble a déjà permis d'obtenir une carotte sédimentaire en une passe de 8,2 m et de travailler dans des tranches d'eau comprises entre 50 cm et 200 m (record à ce jour).

Sur la figure 2b ; sont présentés le dispositif avec un schéma de montage comprenant les différentes pièces à utiliser ainsi que les systèmes de cordages statiques pour maintenir le carottier, le piston stationnaire et actionner les marteaux. Sur les vues des dessins techniques, les dimensions et les poids sont indiqués à des fins de reproductions éventuelles. Il est à noter que le poids de 3,6 kg est celui utilisé pour la carotte pilote obtenue avec un système UWITEC.

Pour ces journées techniques ; nous avons aussi utilisé un autre système de type Nesje modifié pour tube de 90mm (aussi développé par EDYTEM) utilisant les mêmes poids pour les frappes. La figure 2c montre a vue 3D du dispositif. Dans ce cas, une rallonge de carottage classique (norme RD32) vient s'insérer dans la tête du carottier. Des tubes en PVC ou en acier inox peuvent être montés.

Malheureusement, dans le cas des faibles tranches d'eau, le dispositif de carottage s'avère compliqué à utiliser (surtout avec des sédiments dont les propriétés rendent difficile la pénétration des carottes et nécessiterait des développements supplémentaires). Pour cette raison, nous avons testé une technique de carottage par vibration à l'aide d'une aiguille à béton louée (diamètre 40mm, longueur 315 mm, fréquence de vibration 12000 t/min, 2000W, longueur du flexible 3 m ; Fig. 2a). La longueur du flexible impose la limite de profondeur atteignable.

<figure><img src="image_19.png" /><figcaption>Figure 2. matériel de carottage testé : (a) schéma de la tête de l'aiguille à béton ; (b) carottier de type nesjer modifié pour tube de 63mm. Il est à noter que les trois poids sont fendus pour permettre de les mettre ou retirer du carottier (c) carottier de type Nesjer modifié pour tube de 90mm.</figcaption></figure>

## 3 SITE DE CAROTTAGE

Les objectifs étaient de démontrer l'utilisation du matériel décrit précédemment dans le lac souterrain des grottes de Choranche dans le Vercors. En effet, le site répondait à plusieurs contraintes qui justifiaient ce choix. C'est d'abord l'existence d'une série sédimentaire identifiée dans ce lac (épaisseur présumée entre 2,5 et 3,5 m), qui avait fait l'objet de plusieurs investigations, notamment dans le cadre de la thèse d'Anne-Sophie Perroux (2005) et de travaux de recherches publiés (Perroux, 2005, 2006 ; Carcaillet et al., 2007 ; Jaillet et al., 2009 ; Lignier et al., 2012). C'est aussi un site qui s'inscrit dans une grotte touristique (Fig. 1) offrant les commodités d'accès, d'éclairages et d'électricité et qui en outre est clos la nuit permettant de laisser le matériel entre deux séances de travail.

Le lac lui-même mesure une trentaine de mètres de longueur pour une quinzaine de largeur. Il reçoit les eaux de la rivière souterraine de Chevaline qui en crue transporte une charge détritique essentiellement carbonatée
autochtone. Cela veut dire que l'essentiel des dépôts est composé du démantèlement de calcite produite dans la rivière en amont, complété par quelques éléments fins provenant sans doute des soutirages des couvertures non calcaires sur le plateau des Coulmes. Le lac a été perturbé dans les années 1980 par les travaux d'aménagement touristique. Il ne subsiste que des dépôts latéraux et notamment une très large banquette en rive gauche. C'est cette banquette qui avait déjà fait l'objet des investigations précédentes et c'est à nouveau celle-ci qui a été retenue pour cette opération de tests (Fig. 3).

<figure><img src="image_20.png" /><figcaption>Figure 3. (a) Localisation de la salle de la cathédrale et du Lac dans les grottes de Choranche ; (b) Cartographie géomorphologique du lac de la Cathédrale à Choranche et localisation du point retenu pour les tests. Noter la banquette de sédiments anciens laminés faisant l'objet des carottages et, au centre, les sédiments actuels accumulés à la suite de l'épisode de soutirage des années 1980. (Lignier et Jaillet, 2010).</figcaption></figure>

Le point de carottage (cercle rouge sur la figure 2b) s'est avéré trop proche de la berge pour pouvoir être opéré avec le mât pivotant de la mini barge. Nous avons été dans l'obligation d'utiliser le tripode UWITEC avec 2 montants sur la berge et un sur la mini barge, ce qui a alourdi les essais.

## 4 PRINCIPAUX RESULTATS

Nous avons testé onze configurations de ce matériel avec tube PVC de 63 et 90 mm et tube inox de 90 mm de diamètre. La figure 4 présente les huit principales obtenues avec des tubes de 90 mm. Sur cette figure, les mannequins indiquent que les opérateurs ont participé à la masse du système. La figure 4a montre qu'avec la méthode classique (fonçage par battage au mouton) nous avons pu faire qu'une carotte de 56 cm. L'utilisation de tubes PVC sans chemise s'est avérée inefficace du fait de l'absorption de l'énergie de l'aiguille par le tube (Fig. 4b, c et d). Les essais réalisés avec le tube inox de 90 mm se sont avérés beaucoup plus performants excepté lorsque l'aiguille vibrante est fixée sur la tige vissée dans la tête (Fig. 4e ; raccord fileté type R32 « rond » pour tête roto-percutante). L'inefficacité est due à la partie filetée qui se débloque instantanément et absorbe la quasi-totalité de l'énergie. Des mesures ont permis de montrer qu'un desserrage supérieur à 1 mm (qui intervient presque instantanément au démarrage de l'aiguille vibrante) induit sur la tige un jeu axial de 1 mm et un jeu latéral de +/- 0,8°.

Le positionnement de l'aiguille au milieu du tube inox, bien que plus efficace, induit une perte substantielle d'énergie par résonances et/ou interférences due à la réflexion différentielle aux deux extrémités du tube (Fig. 4f). La meilleure performance a été l'obtention d'une carotte de 1,8 m en 1 min et 42 secondes avec un tube inox de 90 mm dans lequel il y a un piston, surmonté d'un poids de 95 kg (15 kg poids du Nesjer et 80 kg d'un opérateur ; Fig. 4g) et l'aiguille fixée perpendiculairement à l'axe de pénétration. Cette technique nous a permis d'atteindre le bedrock à 3,1 m (par l'utilisation complémentaire d'un carottier Niederreiter 63 mm de 2 m). La position de l'aiguille au sommet du tube évite tout risque de blessures sans perte notable d'efficacité (Fig. 4h), et permet de réaliser une carotte en 2 min et 15 secondes dans des sédiments non pénétrables avec les méthodes conventionnelles.

## 5 CONCLUSION

L'ensemble des essais ont permis de démontrer une efficacité de ce système de carottage pour des sédiments difficiles à pénétrer. De plus, la facilité de location des aiguilles vibrantes dans le monde entier n'engendre pas
de surcharge lors des voyages en avion. Toutefois, pour une utilisation pérenne de ce système, un développement mécanique peut être envisagé dans le cadre du réseau de métier du Carottage Continental Français.

<figure><img src="image_21.png" /><figcaption>Figure 4. Principaux résultats obtenus en fonction des configurations d'essais (a – battage au mouton, b-c-d tube PVC 90mm, e-f-g-h tube inox 90mm).</figcaption></figure>

# 6 REMERCIEMENTS

Les auteurs remercient Laurent Garnier, propriétaire des grottes de Choranche, de nous avoir autorisé l'accès pour la réalisation de cette mission. Cet article a été relu par Alain Trentesaux.

# 7 RÉFÉRENCES

Carcaillet, C., Perroux, A.-S., Genries, A., Perrette, Y., 2007. Sedimentary charcoal pattern in a karstic underground lake (Vercors massif, Alps, France) implications for paleo-fire history. The Holocene, 17, 845-850.

Jaillet, S., Lignier, V., 2009. Premières observations géomorphologiques et sédimentologiques dans le siphon de chevaline (Grottes de Choranche, Vercors). Actes des Rencontres d'Octobre. 19ème Rencontres d'Octobre, Spéléo-Club de Paris, St Laurent-en-Royans, pp. 81-88.

Lignier, V., Jaillet, S., Perroux, A.-S., Thomas, M., Malet, E., Deves, G., Morel, L., Delannoy, J.-J., 2012. Dynamique sédimentaire et effets de site en zone noyée du karst : l'exemple du siphon de Chevaline (Grottes de Choranche, Vercors, France). Karstologia, 60, 23-44.

Perroux, A.-S., 2005. Les remplissages détritiques endokarstiques. Contribution méthodologique à la lecture des mémoires paléogéographiques et environnementales. Application aux systèmes karstiques de Choranche (Vercors) et d'Orgnac (Bas-Vivarais). Thèse de l'Université de Savoie, 418 p.

Perroux, A.-S., 2006. Intérêt des sédiments détritiques endokarstiques en tant qu'archive naturelle ? Discussion autour des dépôts lacustres souterrains (Grottes de Choranche – Vercors). Karstologia, 47, 7-20.
# ORGANIC MATTER IN THE SEDIMENTS OF GUAYMAS BASIN, A STUDY BY ROCK-EVAL 7S®

Pauline DODINET<fnref n="1" />,<fnref n="2" />, Isabelle KOWALEWSKI<fnref n="1" />, Eric KOHLER<fnref n="1" />, Herman RAVELOJANOA<fnref n="1" />, Melissa TELLEZ<fnref n="1" />, Wenda DHERBOIS<fnref n="1" />, Violaine LAMOUREUX-VAR<fnref n="1" />, Armelle RIBOULLEAU<fnref n="2" />

Corresponding author: armelle.riboulleau@univ-lille.fr

**Abstract:** The organic matter and sulfur present in the sediments of Guaymas Basin from three sites cored during IODP expedition 385 were characterized using Rock-Eval 7S® in order to study organic matter diagenesis. At the reference site, the downward evolution of total organic carbon (TOC) and total sulfur (TS) contents is comparable to that observed in many marine sediments, i.e. decrease of TOC and opposite increase of TS. This trend is modified at the hydrothermal site, in relation to the migration of fluids. No clear trends are observed at the site containing methane hydrates, probably because of poor sampling resolution.

**Keywords:** Guaymas Basin, Rock-Eval 7S®, organic matter, pyrite, anaerobic oxidation of methane

## 1 INTRODUCTION

Guaymas Basin (Mexico) is a young oceanic basin where organic-rich sediments accumulate rapidly. Sulfide generated by sulfate reduction is present in the sediment, and organic matter sulfurization should be favored because the delivery of detrital iron, that outcompetes organic matter for reaction with sulfide, is relatively low. In parallel, the basin is characterized by a high geothermal gradient and methane of both biogenic and thermogenic origin is present in abundance in the sediments. We aim to understand to which extend organic matter sulfurization influences the generation of biogenic methane. We here present the first step of this project, that corresponds to a characterization of the organic matter present in the sediments.

## 2 MATERIALS AND METHODS

The studied samples were collected during IODP expedition 385 (Teske et al., 2021). Three sites are investigated: Site U1545 where no perturbation is present, Site U1547 which is affected by a thick shallow magmatic intrusion and Site U1552, where the sediment hosts methane hydrates. The studied sediments were collected under a water depth of more than 1500 m. It is considered that the depositional context remained relatively constant during the deposition of the studied intervals, so that changes in organic matter characteristics and sulfur content are considered to result from post-depositional processes.

Sediment samples were immediately placed in an inert atmosphere and frozen after core recovery. Before analysis, the samples were rinsed with deionised water, dried in an oven and finely ground.

Rock-Eval 7S® analysis was applied using a temperature program adapted to recent organic matter and detection of the different sulfur forms (Espitalié et al., 2024). In addition to the usual Rock-Eval parameters describing the organic matter content, the different sulfur forms were quantified using the software Geoworks®.

## 3 RESULTS AND DISCUSSION

### 3.1 Organic carbon and total sulfur

The total organic carbon (TOC) and total sulfur (TS) contents of the sediment show identical trends and values in the upper 70 m at sites U1545 and U1547 (Fig. 1A, B). Maximum TOC values close to 4 wt% are observed in the shallowest samples followed by a rapid decrease to 2 wt% in the upper 50 m and relatively constant values below. In parallel, TS increases rapidly from 0.6 - 1% in the shallowest samples to 1.5 % at ~50 mbsf. Though the involved depth scales can be markedly different between sites, such downwards evolution of TOC and TS contents has long been observed in many marginal and marine sediments (e.g. (Goldhaber and Kaplan, 1974; Berner, 1984), and can be related to organic matter degradation by organoclastic sulfate reducing bacteria. High sulfate reduction rates were measured in surface sediments at

<footnote n="1">¹IFP Energies Nouvelles, Dépt. Sciences Sols et Sous-sols, 1-4 avenue du Bois Préau, 92500 Rueil-Malmaison, France</footnote>

<footnote n="2">²Laboratoire d'Océanologie et de Géosciences, Université de Lille, bâtiment SN5, F-59655 Villeneuve d'Ascq cedex, France</footnote>
site U1545 and at site U1548, located very close to site U1547 (Nagakura et al., 2022), which supports this
interpretation.

A) U1545

<figure><img src="image_22.png" /></figure>

B) U1547

<figure><img src="image_23.png" /></figure>

C) U1552

<figure><img src="image_24.png" /></figure>

**Figure 1.** Vertical variation of Rock-Eval 7S® parameters in the sediment compared to porewater sulfate, sulfide and methane concentrations, and *in situ* temperature (Teske et al., 2021). A) Site U1545. B) Site U1547, C) Site U1552. For each parameter, the scale is the same from A) to C), except for methane concentration.
SMTZ: sulfate-methane transition zone.

Below 70 mbsf, a progressive decrease of TOC values from ~2.2 to ~1.9 wt% and stable TS contents are observed at site U1545 (Fig. 1A). The presence of dissolved methane in the sediment between 50 and 250 mbsf indicates that methanogenesis is responsible for organic matter biodegradation in this depth interval, while the presence of ethane below 350 mbsf suggests that catagenesis might also contribute to the decrease of TOC values in the deepest parts of the core. This influence of catagenesis is supported by the progressive increase of Tmax values downwards at Site U1545. Despite its estimated age of ~400 ky, the deepest analyzed sample presents a Tmax value of 430°C, indicative of the early oil window (Espitalié et al.,
1985) which is consistent with the *in situ* temperature of ~115°C (Fig. 1A). The relative stability of TS content below 50 mbsf suggests that all reactive sulfur is rapidly fixed in the shallow sediments and not affected by deeper processes. At site U1547, a variability of TOC and TS values is observed below 70 mbsf, with several TOC values below 1.5 wt% and TS values higher than 2.0 wt%. Tmax values of the studied samples generally show a progressive increase downwards, indicating that the samples with low TOC and high TS values were not affected by metamorphism related to the upper or lower magmatic intrusions. Nevertheless, the high sulfur content suggests that these sediments were affected by sulfur-rich hydrothermal fluids generated during emplacement of the sills.

At site U1552 a general trend of TOC decrease and TS increase is observed on the studied interval. Nevertheless, values show a high variability in the uppermost 30 m of sediment (Fig. 1C), due to the presence of thick turbiditic deposits. At this site, the rapid TOC decrease and rapid TS increase associated with bacterial sulfate reduction are not observed, probably because of insufficient sampling resolution. Below 30 mbsf, TOC values decrease progressively and TS values are stable, which can be ascribed to methanogenesis as indicated by the presence of methane in the porosity. Methane hydrates were recovered at site U1552 between ~22 and ~32 mbsf (Teske et al., 2021). Fluid circulation associated with these hydrates might also explain the high variability of TOC and TS values in the uppermost 30 m of sediment.

## 3.2 Pyritic and organic sulfur

At site U1545, pyritic sulfur (Spyr) rapidly increases from 0.5 wt% at ~3 mbsf, to 1 wt% at ~6 mbsf. The Spyr content remains close to 1 wt% below, though with variability. This behavior is consistent with rapid formation of pyrite in the shallowest sediment, in relation to the high rate of organoclastic sulfate reduction (Nagakura et al., 2022). Organic sulfur (Sorg) increases from the shallowest sample (0.25 wt%) to a maximum of 1% at ~40 mbsf. Sorg content remains high in the sediments above 100 mbsf and a secondary maximum is observed at 85 mbsf. Below 100 mbsf, Sorg content progressively decreases downwards to reach a value of 0.1 wt% at ~500 mbsf. The increase in Sorg content in the upper 40 m of sediment is consistent with previous observations of diagenetic incorporation of sulfide resulting from bacterial sulfate reduction into the organic matter (Eglinton et al., 1994; Werne et al., 2003). Sulfurization is considered to be mostly associated with organoclastic sulfate reduction (Sinninghe Damsté and de Leeuw, 1990). Nevertheless, the Sorg increase is observed in a depth range where sulfate reduction rate is low (Nagakura et al., 2022), but is overall coincident with the peak of dissolved sulfide centered at the sulfate-methane transition zone (SMTZ; figure 1A). Organic matter sulfurization therefore appears to be mainly associated with anaerobic oxidation of methane and not to organoclastic sulfate reduction (Quijada et al., 2016). The decrease of organic sulfur content below 100 mbsf suggests that organic sulfur is lost during burial. Though the mechanisms involved in this de-sulfurization remain to be understood, one likely explanation is a rapid loss of organic sulfur because of thermal maturation, as indicated by the increase of Tmax values. Maturation experiments showed that the organic sulfur content of rocks significantly decreases during early maturation (Rosenberg et al., 2021).

At site U1547, both Spyr and Sorg increase in the upper 70 m of sediment (Fig. 1B). The increase is more progressive than at Site U1545, and can be mostly ascribed to organoclastic sulfate reduction, as the SMTZ is located deeper in the sediment. Below 70 mbsf, Spyr continues to increase downwards and reaches maximum values of 2.1% above the upper sill. Conversely, the Sorg content decreases and reaches minimum values of 0.15% above the upper sill. Though the Sorg content could have been affected by the thermal influence of the sill, the Tmax values of the sample do not support such conclusion. Therefore, as previously suggested, it appears more likely that the sulfur content and speciation of the samples located above the upper sill was influenced by hydrothermal fluid circulations. Neither sulfate nor sulfide are present in the porewater of sediments located between the upper and lower sills. The Spyr content in this interval is comparable to that observed at 70 mbsf, while the Sorg content is comparable to that of sediments of similar *in situ* temperature at Site U1545. The sediments between the two intrusions overall do not appear to be affected by the thermal influence, or hydrothermal fluids, associated with these intrusions.

At site U1552, an increase of the Spyr content is observed in the upper ~15 m of sediments, coincident with an increase of dissolved sulfide in the porosity. Below this depth, Spyr content fluctuates erratically around 1% while sulfide content in the porosity is negligible. The Sorg content remains below 0.5% in the studied section, showing erratic variations. It is inferred that the sharp geochemical gradients observed at this site, associated with the presence of gas hydrates, only allow efficient formation of pyrite in the shallow sediment but are not favorable to efficient organic matter sulfurization.

## 3.3 Tmax

Tmax is a well-known indicator of thermal maturity of the organic matter that also indirectly reflects its composition and environment of deposition (Espitalié et al., 1985). It is also modified by the presence of labile compounds or of organic sulfur, which shifts the Tmax towards low values (Rosenberg and Reznik,
2021). Tmax values of the shallowest samples are in the range observed for upwelling surface sediments (410-420°C; Wefer et al., 1998; D'Hondt et al. 2003) and decrease to ~380-390°C at intermediate depth (Figures 1A, B), before showing an expected increase with depth. At site U1545, the minimum Tmax values are observed around 80 mbsf, below the SMTZ, at a depth where *in situ* temperature is lower than 30°C and dissolved sulfide concentration in the porewater is low. At site U1547, the minimum Tmax values are also observed around 80 mbsf, above the SMTZ, at *in situ* temperature of ~50°C and high concentration of dissolved sulfide. Though the minimum of Tmax values is not observed at site U1552, a clear decreasing trend is also observed in the 50m of sediment analyzed here. This decrease of Tmax values could reflect the generation of labile microbial biomass in the sediment at intermediate depth. Nevertheless, the very different temperatures and geochemical composition of the sediment at the minimum of Tmax do not give support to this hypothesis. Alternatively, the decrease of Tmax values in the upper sediments could reflect the sulfurization of the organic matter in this interval (Rosenberg and Reznik, 2021).

## 4 CONCLUSION

At the reference site U1545, the downwards evolution of organic matter reflects the progressive influence of bacterial degradation followed by abiotic/thermal transformation. Sulfide released by organoclastic sulfate reduction and anaerobic oxidation of methane seems to mostly react with iron. Organic matter sulfurization is maximum at intermediate depth but decreases below, possibly due to early thermal cracking. This pattern is more or less also observed at the two other sites, though hydrothermal circulations possibly influenced the sulfur content at site U1547. The sediment at site U1552 is more heterogeneous, with the presence of thick clays deposits. Added to the presence of sharp geochemical gradients associated with gas hydrates, our sampling resolution does not allow to identify clear trends in organic matter and sulfur evolution.

## 5 ACKNOWLEDGMENT

The authors would like to express appreciation for the support of IODP France. They also thank the reviewers, Alon Amrani and Mohammed Boussafir, for their constructive comments.

## 6 REFERENCES

* Berner, R.A., 1984. Geochimica and Cosmochimica Acta, 48, 605-615.

* D'Hondt, S.L., Jørgensen, B.B., Miller, D.J., et al., 2003. Proceedoings ODP, Initial Reports, 201. College Station, Texas.

* Eglinton, T.I., Irvine, J.E., Vairavamurthy, A., et al., 1994. Organic Geochemistry, 22, 781-799.

* Espitalié, J., Deroo, G., Marquis, F., 1985. Revue de l'Institut français du Pétrole, 40, 755-784.

* Espitalié, J., Lamoureux-Var, V., Bouton, N., 2024. in: Baudin, F. (Ed.), The Rock-Eval Method: Principles and Application. ISTE Editions, pp. 125-152.

* Goldhaber, M.B., Kaplan, I.R., 1974. in: Goldberg, E.D. (Ed.), The Sea, Vol. 5. Wiley, New York, pp. 569-655.

* Nagakura, T., Schubert, F., Wagner, D., et al., 2022. Frontiers in Microbiology, 13, 845250.

* Quijada, M., Riboulleau, A., Faure, P., et al., 2016. Marine Geology, 381, 54-66.

* Rosenberg, Y.O., Reznik, I.J., 2021. Organic Geochemistry, 162, 104305.

* Rosenberg, Y.O., Reznik, I.J., Vinegar, H.J. et al., 2021. Marine and Petroleum Geology, 124, 104773.

* Sinninghe Damsté, J.S., de Leeuw, J.W., 1990. Organic Geochemistry, 16, 1077-1101.

* Teske, A., Lizarralde, D., Höfig, T., et al., 2021. Proceedings IODP, 385. College Station, Texas.

* Wefer G., Berger W.H., Richter C., et al., 1998. Proceedings ODP, Initial Reports, 175. College Station, Texas.

* Werne, J.P., Lyons, T.W., Hollander, D.J., et al., 2003. Chemical Geology, 195, 159-179.
# CHALK MARL SEAMS: NOT QUITE WHAT THEY SEEM ...

Ophélie FAY<fnref n="1" />, Matheus KROTH<fnref n="2" />, Hannes CLAES<fnref n="3" />, Andrew R. FARRANT<fnref n="4" />, Sarah VANDYCKE<fnref n="1" /> and Mark WOODS<fnref n="4" />

Corresponding author: Ophelie.FAY@umons.ac.be

**Abstract:** The origin of wispy marl seams and nodular chalk has long been debated, particularly regarding their sedimentary versus diagenetic origins. Through scanning electron microscope (SEM) observations of thin sections from the Chalk of Southern England (UK), we gain new insights into the microtexture of nodular chalk, specifically the Lower Turonian Holywell Nodular Chalk and Upper Turonian-Coniacian Lewes Nodular Chalk formations, and marl seams from the Turonian New Pit Chalk Formation. In nodular chalks, microscale observations reveal evidence of pressure dissolution, either overprinting early diagenetic nodules or generating a stylonodular fabric. The darker seams, visible to the naked eye between the nodules, are lower porosity zones with bioclast concentration due to the preferential dissolution of the micritic matrix, cementation, and/or mechanical compaction. Surprisingly, despite their darker colour due to porosity loss, they often contain very little clay. The latter are concentrated in the insoluble residue coating the nodules. Interestingly, in the New Pit Chalk Formation, marl seams do not consist of marl, instead, they are primarily related to the mechanical compaction and pore reduction of the micritic matrix. Low-field nuclear magnetic resonance (NMR) T₂ analysis provides insight into the porous network, where T₂ is a proxy for pore size. Results reveal the T₂ logarithmic mean (T₂lm) equals 34-41ms in white chalk from the New Pit Chalk Formation, while marl seams samples average 18-24 ms, which does not correspond to the signal of typical argillaceous-marly chalk where T₂lm<10 ms. These findings challenge the view that nodular chalk results solely from sedimentary and early diagenetic processes and provide a new perspective on the formation of thin, wispy so-called marl seams. *Marl seams* are likely not of sedimentary origin nor strict dissolution seams but rather correspond to compaction bands or hybrid compaction-dissolution bands. These observations may have important implications for future stratigraphical correlations and fluid flow within the Chalk.

**Keywords:** Nodular chalk, pressure dissolution, microtexture, diagenesis, compaction bands, marl seams

## 1 INTRODUCTION

Marl is defined as a sedimentary rock composed of a mixture of clay (35-65%) and calcium carbonate (65-35%) (Tucker, 2003). In the context of chalk, the term *marl seam* traditionally refers to concentrations of clay that form conspicuous seams within the chalk, a terminology dating back to William Smith, the father of UK geology in the 18th century. These marl seams, observed in cliffs and core samples, appear brown to dark grey and range from a few millimetres to a few centimetres thick in the Anglo-Paris Basin. The origin of marl seams in chalk is still a subject of debate. Mortimore (2011) emphasizes this ongoing controversy: are marl seams the result of pressure dissolution, or are they primary sedimentary structures? Wray and Jeans (2014) argue, based on XRD analysis, that marl seams are sedimentary in origin, as the insoluble residue mineralogical content differs from the white chalk. Conversely, Garrison and Kennedy (1977) describe wispy marl seams as flaser chalk or solution seams, attributing their formation solely to pressure dissolution. Understanding and characterizing marl seams is essential for delineating compartments in chalk reservoirs. These layers may act as mechanical interfaces within groundwater reservoirs and contribute to karst development. Therefore, comprehending their formation processes is crucial for predicting their geographical and stratigraphical distribution.

## 2 GEOLOGICAL CONTEXT AND METHODS

Chalk samples from the Holywell, New Pit and Lewes Chalk formations ranging from Turonian to Coniacian in age, were studied. Samples were collected from a borehole located a borehole on Salisbury Plain, Wiltshire, at the border, at the border between the Southern and Transitional Chalk Provinces, as defined by Mortimore et al. (2001). Twenty standard thin sections (30 mm thick) were prepared. Backscattered electron (BSE) analysis, using a Quanta 600 SEM, at magnifications of 200 to 5,000 times, was used to observe the microtexture, identify non-carbonate minerals and microporosity. NMR transverse relaxation time (T₂) of chalk plugs was measured using a low-field (0.196 T; 8.33 MHz) Halbach-based rock core analyzer at the Sakellariou NMR lab, KU Leuven. NMR T₂ distributions were obtained using the Carr-Purcell-Meiboom-Gill

<footnote n="1">¹University of Mons, UMONS, Mining Engineering Department, Mons, Belgium</footnote>

<footnote n="2">²Department of Earth Sciences, Universiteit Utrecht, Utrecht, The Netherlands</footnote>

<footnote n="3">³Katholieke Universiteit (KU) Leuven, Earth and Environmental Sciences, Leuven-Heverlee, Belgium</footnote>

<footnote n="4">⁴British Geological Survey, Kingsley Dunham Centre, Keyworth NG12 5GG, UK</footnote>
(CPMG) method and provide a proxy for pore size (see Khajooie et al., 2024). Parallely, also water-saturation porosity was measured on the plug-samples.

# 3 RESULTS AND DISCUSSION

## 3.1 Nodular chalks

SEM observations of nodular chalk reveal its microtexture, highlighting the nodules and seams, which appear very dense (white) compared to the surrounding chalk (Fig. 1).

<figure><img src="image_25.png" /></figure>

Figure 1. (A) SEM observation of a thin section from Lewes Chalk showing eogenetic nodules and solution seams. Colour contrast (white seams, dark grey nodules) due to porosity loss in seams. Pressure dissolution indicated by a porous dissolution zone. (B) Close-up view emphasizing the colour contrast between the seams (white) and nodules (dark grey). (C) Detailed image of a solution seam in Lewes Chalk, with insoluble residue concentration, and truncated foraminifera. (D) Interface between a nodule and a seam in Lewes Chalk. (E) Microtexture of a seam showing mechanical compaction and loss of interparticle porosity. (F) Core image of Holywell Chalk with nodular texture. (G) Core image of Holywell Chalk showing coarse facies and nodular texture. (H) SEM observation of a thin section from Holywell Chalk with a diagenetic pseudo-nodule indicated by a bioclast crosscutting both the chalk, seam and nodule. (I) Close-up of the bioclast cross-cutting the seam. (J) Interface between solution seam and nodule in Holywell Chalk, marked by insoluble residue concentration. (K) Microtexture of the chalk matrix in Holywell Chalk, showing primary textural characteristics. (L) Microtexture of a solution seam in Holywell Chalk, showing mechanical compaction and reduced interparticle porosity. Abbreviations: bc – Bioclast, c – Chalk matrix, dz – Dissolution zone, f – Foraminifera, IR – Insoluble residue concentration, n – Nodule, pn – Pseudo-nodule, py – Pyrite, s – Solution seam.
Porosity loss in the seams may be a combination of dissolution of the micritic matrix, cementation and reorganization of the grains by mechanical compaction. The origin of the nodules varies between samples, including both synsedimentary, early diagenetic, and late burial nodules. In all cases, the nodules are characterized by pressure dissolution on their outer edges. Early diagenetic nodules, as described by Garrison and Kennedy (1977), appear dense (light grey, Fig. 1A-C) due to porosity reduction caused by cementation. Around the nodules, the insoluble residue ranges from 10 to 100 µm thick. Considering an averaged insoluble residue of 2% in white chalk (Wray and Jeans, 2014), it would imply that 0.5 mm of chalk must have dissolved to accumulate 10 µm of insoluble residue, or 5 mm for a 100 µm-thick residue. This nodular facies is predominantly encountered in the Lewes Nodular Chalk, particularly near hardgrounds. The rheological contrast between eogenetic nodules and the surrounding chalk concentrates pressure dissolution, sometimes forming microstylolites between nodules. In the Holywell Nodular Chalk, intraclasts are observed, likely related to early sediment remobilization, and concentrate pressure dissolution at their interfaces as well. Microscopic observations reveal that some nodules are strictly diagenetic. In figure 1 (H-I), bioclasts crosscut both the seam and the nodule, indicating the diagenetic origin of the nodule. The micritic matrix has been preferentially compacted both mechanically and by pressure dissolution in the seam (Fig. 1, J-L). Similar facies were described in limestone (Kahsnitz and Willems, 2019) as stylonodular.

Interestingly, nodular chalk formations occur at rheological and mechanical interfaces. The base of the Holywell Nodular Chalk Formation is the Plenus Marls, a global anoxic event. It forms atop the argillaceous Zig Zag Chalk and shows a decrease in nodulation upward. Similarly, the Lewes Nodular Chalk forms atop hardgrounds, with nodulation also decreasing upward. It can be argued that due to the rheological interface and the different mechanical behaviours of the formations, there is a concentration of stress and intensification of pressure, leading to increased mechanical and chemical compaction at these interfaces.

<figure><img src="image_26.png" /><figcaption>Figure 2. NMR T2 signal of marl seams (orange) and white chalk (blue) from the New Pit Formation. Lower T₂ values indicate a reduction in pore size due to mechanical compaction of the microtexture. For comparison, the typical T₂ signal and microtexture of a clay-rich chalk sample (Zig-Zag Chalk Formation) is shown in grey.</figcaption></figure>

## 3.2 Marl seams

Linear structures darker than the host rock are observed on core surfaces. Their internal characteristics cannot be resolved by the naked eye, a hand lens, or even optical microscopy. SEM observations have revealed the intrinsic nature of marl seams. BSE images show that the seams are paler than the surrounding pure chalk due to higher density. Higher magnification reveals compaction of the microtexture and a loss of interparticle porosity (Fig. 2). The calcitic grains forming the microtexture display punctate contacts in the pure chalk but serrate to sometimes coalescent contacts in the marl seams. At the interface between the pure chalk and the compacted chalk, a thin layer of insoluble residue resulting from localized pressure dissolution is sometimes observed. Cements are occasionally seen in marl seam intervals as crystal overgrowths around bioclastic fragments. Porosity measurements reveal that the total sample porosity at the plug scale is reduced by about 8%. However, the average pore size (expressed by T₂) is halved in samples containing marl seams compared to pure chalk (Tab. 1).
The NMR signal from the marl seam samples also shows a distinct pattern compared to those of marly sedimentary origin (Fig. 2). The origin and mechanism behind the formation of marl seams remain to be fully understood. However, our microscale observations provide new evidence about their nature and suggest reconsidering the term “marl seams.” Previous studies (e.g., Wray and Jeans, 2014) have shown that many marl seams contain only 3-10% non-carbonate minerals, yet the term “marl” persists, further confusing the nature of these structures. They are hybrid compaction-pressure dissolution bands. Whether they overprint an initial signal remains to be determined.

<table><caption>Table 1. Petrophysical Properties of New Pit Chalk samples. Porosity, density, and T2lm range of values from pure chalk samples (3) and marl seam samples (3).</caption><thead><tr><th></th><th>Porosity (%)</th><th>Density (g/cm³)</th><th>T2lm (ms)</th></tr></thead><tbody><tr><td>Pure chalk</td><td>41 - 43</td><td>1.57 - 1.61</td><td>34 - 41</td></tr><tr><td>Marl seams</td><td>35 - 38</td><td>1.72 - 1.75</td><td>18 - 24</td></tr></tbody></table>

## 4 CONCLUSION

Marl seams are more complex than they appear at first glance. For decades, geologists have been misled by the darker appearance of these intervals in the field, leading to the assumption that they are *marl*. However, our observations reveal that some of these seams do not contain more clay minerals than the surrounding white chalk. Instead, they all exhibit microtextural scale compaction features, and should be referred to as compaction bands or hybrid compaction-dissolution bands. Petrophysical characterization of these layers indicates lower porosity and, more importantly, smaller pore sizes, which significantly reduce permeability. These low-permeability intervals thus act as barriers to vertical fluid flow and may lead to the development of lateral fluid flow conduits and potential microkarstification atop the compaction bands (commonly known as marl seams) (Farrant et al., 2022). These findings underscore the importance of microscale observations in understanding basin-scale processes. Our research highlights the necessity of investigating the location and timing of compaction bands, as well as the role of early compaction within these intervals prior to rock consolidation. Future modelling efforts should build on our results to address these critical questions.

## 5 ACKNOWLEDGMENT

O.F. and S.V. are researchers at the National Research Foundation of Belgium (FNRS). H.C. was partially supported by the Research Foundation Flanders (FWO) under Grant PorMedNMR -No. G0D5419 N. A.R.F. and M.W. publish with the approval of the Executive Director, British Geological Survey.

## 6 REFERENCES

* Farrant, A.R., Maurice, L., Ballesteros, D. and Nehme, C., 2022. The genesis and evolution of karstic conduit systems in the Chalk. Geological Society of London Special Publications, 517, 15-37.

* Garrison, R.E., Kennedy, W.J., 1977. Origin of solution seams and flaser structure in Upper Cretaceous chalks of southern England. Sedimentary Geology, 19, 107-137.

* Kahsnitz, M.M., Willems, H., 2019. Genesis of Paleocene and Lower Eocene shallow-water nodular limestone of South Tibet (China). Carbonates and Evaporites, 34, 199-218.

* Khajooie, S., Gaus, G., Dohrmann, A.B., Krüger, M. Littke, R., 2024. Methanogenic conversion of hydrogen to methane in reservoir rocks: An experimental study of microbial activity in water-filled pore space. International Journal of Hydrogen Energy, 50, 272-290.

* Mortimore, R., 2011. A chalk revolution: what have we done to the Chalk of England?. Proceedings of the Geologists' Association, 122, 232-297.

* Mortimore, R.N., Wood, C.J., Gallois, R.W., 2001. British upper cretaceous stratigraphy. Geological Conservation Review Series, 23, 558.

* Tucker, M.E., 2003. Sedimentary rocks in the field. John Wiley & Sons.

* Wray, D.S., Jeans, C.V., 2014. Chemostratigraphy and provenance of clays and other non-carbonate minerals in chalks of Campanian age from Sussex, southern England. Clay Minerals, 49, 327-340.
# FLAMBOROUGH HEAD'S UPPER CRETACEOUS CHALK: INSIGHTS INTO A MIXED DEEP-SEA SEDIMENTATION SYSTEM

Ophélie FAËY¹, Eric LASSEUR² and Fabien PAQUET²

¹UMONS, Faculty of Engineering, Mining Engineering Department, Place du parc 20, 7000, Mons, Belgium
²BRGM, 3 avenue Claude Guillemin, BP 6009, 45060, Orléans Cedex 2, France

Corresponding author: ophelie.fay@umons.ac.be

**Abstract:** The outcrops of the Flamborough Head promontory in Yorkshire, UK, including sea cliffs and inland quarries, provide an exceptional view of the Upper Cretaceous sedimentation. These exposures reveal chalks uplifted during the Tertiary inversion. A detailed sedimentological study of the Turonian to Campanian sections, incorporating both macroscopic and microscopic observations, reveals well-preserved sedimentary features consistent with depositional processes in a deep marine environment. The study identified three main types of facies: contourites, turbidites, and hemipelagites. Contourites, influenced by bottom currents, are composed of fine-grained chalk and exhibit thinly laminated beds with planar and undulating laminations, as well as current ripples. These beds, which can reach a total thickness of several dozen meters, exhibit internal organization with topographic compensation. That suggests deposition within larger contourite drifts – as observed in nearby offshore seismic profiles. Turbidites are characterized by massive beds with sharp bases fining upward, laminated deposits, and minimal bioturbation. Hemipelagites, in contrast, are intensely bioturbated mudstones with sparse fauna and show no evidence of current reworking. These deposits in Flamborough Head provides a valuable analogue for the North Sea deposits, offering insights into the dynamics of chalk deposition along the paleo-slope influenced by bottom currents. The identification of these contouritic facies in chalk across the Flamborough Head promontory highlights periods of increased bottom current activity, with hemipelagite-dominated intervals in between. The outcrop presents a unique combination of stylolitization from burial diagenesis and differential weathering, which highlight its sedimentary features. This provides insights into sedimentary processes that are usually hardly observable in chalk from cores. Although contourites geometries are widely reported through seismic data, facies observations are rare. This study not only provides a valuable analogue for North Sea Chalk deposits but also enhances our understanding of the sedimentology and current dynamics within the Chalk bassin.

**Keywords:** chalk facies, contourite, sedimentology, Northern Province, bottom-current

## 1 INTRODUCTION

Recent studies have demonstrated that the Upper Cretaceous chalk deposits in the North Sea and Danish Basins were significantly shaped by contour currents, challenging the traditional perception of chalk as a flat-lying pelagic sediment. Seismic data reveal extensive contourite systems characterized by drifts, moats, and sediment waves, indicating the presence of energetic bottom currents flowing parallel to bathymetric contours, according to Esmerode et al. (2007). Distinguishing contourites from turbidites and hemipelagites is challenging in all sediment types (Hüneke et al., 2020), but it is particularly difficult in fine-grained deposits such as chalk. Previous research on contourite chalk identified through seismic data has shown that it is difficult to differentiate from pelagic chalk based solely on facies analysis (Rasmussen & Surlyk, 2012). The lack of access to contourite facies represents a missing link in understanding the variations in reservoir properties and the internal organization of structures recognized in seismic data. The Flamborough outcrops, ranging from the Turonian to Campanian, provide a unique opportunity to study chalk deposits analogous to those in the subsurface of the North Sea. These outcrops feature deep-sea sediments with alternating contouritic facies, turbidites, and hemipelagites.

## 2 GEOLOGICAL SETTINGS

Flamborough Head, located on the Yorkshire coast in northern England, is a prominent chalk promontory that extends into the North Sea. The cliffs at Flamborough Head provide a continuous section from the base of the Upper Cretaceous Series to the lower part of the Lower Campanian succession (Mortimore, 1986). The deposits of interest in this study correspond to the Burnham and Flamborough formations, as defined by the lithostratigraphy of the Northern Province Chalk in the UK. The Flamborough Chalk Formation is characterized by the absence of flint and the presence of numerous marl layers. This formation is divided into three members: South Landing, Danes Dyke, and Sewerby Steps (Whitham, 1993). Underlying the Flamborough Chalk is the Burnham Chalk Formation, which contains flint and is well-exposed in the cliffs from North Landing to South Landing (Mortimore, 1986).
# 3 RESULTS AND DISCUSSION

## 3.1 Seismic scale

A seismic line acquired 70 km north of Flamborough Head has been analyzed, revealing kilometer-wide architectures typical of contourites, characterized by a topography of ridges and valleys (Fig. 1). The study uncovered various mounded and drift structures, including large erosive formations filled with sediment waves. Some mounded structures show truncation on one flank, while smooth draping occurs on the other side. Additionally, a large channel is inferred, with an erosive structure to the south that is filled by the lateral development of sediment waves.

<figure><img src="image_27.png" /><figcaption>Figure 1. Raw and interpreted seismic section and its location offshore Flamborough Head, UK [modified from BGS geological maps (© UKRI) and EMODnet Geology (2021)]</figcaption></figure>

## 3.2 Outcrop scale

A total of 250 meters were logged in detail, with the deposits classified as biomicrite according to Folk's classification. Three facies were identified as follows (Fig. 2):
- Hemipelagite facies are characterized by fine-grained sediments deposited by slow settling in deep marine environments. These deposits typically exhibit homogeneous intervals with in-situ fall-out fossils, such as sponges and inoceramids, which are almost intact (Fig. 2A).

<figure><img src="image_28.png" /></figure>

Figure 2. (A) Hemipelagites, displaying non fragmented large bioclasts only perturbed by compaction; (B) Turbidites, with sharp base and normal grading; (C) Superimposed 3D dunes with some preserved traction features, and little to no erosion between bedforms; (D) Wavy lamination (HCS like features); (E) Bedforms displaying successive levels of abundances (growing and lowering) of fragmented bioclasts interpreted as the effect of acceleration decelerating bottom currents; (F) Bioclastic lag associated with more energetic bottom currents; C to F are interpreted as contourites deposits; (G) Microfacies of a contourite, with the black arrow indicating the stratigraphic upward direction (H) Microfacies of a turbidite.
- Turbidites beds are 10 to 50cm thick. They are characterized by a sharp lower contact, suggesting a sudden deposition event. A normal grading indicates deposition by decelerating turbidity current. The sedimentary structures suggest that these facies are Ta and Tb terms of a Bouma sequence (Fig. 2B).

- Contourites display sedimentary structures, such, planar to undulating draping laminations, and winnowing features, such as HCS-like features organized as successive beds compensating each other (Fig. 2C-F). Although criteria are often lacking to precisely discriminate between contourites and turbidites, several sedimentary structures exhibit decimetre-thick layers with fragmented bioclasts (predominantly inoceramids) that show varying abundance, typically increasing and then decreasing. These layers are interpreted as records of progressively increasing and decreasing bottom current velocities, indicative of contourite deposition (Fig. 2E). Wavy geometries and laminations are frequently associated with these features, reflecting traction and suspension currents. Occasionally, lags with bioclastic drape are present, containing broken and reworked bioclasts with grain size varying from one lag to another. The largest lags contain pluricentimetric debris, while finer lags contain millimetric bioclasts. The presence of winnowing features suggests a sustained current with varying speeds, rather than a series of single events.

Although turbidites and contourites remain challenging to identify definitively, a trend is observed: contourites and turbidites dominate the South-Landing member at base of the sequence, while within the Sewerby Member, there is a transition from contourite/ turbidite-dominated to hemipelagite-dominated deposits.

## 3.3 Microscale

Optical microscopy does not allow discrimination between the three facies, as all are categorized as mudstone texture according to Dunham classification. However, the most significant findings come from SEM observations of thin sections of contourites using BSE imaging, which provide clear evidence of lamination. This marks the first microscale observation of laminations in chalk, showing a distinct alternation between very small grains (darker intervals) and slightly coarser grains (lighter intervals) belonging to the micarb fraction of the chalky matrix (Fig. 2G). Facies from the base of the turbidite, however, display a more abundant coarser fraction, including foraminifera, calcispheres, and bioclasts (Fig. 2H).

## 3.4 Discussion

The Flamborough Chalk has been buried to a maximum depth of 1200 meters. This burial diagenesis has resulted in the development of stylolites and the concentration of insoluble residues. Stylolites tend to form preferentially along sedimentary interfaces, thereby highlighting sedimentary features in both contourites and turbidites. This unique combination of deep burial stylolitization and outcrop weathering enhances the visibility of sedimentary structures and facilitates the identification of chalk facies. These deposits are typical of deep water and basinal environments, include some turbiditic events and hemipelagites, which have been reworked by more or less active contouritic currents. Mortimore (2019) described the structures within these deposits as resulting from intra-chalk tectonic movements. He suggests that the bedding corresponds to shear zones, which include internally laminated and deformed blocks that have been rotated, linking this deformation to a period before the chalk was fully consolidated. While we concur that synsedimentary faulting occurred in the Flamborough chalk outcrop (Faÿ-Gomord et al., 2017), our sedimentological analysis enables us to link the deposits to deep-sea sedimentation processes and subsequent burial diagenesis. Whitman (1993) had described the Sewerby Member as initially being a thin-bedded, stylolitic chalk that transitions to massive-bedded chalks higher in the succession. This is consistent with our interpretation of the lower part as laminated contouritic deposits, which transition to more homogeneous hemipelagic deposits in the upper part of the formation.

## 4 CONCLUSION

Seismic data and outcrop analyses in the vicinity of Flamborough Head promontory reveal complex deep-sea sedimentation system. Our study reveals the strong influence of bottom currents on deep sea chalk deposition from Turonian to Campanian. Although, end members of contourites from turbidites and pelagites can be identified their precise distinction remains challenging, the findings highlight the possibility of recognizing contourite facies, to better understand reservoir properties and the internal organization of chalk deposits. The Flamborough outcrop provides a valuable analogue for subsurface studies, showcasing alternating contouritic facies, turbidites, and hemipelagites. Future research should focus on improving methods for distinguishing between these deposits and studying outcrop analogues to enhance understanding of subsurface contourite systems and their impact on reservoir properties.

## 5 ACKNOWLEDGMENT

The authors thank the FNRS (Belgium) for supporting the postdoctoral research project of Ophélie Faÿ, and Heiko Hünecke and Thierry Mulder for their reviews of this paper.
6 REFERENCES

EMODnet Geology Consortium, 2021. Seabed Substrates, Sea-floor Geology, Coastal Behavior, Geological Events and Probabilities, Marine Minerals, and Submerged Landscapes. European Marine Observation and Data Network.

Esmerode, E.V., Surlyk, F., Lykke-Andersen, H., 2007. Contourite drifts, moats and channels in the Upper Cretaceous chalk of the Danish Basin. Sedimentology, 54, 405-422.

Faÿ-Gomord, O., Verbiest, M., Swennen, R., 2017. Synsedimentary Polygonal Faults System in Tight Chalk - Multiscale Evidence from Outcrop, Flamborough Head (UK). 79th EAGE Conference and Exhibition, Paris, France, pp. 12-15

Hüneke, H., Hernández-Molina, F.J., Rodríguez-Tovar, F.J., Llave, E., Chiarella, D., Mena, A., 2020. Diagnostic criteria using microfacies for calcareous contourites, turbidites and pelagites in the Eocene-Miocene slope succession, southern Cyprus. Sedimentology, 67, 127-152.

Mortimore, R. N., 2019. Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels, slumps, faults, folds and glacitectonics. Proceedings of the Geologists' Association, 130, 27-65.

Mortimore, R.N., 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, 97, 97-139.

Rasmussen, S.L., Surlyk, F., 2012. Facies and ichnology of an Upper Cretaceous chalk contourite drift complex, eastern Denmark, and the validity of contourite facies models. Journal of the Geological Society, 169, 435-447.

Whitham, F., 1993. The stratigraphy of the Upper Cretaceous Flamborough Chalk Formation north of the Humber, north-east England. Proceedings of the Yorkshire Geological Society, 49, 235-258.
# CARBONATE SOURCES AND DYNAMICS IN COASTAL ENVIRONMENTS: APPLICATION OFF THE COAST OF SAINT-VALÉRY-EN-CAUX (SEINE-MARITIME)

Clément FRIGOLA<fnref n="1" />,<fnref n="2" />, Gwendoline GRÉGOIRE<fnref n="1" />,<fnref n="2" />, Anne MURAT<fnref n="1" />,<fnref n="2" />, Emmanuel POIZOT<fnref n="1" />,<fnref n="2" />, Florian HAITZ<fnref n="1" />, Benjamin LE LOREC<fnref n="1" />,<fnref n="4" />, Ingrid PEUZIAT<fnref n="3" />, Stéphane COSTA<fnref n="3" /> and Yann MÉAR<fnref n="1" />,<fnref n="2" />

Corresponding author: Anne.Murat@lecnam.net

**Abstract:** The Seine-Maritime coastline, shaped by constantly eroding Mesozoic chalk cliffs, experiences significant and poorly understood sandy siltation, particularly to the east of Saint-Valéry-en-Caux. To better determine the origin of this siltation and to follow the sedimentary dynamics, 45 surface sediment samples over a 10 km<fnref n="2" /> area off the coast of Saint-Valéry-en-Caux were collected. Elemental chemical analyses (Ca, Sr, Si, BrOrg) were carried out: (i) on the bulk sediment, and (ii) on separate fractions. The GSTA (Geochemical Sediment Trend Analysis) method was applied to the bulk sediment, while the COVA (COMposition Variability Analysis) method was used for separate fractions. Carbonates produced in situ and likely to be fragmented during transport are excellent indicators of local dynamics. Their stock dynamics were monitored using an innovative approach (PASTA: PArticulate Stock Trend Analysis). The result is a complete picture of sediment dynamics. Three zones can be distinguished: (i) a steeply sloping western zone, in dynamic swell/tide equilibrium, where shells make little cross-shore transport towards the coast, (ii) a gently sloping eastern zone, also in dynamic swell/tide equilibrium, where shells and quartz migrate coastward through the minimum energy zone located at -15 m, and (iii) an intermediate zone, which appears to be much more mixed, with disrupted cross-shore exchange.

**Keywords:** Sediment sources and stocks, carbonate dynamics, coastal environment, COVA method, PASTA method.

## 1 INTRODUCTION

Sediments are mainly composed of two types of particles: silicates and carbonates. Although these two types are often mixed in coastal environments, they differ in both origin and dynamics. Unlike silicate particles, carbonate particles can have multiple origins. They can be lithoclastic, resulting from the erosion of carbonate substrates such as chalk cliffs, or derived from biological in situ production, like shells. Due to their fragility and low density, carbonate particles are prone to fragmentation and are more easily transported by currents. The Seine-Maritime coastline, characterised by Mesozoic chalk cliffs (Letortu et al., 2015; Costa et al., 2019) in constant erosion (from -0.1 to -0.5 m/year), has undergone significant, yet poorly understood, sandy silting, particularly east of Saint-Valéry-en-Caux (Peuziat et al., 2021). To better determine the origin of the siltation and to trace the dynamic of the carbonate particles, this study proposes a dual approach: quantifying carbonates (traced by Ca) and assessing their qualitative variability (Sr/Ca ratio). Strontium and calcium are two quantitative markers of carbonates and are correlated. The Sr/Ca ratio can be used to identify different carbonate origins (Treese et al., 1981; Stoll & Schrag, 2001; Bayon et al., 2007). The variability of these two markers in granulometric and geographical space makes it possible to trace the geographical sources and dynamics of different types of carbonate particles. The first part of the results, based on the GSTA method (Geochemical Sediment Trend Analysis: Baux et al., 2022), presents the analyses carried out on the total sediment. The second part analyses the individual granulometric fractions and their geographical variability (COVA method: COMposition Variability Analysis). Finally, the last section examines carbonates stock dynamics using PASTA (PArticulate Stock Trend Analysis), an innovative approach to sediment transport analysis.

## 2 MATERIALS AND METHODS

The bathymetry of the studied area ranges from 12 to 21 m depth. Analysis of the morphology reveals the presence of two distinct zones: a steeply sloping western zone and a gently sloping eastern zone. A total of 45 subsurface sediment samples were collected with a Sipek grab off Saint-Valéry-en-Caux over a 10 km<fnref n="2" /> area (Fig. 1). The samples were separated by sieving into 20 particle size fractions ranging from less than 50 µm to 2500 µm (log scale of 1.26) (NF ISO 3310-1, 2016). Elemental geochemical analyses were carried out on both the total sample and the separated granulometric fractions using X-ray fluorescence spectrometry.

<footnote n="1">¹Conservatoire National des Arts et Métiers, INTECHMER, Bd de Collignon, 50110 Tourlaville, France</footnote>

<footnote n="2">²Normandie University, UNICAEN, LUSAC, EA4253, 60, Rue Max-Pol Fouchet, CS 20082 Cherbourg, France</footnote>

<footnote n="3">³University of Caen Basse-Normandie, Laboratory LETG-Caen Géophen, Esplanade de la Paix, 14000 Caen, France</footnote>

<footnote n="4">⁴Laboratory of Oceanography and Geosciences, UMR 8187 CNRS, University of Lille, CNRS, ULCO, IRD, 59655 Villeneuve d'Ascq Cedex, France</footnote>
Around sixty marine sediment standards were used to calibrate the analytical method. Additionally, 0.05 g of each subsample was used to measure total organic carbon (TOC) on the fine fraction (<50 µm) by infrared spectrometry (LECO CS 744) after decarbonation with phosphoric acid (H3PO4).

The results obtained on total sediment using the GSTA method and those from the COVA method are presented in the form of maps of spatial variability of element concentration or element concentration ratio. For the PASTA method, calcium stock data were determined using the following formula: (sieve mass (%) × sieve Ca concentration (ppm)). Introducing the mass of the granulometric fractions gives access to the stock of a type of particle. These data describe the geographical variability of the granulometry (mean) of this type of particle. A field of “Finer” vectors indicates the direction of refinement, which can be interpreted as a direction of decrease in particle transport energy.

<figure><img src="image_29.png" /><figcaption>Figure 1: Location of study area and sampled stations.</figcaption></figure>

# 3 RESULTS AND DISCUSSION

## 3.1 Contributions of the GSTA (Geochemical Sediment Trend Analysis) method

The spatial variability in the concentrations of; (i) silicon (Si), an indicator of silicate content, (ii) calcium (Ca), an indicator of carbonate content, (iii) organic bromine (BrOrg), an indicator of organic matter content, and (iv) the Sr/Ca ratio which qualifies the origin of carbonates, is illustrated in figure 2. The variability of the Sr/Ca ratio enables the distinction between carbonates of chalky origin (approximately 1.5) and carbonates derived from shells (greater than 4) (Peuziat, 2022). The concentration ranges of the major elements Si and Ca are identical, varying in opposition from 28 to 7%, corresponding to 60 to 15% SiO2 and 70 to 18% CaCO3. The deposits are therefore primarily composed of carbonates and silicates. The highest silicon concentrations are in the South, but at varying depths, and there is no pattern to the variability of calcium. The Sr/Ca ratio is always greater than 4, indicating that the dominant carbonates are derived from shells throughout the zone. The amount of BrOrg in the total sediment is low, often close to zero with a maximum of 32 ppm. Values above 20 ppm are localised in three patches, which also show an enrichment in silicon. The results obtained on the total sediment do not allow any interpretation of the sedimentary dynamics of the study area.

## 3.2 Contributions of the COVA (Composition Variability Analysis) method

Carbonates represent a target of interest for the COVA methodology, as it allows for the monitoring of both their quantitative (Ca) and qualitative (Sr/Ca) variability in granulometric and geographical spaces.

Figures 3 and 4 illustrate the spatial variability of Ca and Sr/Ca for six distinct granulometric fractions. For the Sr/Ca ratio, the scales remain constant across all granulometric fractions. However, for clarity, each fraction has its own scale for Ca concentration.

A twofold change is evident from 2500 µm to 125 µm, characterized by a decrease in both the range and the maximum value (from a range of 35-9% Ca for the 2000-2500 µm fraction to 9-1% for the 125-160 µm fraction). The coarsest fractions exhibit a greater abundance of carbonates and higher geographical differentiation. The homogenisation of concentrations could be interpreted as indicating more frequent transport of the smallest grains; however, the Sr/Ca ratio provides a different perspective. The three coarsest fractions (Fig. 3, 2500 to 315 µm) are uniformly composed of shells (Sr/Ca ≥ 4), while the finer fractions reveal two origins (Fig. 4, 315-125 µm): shells and chalk. Consequently, there is an absence of
homogenisation for the finest fractions. The decrease in the maximum Ca concentration is associated with the presence of quartz in these finer fractions. The fraction <50 µm is distinguished by high Ca concentrations (range 28-19%) and a higher proportion of chalk than shells.

<figure><img src="image_30.png" /><figcaption>Figure 2: Spatial variability in the concentration (total sediment) of Si (%), Ca (%), BrOrg (ppm) and the Sr/Ca ratio (x1000).</figcaption></figure>

The geographical variability of the 2500 µm to 125 µm fractions generally exhibits higher calcium concentrations offshore in the east (shells) and along the coast in the west (shells for fractions >315 µm and chalk for those <315 µm). The carbonate enrichment of the <50 µm fraction corresponds to the development of a source of coastal chalk to the west, likely originating from the coastal cliffs.

The COVA approach identifies two significant localised sources of carbonates: 315 to 800 µm shells in the offshore area to the east, and 315 to <50 µm chalk from the cliffs on the coast to the west.

<figure><img src="image_31.png" /><figcaption>Figure 3: Spatial variability of Ca concentration (%) and Sr/Ca ratio (x1000) for fractions 2000-2500 µm, 630-800 µm and 315-400 µm.</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>Figure 4: Spatial variability of Ca concentration (%) and Sr/Ca ratio (x1000) for fractions 250-315 µm, 125-160 µm and < 50 µm.</figcaption></figure>

## 3.3 Contributions of the PASTA (PArticular Stock Trend Analysis) method

The PASTA methodology facilitates the visualisation of the current granulometric gradients within the stocks of a specific grain type using vectors. Figure 5 illustrates the 'finer' gradients of the carbonate stock. Inshore vectors are indicated in red, offshore vectors in blue.

<figure><img src="image_33.png" /><figcaption>Figure 5: Finer gradients of the carbonate stock: inshore red vectors correspond to the prevailing action of tides; offshore blue vectors correspond to the prevailing action of swells. Spatial variability BrOrg/TOC (10-4) on fractions < 50 µm. The dashed line indicates the dominant swell/tide boundary.</figcaption></figure>
The area of convergence between the two vectors fields is delineated by a black dotted line, indicating the region where carbonates are the finest in size, at an approximate depth of 15 metres. This zone of reduced energy activity is characterised by the convergence of two distinct refinement gradients: (i) a gradient originating from the open sea and terminating at the coast (red vectors), which corresponds to the prevailing action of tide (Larsonneur, 1971; Levoy and Larsonneur, 1992; Guillen and Hoekstra, 1996; Baux et al., 2019), and (ii) a gradient extending from the coast to the open sea (blue vectors), which corresponds to the prevailing action of swells. It appears that a state of dynamic equilibrium has been established for both hydrological parameters. Geographically, three zones can be distinguished: (i) in the west, the dominant swell/tide boundary is at a depth of approximately 15 metres, (ii) in the east, it is at the same depth, (iii) in the centre, it approaches the coast. Baux et al. (2019) previously identified a similar dynamic equilibrium with two gradients related to the bathymetry which converge towards the 15m depth in the south of Antifer Harbour (70 km further south).

A comparison of the spatial evolution of the carbonate stock with that of BrOrg/TOC (fraction < 50 µm) reveals that the distributions are highly analogous, except in the central zone. The BrOrg/TOC ratio can be used to trace the marine or continental origin of organic matter (Mayer et al., 2007). Marine organic matter is enriched in bromine. A BrOrg/TOC ratio greater than $100 \cdot 10^{-4}$ corresponds to marine organic matter and less than $20 \cdot 10^{-4}$ to continental organic matter. Organic matter of continental origin is enriched in the coastal zone, while organic matter of marine origin is enriched in the offshore zone. Mixing is minimal in the eastern and western zones, and higher in the central zone. Two independent parameters (carbonates stock and organic matter content) demonstrate a change in the behaviour of sediment dynamics at Saint Valery-en-Caux.

The carbonate stock is in dynamic equilibrium and displays two gradients that converge at a depth of about 15 metres, irrespective of the bottom slope.

The variability in organic matter, as depicted by BrOrg/TOC ratio, reveals that the mix of marine and continental organic matter is less pronounced in the eastern and western zones. This phenomenon can be attributed to the well-marked gradients of dominant wave/tide action in these regions. In contrast, the central zone exhibits a higher degree of mixing between these two sources of organic matter.

## 4 CONCLUSIONS

The GSTA method applied on total sediment does not provide meaningful insights into the sources and sedimentary dynamics of the particles making up the deposits. In contrast, the COVA approach provides more interpretable data revealing the presence of two distinct carbonate origins: shell debris and chalk. The local sources and the particle sizes of these two types of carbonates were determined.

The PASTA method then complemented these approaches by assessing the dynamics and trajectories of the carbonate stock.

As a result, a comprehensive diagram of sedimentary dynamics can be established, distinguishing three distinct zones: (i) a steeply sloping western zone, in dynamic swell/tide equilibrium, where marine shells make little progress coastward (cross-shore transport); (ii) a gently sloping eastern zone, also in swell/tide equilibrium, where shells and quartz migrate coastward through the minimum energy zone at -15 metres; (iii) an intermediate zone which appears to be much more mixed, with disrupted cross-shore exchanges.

The sandy siltation observed east of Saint-Valéry-en-Caux corresponds to the influx of a sedimentary stock composed of shells and quartz. These sediments move shoreward through cross-shore transport and successfully traverse the threshold of dominant swell/tide action, likely facilitated by the gentler slope.

## 5 ACKNOWLEDGMENTS

This research was supported by a grant from the Région Basse Normandie and the Communauté d'Agglomération du Cotentin. Sediment samples were obtained during the CAUXySTA cruise. The authors would like to thank the Flotte Océanographique Française and the crew of R/V Côtes de la Manche that make the CAUXySTA cruise possible (https://doi.org/10.17600/18002491). They also thank Alain Trentesaux for reviewing the manuscript.

## 6 REFERENCES

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# WEICHSELIAN LATEGLACIAL AND HOLOCENE FLUVIAL DYNAMICS VARIABILITY IN THE SOMME BASIN: THE EVOLUTION OF AN ALLUVIAL PEATLAND

Chloé GARCIA<fnref n="1" />, Boris BRASSEUR<fnref n="1" />, Agnès GAUTHIER<fnref n="2" />, Lou-Anne MATHIEU<fnref n="2" />, Dierk MICHAELIS<fnref n="3" />, Jérémy BACON<fnref n="1" />,<fnref n="4" />, Ségolène SAULNIER-COPARD<fnref n="2" /> and Pierre ANTOINE<fnref n="2" />

Corresponding author: garcia.chloe@u-picardie.fr

**Abstract:** The Somme basin is one of the most extensive alkaline alluvial peatland systems in North-West Europe. The Weichselian Lateglacial and Holocene peat sequences from this basin have remarkable potential for palaeoenvironmental reconstructions, to understand the long-term influence of climate and anthropogenic forcing on fluvial dynamics. New field investigations carried out since 2021 are based on two high-resolution stratigraphic transects (120 boreholes), soil and geochemical characterisation, identification of pollen and plant macroremains, and 50 radiocarbon dates. The results, supplemented by a synthesis of the data from the various archaeological excavation programmes undertaken since the 1990s, highlight the response of fluvial and peat systems to climatic and anthropogenic environmental change, and provide a particularly robust and detailed outline of the evolution of the valley floor sedimentary infilling. The dynamics and nature of the deposits were determined by climatic changes until the mid-Holocene. Fluvial dynamics constrain the accumulation of peat, temperate periods were particularly favourable to its development: reduction of water flow, detrital impute and particle size, and the development of vegetation. The oldest peat deposits were confined to the abandoned river channels during the Bølling and the Allerød, then peat progressively filled the entire valley floor during the Early and Middle Holocene. A reactivation of fluvial dynamics is recorded in the basin from 7800 cal BP with the establishment of deep erosive channels. From this time onwards, environmental responses vary across the basin. Anthropogenic influences had an increasingly pronounced impact and varied within the basin: soil erosion on slopes, management of alluvial water tables (increased by dams or decreased by peat extraction), causing diachronic interruptions in peat accumulation processes and even peat degradation.

**Keywords:** alluvial peatland, river dynamic, chalk basin, climate variation, anthropisation.

## 1 INTRODUCTION

Peatlands are exceptional archives for palaeoenvironmental reconstruction. In the chalk valleys of north-western Europe, alluvial alkaline peatlands have recorded fluvial dynamics and past ecosystems. Since the Lateglacial, several initiation periods of peat formation have been recorded in Europe: either associated with favourable climatic conditions (rise in temperature, intensification of precipitation with the return of oceanic conditions), as during the Lateglacial temperate interstadials and during the first half of the Holocene. Or with human-induced modifications to the hydrography, as in the second half of the Holocene (Ruppel et al., 2013).

The Somme basin, with its remarkable historical archaeological records and existing stratigraphic data on the valley floor infillings, represents a particularly important case study for investigating the spatial impacts of climatic and anthropogenic changes and modifications on the river dynamics since the Lateglacial.

## 2 MATERIALS AND METHODS

Data on stratigraphy, chronology and palaeoenvironmental conditions from the Weichselian Lateglacial period onwards provide a framework for identifying patterns of change in the fluvial dynamics of the catchment. The data presented here originate from two recently studied stratigraphic transects: Tirancourt (Middle Somme Valley, Garcia et al., 2024) and Morcourt (Upper Somme Valley, Garcia et al., 2025).

In addition, the sites are compared with former studies throughout the basin (Fig. 1), including in the Selle tributary: in Conty (Antoine et al., 2012) and Renancourt (Antoine and Ducrocq, 1997), both situated within

<footnote n="1">¹ EDYSAN (Ecologie et Dynamiques des Systèmes Anthropisés) UMR 7058 CNRS - UPJV, UFR de pharmacie, 1 rue des Louvels, FR-80037, AMIENS, cedex 1</footnote>

<footnote n="2">² Laboratoire de Géographie Physique, Environnements quaternaires et actuels UMR 8591 CNRS- Univ. Paris 1 & UPEC, 2 rue Henri Dunant, FR-94320, THIAIS.</footnote>

<footnote n="3">³ Institute for Botany and Landscape ecology, University of Greifswald, Greifswald, Germany</footnote>

<footnote n="4">⁴ Ecologie des Hydrosystèmes Naturels et Anthropisés, UMR 5023 CNRS-Université Lyon 1, 3-6 rue Raphaël Dubois, Bâtiments Darwin C. & Forel, FR-69622, VILLEURBANNE.</footnote>