# Late Holocene paleoenvironmental and paleoclimatic reconstruction inferred from Sidi Ali El Mekki lagoon in the northeast of Tunisia.

Wiem Hamzaoui<fnref n="1" />,<fnref n="2" />, Meryem Mojtahid<fnref n="1" />, Soumaya Boussetta<fnref n="2" />, Nejib Kallel<fnref n="2" />

The Mediterranean is considered as one of the hotspot regions with respect to the climatic changes. Particularly, its coastal area is affected not only by natural hazards but also by the increase of population and infrastructure. Previous studies (Sabatier et al., 2008, Dezileau et al., 2011, affouri et al., 2017, BS Kohila et al., 2023, and Khalfaoui, O., et al. 2023) have shown that lagoon sediment archives are sensitive to climatic change, and could record efficiently paleoenvironmental evolution.

In this context, our study aims to reconstruct the environmental evolution of the Sidi Ali El Mekki lagoon over the past ~1600 years and makes the link with natural climate variability and human activities. Using a multidisciplinary approach that integrates benthic foraminiferal analysis, sedimentology, geochemistry, and a chronology derived from radiocarbon (¹⁴C) dating, we recorded along the core GEMP1 collected from Sidi Ali El Mekki lagoon a significant shift in sedimentary dynamics and benthic fauna assemblages across key climatic periods: the Dark Age (DA), the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Industrial Period (IP).

During the Dark Age (393–1100 AD), grain size analyses showed low clay content. Geochemical data indicated elevated levels of calcium (Ca) and strontium (Sr), interpreted as a high contribution of biogenic marine material, while low concentrations of iron (Fe) and titanium (Ti) pointed to minimal terrestrial input. We assume that at the time, the Sidi Ali El Mekki lagoon functioned as an open bay, maintaining regular exchanges with the sea. These conditions favored the proliferation of benthic foraminifera, with Haynesina depressula dominating the foraminiferal assemblage. While, the Medieval Climate Anomaly (1100–1690 AD) is characterized by an increase in fine terrigenous input, particularly iron (Fe) and titanium (Ti) accompanied with an increase in clay content and sedimentation rates. This reflects an increase in continental influence, probably in link with more humid conditions over the continent. These changes led to an intriguing significant decline in benthic foraminiferal density, likely due to the dilution effect caused by increased terrestrial inputs or by unfavorable ecological conditions. By 1690 AD, during the Little Ice Age (LIA), we recorded a sharp drop in clay content, along with a significant increase in terrigenous input and a decrease in strontium (Sr) and calcium (Ca) concentrations. These changes were concomitant to the dominance of Ammonia morphogroup tepida, a morphogroup well-adapted to fluctuating salinity and organic-rich sediments. Our results may indicate that the lagoon transitioned into a closed system, which led to reduced water exchange with the sea and an increase in internal sedimentation. Finally, during the Industrial Period (post-1900), we recorded increased concentrations of iron (Fe) and titanium (Ti), indicating enhanced terrestrial input. We also recorded a decline in benthic foraminiferal biodiversity. These changes can be linked to enhanced human activities that characterized this period such as agriculture and urbanization that may have accelerated erosion and sediment input and with it pollutants input.

<footnote n="1">¹ Laboratoire de Planétologie et Géosciences (LPG Nantes) — Université Angers, Le Mans Université, Nantes
Université, CNRS, CNES — 44322 Nantes Cedex 3, France</footnote>

<footnote n="2">² Laboratoire Géoressources, Matériaux, Environnements et Changements Globaux (GEOGLOB) — Université de Sfax —
Faculté des Sciences de Sfax, Sfax, 3000, Tunisie</footnote>
# Impact of riverine sediment mineralogy on seawater Nd isotope compositions in the northeastern part of the Indian Ocean during the last two glacial cycles

Yi Huang<sup>1,2</sup>, Christophe Colin<sup>1</sup>, Franck Bassinot<sup>3</sup>, Zhaojie Yu<sup>4</sup>, Quentin Dubois-Dauphin<sup>1</sup>, Arnaud Dapoigny<sup>3</sup>, David J. Wilson<sup>5</sup>, Germain Bayon<sup>6</sup>

Radiogenic neodymium isotope compositions (eNd) are used as a tracer for water mass circulation and continental weathering at different timescales. However, uncertainties remain in the relative roles of these two factors in driving past seawater eNd variability in settings under the influence of terrestrial or riverine sediment inputs. In this study, Nd isotopes of mixed planktonic foraminifera species and δ¹⁸O and δ¹³C of *Cibicidoides wuellerstorfi* were analyzed on three cores from the northeastern Indian Ocean to better assess the impact of lithogenic inputs from Himalayan rivers and deep-water hydrological changes on the past eNd distribution in the Bay of Bengal (BoB). Our eNd data indicate relatively homogenous and radiogenic values (from

-8.4 to -7.5) during glacial periods in the BoB, similar to the composition of glacial water masses of the Southern Ocean. In contrast, interglacials were characterized by more unradiogenic eNd and a pronounced north-south gradient of ~4.5 eNd units (from -12.9 to -8.5) in bottom water, similar to the present-day distribution in the BoB, pointing to a strong lithogenic control by seawater-particulate interactions. Notably, this significant decoupling of the local Nd isotope signature from the Southern Ocean composition occurred when Himalayan riverine inputs were dominated by the erosion of Indo-Gangetic plain soils during interglacial periods, whereas the preferential delivery of fresh primary mineral assemblages during glacial periods appears to have had little impact on Nd exchange with seawater. These findings provide direct evidence that the degree of seawater-particulate exchange at continental margins is governed by the mineralogy of riverine inputs, with further implications for the use of Nd isotopes as palaeoceanographic tracers.

<footnote n="1">¹ Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">² School of Geosciences and Info-Physics — Central South University — 410083, Changsha, China</footnote>

<footnote n="3">³ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="4">⁴ Key Laboratory of Marine Geology and Environment, Institute of Oceanology — Chinese Academy of Science — Qingdao, China</footnote>

<footnote n="5">⁵ Institute of Earth and Planetary Sciences, — University College London, Birkbeck, University of London — Gower Street, London, WC1E 6BT, UK., Royaume-Uni</footnote>

<footnote n="6">⁶ Laboratoire Géosciences Océan (GEO-OCEAN) — Université de Brest, CNRS, IFREMER, Université Bretagne-Sud, SHOM — 29280 Plouzané, France</footnote>
# $\delta^{18}$O of oak cellulose as useful proxy for reconstructing past summer drought in central France

Charlie Hureau<fnref n="1" />,<fnref n="2" />, Valérie Daux<fnref n="2" />, Monique Pierre<fnref n="2" />, Michel Stievenard<fnref n="2" />, Edouard Régnier<fnref n="2" />, Emmanuèle Gautier<fnref n="1" />

Understanding natural climate variability by documenting it during the preindustrial period is crucial for assessing climate variability and its drivers, improving the quality of simulation models, and specifying the anthropogenic contribution to climate change. In Europe, tree ring width (TRW) and maximum latewood density (MXD) have been widely used to reconstruct past temperature and precipitation variations, particularly in high-altitude or high-latitude regions. However, to correct age-related growth effects, analyses based on TRW and MXD require standardization procedures, which can affect long-term, low-frequency signals.

The study of variations in the oxygen isotopic composition in tree-ring cellulose ($\delta^{18}$O) offers a promising alternative. This isotopic proxy is sensitive to both temperature and water stress and is less or even not affected by age-related growth effects. Several climate reconstructions, based on stable isotopes or tree-ring width and covering the past millennium, exist across Europe. However, none have been conducted for low-altitude regions in France. Our long-term goal is to fill the gap in isotopic data, for the 1200-1400 CE period, of the quasi-millennial $\delta^{18}$O chronology (1046-2007 CE) already existing in France (Etien et al., 2008; Labuhn et al., 2016).

Yet, in dendroclimatology as in dendroisotopy, climate reconstructions require regional transfer functions based on modern reference chronologies. Here, we present a new $\delta^{18}$O chronology from oak trees sampled in the Bertranges forest (Nièvre department, central France) over the period 1910-2023 CE, enabling us both to extend the millennial chronology currently under construction and to update isotope-climate relationships during the instrumental period.

The relationships between $\delta^{18}$O and monthly or seasonal instrumental data were assessed using bootstrapped Pearson correlations for the period 1910-2023. We confirm that oak cellulose $\delta^{18}$O is a reliable proxy for reconstructing climate variables, including July maximum temperature ($r$ = 0.65, $p$ < 0.001), June-July precipitation ($r$ = -0.63, $p$ < 0.001) and, especially, June-July drought conditions (SPEI; $r$ = -0.75; $p$ < 0.001). Furthermore, our chronology aligns well with previous research in northern, western and southwestern France as well as central England. Additionally, spatial correlations with CRU TS4.08 SPEI gridded data highlight the strength of the isotopic signal, the robustness of the statistical relationships, and the regional climate coherence over Western Europe. These results provide a solid foundation for reconstructing past hydroclimatic variability in central France between 1200 and 1400 CE, offering an unprecedented insight into the climatic transition from the Medieval Climate Anomaly to the Little Ice Age in the lowland regions of France.

<footnote n="1">¹ Laboratoire de géographie physique : Environnements Quaternaires et Actuels (LGP) – CNRS, Université Panthéon-Sorbonne, Université Paris-Est Créteil Val-de-Marne – 94320 Thiais, France</footnote>

<footnote n="2">² Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>
Labuhn, I., Daux, V., Girardclos, O., Stievenard, M., Pierre, M., & Masson-Delmotte, V. (2016). French summer droughts since 1326 CE: a reconstruction based on tree ring cellulose δ18O. Climate of the Past, 12, 1101–1117.
Applicability of bacterial membrane lipids as
environmental proxies in lacustrine settings from
French Alps and Chilean regions

Sai Ke <fnref n="1" />, Judith Lemarié <fnref n="1" />, Sergio Contreras <fnref n="2" />, Pierre Sabatier <fnref n="3" />, Charline Giguet-Covex <fnref n="3" />, Erwan Messager <fnref n="3" />, Andréa Julien <fnref n="3" />, Arnaud Huguet<fnref n="1" />

Studies on paleoclimate have revealed the unique mechanisms of climate system and have been used
to improve our predictions of the future climate. Current paleoclimate records are mostly based upon
marine archives, due to the stable deep-sea sedimentation environments. High-resolution paleoclimate
reconstruction of lakes and terrestrial areas are still scarce. To obtain more reliable knowledge on the
past climate, it is essential to solve this orphan state.

Lipids preserved in sediments are widely applied to paleoclimate reconstructions, including 3- hydroxy
fatty acids (3-OH FAs). These molecules produced by gram-negative bacteria can be distinguished into
the normal (straight chain), iso (methyl on the penultimate position), and anteiso (methyl on the
antepenultimate position) groups. Since 2016, several regional and global calibrations between 3-OH-FA
relative abundance and temperature/pH have been published based mainly on soil samples from global
mountainous regions (1). However, the potential of 3-OH FAs as temperature and pH proxies in lakes has
only been scarcely investigated, despite the interest of lacustrine cores for high resolution
paleoreconstructions. The aim of this work was to investigate the applicability of 3-OH FAs as temperature
and pH quantitative reconstitution in lakes.

3-OH FAs were analyzed in sediments from 50 lakes of the French Alps and 20 lakes in southern Chile (Araucania and Los Lagos). As comparison, soils of the same regions are also analysed, if not previously reported (1). The difference of 3-OH FA distribution of lakes and soils is significant. This suggests that the 3-OH FAs can be produced in situ (in the water column and/or sediments) in lakes and vary from terrestrial-origin compositions. In contrast with soils, no linear correlation was observed between 3-OH FA distribution and MAAT or pH in the French and Chilean lakes. To take into account the complex effect of multiple environmental parameters on the 3-OH FA relative abundance in lakes, non-linear, machine-learning algorithms were used. We combined our lacustrine data to those available for 20 Chinese lakes (2) and tested different models. One machine learning algorithm (random forest) allowed to obtain strong (R2 > 0.6) and significant correlations between 3-OH FA relative abundance and either MAAT or pH, with reasonable prediction accuracy (root mean square error of 3.5°C for MAAT and 0.3 for pH).

<footnote n="1">¹ Milieux Environnementaux, Transferts et Interactions dans les hydrosystèmes et les Sols (METIS) — CNRS, EPHE
Paris, Sorbonne Université, Université Paris sciences et lettres — 4 place Jussieu, 75252 Paris Cedex 05, France</footnote>

<footnote n="2">2 Universidad Católica de la Santísima Concepción — Concepción, Chili</footnote>

<footnote n="3">³ Environnements, Dynamiques et Territoires de la Montagne (EDYTEM) — CNRS, Université Savoie Chambéry,
Ministère de la Culture — 73376 Le Bourget du Lac Cedex, France</footnote>
This shows the potential of 3-OH FAs a temperature and pH proxies in lakes.

Références : (1) Véquaud et al., 2021. Biogeosciences, 18(12), 3937-3959. (2) Yang et al., 2021. Organic Geochemistry 160, 104277.
Modern planktonic foraminifera communities in the
Bay of Bengal

Rose Manceau ¹, Meryem Mojtahid ¹, Gianluca Marino ², Eelco Rohling ³,⁴

¹ Laboratoire de Planétologie et Géosciences (LPG Nantes) — Université Angers, Le Mans Université, Nantes
Université, CNRS, CNES — 44322 Nantes Cedex 3, France

²Centro de Investigación Mariña, GEOMA, Palaeoclimatology Lab – Universidade de Vigo — Vigo, 3610, Espagne

³Department of Earth Sciences – Utrecht University — Vening Meinesz building A, 3584 CB Utrecht, Pays-Bas

⁴Ocean and Earth Science, National Oceanographic Centre Southampton – University of Southampton, Southampton
SO14 3ZH, Royaume-Uni

Planktonic foraminifera are widely used as proxies for reconstructing past climate using both test geochemistry and assemblages. Here we evaluate the modern planktonic foraminifera communities in the Bay of Bengal (BOB), where hydrography is influenced by monsoon precipitation and river runoff. We use available plankton tow data from the BOB (Chaabane et al. 2023) to investigate: (1) the spatial and vertical distribution of planktonic foraminifera; and (2) test to which extent modern assemblages reflect hydrography in the BOB. Preliminary results show that the modern planktonic foraminifera assemblages in the Bay of Bengal consist of typical tropical communities, dominated by species such as *Neogloboquadrina dutertrei*, *Globigerinoides ruber*, *Trilobus sacculifer*, *Globorotalia cultrata*, *Globigerinita glutinata*. In terms of spatial distribution, certain species tend to be more abundant in coastal areas (e.g., *N. dutertrei*) while others are more common in the open ocean (e.g., *G. ruber*). The vertical distribution is characterised by a high abundance of *N. dutertrei*, *G. ruber*, and *T. sacculifer* in the first 40 m of the water column.
Vers l'utilisation des alcénones lacustres comme
paléothermomètre dans les lacs de moyennes
latitudes : établissement d'une calibration en Suisse

Céline Martin <fnref n="1" />,<fnref n="2" />, Nora Richter <fnref n="1" />, Linda Amaral-Zettler <fnref n="3" />, Francesco Pomati <fnref n="1" />,
Nathalie Dubois <fnref n="1" />

Connaître l'évolution des températures dans le passé, au-delà des enregistrements historiques, est
indispensable pour comprendre la dynamique à long terme du climat et tester la capacité des modèles
climatiques à reconstituer la variabilité naturelle du climat. Pourtant, il existe encore peu de
reconstructions quantitatives des températures continentales en Europe au cours de l'Holocène, les 12
derniers milliers d'années. De plus, les paléothermomètres existants donnent accès aux températures
moyennes annuelles ou aux températures d'été ce qui limite notre compréhension de la variabilité du
climat durant les saisons intermédiaires et l'hiver. Les alcénones sont des lipides sensibles à la
température produits par les algues Isochrysidales. Ils sont utilisés depuis plusieurs décennies pour
reconstituer quantitativement les changements de température des eaux de surface dans l'océan. Bien
que les alcénones ne soient pas présents dans tous les lacs, ils sont de plus en plus signalés dans les lacs
salés et d'eau douce de l'hémisphère nord. Les alcénones lacustres ont déjà été utilisés avec succès pour
reconstruire les températures dans le passé dans des lacs de hautes latitudes. En fonction de la saison du
dégel des lacs, ils enregistrent les températures de printemps ou d'été. Le suivi temporel du lac St Moritz,
un lac d'altitude au Sud-Est de la Suisse, a montré que le pic de production des alcénones intervient au
printemps dès le dégel du lac. L'établissement d'une calibration in situ entre les indices calculés à partir
des alcénones (UK37 et UK"37) et les températures de l'eau mesurées pendant le suivi devrait nous
permettre d'appliquer ce nouveau paléothermomètre dans le passé pour reconstruire les températures
de printemps. En parallèle, les analyses génétiques de la communauté d'Isochrysidales ainsi que le suivi
des paramètres physico-chimiques de l'eau du lac et de la succession des groupes de phytoplancton vont
apporter de nouvelles informations sur l'écologie des producteurs d'alcénones et les facteurs contrôlant
le timing du bloom. Cette étude va permettre de poser les bases de l'utilisation des alcénones lacustres
comme proxy de température dans les lacs d'Europe centrale.

<footnote n="1">¹ Institut Fédéral Suisse des Sciences et Technologies de l'Eau (EAWAG) – Suisse</footnote>

<footnote n="2">² Institut des sciences de la Terre d'Orléans (ISTO) — Université d'Orléans, CNRS, BRGM — 45071 Orléans Cedex 2,
France</footnote>

<footnote n="3">³ Institut royal néerlandais de recherche sur la mer (NIOZ) – Pays-Bas</footnote>
# Age constraints on Antarctic ice-sheet meltwater pulses during the Last Interglacial Period

Maddalena Passelergue<fnref n="1" />, Russell Drysdale<fnref n="1" />, Isabelle Couchoud<fnref n="2" />,
John Hellstrom<fnref n="1" />, Pauline Treble<fnref n="3" />, Alan Greig<fnref n="1" />

According to recent IPCC estimations based on the intermediate scenario, global warming will likely result in a 0.8-1.5 m sea-level rise by 2100. Since polar ice sheets are the major potential contributors to sea-level rise, it is crucial to understand their response to past warmer periods for better constraining ice-sheet and climate models used for future predictions. The Last Interglacial (LIG; 129 to 116 ka BP) is a suitable period for this approach since temperatures have been estimated at 1.5 C above pre-industrial temperature, which is likely to be reached by the middle of the century. During the LIG, several Meltwater Pulses (MWPs) occurred, as identified in ocean-sediment cores from both Hemispheres. However, the timing and boundary conditions of Antarctic MWPs remain uncertain due to poorly constrained chronologies. Here we will present preliminary results of a project aimed at reducing the chronological uncertainties of LIG ice-sheet MWPs by linking marine sediment records from the Southern Hemisphere with Australasian speleothems. These paleoclimate archives are datable with high precision and have a proven track record at registering climate perturbations triggered by high-latitude meltwater releases. Atmospheric changes related to MWPs will be identified through a high-resolution multiproxy analysis of the speleothems, and their timing constrained with high-density U-Th dating and Synchrotron-X-ray Fluorescence microscopy lamina counting. Synchronisation of speleothem and ocean-sediment core time-series will allow us to produce a new precise chronology for Southern Hemisphere ocean-sediment records that preserve LIG MWPs. Furthermore, the well-constrained chronologies will be used to resolve the phasing of MWPs from both SH and NH ice sheets, which is crucial information for testing ice-sheet model simulations and for evaluating future responses to global warming.

<footnote n="1">¹ School of Geography, Earth and Atmospheric Sciences — University of Melbourne — Parkville 3010, VIC, Australie</footnote>

<footnote n="2">2 Environnements, Dynamiques et Territoires de la Montagne (EDYTEM) — CNRS, Université Savoie Chambéry, Ministère de la Culture — 73376 Le Bourget du Lac Cedex, France</footnote>

<footnote n="3">³ Institute for Environmental Research, Australian Nuclear Science and Technology Organisation – Lucas Heights 2234, NSW, Australie</footnote>
# Fractionnement du dioxygène lors des processus biologiques : application à la reconstruction de la productivité passée de la biosphère

Clémence Paul<fnref n="1" />, Clément Piel<fnref n="2" />, Joana Sauze<fnref n="2" />, Amaëlle Landais<fnref n="1" />

La composition isotopique du dioxygène piégé dans les bulles d'air des carottes de glace peut être utilisée pour reconstituer la productivité globale de la biosphère dans le passé ainsi que pour documenter l'évolution du cycle de l'eau aux basses latitudes. Le travail de cette thèse a été d'améliorer l'interprétation du δ¹⁸O de l'O₂. Pour cela, nous avons affiné la détermination des différents coefficients de fractionnement associés au cycle de l'oxygène au niveau de la biosphère terrestre, c'est-à-dire pendant les processus de respiration et de photosynthèse. Afin de quantifier ces coefficients de fractionnement, nous avons développé un nouveau dispositif expérimental qui repose sur un système de chambres biologiques fermées multiplexées dans lesquelles tous les paramètres environnementaux sont contrôlés, stabilisés et mesurés. Ces chambres biologiques fermées ont un volume de 120 L permettant d'étudier un système " plante + sol ". La première étape a été de développer un prototype avec une seule chambre biologique et de faire unepremière étude sur de la fétuque (Festuca arundinacea). Ensuite, un deuxième développement a permis de travailler sur plusieurs chambres biologiques via un système multiplexé. Ensuite, un deuxième développement a permis de travailler sur plusieurs chambres biologiques via un système multiplexé. Nous avons aussi développé un instrument de spectrométrie optique (SARA-O2) afin de mesurer en continu la concentration et composition isotopique de l'O₂ pendant les expériences en chambres multiplexées. Ceci remplace la mesure par spectrométrie de masse d'échantillons prélevés dans les chambres de façon manuelle. Ce système a permis d'étudier les coefficients de fractionnement pour plusieurs plantes (fétuque, bananier, laurier, maïs). Nous avons notamment mis en évidence un fractionnement isotopique associé à la photosynthèse terrestre. Les fractionnements biologiques trouvés lors de ces expériences ont finalement été intégrés dans des calculs globaux de composition isotopique du dioxygène de l'air et comparés aux mesures effectuées sur la dernière déglaciation. Ceci permet de conclure que les variations passées de δ¹⁸O de l'O₂ dans l'atmosphère sont principalement liées aux variations du cycle hydrologique aux basses latitudes.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² Écotron européen de Montpellier — CNRS — 34980 Montferrier sur Lez</footnote>
The Mid-Pleistocene transition enigma with
combination of planktonic and benthic clumped
isotopes and Os measurements

Marion Peral <fnref n="1" />, Inigo A. Müller <fnref n="2" />, Lucien Nana Yobo <fnref n="3" />, Thibaut Caley <fnref n="1" />,
Bruno Malaize <fnref n="1" />, Thomas Extier <fnref n="1" />, Dominique Blamart <fnref n="4" />, Franck Bassinot <fnref n="4" />,
Mathieu Daëron <fnref n="4" />, Steven Goderis <fnref n="2" />, Philippe Claeys <fnref n="2" />

The Mid-Pleistocene transition (MPT) is marked by a progressive increase of glacial-interglacial cycle
amplitude, a shift of the climatic response from a 41-ka cycle dominated to a 100 ka-cycle, a prominent
asymmetry in large glacial inceptions and an extension of glaciation. This transition is associated with a
cooling of the sea surface temperatures and an increase of the atmospheric CO2 concertation, that could
be associated with a change in the ice-sheet volume. One of the hypotheses to explain the MPT transition
is the regolith hypothesis, based on the basal erosion of glaciers, resulting in changes in weathering and
in ice-sheet volume. Here, we present two new clumped isotope records covering the MPT: (1) the core
MD96-2048 from the southwest sector of the Indian Ocean, in which we analysed a surface-living
planktonic foraminifer; and (2) the "Shackleton" site (IODP Expedition 397 Site U1385) in the Northeast
Atlantic Ocean, with a focus on the benthic foraminifera. The clumped isotope data are combined with
Os measurements to track potential weathering changes.

The clumped isotope thermometer (Δ47) is not affected by secondary biases, allowing, in theory, accurate reconstructions of water temperatures. Our results show colder Δ47 -temperatures compared to the other foraminiferal thermometers (transfer function and Mg/Ca) and organic thermometers (TEX86 and UK’37). Also, no obvious cooling at the ocean surface is observed with Δ47 during glacials and interglacials, but an increase in the amplitude difference of temperatures between the glacial periods before and after the MPT.

The Δ47 also has the advantage of being independent from the isotopic composition of the seawater (δ18Osw). As a consequence, by combining Δ47 and δ18O from foraminifera, the δ18Osw can be reconstructed. Our data are compared to osmium (Os) isotope measurements to observe potential changes in weathering intensity and ice sheet dynamics. Thanks to this unique combination of Os isotope and Δ47, we can test the many hypothesis around the enigma of the Mid-Pleistocene transition.

This work is still ongoing, the data that will be presented are still ongoing.

<footnote n="1">1 Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>

<footnote n="2">2 Vrije Universiteit — Bruxelles, Belgique</footnote>

<footnote n="3">³ Texas AM University – États-Unis</footnote>

<footnote n="4">⁴ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>
# Vegetation and climate changes at the Early to Late Pliocene Transition in Eurasia: the case of Lake Burdur in South-Western Turkey

Mary Robles<fnref n="1" />, Valérie Andrieu<fnref n="1" />, Pierre Rochette<fnref n="1" />, Séverine Fauquette<fnref n="2" />,
Odile Peyron<fnref n="2" />, François Demory<fnref n="1" />, Eliane Charrat<fnref n="3" />, Jérémy Jacob<fnref n="4" />,
Mehmet Cihat Alçiçek<fnref n="5" />

The Pliocene Epoch (5.33-2.58 Ma) is particularly interesting for understanding future climate change linked to the increase in greenhouse gases. Around the Mediterranean basin, the Early to Late Pliocene transition (~3.6 Ma) was marked by the establishment of a Mediterranean climate with a seasonal rhythm, including summer droughts and cool, wet winters. In contrast to the western Mediterranean, the eastern part of the Mediterranean area is poorly documented regarding vegetation and climate changes during this period. The Eastern Mediterranean is considered a biodiversity hotspot and an important refuge area for plants. This region is also the cradle of agriculture at the beginning of the Holocene and a major center for crop plants diversification. The domestication of Poaceae by humans and thus the appearance of cereals is attested at around 12,000 cal BP in the Fertile Crescent. However, cereal harvesting is evidenced 23,000 cal BP in the Levant (Ohalo II), and the presence of pollen of Cerealia-type (proto-cereals) is attested in Lake Acigöl, South-Western Turkey since 2.3 Ma (Andrieu-Ponel et al., 2021). As proposed by Andrieu-Ponel et al. (2021), the appearance of proto-cereals could be linked to the pressure of large herbivore herds on steppe ecosystems, leading to gene mutations of Poaceae.

As part of the ANR project FOOD-RE, the environmental and climate study of the Early-Late Pliocene transition of Lake Burdur (ca. 30 km west of Lake Acigöl) is currently being conducted. The aim of this study is to (1) reconstruct vegetation, lake dynamics, and water level changes based on pollen and Non-Pollen Palynomorphs (NPPs), (2) morphologically characterize the pollen grains of large Poaceae (Cerealia-type), and (3) quantitatively reconstruct the climate using a multimethod approach (Modern Analogue Technique, Weighted Averaging Partial Least Squares regression, Random Forest, Boosted Regression Trees and Climatic Amplitude Method).

Our first results indicate that, during the Early to Late Pliocene Transition, the vegetation was dominated by steppes with Poaceae, Artemisia, and Amaranthaceae. After 3.8 Ma, arboreal taxa significantly decreased, and there is the alternation of steppe grasslands with deciduous Quercus and steppes dominated by Amaranthaceae. The lacustrine ecosystem was characterized by semi-aquatic vegetation and freshwater algae, which exhibit alternating oligotrophic and eutrophic conditions. Pollen of large Poaceae (Cerealia-type) is recorded in the series, but the percentages are lower than at Acigöl (Andrieu-Ponel et al., 2021), not exceeding 3.5%. Morphological characteristics of pollen of large Poaceae

<footnote n="1">¹ Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE) – Collège de France, CNRS, IRD, Aix Marseille Université, INRAE – 13545 Aix-en-Provence, France</footnote>

<footnote n="2">² Institut des Sciences de l'Évolution de Montpellier (ISEM) – CNRS, EPHE Paris, IRD, Université de Montpellier, Université Paris sciences et lettres, CIRAD, INRAP – 34095 Montpellier Cedex 05, France</footnote>

<footnote n="3">³ Institut méditerranéen de biodiversité et d'écologie marine et continentale (IMBE) – Avignon Université, CNRS, IRD, Aix Marseille Université – Aix-en-Provence, France</footnote>

<footnote n="4">⁴ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="5">⁵ Department of Geology — Pamukkale University — Turquie</footnote>
from Burdur are similar to pollen from domesticated cereals in recent periods and do not permit a clear
distinction between pollen of wild and domesticated Poaceae. Climate reconstructions show similar
trends among different methods, and the reconstructed values during the Early-Late transition are close
to present-day values. Following a climatic optimum of precipitation and temperature, climate
reconstructions show an alternation between cool, wet conditions and warm, dry conditions after 3.8
Ma. Around the Mediterranean basin, climate reconstructions indicate warmer conditions with
contrasting precipitation patterns between 3.3 and 3 Ma.

Andrieu-Ponel et al., 2021. Continuous presence of proto-cereals in Anatolia since 2.3 Ma, and their possible co-evolution with large herbivores and hominins. Sci Rep 11, 8914.
# Electrospray-Orbitrap: a groundbreaking new tool for clumped isotopologue analysis of ice core oxyanions

Jack Saville¹, Julien Witwicky¹, Elsa Gautier¹, Joel Savarino¹

¹ Institut des Géosciences de l'Environnement (IGE) – Institut polytechnique de Grenoble, CNRS, IRD, Grenoble Alpes, INRAE – 38400 Saint-Martin d'Hères, France

Despite decades of research, the relationship between climate and atmospheric chemistry remains cloudy. One promising research pathway involves isotopic analyses of nitrate and sulphate oxyanions from ice cores, which hold valuable clues about the past oxidative capacity of Earth's atmosphere. However, traditional bulk isotope analysis techniques for oxyanions, such as IRMS and ICP-MS, fail to extract sufficient isotopic information to demystify the link between climate and atmospheric oxidation.

Recent enhancements in Electrospray-Orbitrap mass spectrometry provide fresh hope. Unlike existing methods that break down molecules for bulk isotope analysis, Electrospray-Orbitrap keeps molecules intact and directly quantifies isotopologue ratios, preserving the sample's finer isotopic details. This innovation paves the way to a new era in stable isotope analysis, moving beyond traditional bulk isotope ratios (e.g. δ¹⁵N-NO₃⁻) and towards isotopologue-specific quantifications (e.g. δ¹⁵N¹⁸O¹⁶O₂⁻).

We demonstrate Electrospray-Orbitrap's ability to sensitively and simultaneously quantify an extended range of nitrate and sulphate isotopologue ratios at high mass-resolution, including the previously unmeasurable doubly-substituted isotopologues. This breakthrough enables the first-ever clumped isotopologue analysis of oxyanions, offering a wealth of new ice core proxies that may illuminate past atmospheric oxidative capacity.

Despite these significant advantages, questions remain regarding the accuracy and precision of Orbitrap measurements compared to the gold-standard IRMS and ICP-MS methods. To validate its performance, we present a comprehensive comparison between measurements obtained using Electrospray-Orbitrap and these gold-standard techniques.
# Status of the in situ <sup>14</sup>C extraction system at CEREGE (Aix en Provence, France) – An emerging tool for dating Holocene deglaciation

Irene Schimmelpfennig¹

¹Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE) – Collège de France, CNRS, IRD, Aix Marseille Université, INRAE – 13545 Aix-en-Provence, France

Thanks to its relatively short half-life of 5.7 kyr, cosmogenic <sup>14</sup>C, produced in situ in quartz, provides the opportunity to investigate exposure histories and surface processes on time scales up to 25 ka ago. In situ <sup>14</sup>C particularly boosts the cosmogenic toolbox, because it can be coupled with the longer-lived <sup>10</sup>Be and <sup>26</sup>Al in the same samples to constrain complex exposure, burial and erosion histories, such as those related to glacier fluctuations over the Holocene. In particular, the emerging approach of paired <sup>10</sup>Be-<sup>14</sup>C dating of recently deglaciated bedrock can be applied to constrain the duration of glacier recession during Holocene warm periods, thus complementing the precise <sup>10</sup>Be moraine records that are becoming increasingly abundant for the Holocene (e.g., Schimmelpfennig et al., Clim. Past 18, 23-44, 2022). Also, in the polar regions, in situ <sup>14</sup>C provides the opportunity of overcoming the "isotope inheritance" problems often encountered with <sup>10</sup>Be and <sup>26</sup>Al, when dating Holocene deglaciation of glacial surfaces of cold-based glaciers. Techniques to extract in situ <sup>14</sup>C in the form of CO<sub>2</sub> gas from quartz have been developed and refined for more than 20 years (e.g. Lifton et al., GCA 65, 1953-1969, 2001; Lifton et al., Geochron. 5, 361-375, 2023). However, extracting the minute amounts of in situ <sup>14</sup>C from quartz remains challenging because of 1- the risk of contamination with the ubiquitous atmospheric <sup>14</sup>C and 2- potentially incomplete extraction and recovery of all carbon species released from the heated quartz grains. Based on a modified version of the first extraction line design at ETH Zurich (Hippe et al., Quat. Geochron. 4, 493-500, 2009; Hippe et al., NIMB 294, 81-86, 2013), we implemented the first French in situ <sup>14</sup>C extraction system at CEREGE: a vertical resistance furnace for heating 5 g quartz samples at 1650 C under (ultra-)high vacuum in a sapphire furnace tube, connected to a stainless-steel gas purification line. The extracted and purified CO<sub>2</sub> is not graphitized but directly measured at the AixMICADAS Facility at CEREGE (Bard et al., NIMB 361, 80-86, 2015). We will show our results of procedural blanks and interlaboratory quartz samples that are comparable to those from other in situ <sup>14</sup>C facilities.
# Thule/Iñupiat architectural wood, dendroclimatology and oxygen isotopes: a window into the Medieval Climate Anomaly (11th-12th centuries) in northwest Alaska?

Juliette Taieb<fnref n="1" />,<fnref n="2" />, Valérie Daux<fnref n="2" />, Claire Alix<fnref n="1" />

In northwest Alaska, Thule/Iñupiat cultural changes observed in the archaeological record at the beginning of the 2nd millennium are often interpreted as responses to climatic and environmental variations. However, local paleoclimatic data for the Medieval Climate Anomaly (MCA, 9th-13th centuries CE), often considered a relatively warm period outside of Alaska, are scarce and offer limited temporal and spatial resolution.

Well-preserved in the frozen coastal deposits of northwest Alaska, timbers used to build Thule/Iñupiat winter houses represent a valuable resource for documenting climate variations at a local and annual scale. Composed mainly of white spruce (Picea glauca) driftwood that originated from the subarctic forests of interior and northwest Alaska, these remains offer a unique opportunity for reconstructing the climate variations at the onset of the 2nd millennium.

Conventional dendrochronology studies (analysis of ring-width variations) in the region revealed a complex relationship between summer temperatures and the growth of modern white spruce. Consequently, because of this complexity and the scarcity of archaeological samples from the MCA time period, using the available samples did not allow us to reconstruct the climatic conditions of that period.

However, in south Alaska and northwestern Canada, analyses of modern spruce tree-ring series show a strong correlation between climatic variations and the oxygen isotopic composition (δ¹⁸O) of individual tree-rings, even with limited sampling. To assess the potential of the isotopic approach we had the opportunity to analyse well-preserved and well-documented architectural driftwood timbers from recently excavated house feature at a site in northwest Alaska dating to the MCA.

Results from five archaeological wood disks and six modern wood cores exhibit isotopic coherence. Their δ¹⁸O signals were averaged to create both a modern chronology (1945-2002 CE) and an archaeological chronology (1042-1123 CE). We identified a correlation between modern δ¹⁸O and the July Standardized Precipitation and Evapotranspiration Index (SPEI4), an indicator of spring and summer moisture conditions that reflects the influence of humidity on vegetation. The isotopic reconstruction based on this correlation showed that conditions between 1042 and 1123 were generally wetter than in the modern period, with a gradual trend towards slightly drier conditions over these 80 years. This study offers a unique insight into the specific hydroclimatic conditions of the MCA, thus contributing to our understanding of the immediate environment of the Thule/Iñupiat people.

<footnote n="1">¹ Archéologie des Amériques (ArchAm) — CNRS, Université Panthéon-Sorbonne Paris 1, Ministère de la Culture — 75004 Paris, France</footnote>

<footnote n="2">² Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>
Multiproxy analyses for a network of firn cores
covering the last 40 years from coastal Adélie Land

Titouan Tcheng¹, Elise Fourré¹, Léa Baubant¹, Coralie Lassalla-Bernard¹,
Roxanne Jacob¹, Frédéric Prié¹, Frédéric Parrenin²,
Olivier Jossoud¹, Bénédicte Minster¹, Cécile Agosta¹, Christophe Leroy-
Dos Santos¹, Mathieu Casado¹, Marie Bouchet¹, Vincent Favier²,
Olivier Magand², Emmanuel Le Meur², Ghislain Picard², Alexandre
Cauquoin⁴, Martin Werner⁵, Amaëlle Landais¹, Bruno Jourdain²,
Laurent Arnaud²

¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France

² Institut des Géosciences de l'Environnement (IGE) – Institut polytechnique de Grenoble, CNRS, IRD, Grenoble Alpes, INRAE – 38400 Saint-Martin d'Hères, France

⁴Institute of Industrial Science – Meguro City, Tokyo 153-8505, Japon

5 Alfred Wegener Institute for Polar and Marine Research –Allemagne

Water stable isotopes signals recorded in snow, firn and ice cores were successfully used to investigate past temperatures on glacial/interglacial scales (Jouzel and Masson-Delmotte 2010, Dansgaard, 1964). However, as evidenced by Goursaud et al. (2018) in coastal Adélie Land, many uncertainties hampered the interpretation of water isotope records at sub-annual to decadal resolution as a proxy of past temperature variations only (Goursaud et al. 2018). Condensation, sublimation and/or redistribution of snow triggered by strong katabatic winds as well as precipitation intermittencies, origin of moist air masses bringing precipitation and diffusion within firn lessen the representativity of a single isotopic profile to reconstruct past temperature in this region (Grazioli et al. 2017, Khale et al. 2018, Picard et al. 2019, Casado et al. 2020, Hirsch et al. 2023). In order to mitigate the non-representativity of a single isotopic profile, a solution consists in averaging several records to increase signal to noise ratios. However, to do so, it is necessary to provide a good correspondence between the different cores of interest.

In this study, we make good use of water stable isotopes and major chemistry records from 9 firn core (20 to 40m deep) drilled at 3 sites (so called D47, Stop5 and Stop0) during the ASUMA campaign. These sites display a high mean accumulation rate of about 250 mm.weq/year and a wide range of environmental conditions with elevation ranging from 1550m to 2460m, distance from coast ranging from 103km to 423km and different katabatic winds influence. In particular, we use the IceChrono1 probabilistic model with water stable isotopes signals and major chemistry records coupled with beta-gamma and RADAR data to obtain the best correspondence between the different cores. We then quantify how the stacking of several cores permit to increase the signal to noise ratio at the different sites and can provide a faithful record to document variations of the temperature and/or atmospheric water cycle over the last decades in this region.
# Circulation changes in the Mediterranean Sea over the last 30,000 years – constraints from deep-water Nd isotopes

Jiawang Wu<fnref n="1" />, Yan Zhang<fnref n="1" />, Christophe Colin<fnref n="2" />, Guohui Gao<fnref n="2" />, Huang Huang<fnref n="3" />, Marcus Gutjahr<fnref n="3" />, Qiong Wu<fnref n="4" />, Pahnke Katharina<fnref n="5" />, Gert De Lange<fnref n="6" />

The circulation in the Mediterranean Sea is thought to play a key role in the rhythmic deposition of organic-rich sedimentary layers, known as sapropels, which signify anoxic deep-water phases in the geologic past. However, the intricate interactions among climatic and oceanographic processes related to circulation dynamics, surface hydrology, and sea level changes are still highly debated. Here, we present a deep-water record of Nd isotope composition (εNd) for the eastern Mediterranean Sea (EMS) over the past ~30,000 years, using the reliable archives of fish teeth/debris and foraminifera shells. The seawater εNd record of core CP10/11, collected from the open Ionian Sea at 1,501 m water depth, reflects the evolution of deep-water properties in the EMS. By combining our data with published authigenic εNd records and applying a refined box model of the Nd budget, we constrain the Mediterranean circulation variations since the Last Glacial Maximum (LGM), including sapropel S1.

During the LGM, unradiogenic εNd values (-7.4 to -6 ± 2) indicate a 44% ± 2% decline in water exchange across the Strait of Sicily, caused by the lower global sea level. An increasing trend of εNd towards the time of S1 formation, despite a rising sea level and hence gradual increase in exchange with the Atlantic, is largely attributed to enhanced Nile runoff that is the only significant source of radiogenic εNd. However, the stepwise increase of deep-water εNd is not concurrent with Nile records. This is likely due to a "reservoir effect" of EMS deep waters in response to surface freshening. The latter is primarily caused by the increased inflow of fresher, unradiogenic Atlantic water driven by deglacial sea-level rise.

During the S1 period, constant and radiogenic εNd values (-5.0 to -4.7 ± 0.3) which are also found at other cores, indicate a basin-wide stagnating condition at > 800 m depth in the EMS. Such deep-water stagnation initiated ~1,000 years prior to the onset of S1, linked to intensified Nile runoff and rapid sea-level rise at ~11 kyr. It gradually attenuated after ~8.2 kyr and terminated by a full deep-water renewal indicated by a shift to less radiogenic εNd. Our EMS deep-water εNd record show a close correspondence with intermediate-water record of core MD01-2472 in the western Mediterranean Sea (WMS), reflecting the EMS-WMS water exchange. For the S1 period, a 50% decrease in this exchange and decoupling of deep and intermediate waters between the EMS and WMS, are derived with a 2-fold increase in Nile discharge. Our findings highlight the dominant role of the Nile River in shaping the εNd distribution and circulation in the EMS.

<footnote n="1">¹ Sun Yat-Sen University – Chine</footnote>

<footnote n="2">² Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="3">³ GEOMAR – Allemagne</footnote>

<footnote n="4">⁴ Hohai University – Chine</footnote>

<footnote n="5">⁵ Carl-von-Ossietzky Universität, Oldenburg – Allemagne</footnote>

<footnote n="6">⁶ Utrecht University – Pays-Bas</footnote>
# Neodymium isotope variations in the South China Sea: implications for water mass exchange and particle dissolution

Qiong Wu<fnref n="1" />, Christophe Colin<fnref n="2" />, Zhifei Liu<fnref n="3" />, Eric Douville<fnref n="4" />, Yulong Zhao<fnref n="3" />, Jiawang Wu<fnref n="5" />, Arnaud Dapoigny<fnref n="4" />, Louise Bordier<fnref n="4" />, Pengfei Ma<fnref n="3" />, Yi Huang<fnref n="2" />

Neodymium isotopic composition (eNd) has been used as a quasi-conservative proxy for tracking changes of ocean circulation in the past. However, eNd distributions in the ocean and underlying controlling mechanisms are still not well understood. As the largest marginal sea in the western Pacific Ocean, the South China Sea (SCS) is an ideal region to study the processes that potentially influence dissolved eNd variations and its marine cycling. Here we present eNd obtained from six stations spread across the SCS. The results show large variations of eNd (–6.7 to –2.8) and high Nd concentrations in the surface water, indicating the influence of dissolution of riverine and marine particles. The SCS deep water shows a narrow eNd range from –4.3 to –3.4, showing the presence of the North Pacific Deep Water in the deep SCS. The intermediate water in the SCS is characterized by a more negative eNd signal (–4.2 to –3.4) than its counterpart in the West Pacific (–3.5 to –2.8), implying alterations by deep water through three-dimensional overturning circulation from the northern to southern SCS below ~500 m. In addition, the Nd contributions of external sources were quantitatively estimated for the SCS. The dissolution of particles from the SCS surrounding rivers and continental margins may play an important role in providing additional Nd to the SCS surface water.

<footnote n="1">¹College of Oceanography — Hohai University — China</footnote>

<footnote n="2">²Géosciences Paris Saclay (GEOPS) — Université Paris-Saclay, CNRS — Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="3">³State Key Laboratory of Marine Geology — Tongji University — China</footnote>

<footnote n="4">⁴Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="5">⁵School of Marine Sciences — Sun Yat-Sen University — China</footnote>
# 11 - Événements climatiques extrêmes et leurs impacts à différentes échelles de temps : fréquence, intensité et processus
# Évolution des Pluies Extrêmes dans le Bassin Versant de la Rivière San-Pédro au Barrage Hydroélectrique de Fayé

Franck Hervé Akaffou <fnref n="1" />, Salomon Obahoundje <fnref n="2" />, Bérenger Koffi <fnref n="3" />,
Loukou Alexis Brou <fnref n="1" />, Wawogninlin Brice Coulibaly <fnref n="1" />, Kouakou Lazare Kouassi <fnref n="1" />,
Arona Diedhiou <fnref n="4" />

La vulnérabilité de l'Afrique de l'Ouest aux inondations pourrait s'accroître dans les années à avenir en raison des effets des changements climatiques. Ainsi, il est urgent de mieux appréhender l'occurrence et la sévérité des pluies extrêmes afin de développer des stratégies d'adaptation. L'objectif de cette étude est de caractériser l'évolution des pluies extrêmes dans le bassin versant de la rivière San-Pédro au barrage hydro-électrique de Fayé. Pour ce faire, les données pluviométriques du produit Climate Hazard Group Infrared Precipitation with Station (CHIRPS) ont été utilisées comme observation pour calculer sept indices de pluie (CWD, PRCPTOT, R1mm, R99pTOT, Rx1day, Rx5day, et SDII) à l'échelle annuelle et durant les deux saisons pluvieuses (avril-juillet et octobre-novembre) sur la période 1982-2021. Le test statistique de Mann-Kendall a été appliqué afin d'analyser la significativité de la tendance. Les résultats montrent que les indices ont globalement connu une tendance haussière non-significative. Les valeurs d'indices affichées à l'échelle annuelle, sont également enregistrées au cours de la grande saison pluvieuse (avril-juillet), au cours de laquelle tombe environ 2/3 de la précipitation annuelle. En ce qui concerne la distribution spatiale des indices, les résultats révèlent une variation spatiale similaire à l'échelle annuelle et pendant grande saison pluvieuse. Les valeurs les plus élevées d'indices sont observées dans la partie ouest du bassin versant d'étude, où la pluviométrie est plus élevée en raison de la présence du Parc National de Taï. Ainsi, les pluies extrêmes ont varié proportionnellement à la pluviométrie totale. Cette tendance haussière des pluies extrêmes pourrait engendrer des crues importantes qui vont inonder le barrage de Fayé. La mise en place de projection des indices pourrait aider le gestionnaire de ce barrage dans l'anticipation des inondations.

Mots clés : Indice de pluie, Pluie extrême, Inondation, CHIRPS, Barrage hydroélectrique de Fayé

<footnote n="1">1 Université Jean Lorougnon Guédé – Côte d’Ivoire</footnote>

<footnote n="2">2 International Water Management Institute – Ghana</footnote>

<footnote n="3">3 Institut National Polytechnique Félix Houphouët Boigny – Côte d’Ivoire</footnote>

<footnote n="4">4 Université Grenoble Alpes – Université Grenoble Alpes – France</footnote>
Reconstruction of extreme flood events in the
southeast Tunisia during the two last millennium
from El Bibane lagoon sediment cores

Aida Affouri <fnref n="1" />, Soumaya Boussetta <fnref n="1" />, Nejib Kallel <fnref n="1" />, Laurent Dezileau <fnref n="3" />

The extreme events such as coastal storms or flood events are considered among the most destructive and dangerous natural disasters that caused significant human and economic damages along coastal regions. Hence, several studies have focused on those extreme meteorological events and on their link with climate change (Sabatier et al., 2008, Dezileau et al., 2011, Affouri et al., 2017, and BS Kohila et al., 2023). It has been demonstrated that lagoon sediments archives have the potential to reconstruct these events during the late Holocene.

In this context our study aims to reconstruct the historical extreme flood-event record during the two last millennium using a multi-proxy approach based on geochemical, sedimentological, and geochronological analysis of three sediment cores BL12-5, BL12-7 and BL12-10 collected in the El Bibane coastal lagoon, southeast of Tunisia. Our results display the occurrence of past floods levels in the lagoonal sedimentary column, which can reach even ten events in some cores. They are characterized by fine material such as clay and silt and their geochemical signature are marked by high values of the elemental ratios Ti/Ca and Fe/Ca. They are probably the result of enhanced precipitation that increased the terrigenous material supply into the El Bibane lagoon by the Fessi River.

The chronology of our cores has been established using the ¹³⁷Cs and ²¹⁰Pb measurements for the uppermost 30 cm of three cores and by ¹⁴C dates. We have then compared our reconstructed palaeoflood events with those described in historical archives (DGRE, 2000; Boujarra and Ktita, 2009; Fehri, 2014). Moreover, a good agreement is observed between instrumental records and recent flood events recorded in the El Bibane Lagoon. On the other hand, comparison with North Atlantic and regional paleoclimatic records (Bond et al., 2001, Marquer et al., 2008, and Steinhilber et al., 2009) displays that the periods of increased flood frequency coincide with the two major cooling intervals in the Northern Hemisphere (Bond events NAC-0 and NAC-1) which are associated also to low solar irradiance.

<footnote n="1">¹ Laboratoire Géoressources, Matériaux, Environnements et Changements Globaux (GEOGLOB) — Université de Sfax —
Faculté des Sciences de Sfax, Sfax, 3000, Tunisie</footnote>

<footnote n="3">³ Morphodynamique Continentale et Côtière (M2C) — Université de Caen Normandie, Université Rouen, CNRS —
14000 Caen, France</footnote>
East Asian climate and western Pacific warmpool
controls on the typhoon activities affecting Taiwan
during the late Holocene

Joffrey Bertaz<sup>1,2,3</sup>, Kevin Jacq<sup>4,5</sup>, Zhifei Liu<sup>2</sup>, Christophe Colin<sup>1</sup>, Maxime Debret<sup>5</sup>,
Hongchao Zhao<sup>2</sup>, Quentin Dubois-Dauphin<sup>1</sup>, Franck Bassinot<sup>6</sup>, Andrew Tien-
Shun Lin<sup>7</sup>

The reconstruction of paleo-typhoon activity in East Asia, with associated climatic and environmental
forcing mechanisms, is a crucial element in understanding the evolution of typhoon frequency and
intensity in the context of ongoing anthropogenic global warming. In this study, we combined high-
resolution non-destructive analyses of Fe/Ca and Si/Al by X-ray fluorescence (XRF) core scanning and
darkness (Rmean) and clay-fraction content (Clay R2200) by hyperspectral imaging with punctual
measurements of X-ray diffraction (XRD) bulk mineralogy and laser grain size of fine laminate (millimetric
to centimetric) sediments of Core MD18-3574, which was collected on the western levee of the Gaoping
submarine canyon off Taiwan in the northeastern South China Sea. The main goal of this study is to
establish the frequency and intensity of typhoon activity over the past 4500 yrs. Core MD18-3574
sediments consist mainly of fine silt (mean grain size ~13.4 µm) presenting relatively fine-grained and
coarse-grained laminations. Slightly thick coarse-grained laminae display a fining-upward and asymmetric
sorting of typical turbidite deposits. Principal component analysis (PCA) indicates that the darker laminae
are composed of coarser sediments with high Si/Al (quartz and feldspar-rich) and Clay_R2200 values and
low Fe/Ca (calcite-rich) which are the result of gravity flows. Conversely, lighter laminae are composed of
finer sediments with low Si/Al (muscovite and illite-rich), Clay_R2200 and high Fe/Ca, indicative of
hemipelagic deposition. This interpretation was subsequently extended to the core scale to identify
gravity flow deposits. Consequently, approximately 2,400 dark laminae were identified, which were
attributed to hyperpycnal flow and were mainly induced by high-intensity typhoons that affected
Taiwan. We thus calculated, for the first time, a detailed record of the frequency of typhoons, indicating
a decrease in their frequency since 4 cal kyr BP in response to a reduction in sea surface temperature in
the western Pacific warm pool and an intensification of the East Asian winter monsoon. The intensity of
tropical storms deduced from the thickness of turbidites has increased since 2 cal kyr BP in La Niña-like
periods. This indicates that the surge of super-typhoons hitting Taiwan could be triggered by El Niño–
Southern Oscillation (ENSO) variability. It can therefore be inferred that the tropical storm activity has
been controlled by ENSO conditions of the western Pacific warm pool and the East Asian monsoon system
during the late Holocene. The findings of this study contribute to our understanding of how ongoing
climate change may affect storm activity in the western Pacific Ocean.

<footnote n="1">¹ Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">² State Key Laboratory of Marine Geology – Shanghai 200092, China</footnote>

<footnote n="3">³ Institut des Sciences de la Terre (ISTerre) — CNRS, Université Savoie Chambéry, IRD, Université Grenoble Alpes, Université Gustave Eiffel — 38058 Grenoble Cedex 9, France</footnote>

<footnote n="4">⁴ Laboratoire Commun SpecSolE — Envisol CNRS, Université Savoie Mont Blanc — Chambéry, 73000, France</footnote>

<footnote n="5">⁵ Morphodynamique Continentale et Côtière (M2C) — Université de Caen Normandie, Université Rouen, CNRS — 14000 Caen, France</footnote>

<footnote n="6">⁶ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="7">⁷ Department of Earth Sciences-NCU – Taoyuan 32001, Taiwan</footnote>
# The Urban Heat Island of Paris megacity as a natural experiment to study forest responses to warming

Jonathan Barichivich<fnref n="1" />, Valérie Daux<fnref n="1" />, Olivier Mestre<fnref n="2" />

Altered biogeochemical cycles, with higher CO₂ concentrations and warmer atmosphere than in their rural surroundings make cities a natural climate change experiment. Current conditions in metropolitan regions simulate some of those that natural ecosystems will experience with similar intensity in the near future, notably warming. In our study, we explore whether the urban-rural gradient between the Paris megacity and the Fontainebleau Forest can be leveraged to study both short-term forest responses (e.g., growth extremes) and long-term adaptations (e.g., water use efficiency, stomatal responses) to climate change, in particular to warming and increasingly intense heatwaves. Since the summer of 2019, we have monitored the intensity of the urban heat island (UHI) and its variations during repeated heatwaves. We have also quantified the physiological responses of vegetation specifically growth and water use efficiency to the development of the UHI by comparing the growth and carbon and oxygen isotopic composition in oak tree rings from the Fontainebleau Forest, and from Vincennes and Montsouris parks, over the period 1950-2018. In this presentation, we will discuss to what extent urban trees and forests in Paris can be used as indicators of long-term warming effects on tree physiology. We will also examine how the UHI modulates the intensity and duration of heatwaves in comparison to a natural forest environment. Our findings will contribute to refine the approach to use cities as a unique setting to project long-term responses of trees to warming.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² Centre national de recherches météorologiques (CNRM) — CNRS, Météo-France — 31507 Toulouse Cedex, France</footnote>
DISTRIBUTIONS DE TEMPÉRATURES À DIFFÉRENTS
NIVEAUX DE RÉCHAUFFEMENTS GLOBAUX EN
SIMULATIONS TRANSITOIRES VERSUS STABILISÉES

Isaline Bossert ¹, Roland Séférian ¹, Yeray Santana-Falcon ¹

¹ Centre national de recherches météorologiques (CNRM) — CNRS, Météo-France — 31507 Toulouse Cedex, France

Les vagues de chaleur marine, qui se définissent par des périodes de températures océaniques anormalement élevées, ont des effets dévastateurs sur les écosystèmes marins. Avec le réchauffement climatique, leur intensité, leur fréquence et leur étendue devraient continuer à croître, entraînant des changements dans les conditions environnementales à travers tous les bassins océaniques. De plus, certaines études mettent en avant le rôle crucial des variations de température, par rapport aux changements de moyenne, sur la viabilité des espèces. En effet, l'augmentation des extrêmes de température peut être attribuée soit à un déplacement global de la distribution vers des valeurs plus élevées, soit à des modifications de la forme de cette distribution, y compris sa variabilité et son asymétrie. La plupart des connaissances actuelles sur ces extrêmes sont basées sur des états transitoires de notre système Terre, obtenus par le biais d'observations ou de modèles. Néanmoins, cette approche pourrait sous-estimer la réponse de l'océan à un niveau de réchauffement donné, la variabilité interne pouvant atténuer voire masquer les événements les plus extrêmes. Dans cette étude, nous examinons la réponse des vagues de chaleur marine à un climat stabilisé. Dans ce cadre, nous explorons de nouveaux types de scénarios futurs idéalisés stabilisant le réchauffement à différents niveaux de réchauffement globaux. Nous posons la question de la plus-value des simulations stabilisées dans l'analyse des distributions de température par rapport aux simulations transitoires en étudiant les modes caractéristiques de ces distributions.
Can we detect changes in tropical cyclone related
hazards?

Stella Bourdin <fnref n="1" />, Faranda Davide <fnref n="2" />, Suzana Camargo <fnref n="3" />, Chia-Ying Lee <fnref n="3" />, Jonathan
Lin <fnref n="4" />, Mathieu Vrac <fnref n="2" />, Pradeebane Vaittinada Ayar <fnref n="2" />

Tropical Cyclones (TCs) pose a great threat to human lives and assets, being the second deadliest and most costly weather-related disaster. Yet, it is not clear how climate change might affect these phenomena. Part of this deficiency is due to a short observational record, dating back only to the satellite era. In this study, we make use of synthetic track generators to go beyond the limitations of the past record.

Our work aims to identify whether a change in hazard over the last decades is detectable for 9 case
studies of impactful tropical cyclones in the North Atlantic. To do so, we find analogues of our studied
storms and see how their properties have changed over time. Starting from the ClimaMeter/flow-
analogue method, we show how changing the analogue selection, and using synthetic track generators
allow us to access more analogues, yielding a more robust statistical analysis.

Our analysis shows that there has been little change in tropical cyclones' number, intensity, translation speed, or seasonality over the last decades. On the contrary, precipitation is already seen to increase.

<footnote n="1">¹ Department of Atmospheric, Oceanic and Planetary Physics (AOPP) — Department of Physics, University of Oxford — Clarendon Laboratory, Parks Road, Oxford OX1 3PU, Royaume-Uni</footnote>

<footnote n="2">² Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="3">³ Lamont-Doherty Earth Observatory (LDEO) — University of Columbia — Palisades, New York 10964, États-Unis</footnote>

<footnote n="4">⁴ Cornell University – Ithaca, New York 14853, États-Unis</footnote>
# Analysis of marine heatwaves along the central and northern coast of São Paulo (SE Brazil) over the last four decades

Breylla Campos Carvalho <fnref n="1" />,<fnref n="2" />, Áurea Maria Ciotti <fnref n="2" />

Marine heatwaves (MHW) can be considered extreme events in which seawater temperatures exceed the historical average and persist for more than five days. The thermal stress caused by the anomaly impacts marine biodiversity (e.g. coral bleaching or changes in the vertical distribution of organisms). In recent years, the number of studies on global MHWs have grown worldwide but there is still a knowledge gap in some oceanic sectors, such as the coast of São Paulo (SE Brazil). The São Paulo coast is of significant ecological vulnerability as it concurrently houses tens of marine environmental conservation units and intense commercial and economic activities, such as the largest port in Latin America. This study aims to analyze the occurrence of MHWs in this important area (central and northern coast of São Paulo (-47.0 to -44.5 W, -24.5 to -23.0 S), using daily data from the Operational Sea Surface Temperature and Ice Analysis (OSTIA), via the Copernicus Marine Service (CMEMS), from 1982 to 2021. The OSTIA provides sea surface temperature (SST) obtained from satellite data from the GHRSSST project and in situ observations, with an approximate spatial resolution of 6 km. The MHWs were detected when temperatures exceeded the 90th percentile of the climatology. Subsequently, the data was analyzed both spatially (pixel-by-pixel) and temporally (means of the entire study area) The MHW events showed on average higher intensity in the central area of the inner continental shelf (between the 20 and 50 m isobaths), while in the regions near the coast (depths of up to 20 m)

the events were more frequent per year, and tended to last longer. Of the 95 MHW events recorded, the strongest occurred between 14/Dec/2018 and 06/Feb/2019. Between 2002 and 2005 and 2019 and 2020, there were more than four events per year, with the events in 2019 and 2020 being more intense (up to 30.6 °C) and of longer duration (average of 20 days), for a total of 16 MHW events. From 1995 onwards, the Atlantic Multidecadal Oscillation became positive, at the same time, the climate anomaly reached 0.35 °C and continued to increase. From 2002 to 2021, the positive temperature anomalies almost always coincide with El Niño events. These results agree with other investigations worldwide showing an increase in the intensity, duration, and frequency of MHWs in recent years for this region. The increase in SST, which in turn has implications for marine biodiversity such as the intense 2019 bleaching of several coral reefs, including those on the north coast of São Paulo. This work will be used to identify the areas most vulnerable to extreme MHW events, as well as help to understand their implications for the region's marine biodiversity.

<footnote n="1">¹ Universidade Federal de São Paulo (UNIFESP) – Brésil</footnote>

<footnote n="2">² Universidade de São Paulo (USP) – Brésil</footnote>
RHITA: a framework for real-time and automated
detection of hazards

Greta Cazzaniga <fnref n="1" />, Adrien Burq <fnref n="1" />, Mathieu Vrac <fnref n="1" />, Davide Faranda <fnref n="1" />,<fnref n="2" />

Event identification and tracking tools have become essential in weather and climate research due to
the rapid growth of weather and climate model data. These algorithms automate the detection of
extreme weather events and hazardous phenomena, along with their corresponding features, thereby
enhancing predictive capabilities. Our study introduces RHITA (Real-time Hazards Identification and
Tracking Algorithm), a real-time, open, and flexible framework designed for the real-time and automated
identification and tracking of various hazard types from a Lagrangian perspective. RHITA spans multiple
spatial and temporal scales, providing insights into the dynamics and behavior of hazardous events. This
framework applies to a wide range of scenarios, from historical hazard analyses to the evaluation of
future climate scenarios. Additionally, it facilitates the investigation of weather patterns that give rise to
these hazards and enables the assessment of their impacts. The algorithm relies on connected
components and distance thresholds to identify and track events in space and time, allowing for event
merging and splitting dynamics over time. Furthermore, it characterizes events based on diverse features
such as duration, volume, intensity, trajectory, and rarity. The framework includes visualization tools for
displaying event trajectories and the three-dimensional evolution of the events. It also enables statistical
analyses of detected time series derived from historical records and climate model outputs (e.g.,
identifying trends, exploring correlations between feature pairs, and analyzing feature distributions).
RHITA can be applied in real-time by exploiting MSWX datasets, a reanalysis dataset with short latency
and high update frequency, making it useful for operational purposes. We applied RHITA to detect major
weather-related hazards such as heatwaves, cold spells, heavy precipitation events, hydrological
droughts, and even combinations of these events, known as compound events.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² Laboratoire de météorologie dynamique (LMD) — CNRS, ENS Paris, École Polytechnique, Sorbonne Université, Institut polytechnique de Paris, Université Paris sciences et lettres, École des Ponts ParisTech — 91128 Palaiseau Cedex, France</footnote>
# Flood Risk Assessment, Using HEC-RAS, GIS and remote sensing, a Case Study in an Arid Environment of Southeast Morocco

Khadija Diani¹

¹LO3G Laboratory, Department of Earth Sciences, Faculty of Sciences — Mohammed V-Agdal University — Rabat, Maroc

Since the early 1980s, several regions of Morocco have experienced alternating series of wet and dry periods related to climate change, with significant consequences for the country's economy. This study aims to map flood hazards by using a hydraulic modeling approach, with a specific focus on the Oued (river) Zaouiat Sidi Hamza passing through Zaouiat Sidi Hamza township belonging to the High Ziz watershed (province of Midelt, region of Drâa-Tafilalet, Morocco). Hydraulic modeling and flood simulation was carried out followed by mapping of the flood hazard. The Oued Zaouiat Sidi Hamza River can cause torrential flooding issues, particularly during periods of heavy storms, impacting agricultural land and certain buildings on both riverbanks. All the cultivated areas developed alongside the river are at risk of flooding. Moreover, undermining and erosion of the river's banks lead to rising high-water levels and flooding agricultural terraces. The use of geographic information systems (GIS) and hydraulic modeling tools is essential to gain a deeper understanding of this situation. Also, the mapping of lineaments and fractures in the study area, carried out by satellite image processing, helped to quantify the directional influence of these lineaments on the flow of the hydrographic network, on the orientation of the circulation of surface water, and on the storage of groundwater. According to this study, the most vulnerable areas to flooding and erosion are those that receive the most water in the basin and have steep slopes. Decision-makers can use the results to produce development plans tailored to combat these risks and raise awareness among the population concerned, improving water resource evaluation and forecasting, as well as improving agricultural production.
# ClimaMeter: contextualizing extreme weather in a
changing climate

Faranda Davide¹

¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France

Climate change is a global challenge with multiple far-reaching consequences, including the intensification and increased frequency of many extreme-weather events. In response to this pressing issue, we present ClimaMeter, a platform designed to assess and contextualize extreme- weather events relative to climate change. The platform offers near-real-time insights into the dynamics of extreme events, serving as a resource for researchers and policymakers while also being a science dissemination tool for the general public. ClimaMeter currently analyses heat- waves, cold spells, heavy precipitation, and windstorms. This paper elucidates the methodology, data sources, and analytical techniques on which ClimaMeter relies, providing a comprehensive overview of its scientific foundation. We further present two case studies: the late 2023 French heatwave and the July 2023 Storm Poly. We use two distinct datasets for each case study, namely Multi-Source Weather (MSWX) data, which serve as the reference for our rapid-attribution protocol, and the ERA5 dataset, widely regarded as the leading global climate reanalysis. These examples highlight both the strengths and limitations of ClimaMeter in expounding the link between climate change and the dynamics of extreme-weather events.
# Fingerprint of Extreme Precipitation Events on Erosion: From Decadal to Millennial Timescales

Éric Gayer¹, Laurent Michon¹

¹ Institut de physique du Globe de Paris (IPGP) – CNRS, Institut de Physique du Globe de Paris, Université Paris Cité, Université La Réunion – 75238 Paris Cedex, France

Extreme precipitation events play a pivotal role in shaping Earth's surface by influencing hillslope processes and sediment transport in rivers. Although the impact of such meteorological events is evident during floods, their role in the long-term evolution of landscapes remains less clear. In this study, we use the unique climatic setting of Réunion Island - characterized by a steep precipitation gradient between its windward and leeward sides, as well as intense cyclonic activity - to investigate the impact of extreme precipitation on landscape evolution over short-term (daily to decadal) and long-term (millennial) timescales. To understand how these extreme precipitation events influence landslides and sediment transport at the short-time scale, we focused on an historical catastrophic rock avalanche that occurred in May 1965 in La Rivière des Remparts, a deeply incised valley of the Piton de la Fournaise volcano. The avalanche occurred one day after heavy rainfall, and was subsequently interpreted as a mud/debris flow event. Using photogrammetric techniques applied to historical aerial images, we reconstructed nine DEMs of the entire canyon over a six-decade period. By analyzing differences in DEMs and estimating sediment volumes, we assessed the spatio-temporal evolution of the canyon and the drainage of sediment from major landslides and rock avalanches between 1950 and 2011. Our results indicate that the 1965 landslide introduced nearly 60 Mm³ of sediment into the river, with two additional catastrophic events in 1995 and 2001 contributing a further 20 Mm³. We estimate that 40 Mm³ (95 Mt) of sediment was transported out of the watershed between 1950 and 2011 via bedload waves, from a total input of 80 Mm³. Furthermore, we modeled the flow dates of this ephemeral river and found that this sediment export occurred over only 391 days across the 62-year period, at an average rate of 0.1 Mm³/day (0.25 Mt/day). Our investigation confirms that sediment transport coincided with officially recorded extreme meteorological events, such as cyclones, and was predominantly concentrated on days corresponding to high-percentile precipitation events. This demonstrates that sediment transport primarily occurs during intense rainfall periods. Our findings confirm the general notion that the majority of erosional work occurs through discrete, short-term events such as floods and landslides, with varying magnitudes and frequencies. However, on longer timescales, erosion is often treated as a continuous process, and the influence of temporal rainfall variability on long-term landscape evolution remains unclear. By comparing erosion rates (0.8 to ~10 mm/yr over ~70 ka), estimated from paleo-volcanic reconstructions across Réunion's steep rainfall gradient, with geostatistical analysis of daily rainfall records, we reveal that erosion rates are highest in the driest areas, where the difference between extreme rainfall events and average precipitation is greatest. This suggests that extreme precipitation events leave an imprint on long-term landscape evolution, but that erosion is driven not only by these extreme events, but also by background rainfall-induced factors, such as humidity and dryness, which influence the erosion threshold of Earth's Critical Zone.
# Érosion littorale et évènements extrêmes : le cas de Saint-Martin (Caraïbes) et de l'ouragan Irma (2017)

Julien Gargani<fnref n="1" />, Gwenael Jouannic<fnref n="2" />, Kelly Pasquon<fnref n="2" />, Benjamin Piccinini<fnref n="2" />, Élise Banton<fnref n="3" />, Oscar Navarro<fnref n="3" />

La montée du niveau marin, l'augmentation de l'intensité des évènements extrêmes et l'évolution littorale durant les ouragans sont documentés de façon toujours plus précise. Cependant ce qui se passe en termes d'évolution du littoral et de transport sédimentaire juste après les évènements extrêmes a été beaucoup moins étudié et est moins bien contraint. Cette étude vise à comprendre la dynamique de l'érosion et de la sédimentation juste après un ouragan à partir du cas de Saint-Martin dans les Caraïbes, où plusieurs ouragans ont eu lieu durant les dernières décennies (1,2). Sur cette ile, les ouragans ont conduit à des destructions significatives (3), du fait des nombreuses constructions sur le domaine littoral (4), ainsi que sur le reste du territoire.

A partir de l'analyse d'images satellites, de photos aériennes, d'une étude de terrain et de modélisations numériques, conduites notamment durant l'ANR RELEV, nous proposons de décrire les cycles de l'érosion sur le domaine littoral (amplitude, taux) et les mécanismes de transport observés à Saint-Martin. De façon plus appliquée, nous discutons la pertinence du choix des indicateurs d'érosion utilisés pour caractériser l'érosion littoral (limite de végétation, limite du " jet de rive ",...), ainsi que comme outil d'aide à la décision et à la règlementation.

<footnote n="1">1 Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">2 Centre d'Études et d'Expertise sur les Risques, l'Environnement, la Mobilité et l'Aménagement - Direction Ouest (CEREMA) — Direction Ouest, BP 46223 - 44262 Nantes Cedex 2, France</footnote>

<footnote n="3">3 Détection, évaluation, gestion des Risques Chroniques Émergents (CHROME) – Université de Nîmes – Université de Nîmes, 30021 Nîmes Cedex, France</footnote>

<footnote>(1) Pasquon et al., Evolution de l'urbanisme et exposition au risque cyclonique à Saint-Martin de 1954 à 2017. Vertigo, 22, 2022</footnote>

<footnote>(2) Jouannic et al., Recovery of the Island of Saint Martin after Hurricane Irma: An Interdisciplinarity Perspective. Sustainability, 12(20), 8585, 2020.</footnote>

<footnote>(3) Gargani, Impact of major hurricanes on electricity production. Int. J. Disaster Risk Reduc., 67, 2022</footnote>

<footnote>(4) Pasquon et al., Vulnerability to marine inundation in Caribbean islands of Saint-Martin and Saint-Barthélemy. Int J. Disaster Risk Reduc., 78, 2022</footnote>
# L'activité des typhons extrêmes dans l'ouest du Pacifique Nord lors des 4 dernières décennies (1980-2019)

Karl Hoarau ¹,³, Pierre Camberlin ², Freddy Vinet ³, Monique Gherardi ³

¹ Laboratoire de géographie et d'aménagement (PLACES) — CY Cergy Paris Université — 95011 Cergy Pontoise, France

² Biogéosciences – Université Bourgogne Dijon, EPHE Paris, Université Bourgogne Franche-Comté, CNRS, Université Paris sciences et lettres, Institut national d'enseignement supérieur pour l'agriculture, l'alimentation et l'environnement — 21000 Dijon, France

³ Laboratoire de Géographie et d'Aménagement de Montpellier (LAGAM) — Université Paul Valéry Montpellier 3 – 34199 Montpellier cedex 5, France

D'après la base IBTrACS, l'ouest du Pacifique Nord (100°E à 180°) est le bassin océanique le plus actif avec 30% de l'activité cyclonique sur la période 1980 à 2019. Dans le débat actuel sur le réchauffement atmosphérique et l'évolution du nombre global des cyclones tropicaux, certaines études ont mis en évidence une augmentation de la fréquence des cyclones des catégories 4 et 5 dans les 6 bassins océaniques.

Nous avons voulu déterminer si une telle tendance existait dans le plus grand bassin cyclonique du globe avec les typhons d'une intensité d'au moins 155 nœuds (80 m/s). Ces phénomènes extrêmes (le haut de la catégorie 5) nécessitent un contenu thermique océanique élevé ainsi que des conditions atmosphériques optimales. La base IBTrACS indique que 42 cyclones de cette intensité se sont formés sur le globe sur les 4 dernières décennies (1980-2019). 27 (64%) se sont produits dans l'ouest du Pacifique Nord sur cette période. IBTrACS donne l'évolution suivante du nombre décennal pour les typhons d'au moins 155 noeuds : 1 pour 1980-1989, 8 pour 1990-1999, 5 pour 2000-2009, et 13 pour 2010-2019. Le nombre significativement plus faible de la décennie 1980-1989 est d'autant plus surprenant que tous les typhons de la période 1980-1987 ont bénéficié des reconnaissances aériennes. Afin d'apprécier la qualité de ces données représentées par le vent maximal soutenu sur 1 minute, nous avons procédé à la ré-analyse de l'intensité des typhons observés par avion et de ceux ayant évolué de 1988 à 2019. Cette ré-analyse a été effectuée avec la méthode de Dvorak (1984) qui utilise les images en infrarouge thermique des satellites météorologiques.

Les résultats de la ré-analyse aboutissent à un nombre significativement plus élevé des typhons extrêmes dans l'ouest du Pacifique Nord pour la période 1980 à 2019, 72, par rapport aux données d'IBTrACS, 27. S'il n'existe pas de tendance à la hausse du nombre des typhons extrêmes, leur durée a augmenté sur les 4 dernières décennies. De même, les typhons extrêmes de la décennie 2010-2019 se sont intensifiés de manière plus rapide et ont atteint leur intensité maximale à une latitude plus élevée que ceux des décennies précédentes. En l'absence d'une diminution du cisaillement vertical du vent (entre 200 et 850 hPa) lors des 40 dernières années, ces observations peuvent s'expliquer, en grande partie, par la hausse de la température de l'océan dans l'ouest du Pacifique Nord en lien avec le réchauffement atmosphérique. 60% des typhons extrêmes ont évolué en fin de saison d'octobre à novembre, et 32% sont intervenus en pleine saison de juillet à septembre. Les épisodes El Niño ont produit deux fois plus de typhons extrêmes (26) que les phases La Niña (13) alors que le nombre pour les périodes neutres a été de 33 typhons. Lors d'un épisode El Niño significatif, les typhons se forment plus à l'est, et ayant une trajectoire plus longue, ils ont plus de temps pour rencontrer des conditions favorables à une intensification plus marquée et atteindre une intensité plus forte.
# End-of-century marine heatwaves in the California Current System in a regional climate model

Eike E. Köhn ¹, Gesa Eirund ¹, Matthias Münnich ¹, Nicolas Gruber ¹

¹ Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics — ETH Zurich — Zurich, Suisse

With ongoing ocean warming, marine heatwaves (MHWs) like the impactful Blob in the Northeast Pacific in 2014-2016 are expected to become more frequent and intense in the future. Yet, how these MHWs will affect the biologically productive and economically important California Current System (CalCS) is still uncertain, though addressing this question could guide future policy-making and ecosystem management. In this study, we examine the future of MHWs in the CalCS by the end of the 21st century using a high-resolution, regionally coupled ocean-atmosphere model (ROMSOC). Using the pseudo-global warming (PGW) approach under the SSP5-8.5 and SSP2-4.5 scenarios, we simulate what the Blob would look like at the end of the 21st century (pseudo years 2096-2106), assuming large-scale weather patterns similar to those observed from 2011 to 2021. We find that under the SSP5-8.5 scenario, the future CalCS is not only warmer by 2°C on average, but this warming is more pronounced during the summer months (June-October), with temperatures increasing by up to 3°C. This means that the already warmest months of the year are warming more rapidly than other months. The amplified summer warming is linked to a shoaling mixed layer depth (MLD) and a weakening of summer upwelling winds. Applying the seasonally varying 90th percentile extreme threshold from the present-day 2011-2021 period to the SSP5-8.5 future scenario (fixed baseline), we find a large increase in the frequency and intensity of extremes throughout the year. In contrast, using a moving baseline - by adjusting the present-day extreme threshold for the annual mean future warming - we primarily see an increase in the frequency and intensity of extremes in the summer months. This change is driven by the amplified seasonal cycle and intensified short-term variability of sea surface temperature anomalies. These findings suggest that marine organisms adapted to cooler summer temperatures due to upwelling may be pushed beyond their thermal tolerances during summer MHWs, adding stress to the future CalCS ecosystem. Furthermore, our results indicate that climate change mitigation, specifically transitioning from the SSP5-8.5 to the SSP2-4.5 scenario, could significantly reduce the number and intensity of extremes affecting the CalCS at the end of the 21st century.
# Spatiotemporal Patterns of Dry and Wet Spells in Ghana's Savanna and Transitional Zones: A 40-Year Analysis

Salomon Obahoundje ¹, Seifu A. Tilahun ¹, Petra Schmitter ¹

¹ International Water Management Institute (IWMI) – Ghana

West Africa is highly vulnerable to climate change and variability, with increasing droughts and floods, particularly driven by changes in dry and wet spells. This study examines the spatiotemporal variability of dry and wet spells across the northern agroecological zones of Ghana- namely, the Sudan Savanna, Guinea Savanna, and Transitional zones-over the past 40 years (1984–2023). Daily precipitation data from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) were used to assess trends in critical indices, including consecutive dry days (CDD), consecutive wet days (CWD), total precipitation (PRCPTOT), rainfall intensity (SDII), and rainy days (RR1) over the rainy season from April to October.

The results highlight a slight decrease in maximum CDD and a significant decrease in maximum CWD in the Sudan Savanna, accompanied by a slight increase in RR1 and SDII. This has led to a modest increase in precipitation during the rainy season (April to October). In contrast, the Guinea Savanna shows steady trends in both maximum CDD and CWD, with a slight decline in RR1 and SDII, resulting in a reduction in PRCPTOT during the same period. The Transitional zone experiences the most pronounced changes, with a significant increase in both maximum CDD and CWD, coupled with a substantial decrease in RR1 and SDII. These changes suggest increasing dryness and variability in rainfall intensity.

In recent decades, a decadal analysis reveals a trend toward more frequent but shorter dry and wet spells in all three agroecological zones. These findings provide critical insights into the evolving climate dynamics in northern Ghana and underscore the need for adaptive strategies to mitigate the impacts of increasing climate variability on agriculture and water resources.
# Etude multiscalaire des dépôts sédimentaires du lac du Lauzet (Vallée de l'Ubaye, Alpes du Sud) : évolution de l'aléa hydro-gravitaire au cours de l'Holocène.

Eah Opitz<fnref n="1" />, Elodie Brisset<fnref n="1" />,<fnref n="2" />, Frédéric Guiter<fnref n="2" />, François Demory<fnref n="1" />,
Francoise Chalié<fnref n="1" />, Hanane Idabdellah<fnref n="1" />, Doriane Delanghe<fnref n="1" />, Kasuyo Tachikawa<fnref n="1" />,
Laurence Vidal<fnref n="1" />

Les milieux de montagne sont particulièrement concernés par les risques hydro-gravitaires. Les crues torrentielles, glissements et coulées de débris, et d'avalanche sont régulièrement dévastateurs dans les Alpes françaises. Ces phénomènes sont difficiles à anticiper et pourraient voir leur fréquence et leur ampleur augmenter sous l'effet du changement climatique (GIEC, 2021). Leurs récurrence et intensité sont modulées par des facteurs locaux très spécifiques : climat régional, géologie, topographie, taille des bassins versants, usage des sols.

Les sédiments lacustres de montagne constituent de précieux enregistrements de l'occurrence des évènements hydro-sédimentaires passés (Sabatier et al., 2022) qui, mis en perspective des trajectoires hydrologiques et biologiques locales, permettent de comprendre les facteurs d'atténuation ou d'exacerbation des aléas hydro-gravitaires à l'échelle millénaire.

Cette étude se focalise sur la séquence lacustre du Lauzet (44 25'37"N, 6 26'02"E), prélevée en base vallée du bassin versant de l'Ubaye (Alpes du Sud) à 890 mètres d'altitude. La séquence composite LAU21 d'une longueur de 15 mètres couvrant les derniers 14 000 ans (datations carbone 14).

Ce travail présente les résultats obtenus sur deux sections sédimentaires couvrant la période Mid-Holocène. Une approche multiscalaire, combinant des analyses lithologiques, granulométriques, géochimiques, géophysiques et l'observation micrométrique de lames minces, a été conduite. Cet ensemble de données a permis de proposer une caractérisation précise d'une sédimentation continue, composée des fractions autochtone endogène/biogène et allochtone détritique. Un certain nombre de dépôts sont caractéristiques d'apports détritiques instantanés qui s'intercalent avec de dépôts finement laminés, dont l'analyse micrométrique pourrait révéler l'existence d'une signature saisonnière. A ce stade, nos observations permettent de distinguer (il sera possible de les compter) deux types de dépôts instantanés associés à des crues torrentielles, et à des coulées de débris et/ou d'avalanches. Ces résultats, lorsqu'ils sont replacés dans un cadre temporel précis et comparés aux analyses palynologiques (réalisées sur la même séquence), permettent de discuter les interactions entre les dynamiques de végétation locale et les aléas hydro-gravitaires à l'échelle du bassin versant. Trois périodes sont mises en évidence : i) une sédimentation dominée par les dépôts instantanés marque la période qui précède le Mid-Holocène ; ii) entre 8 000 et 3 000 cal. BP environ, un couvert végétal forestier dense est présent et la sédimentation est dominée par des dépôts laminés, et organiques ; iii) après 3000 cal. BP, l'ouverture des paysages et l'augmentation des marqueurs pollinique rudéro-anthropiques correspond à un retour de la sédimentation détritique, et des dépôts instantanés particulièrement épais. Ces premiers résultats

<footnote n="1">1 Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE) – Collège de France, CNRS, IRD, Aix Marseille Université, INRAE – 13545 Aix-en-Provence, France</footnote>

<footnote n="2">2 Institut méditerranéen de biodiversité et d'écologie marine et continentale (IMBE) – Avignon Université, CNRS, IRD, Aix Marseille Université – Faculté St-Jérôme, 13397 Marseille Cedex, France</footnote>
illustrrent le potentiel de cette archive pour documenter l'évolution millénaire des aléas hydrogravitaires
dans les Alpes méridionales.

- Mots clé : Aléa hydro-gravitaire, Paléoenvironnements, Sédiments lacustres, Alpes du Sud, Holocène.
# A distance learning algorithm for analog methods

Paul Platzer<sup>1</sup>, Pierre Tandeo<sup>2</sup>, Arthur Avenas<sup>3</sup>, Bertrand Chapron<sup>1</sup>,
Léo Vinour<sup>4</sup>, Lucas Drumetz<sup>2</sup>, Alexis Mouche<sup>1</sup>

Analogs are similar states of a system, occurring at remote times within independent numerical simulations or previous observations. This concept was developed in atmospheric sciences and further used in atmospheric and ocean sciences for forecasting, downscaling, upscaling, extreme event attribution, among others.

The distance is a key feature of analog methods. Most studies are based on the Euclidean distance or other pre-defined metrics. In this investigation, we leverage distance learning algorithms originally designed for classification and regression and adapt them for statistical forecasting objectives, using in particular the continuous-ranked probability score as a loss function. To our knowledge, this is the first time that the gap between distance learning and analog methods is bridged.

Our algorithm learns a linear transformation of the feature variables that is applied ahead of a classical Euclidean-distance-based analog ensemble methodology. This allows to jointly optimize three key hyperparameters of analog methods: the feature space, the distance, and the number of analogs used. In particular, this algorithm allows to reduce the feature space dimension while keeping analog ensemble performances as high as possible, a key requirement for small and medium-sized datasets.

We test our algorithm on the three-variable Lorenz system and on a small-size tropical cyclone dataset from meteorological agencies. Experiments on the Lorenz system suggest that the optimal distance strongly depends on the forecast horizon and the number of available data, and that our algorithm allows for reasonable performances of analog ensemble methods even for small-size datasets. For low-size synthetic Lorenz system datasets, we observe strong variations of the optimal distance with catalog size, followed by stabilization above a given threshold and eventually convergence. We also observe the growth of the scale of the optimal distance with forecast horizon, indicating that a more severe selection of analogs is needed for long-term forecasts. We finally show that a CRPS-based optimization allows to have better uncertainty quantification from analog ensembles compared to RMSE-based optimization.

We use our algorithm with IBTrACS tropical cyclone data to weight input variables in the case of TC intensity forecasting. Our algorithm allows for significant improvement with respect to a baseline of analog forecast where all input variables are given equal weights. Even without regularization our algorithm can remove irrelevant input variables. To further reduce the number of variables we add a regularization term, allowing to reveal which variables contribute the most to the optimal definition of distance for analog ensemble forecasting of TC intensity.

Our approach is not limited to forecasting and can assist the search for optimal hyperparameters of any analog method, enhancing exploration possibilities and improving overall performances.

<footnote n="1">1 Laboratoire d'océanographie physique et spatiale (LOPS) — Université de Brest, IRD, IFREMER, CNRS — 29280 Plouzané, France</footnote>

<footnote n="2">2 Laboratoire des Sciences et Techniques de l'Information, de la Communication et de la Connaissance (LAB-STICC) — Université de Bretagne Occidentale, Université de Bretagne Sud, CNRS, IMT Atlantique, ENI Brest, ENSTA Bretagne — Technopôle Brest-Iroise, 29238 Brest Cedex 3, France</footnote>

<footnote n="3">3 ESA – Italie</footnote>

<footnote n="4">4 France Energies Marines – Technopole Brest-Iroise, 29280 Plouzané, France</footnote>
Sampling effects on weather regime dimension
estimates

Paul Platzer ¹, Bertrand Chapron ¹

¹ Laboratoire d'océanographie physique et spatiale (LOPS) — Université de Brest, IRD, IFREMER, CNRS — 29280 Plouzané, France

Large-scale atmospheric circulation is driven by recurring patterns called " weather regimes ". In the last decade, the multi-fractal formalism of dynamical systems has allowed to relate typical values of local dimension to specific weather regimes. It was shown that the local dimension decreases near peak weather regime index, and increases near regime-transitions, confirming the physical grounding and dynamical footprint of weather regimes.

This work is separated in two parts. First, we show that variations in local dimension may originate
from uneven sampling of phase-space, by studying absolutely continuous random variables. For the latter
and in the absence of zeros and singularities, the theoretical value of this local dimension is constant and
equals the phase-space dimension. However, due to uneven sampling across the dataset, practical
estimations of the local dimension may diverge from this theoretical value, depending on both the phase-
space dimension and the position at which the dimension is estimated. To explore such variations of the
estimated local dimension of absolutely continuous random variables, approximate analytical
expressions are derived and further assessed in numerical experiments. These variations are expressed
as a function of 1. the random variables' probability density function, 2. the threshold used to compute
the local dimension, and 3. the phase-space dimension. Largest deviations are anticipated when the
probability density function has a low absolute value, and a high absolute value of its Laplacian. Numerical
simulations of random variables of dimension 1 to 30 allow to assess the validity of the approximate
analytical expressions. These effects may become important for systems of moderately-high dimension
and in case of limited-size datasets.

Second, we propose a methodology to evaluate the effect of uneven sampling on dimension estimates from reanalysis data, namely ERA5 500-mb geopotential height over the North-Atlantic ocean and Europe. We build " stochastic twins " of the atmosphere in the space of truncated empirical orthogonal function, based on the Gaussian-mixture-model fit used to define weather regimes from ERA5-reanalysis 500mb-geopotential height. By estimating sampling-driven variations of local dimension using stochastic twin samples, one can explain more than 50% of the variations of local dimension computed from atmospheric data alone. The dimension estimated from stochastic twins bears similar properties to that of atmospheric data in terms of decreasing dimension near weather regime peak index, however with a weaker inter-regime variability. This methodology can be applied to any observed system whose sampling distribution can be approximated. It can be generalized to separate sampling-density-driven and multi-fractal-driven variations of the local dimension, in an attempt to better estimate actual variations of the number of degrees of freedom.
# Étude expérimentale de l'impact d'évènements pluvieux extrêmes sur l'érosion de technosols.

Gabriel Portzer<sup>1,2</sup>, Jean-Louis Grimaud<sup>2</sup>, Olivier Stab<sup>2</sup>, Jean-Alain Fleurisson<sup>2</sup>, Samuel Abiven<sup>3</sup>, Albert Marchiol<sup>4</sup>, Frédéric Ego<sup>4</sup>, Philippe Pellerej<sup>1</sup>, Yara Maalouf<sup>1</sup>, Nicole Khoueiry<sup>1</sup>

Le changement climatique entraîne une recrudescence des événements extrêmes tels que des précipitations intenses, des tempêtes et des sécheresses prolongées. Ces phénomènes peuvent accentuer l'érosion des sols, notamment dans les régions où les infrastructures doivent maintenir leur intégrité sur le long terme. Dans cette étude, nous nous focalisons sur la zone du Cotentin, située en Normandie. Les projections climatiques du Laboratoire des Sciences du climat et de l'Environnement (LSCE) pour la région prévoient des étés plus secs et des périodes de précipitations intenses de l'automne au printemps, qui sont propices à l'augmentation des taux d'érosion des sols.

Afin de regarder la réponse érosive aux événements anticipés par les modèles climatiques, nous avons développé un dispositif expérimental d'érosion installé dans une chambre climatique. Les technosols de la zone ont été échantillonnés sur le terrain et caractérisés (densités, humidité, distribution granulométrique et teneur en matière organique du sol), afin de reproduire au mieux leur état en laboratoire. En s'appuyant sur l'analyse statistique réalisée sur les jeux de données météorologiques locales, nous avons extrait les événements pluvieux d'intensité décennale, centennale, millénale et décamillénale. En utilisant les capacités des chambres climatiques du laboratoire Ecotron à Nemours (France) nous testons également plusieurs niveaux d'humidité du sol avant simulation : très humide, modérément humide et sec. Chaque expérience est répétée plusieurs fois afin d'évaluer l'effet de "mémoire" de la topographie sur l'érosion.

L'érosion est quantifiée en mesurant les concentrations de sédiments dans les eaux de ruissellement et d'infiltration collectées à la sortie du modèle couplées à des acquisitions topographiques réalisées par photogrammétrie.

<footnote n="1"><sup>1</sup> WSP France - 40 Avenue des Terroirs de France, 75012 Paris, France</footnote>

<footnote n="2"><sup>2</sup> Centre de Géosciences (GEOSCIENCE) — ENSM Paris, Université Paris sciences et lettres — 77300 Fontainebleau</footnote>

<footnote n="3"><sup>3</sup> Centre de recherche en écologie expérimentale et prédictive - Ecotron Ile de France (CEREEP) — CNRS, ENS Paris, Université Paris sciences et lettres — 77140 Saint-Pierre les Nemours, France</footnote>

<footnote n="4"><sup>4</sup> Agence Nationale pour la Gestion des Déchets Radioactifs (ANDRA) – ANDRA – Centres Industriels de l'Andra dans l'Aube (CSA et Cires), 10200 Soulaines-Dhuys, France</footnote>
# Hurricane Irma: an unprecedented event over the last 3700 years?

Pierre Sabatier<fnref n="1" />, Maude Biguenet<fnref n="2" />,<fnref n="1" />, Eric Chaumillon<fnref n="2" />, Fabien Arnaud<fnref n="1" />, Thibault Coulombier<fnref n="2" />, Nathalie Feuillet<fnref n="6" />

Low-lying coasts and small islands, such as in the Lesser Antilles, are particularly vulnerable to extreme climatic event such as hurricane. In September 2017, category 5 Hurricane Irma, with winds up to 360 km.h-1, hit the northern Caribbean islands and caused the destruction of 95% of the structures on Barbuda Island. We investigated the geomorphological impacts and the sedimentological record related to the storm surge of hurricane in Barbuda's Codrington Lagoon. Following Hurricane Irma, two wide inlets developed across the Codrington sandy barrier. One of these inlets was enlarged and was still open 4 years later. From available data, it seems that this barrier remained continuous for the last 250 years before Hurricane Irma. At a longer timescale, very high-resolution seismic exploration combined with sediment cores sampled in Codrington Lagoon were used to investigate Irma deposits and environmental changes for the last 3700 years. The evolution from a low-energy small and shallow lagoon to the modern wide and relatively high-energy lagoon recorded by its sediment-fill was related to both long-term sea-level rise and subsidence. The top of the lagoon fill consists of a thick and extensive sand sheet recording an abrupt increase in energy. Given (1) its location at the top of the cores ; (2) its very recent age, supported by short-lived radionuclide data ; (3) large inlets opening and barrier erosion after Irma that imply a large sand supply to the lagoon ; this sand sheet was attributed to Hurricane Irma. From our cores, it appears that this deposit is unique over more than 3700 years. Both the opening of a new inlet and the thick upper sand sheet support the exceptional character of Irma at the scale of centuries to millennia. Our study reinforces the idea that Hurricane Irma was exceptional in terms of intensity and may be associated with global warming.

<footnote n="1">¹ Environnements, Dynamiques et Territoires de la Montagne (EDYTEM) — CNRS, Université Savoie Chambéry, Ministère de la Culture — 73376 Le Bourget du Lac Cedex, France</footnote>

<footnote n="2">² Littoral, Environnement et Sociétés (LIENSs) — CNRS, La Rochelle Université – Technoforum, 17031 La Rochelle, France</footnote>

<footnote n="6">⁶ Institut de physique du Globe de Paris (IPGP) — CNRS, Institut de Physique du Globe de Paris, Université Paris Cité, Université La Réunion — 75238 Paris Cedex, France</footnote>
13 - Variabilité climatique décennale à pluri-centennale dans le passé et implications pour la prévision du climat et les services climatiques
# Variabilité Climatique Pluri-Centennale en Europe
durant l’Holocène : Implications pour les Projections
Climatiques futures

Amen Al-Yaari<fnref n="1" />, Didier Swingedouw<fnref n="1" />, Pascale Braconnot<fnref n="2" />, Paul Lincoln<fnref n="3" />, Laura Boyall<fnref n="3" />, Olivier Marti<fnref n="2" />, Thibaut Caley<fnref n="1" />, Thomas Extier<fnref n="1" />, Celia Martin-Puertas<fnref n="3" />

Le climat varie naturellement, et ses fluctuations interagissent avec les effets du changement climatique anthropique causé par les émissions massives de gaz à effet de serre depuis la révolution industrielle. Bien que le réchauffement observé au cours du siècle dernier soit principalement attribué à ces gaz à effet de serre, les simulations historiques et futures sont marquées par une grande incertitude. Depuis les années 1980, l'Europe s'est réchauffée plus rapidement que la moyenne globale, avec une augmentation des températures moyennes annuelles deux fois supérieure à la moyenne mondiale (Organisation météorologique mondiale). Cependant, la moyenne d'ensemble des simulations des modèles GCM (General Circulation Models) de CMIP6 (Coupled Model Intercomparison Project Phase 6) tend à sous-estimer ce réchauffement par rapport aux observations. Dans ce contexte, notre étude se concentre sur la variabilité interne du climat, et son rôle potentiel dans le réchauffement marqué de l'Europe de ces dernières décennies. Nous nous concentrons en particulier sur les fluctuations climatiques sur des échelles de temps pluri-centennales. Pour comprendre ce type de fluctuation très basse fréquence, une longue période de temps est nécessaire, c'est pourquoi nous nous sommes focalisés sur l'Holocène. Nous avons comparé des simulations transitoires de plusieurs modèles, ainsi que plusieurs reconstructions paléoclimatiques et réanalyses (intégrant modèles et données de la base TEMP12k) en Europe pour :

(i) Évaluer les principales caractéristiques de la variabilité climatique à travers ces différentes sources d'informations paléoclimatiques.

(ii) En déduire l'impact potentiel d'une telle variabilité naturelle sur les fluctuations récentes observées et celles à venir.

Nos premières analyses ont révélé des pics de variabilité situés entre 100 et 250 ans dans les reconstructions, ainsi que dans certaines simulations, que nous avons associés à la circulation méridienne de retournement dans l'Atlantique Nord (AMOC). Cette variabilité multi-centennale présente une signature spatiale distincte et se manifeste dans toute l'Europe. En utilisant le grand ensemble de simulations historiques prolongées d'IPSL (IPSL-EHS), nous en avons déduit que le réchauffement très marqué observé en Europe depuis 1980, faiblement capturé par la moyenne multi-modèle de CMIP6, pourrait contenir une composante naturelle importante qui amplifie ces tendances. Ces résultats offrent de nouvelles perspectives sur les tendances climatiques historiques et leurs projections futures en Europe. En particulier, la variabilité naturelle pourrait avoir amplifié les récentes périodes de réchauffement en Europe et pourrait continuer à influencer ces tendances jusqu'au milieu du 21ème siècle. Ces résultats soulignent l'importance de bien prendre en compte la variabilité interne pour réduire l'incertitude des projections climatiques.

<footnote n="1">1 Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>

<footnote n="2">2 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="3">³ Royal Holloway — University of London] – Egham, Surrey, TW20 0EX, Royaume-Uni</footnote>
# Les forçages externes comme moteurs des tendances décennales de l'Oscillation Nord-Atlantique

Ramdane Alkama<fnref n="1" />, Didier Swingedouw<fnref n="1" />, Juliette Mignot<fnref n="2" />, Guillaume Gastineau<fnref n="2" />, Jérôme Ogée<fnref n="3" />

Comprendre et anticiper les phénomènes atmosphériques tels que l'Oscillation Nord-Atlantique (NAO) est essentiel pour prévoir les impacts climatiques en Europe et développer des services climatiques adaptés. De 1960 à 2005, les tendances décennales de la NAO ont été prédictibles avec succès grâce à un grand nombre de simulations. Cependant, la précision des prévisions s'est détériorée pour les périodes plus récentes, soulevant des doutes quant à la fiabilité des systèmes de prévision à long terme.

Pour améliorer cette fiabilité, une nouvelle méthode a été développée à partir du système de prévision décennale du CMIP6, permettant d'affiner les prévisions de la NAO même pour les périodes récentes. Toutefois, une diminution de la précision après 2014 a été observée, coïncidant avec l'arrêt de l'utilisation des forçages externes observés dans les simulations, comme les aérosols d'origine humaine et les gaz à effet de serre.

L'analyse des simulations historiques du CMIP6, et plus spécifiquement du projet DAMIP, a révélé que les aérosols d'origine humaine jouent un rôle clé dans la variabilité inter-décennale de la NAO, tandis que les gaz à effet de serre influencent davantage les tendances à long terme.

En conclusion, cette étude souligne l'importance des forçages externes dans la prévisibilité de la NAO et propose des méthodes de post-traitement alternatives pour améliorer la précision des prévisions décennales futures.

<footnote n="1">¹ Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>

<footnote n="2">² Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN) – Sorbonne Université, CNRS, MNHN, IRD – 4 place Jussieu - 75252 Paris Cedex 05, France</footnote>

<footnote n="3">³ Interaction Sol Plante Atmosphère (ISPA) — ENSA Bordeaux, INRAE — 33883 Villenave d'Ornon, France</footnote>
La variabilité du climat à l'échelle humaine

Ariane Burke <fnref n="1" />, Masa Kageyama <fnref n="2" />, Mathieu Vrac <fnref n="2" />

Dans le contexte actuel prédire l'impact écologique du réchauffement planétaire est à notre portée mais prédire son impact humain est plus contentieux. Les archéologues disposent d'un registre archéologique qui permet théoriquement de cerner les sources de vulnérabilité et de résilience de populations passées face au changement climatique. Les collaborations entre paléoclimatologues et archéologues nous permettent donc d'explorer la relation entre le climat et l'humain dans des contextes analogiques à celles qui sont sur le point de se réaliser dans le futur proche. Nous pouvons ainsi documenter les solutions recherchées par diverses sociétés humaines dans le passé pour parer les défis climatiques et leurs retombées, réussites comme échecs. L'échelle de temps à laquelle les humains perçoivent et réagissent au changement climatique est une question clé qui ressort de ces collaborations. Dans cette communication nous décrivons comment nous avons quantifié la variabilité climatique à l'échelle mensuelle/saisonnière afin d'explorer son impact sur les décisions prises par des populations humaines vivant en Europe au cours du Pléistocène récent.

<footnote n="1">1 Anthropologie – Université de Montréal — Centre-Ville Montréal, QC Canada H3C 3J7, Canada</footnote>

<footnote n="2">2 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>
High resolution paleo-hydrological record from
carbonate deposits in the Roman aqueduct of
Traconnade (Aix-en-Provence, SE France).

Christelle Claude ¹, Laurence Vidal ¹, Cees Passchier ², Bernard Angeletti ¹,
Abel Guihou ¹, Gül Sürmelihindi ³, Angèle Ricolleau ⁴, Ghislain Gassier ¹, Marc
Panneau ⁵, Corrine Sonzogni ¹, Doriane Delanghe ¹, Pierre Deschamps ¹, Marc
Fuhry ⁶, Bernard Fino ⁶, Philippe Leveau ⁷, Lionel Marié ¹, Nuria Nin ⁸,⁹

1 Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE) – Collège de France, CNRS,
IRD, Aix Marseille Université, INRAE – 13545 Aix-en-Provence, France

2 Department of Earth Sciences — University of Mainz — Allemagne

3 School of Archaeology — University of Oxford — Royaume-Uni

4 Centre Interdisciplinaire de Nanoscience de Marseille (CINaM) — CNRS, Université Aix-Marseille, 13288 Marseille
Cedex 09, France

5 Institut de recherche sur l'architecture antique (IRAA) — CNRS, Université Aix-Marseille, INRAP, Université Pau et
Pays de l'Adour — 13100 Aix-en-Provence, France

6 Service d'archéologie d'Aix en Provence – 13290 Aix-en-Provence, France

7 Laboratoire Méditerranéen de Préhistoire Europe Afrique (LAMPEA) — Aix Marseille Université, CNRS, Ministère de la
Culture et de la Communication — MMSH 5 rue du château de l'horloge, 13094 Aix-en-Provence, France

8 Direction Archéologie et Muséum d'Aix-en-Provence (DAMVA) — 13616 Aix-en-Provence Cedex 1, France

9 Archéologie des sociétés méditerranéennes (ASM) — CNRS, Ministère de la Culture, Université Paul Valéry
Montpellier 3, INRAP — Université Paul-Valéry, 34090 Montpellier, France

Carbonate deposits in Roman aqueducts are potential valuable archives of the paleo-hydrological record. Here we present a new high-resolution record from carbonate deposits in the aqueduct of Traconnade built for Aix-en-Provence, the Roman city of Aquae Sextiae. As its name indicates, Aquae Sextiae was built around an abundant source of water. The Traconnade aqueduct carbonate provides sufficient temporal resolution to explore environmental variations on a seasonal scale. Through a combination of micromilling and in situ analysis (LA–MC–ICPMS) on thin sections, we measured geochemical variations (stable isotopes and trace elements) and performed sampling for absolute U-Th dating. Geochronological results based on classical correction and isochron along with archaeological observations confirm that the carbonate concretion was deposited in Roman times on the walls of the Traconnade aqueduct, built no later than AD 140. Carbonate δ¹⁸O records document the presence of clear geochemical cycles that were mainly controlled by seasonal variations in temperature and δ¹⁸O from water circulating in the aqueduct. Minima and maxima in δ¹⁸O values were attributed to summer and winter conditions, respectively in agreement with the modern spring’s stable isotope values. The other tracers (Mg/Ca, Sr/Ca and δ¹³C) also show marked cyclicities with maximum values occurring in summer. The enrichment in trace elements and δ¹³C were linked to processes occurring during dry periods (1) prior precipitation of calcite in the epikarst, and/or (2) a greater degree of CO₂ degassing. The geochemical variations in Ba/Ca, U/Ca and Rb/Ca behave differently and their co-variations may reflect the water-rock interactions in the karst system. We highlight that these tracers can be used as a proxy for intense precipitation events. These results suggest the occurrence of a double rainy season (spring and autumn) in the Aix-en-Provence region, similar to present-day climatic conditions. Our study confirms the high potential of aqueduct concretions for paleo-hydrological studies.
Palaeoclimate synthesis of the central Mediterranean
area from pollen data

Léa D'oliveira¹, Sébastien Joannin¹, Guillemette Menot²,³, Nathalie Combourieu-Nebout⁴, Lucas Dugerdil¹,⁵, Marion Blache⁶, Elisabetta Brugiapaglia⁷, Assunta Florenzano⁸, Alessia Masi⁹, Anna Maria Mercuri¹⁰, Mary Robles¹¹, Laura Sadori⁹, Marie Balasse¹², Odile Peyron¹

1 Institut des Sciences de l'Évolution de Montpellier (ISEM) – CNRS, EPHE Paris, IRD, Université de Montpellier, Université Paris sciences et lettres, CIRAD, INRAP – 34095 Montpellier Cedex 05, France

2 Laboratoires Mixtes Internationaux : Dynamique des écosystèmes continentaux d'Afrique Centrale (LMI DYCOFAC) – IRD, Université de Yaoundé 1, IRGM – BP1857, Yaoundé, Cameroun

3 Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE) - CNRS, ENS Lyon, Université Claude Bernard Lyon 1, Université Jean Monnet Saint-Etienne - 69622 Villeurbanne Cedex, France

4 Histoire Naturelle de l'Homme Préhistorique (HNHP) — CNRS, Université de Perpignan via Domitia, MNHN, IRD — 75013 Paris Cedex, France

5 Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE) - CNRS, ENS Lyon, Université Claude Bernard Lyon 1, Université Jean Monnet Saint-Etienne - 69622 Villeurbanne Cedex, France

6 Université du Québec en Abitibi-Témiscamingue (UQAT) - 445 Boulevard de l'université, Rouyn-Noranda, Canada
7 University of Molise (UNIMOL) - Via Francesco De Sanctis, 1 - Campobasso, Italie

8 Laboratorio di Palinologia e Paleobotanica, Dipartimento di Scienze della Vita — Università di Modena e Reggio Emilia — Italie

9 Università degli Studi di Roma "La Sapienza" (UNIROMA) - Piazzale Aldo Moro 5, 00185 Roma, Italie

10 Università degli Studi di Modena e Reggio Emilia (UNIMORE) - 41121 Modena Reggio Emilia, Italie

11 Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE) - Collège de France, CNRS, IRD, Aix Marseille Université, INRAE - 13545 Aix-en-Provence, France

12 Archéozoologie, archéobotanique, Sociétés, pratiques et environnements (AASPE) — CNRS, MNHN, INRAP — 75231 Paris cedex 05, France

Many studies suggest that the Holocene is characterized by longitudinal and latitudinal complex and different climatic trends in the central Mediterranean region. This particularity in the regional climate system has been highlighted by multiple proxies showing contrasting trends of climate variations. Chironomid temperature-based reconstructions indicate the presence of a thermal optimum in Italy (10,000 – 7,000 cal. BP), contradicting the pollen-inferred quantification of Northern Europe. Those discrepancies point to the need for further investigation concerning the Holocene climate of the Mediterranean region. To date, various studies have attempted to quantify the climate signal through different approaches without being able to propose a consistent climate reconstruction between proxies and sites.

In this context, a pollen-based multi-method approach has been developed to quantify the Holocene climate in the central Mediterranean through a regional approach using multiple sites in Southern France and Italy. We show the results here based on various sites extracted from international pollen databases (NEOTOMA or European Pollen Database). The pollen-inferred climate is quantified by a multi-method approach, using four different methods (MAT, WA-PLS, BRT and RF) and three modern pollen databases (regional and global).

This study highlights the existence of strong inter-regional differences in the Holocene climate in the central Mediterranean. The results show that not all of the northern Mediterranean basin is marked by a Holocene thermal optimum and that there is strong spatial variability. Western southern France seems to show the presence of a thermal optimum, but this pattern is not observed in the rest of the Mediterranean basin, particularly in southern Italy. Southern France and northern Italy show more variable climatic patterns, where altitude seems to play a particular role. Seasonality also appears to be
an important aspect of the Mediterranean climate, and its study enables us to highlight different climatic
dynamics and gain a better understanding of the particular characteristics of the Holocene climate in this
region.
Climate changes during the Holocene in central
Mediterranean area reconstructed from the new
Monticchio record (Potenza, Italy) using a coupled
pollen and lipid biomarkers (brGDGT) approach

Léa D'oliveira <fnref n="1" />, Odile Peyron <fnref n="1" />, Sebastien Joannin <fnref n="2" />,<fnref n="1" />, Nathalie Combourieu-Nebout <fnref n="3" />, Lucas Dugerdil <fnref n="1" />,<fnref n="4" />, Sébastien Nomade <fnref n="5" />, Alison Pereira <fnref n="6" />, Marie Balasse <fnref n="7" />, Guillemette Menot <fnref n="8" />,<fnref n="4" />

Past climatic changes in southern Europe are characterised by a strong spatial and temporal
heterogeneity whose patterns are still poorly understood. This is particularly true for Italy, where several
studies have highlighted north-south regional patterns during the Holocene. The distinct characteristics
of the regional climate system have been highlighted by various proxies that reveal differing climate
trends and rapid climate events. Temperature reconstructions based on chironomid data suggest a
thermal optimum in North Italy during the Middle Holocene, which contrasts the model-based
assessments from the central Mediterranean region. Pollen data and lake-levels have shown opposite
climatic dynamics north and south of 40 N latitude, with maximum humidity between 8000 and 4000 cal
BP; this process reverses around 4000 cal BP. These inconsistencies highlight the necessity for additional
research into the Holocene climate of the Mediterranean region.

The Lago Grande di Monticchio (Potenza, Italy) is an emblematic sequence in the Mediterranean region and has already been the subject of several studies, focusing on tephrochronology, geochemistry, palynology and palaeoclimatology. Here, a multi-proxy approach combining pollen and lipid biomarkers (branched Glycerol Dialkyl Glycerol Tetraethers, brGDGTs) is applied to a new record in the Lago Grande di Monticchion, to reconstruct at high resolution the climate changes over the last 13,000 years in southern Italy. To provide reliable climate reconstructions, we have performed a multi-method approach based on transfer functions applied to pollen data (MAT, BRT) and lipid biomarkers (brGDGTs).

Preliminary results indicate that pollen and brGDGT-inferred climate trends are consistent between the pollen and brGDGT reconstructions. The Lateglacial and Early Holocene periods are characterised by colder conditions and are followed by a relatively warm Middle Holocene. The Late Holocene is characterised by a cooling to present-day conditions. Altogether, our preliminary results from the Lago Grande di Monticchio sequence reveal coherent trends between independent proxies, supporting the reliability of our reconstruction.

<footnote n="1">¹ Institut des Sciences de l'Évolution de Montpellier (ISEM) – CNRS, EPHE Paris, IRD, Université de Montpellier, Université Paris sciences et lettres, CIRAD, INRAP – 34095 Montpellier Cedex 05, France</footnote>

<footnote n="2">2 School of Geography and earth sciences – Canada</footnote>

<footnote n="3">3 Histoire Naturelle de l'Homme Préhistorique (HNHP) — CNRS, Université de Perpignan via Domitia, MNHN, IRD — 75013 Paris Cedex, France</footnote>

<footnote n="4">4 Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE) - CNRS, ENS Lyon, Université Claude Bernard Lyon 1, Université Jean Monnet Saint-Etienne - 69622 Villeurbanne Cedex, France</footnote>

<footnote n="5">5 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="6">6 Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="7">7 Archéozoologie, archéobotanique, Sociétés, pratiques et environnements (AASPE) — CNRS, MNHN, INRAP — 75231 Paris cedex 05, France</footnote>

<footnote n="8">8 Laboratoires Mixtes Internationaux : Dynamique des écosystèmes continentaux d'Afrique Centrale (LMI DYCOFAC) – IRD, Université de Yaoundé 1, IRGM – BP1857, Yaoundé, Cameroun</footnote>
# Evaluation and Attribution of a Warm Winter Bias Over Arctic Sea Ice in a Climate Model

Nicolas Michalezyk<fnref n="1" />, Guillaume Gastineau<fnref n="1" />, Martin Vancoppenolle<fnref n="1" />, Clément Rousset<fnref n="2" />

Climate models often show important winter near-surface temperature biases over Arctic sea ice, increasing uncertainties in future sea ice and Arctic climate projections. Such biases reflect difficulties in representing air-sea-ice interactions, and limited observations in the Arctic. Mitigating them in future model versions requires proper evaluation and understanding of their origin. The climate variability in the models is strongly linked to the mean state simulated. In this contribution, we compare historical simulations using atmosphere-only and coupled configurations of the IPSL-CM6A-LR climate model regarding near-surface air temperature and find substantial differences. To establish a reliable baseline for evaluating simulations, we identified a linear relationship between ERA5 near-surface air temperature values and their bias relative to in situ observations from Soviet North Pole drift stations. This relationship is then used to correct the bias in ERA5 data. We find the winter near surface temperature bias in the atmosphere-only IPSL-CM6A-LR configuration turns from cold to warm once ERA5 is linearly corrected, reaching +2.2°C over Arctic multiyear ice. The bias increase to +4.8°C in the fully-coupled configuration. A pan-Arctic energy budget evaluation indicates that excess poleward atmospheric heat transport is the most likely cause of the warm bias. In the coupled configuration, presumably due to interactive sea ice and ocean components, we find compensation effects between high atmospheric heat transport, low poleward oceanic heat transport and surface energy budget. This translates to a weaker-than-expected heat release from the ocean to the atmosphere. The methods developed here could be used in multi-model evaluations to further progress in understanding and reducing biases in climate models, and to diminish the uncertainties weighing on the future climate.

<footnote n="1">¹ Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN) – Sorbonne Université, CNRS, MNHN, IRD – 4 place Jussieu - 75252 Paris Cedex 05, France</footnote>

<footnote n="2">² Institut Pierre-Simon-Laplace – Sorbonne Université – France</footnote>
# Sensibilité à la prise en compte du niveau marin dans les simulations du climat du dernier interglaciaire

Gilles Ramstein<fnref n="1" />, Manua Ewart<fnref n="1" />, Sébastien Nguyen<fnref n="1" />, Zhongshi Zhang<fnref n="2" />

La remontée du niveau marin (GMSL global mean sea level) est perçue comme une conséquence du changement climatique. Celle-ci est d'abord lié à la dilatation des molécules d'eau, mais ce processus va être bientôt largement dominé par la contribution en eau douce liée à la fonte partielle des calottes de glace, à la fois celle du calotte Groenland mais également celle de l'Antarctique. Or il a été récemment montré que la remontée du niveau marin pouvait devenir un facteur forçant du changement climatique.

Cet effet a été illustré dans le cadre du dernier Interglaciaire (LIG pour Last Inter Glacial à 130-115 ka BP) où le niveau marin global était plus élevé (de 3 à 9 m), du fait de la réduction importante des volumes des calottes du Groenland et de l'Antarctique de l'ouest. Les forçages climatiques " classiques " pour simuler le dernier interglaciaire utilisées dans PMIP4 (Otto-Bliesner CP2021) sont d'abord les paramètres orbitaux, en particulier, la précession qui donne une forte saisonnalité et les concentrations à gaz à effet de serre ($CO_2$, $CH_4$ et $N_2O$) qui sont peu différentes de celles de l'Holocène.

La prise en compte du niveau marin du LIG a, pour la première fois, été simulée (Zhang 2023) avec le GCM NORESM versionF1 et a montré une forte sensibilité climatique avec en particulier

(1) une amélioration sensible des températures de surface de l'océan comparées aux données dans l'hémisphère sud ;

(2) une augmentation marquée des températures dans le détroit de Béring.

Il était donc important de tester ce même forçage avec un autre GCM pour évaluer la robustesse de ce résultat.

Nous montrons à travers une série de simulations de sensibilité avec le modèle de l'IPSL (CM6LR) que

(1) La prise en compte du GMSL est déterminante pour simuler le climat du LIG ;

(2) la réponse des 2 modèles présente des caractéristiques communes, mais aussi des différences importantes. Celles-ci sont principalement liées aux caractéristiques propres de chaque modèle dans leurs simulations du climat pré-industriel (e.g. AMOC) et à la distribution spatiale utilisée, en particulier pour le détroit de Béring.

Enfin pour notre interglaciaire, la prise en compte d'une augmentation du niveau marin, au cours de ce siècle (0.5-1 m) et à l'échelle de 2300 (jusqu'à 13m) (résumé pour décideurs du W1 du GIEC, figs. 8d et 8e) pourrait devenir un facteur déterminant du changement climatique à travers des processus linéaires et non linéaires, liés en particulier aux effets de seuils produits par l'augmentation du niveau marin dans les détroits sur la circulation océanique en Arctique et en Atlantique nord.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² Université de Wuhan — Chine</footnote>
# A Regime View of ENSO Flavors Through Clustering in CMIP6 Models

Pradeebane Vaittinada Ayar <fnref n="1" />, David Battisti <fnref n="2" />, Camille Li <fnref n="3" />, Martin King <fnref n="4" />, Mathieu Vrac <fnref n="1" />, Jerry Tjiputra <fnref n="5" />

El Niño-Southern Oscillation (ENSO) flavors in the tropical Pacific are studied from a regime perspective. Five recurring spatial patterns or regimes characterizing the diversity of ENSO are established using a clustering approach applied to the HadISST sea surface temperature (SST) anomalies. Compared to previous studies, our approach gives a monthly characterization of the diversity of the warm and cold phases of ENSO established from observations but commonly applied to models and observations. Two warm (eastern and central El Niño), two cold (basin wide and central La Niña) and a neutral reference regimes are found. Simulated SST anomalies by the models from the latest Coupled Model Intercomparison Project Phase 6 are then matched to these reference regimes. This allows for a consistent assessment of the skill of the models in reproducing the reference regimes over the historical period and the change in these regimes under the high-warming Shared Socio-economic Pathway (SSP5.8.5) scenario. Results over the historical period show that models simulate well the reference regimes, with some discrepancies. Models simulate more intense and spatially extended ENSO patterns and have issues in capturing the correct regime seasonality, persistence, and transition between regimes. Some models also have difficulty simulating the frequency of regimes, the eastern El Niño regime in particular. In the future, both El Niño and central La Niña regimes are expected to be more frequent, accompanied by a less frequent neutral regime. The central Pacific El Niño and La Niña regimes are projected to increase in amplitude and variability.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² University of Washington — Box 351640 Seattle, WA 98195, États-Unis</footnote>

<footnote n="3">³ University of Bergen and the Bjerknes Centre for Climate Research – Jahnebakken 5 5007 Bergen, Norvège</footnote>

<footnote n="4">⁴ Geophysical Institute [Bergen] (GFI/BiU) – Universitetet i Bergen, Geofysisk institutt, Allégaten 70, 5020 Bergen, Norvège</footnote>

<footnote n="5">⁵ NORCE Climate Bjerknes Centre for Climate Research – Jahnebakken 5 5007 Bergen, Norvège</footnote>
# 14 - Variabilités climatiques et cycle du carbone au cours du Quaternaire
# Réponse de la végétation et du climat dans la péninsule coréenne pendant les Interstadiaires du Groenland 8 et 12

Léa Baubant<fnref n="1" />, Coralie Zorzi<fnref n="2" />, Jong Hyun Kim<fnref n="3" />, María Sánchez Goñi<fnref n="4" />

La dernière période glaciaire a été marquée par une succession rapide d'interstadiaires (phases tempérées, GI) et stadiaires (phases froides, GS) au Groenland. Les mécanismes modulant la variabilité rapide du climat sont encore mal compris en Asie de l'Est, une région influencée par les moussons d'été et d'hiver, qui apportent respectivement des conditions chaudes/humides et froides/sèches. Malheureusement, les modèles climatiques actuels ne représentent pas encore avec fiabilité la distribution et la saisonnalité des précipitations dans cette région face au changement climatique. Ce travail étudie les changements de végétation et du climat sur la péninsule coréenne pendant les Interstades de Groenland (GI)-8 (-38.3 à -36.6 ka) et GI-12 (-44.3 à -46.8 ka). Un interstadiaire est caractérisé par un réchauffement rapide suivi d'un refroidissement progressif, avec une température relativement élevée. Ces GI sont des analogues stratégiques pour comprendre la réponse régionale des écosystèmes face à un réchauffement abrupt. Ces deux périodes partagent une forte hausse des températures au Groenland et une insolation similaire, mais diffèrent par leurs configurations orbitales.

L'analyse palynologique de la carotte marine ES14-GC01 montre que lors des réchauffements au Groenland, la végétation coréenne réagit par une expansion des forêts tempérées de feuillus et une diminution des conifères boréaux. Cela suggère une hausse des températures et des précipitations, en accord avec le δ¹⁸O des spéléothèmes de Hulu Cave et la taille des grains de loess de Gulang, indiquant une réponse de la végétation à la variabilité du système de mousson Est-asiatique. Cependant, les assemblages polliniques diffèrent entre les deux GIs. Les forêts tempérées de feuillus et de conifères sont davantage développées durant le GI-12, tandis que le GI-8 se distingue par une forêt chaude-tempérée et une plus grande abondance de taxons steppiques et de conifères boréaux. Le développement de ces taxons suggère une augmentation de la saison humide estivale et un allongement de la saison sèche hivernale, reflétant un contraste saisonnier plus marqué, lié au minimum de précession pendant le GI-8. En revanche, le GI-12, avec un maximum d'obliquité, favorise un ensoleillement annuel plus important et des hivers plus doux, ainsi qu'une intensification de la mousson d'été, entraînant un développement plus fort des forêts. Ces réchauffements initiaux favorisent l'expansion forestière sur la péninsule coréenne, mais l'ampleur et la composition des forêts sont modulées par les paramètres orbitaux.

<footnote n="1">1 PhD Track Graduate School Géosciences, Climat, Environnement, Panètes – Université Paris-Saclay – France</footnote>

<footnote n="2">2 Centre of Marine Sciences - Instituto Português do Mar e da Atmosfera (CCMAR-IPMA) – Portugal</footnote>

<footnote n="3">3 Centre for Glacial and Cryospheric Research - Korea Polar Research Institute – Corée du Sud</footnote>

<footnote n="4">4 Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>
Holocene Integrative Vegetation Evolution (HIVE) :
dynamical modelling with different models under
transient climate conditions in Europe

Isabeau Bertrix <fnref n="1" />, Didier M. Roche <fnref n="1" />, Hisashi Sato <fnref n="2" />, Nicolas Viovy <fnref n="1" />

Climate always had a strong influence on both vegetation cover and human population size, which in
turn is a big influencer of landscape. Indeed, if we look at the Holocene period, human activities such as
deforestation, hunting and agriculture, have led to substantial changes in the European vegetation cover
(Nikulina et al 2024), as well as a rise in CO2 levels more recently (Petit et al, 2020). Moreover, land cover
is an integral component of the climate system that can lead to two types of feedback loops:
biogeophysical (albedo, thermal conductivity) and biogeochemical (carbon dioxide absorption or release
during photosynthesis or decomposition). Human societies impact on vegetation thus modify the climate
directly and through the mediation of the carbon cycle. Additionally, for a given set of climate and human
impact conditions the vegetation cover is not ubiquitous but is also a result of the past history.

A good candidate to solve this three-body problem is the comparison between pollen-based reconstructions and Dynamical Global Vegetation Models (DGVMs) simulations. Indeed, pollen-based reconstructions represent the vegetation effectively affected by human activities and climate conditions, whereas the DGVMs simulations represent the Potential Natural Vegetation (PNV) that could have lived in the absence of homo sapiens. DGVMs use climatic data as inputs, so they can be sensitive to climatic variations too.

We aim at exploring the difference between the results of two DGVMs model, SEIB-DGVM and ORCHIDEE-DGVM, compared to pollen-based reconstruction.

The DGVMs simulations were running using climatic inputs from the intermediate complexity climatic model iLOVECLIM, using a downscaling approach to increase the spatial resolution and a bias correction method to improve the climate representation over the European area. Using steps of 300 years time windows periods from 8.5K bp to the preindustrial period, we present the results of those simulations and the comparison with the pollen database as well as comparison between the two DGVMs to assess the inter-model dispertion. The immediate outcome of our work is to validate a generic methodology to intercompare models outputs taking into account their differences in processes, complexity etc., as well as to infer what is the robust signal that can be extracted from the data – model comparison. In a second step, this will lead to more robust estimations of vegetation response in a climate change context and particularly in the view to help us understand how our species has shaped the lands even before agricultural times.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² Department of Environmental Geochemical Cycle Research — Japan Agency for Marine-Earth Science and Technology — Japon</footnote>

<footnote>Nikulina A., MacDonald K., Zapolska A., Serge M. A., Roche D. M., Mazier F., Davoli M., Svenning J., Wees D, Pearce E. A., Fyfe R., Roebroeks W., and Scherjon F.. Hunter-gatherer impact on european interglacial vegetation: A modelling approach. Quaternary Science Reviews, 324:108439, 2024. ISSN 0277-3791. doi: https://doi.org/10.1016/j.quascirev.2023.108439. URL</footnote>

<footnote>Jean-Robert Petit and Dominique Raynaud. Forty years of ice-core records of CO2., 579(7800):505–506, March 2020. doi:10.1038/d41586-020-00809-8.</footnote>
# Changements environnementaux et climatiques au cours des MIS 37-31 (~1,25~1,06 Ma) en Méditerranée occidentale : vers une meilleure compréhension de l'occupation tardive de l'Europe de l'Ouest par les Hominines

Maé Catrain <fnref n="1" />, Nathalie Combourieu-Nebout <fnref n="1" />, Vincent Lebreton <fnref n="1" />,
Séverine Fauquette <fnref n="2" />, Odile Peyron <fnref n="2" />, Sébastien Joannin <fnref n="2" />, Morgane Fries <fnref n="3" />,
Viviane Bout-Roumazeilles <fnref n="4" />, Lionel Dubost <fnref n="1" />, Patricia Richard <fnref n="3" />,
Jeanne Lepelletier <fnref n="1" />, Marie-Hélène Moncel <fnref n="1" />

Les premières traces de peuplement de l'Europe occidentale sont datées de 1.4 et 1.2 Ma en Espagne (Fuente Nueva 3 et Sima del Elefante) et sont bien postérieures à celles du Caucase (1.8 Ma) et de l'Asie (2 Ma). Cette colonisation tardive coïncide avec le début de la Early Middle Pleistocene Transition (EMPT, 1.4 à 0.4 Ma), au cours de laquelle les cycles climatiques présentant une période de 41 ka sont remplacés progressivement par des cycles de 100 ka forcés non plus par l'obliquité mais par l'excentricité. Cette période de temps est climatiquement complexe, mais très peu documentée. Il est donc essentiel de comprendre les changements climatiques et environnementaux survenus au cours de l'EMPT, ce qui permettra de mieux appréhender ces trajectoires différentes de peuplement. Cette étude est basée sur des analyses palynologiques du site Leg 161 ODP 976 dans la mer d'Alboran, au sud de la péninsule ibérique. Cette séquence marine est le seul enregistrement continu pour cette période en Méditerranée occidentale.

Les nouvelles données polliniques de la carotte ODP 976 couvrent les stades isotopiques marins (MIS) 37-31 (1,25 - 1,06 Ma) et améliorent notre compréhension des environnements méditerranéens au Pléistocène inférieur. Notre étude est multi-proxies : données polliniques, assemblages de foraminifères planctoniques, analyse de la composition des argiles, permettant ainsi une reconstruction des changements climatiques au cours des différentes périodes glaciaires et interglaciaires. Cette étude pluridisciplinaire propose une vision détaillée de la dynamique climatique du sud-ouest de la Méditerranée au cours des cycles à 41 ka.

Les résultats montrent que les transitions entre les conditions glaciaires et interglaciaires sont caractérisées par des successions récurrentes de la végétation, qui seront comparées à celles des séquences continentales clés en Méditerranée. Ils soulignent également des corrélations entre les changements observés dans les courbes polliniques et celles des assemblages de foraminifères planctoniques, tandis que les données issues de l'analyse des argiles dans les sédiments indiquent la provenance des dépôts en lien avec des variations des flux atmosphériques. Ces recherches viennent

<footnote n="1">1 Histoire Naturelle de l'Homme Préhistorique (HNHP) — CNRS, Université de Perpignan via Domitia, MNHN, IRD — 75013 Paris, France</footnote>

<footnote n="2">2 Institut des Sciences de l'Évolution de Montpellier (ISEM) — CNRS, EPHE Paris, IRD, Université de Montpellier, Université Paris sciences et lettres, CIRAD, INRAP — 34095 Montpellier Cedex 05, France</footnote>

<footnote n="3">3 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="4">4 Laboratoire d'océanologie et de géosciences (LOG) — CNRS, Université Littoral-Côte d'Opale, Université de Lille, IRD — 59650 Villeneuve d'Ascq, France, France</footnote>
préciser le cadre environnemental et climatique contemporains des premiers peuplements de l'Europe occidentale et permet de discuter les changements des comportements techniques et de subsistance ainsi que de la mobilité des premiers Hominines pendant cette période clé de l'EMPT.
# Towards a continuous single site record of Quaternary CO₂

Thomas Chalk<fnref n="1" />, Rachel Brown<fnref n="1" />, Elwyn De La Vega<fnref n="2" />, Paul Wilson<fnref n="3" />,
Gavin L. Foster<fnref n="3" />

Ice core records of trapped past CO₂ span the last 800,000 years in a continuous fashion, less than half of the Quaternary epoch. Beyond that exist snapshots of discontinuous ‘blue ice’ where older ice preserves deeper time as discrete datapoints. Marine records comprising boron isotope ratios from planktic foraminifera have the proven potential to substitute for direct CO₂ measurements in the geological past, and are especially powerful over the Quaternary. One marine site in particular, ODP Site 999, has been extensively proven for the Plio-Pleistocene, but not yet complete. Here we explore the comparison of existing ice core and δ¹¹B-derived records of past CO₂, where they co-exist in the latest Pleistocene to exploring the potential implications arising from this comparison – in particular over the interval of the discontinuous ‘blue ice’. Marine records are capable of providing high fidelity reconstructions of pH and therefore CO₂ (albeit less so than that of the ice cores), but they can also add much needed context and information on the mechanisms of CO₂ change where they diverge from the atmosphere. There is much to be learned via the extensive proxy-training period with the ice cores interval, which we then seek to apply over the entire Pleistocene to connect with the Pliocene.

<footnote n="1">¹ Centre de Recherche et d'Enseignement de Géosciences de l'Environnement (CEREGE) – Collège de France, CNRS, IRD, Aix Marseille Université, INRAE – 13545 Aix-en-Provence, France</footnote>

<footnote n="2">² University of Galway – Irlande</footnote>

<footnote n="3">³ University of Southampton – Royaume-Uni</footnote>
Environmental changes during Heinrich Stadial 4 and
DO 8 in SW Mediterranean: insights from new high-
resolution palynological analyses and climate
reconstructions.

Liz Charton <fnref n="1" />,<fnref n="2" />, Nathalie Combourieu-Nebout <fnref n="1" />, Adele Bertini <fnref n="2" />, Odile Peyron <fnref n="3" />,
Marie-Hélène Moncel <fnref n="1" />, Vincent Lebreton <fnref n="1" />

During the last glacial period (MIS 4-2), the Northern hemisphere experienced submillenial climate oscillations (Dansgaard-Oeschger cycles) as evidenced through the study of Atlantic sedimentary cores and the Greenland ice records. Heinrich Stadials were described as phases of extreme and abrupt shift toward cold and dry conditions on the continent, linked with major iceberg discharges in the North Atlantic. Palynological studies of marine cores from the Iberian margins already evidenced vegetation changes associated to the submillenial climate variability and Heinrich events, but generally with low resolution. Leg 161 ODP 976 marine core is located in the Alboran Sea, 50km south of Malaga, and already provided important insights into the fast climate variability of the Last Glacial. Here, we report the results of ultra-high resolution palynological analyses for the interval between 41-34 ka BP including the Heinrich Stadial 4 (HS4) and the Dansgaard-Oeschger 8 (DO8). An updated age model was built using radiocarbon ages and calibration with the Greenland isotopic chronology. Vegetation and climate changes are discussed on the basis of 1) pollen counting and identification, and 2) multiple methods of pollen- based climate reconstructions: Modern Analogue Technique (MAT), Weighted Averaging Partial Least Squares regression (WA-PLS), and machine learning methods as Random Forest (RF) and Boosted Regression Trees (BRT).Results show the fast spread at centennial-scale of semi-arid vegetation with the main taxa Artemisia, Amaranthaceae and Ephedra reaching more than 60% of the total pollen sum during the main phase of the HS4. The transition to the following DO 8 climate amelioration is progressive, displaying a major instability with a vegetation succession that mimics the pattern of the last deglaciation. The climate reconstruction shows cold conditions during HS4 and enhanced aridity mostly linked to the winter rainfall deficit caused by the abrupt disturbance of the Mediterranean seasonal regime of precipitations. The regional comparison with available palynological and paleoclimatic records from the western Mediterranean and the Atlantic sheds light on the spatial expression of the arid event from both sides of the Gibraltar Strait. The typical three-phased pattern for the HS4 described in Greenland (and in Brazil from speleothems) is also identified in SW Mediterranean records, supporting the connection between

<footnote n="1">¹ Histoire Naturelle de l'Homme Préhistorique (HNHP) — CNRS, Université de Perpignan via Domitia, MNHN, IRD — 75013 Paris, France</footnote>

<footnote n="2">2 Dipartimento di Scienze della Terra — Università degli Studi di Firenze — Via Pira 4, 50121 Firenze, Italy, Italie</footnote>

<footnote n="3">³ Institut des Sciences de l'Évolution de Montpellier (ISEM) - CNRS, EPHE Paris, IRD, Université de Montpellier, Université Paris sciences et lettres, CIRAD, INRAP - 34095 Montpellier Cedex 05, France</footnote>
Greenland and lower latitudes hydroclimatic changes while the north Atlantic cores record a two-phased pattern. The comparison also evidences a westward trend in aridification in the Iberian Peninsula toward the Mediterranean: a more intense drying is reconstructed in the SW Mediterranean region compared to the Atlantic, with a drop in precipitations of 100 to 400 mm during the main phase of the event. The Alboran Sea is confirmed to be a critical area at the interface of the Atlantic and Mediterranean influences, recording the particular expression of fast climate oscillations in the westernmost part of the Mediterranean. The results of this study are of particular relevance for the debate on the impact of fast climate variability on the dynamics of Neanderthal and Homo sapiens populations during MIS3 in Southern Europe.
# Terrestrial biosphere evolution during Termination V: a model-data comparison

Thomas Extier<fnref n="1" />, Audrey Zimmer<fnref n="1" />, Stéphanie Duchamp-Alphonse<fnref n="2" />, Antoine Séjourné<fnref n="2" />, María Sánchez Goñi<fnref n="1" />

Glacial terminations of the past 800,000 years are key periods associated with a rise of 50-100 ppm of the atmospheric carbon dioxide concentrations ($pCO_2$) within a few thousand years. However, the mechanisms that led to these deglacial transitions remain elusive even if evidence suggests that terrestrial biosphere may have played a key role in regulating the $pCO_2$. At the late Pleistocene, the Termination V (TV ~430 ka) is characterized by a minimum in eccentricity together with unprecedented increases in sea level, Antarctic air temperature and $pCO_2$. It is also associated with a 20% increase of global primary productivity compared to other terminations (Brandon et al., 2020; Yang et al., 2022) as well as the likely development of the temperate and boreal forests, based on a pollen database (Hes et al., 2021). We present here an updated version of the compilation of terrestrial biosphere records, including new pollen and $\delta^{13}$Cwax records, qualitatively interpreted as indirect marker of terrestrial primary productivity changes. In addition, we performed a transient simulation between 460 and 350 ka, bracketing MIS12-10 and hence TV, with the coupled model of intermediate complexity iLOVECLIM to investigate vegetation patterns. Results show a strong increase of 60 PgC in terrestrial productivity during TV, mainly due to the development in the northern hemisphere of mid to high latitudes shrub and temperate forests. These biomes suggest warmer and wetter conditions during the MIS11, compared to MIS 12, together with an ice-sheet decrease of 25%. One cannot exclude that the low eccentricity context that characterizes this period also allows for conducive conditions for a wider vegetation cover. All together, these results indicate a higher terrestrial productivity by ~30% during the TV, supporting the hypothesis of a larger contribution of the terrestrial biosphere to the global primary productivity at that time (Yang et al., 2022).

<footnote n="1">¹ Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>

<footnote n="2">² Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>
Réponse de la végétation méditerranéenne aux
cycles climatiques millénaires d'après un nouvel
enregistrement pollinique marin du golfe du Lion

Tiffanie Fourcade <fnref n="1" />,<fnref n="2" />, Maria Fernanda Sanchez Goñi <fnref n="2" />

Les archives polliniques provenant de la marge de l'Europe occidentale indiquent que la végétation régionale oscillait entre forêt et steppe au cours de la dernière période glaciaire en réponse à la variabilité climatique multi-centennale, en particulier les cycles de Dansgaard-Oeschger (D-O) et les événements de Heinrich (HE). Cependant, la dynamique de la végétation dans le nord-ouest de la région méditerranéenne pendant cette période reste mal comprise, en raison de la nature fragmentaire des séquences disponibles.

Dans cette étude, nous présentons un nouvel enregistrement pollinique marin, provenant du golfe du Lion (MD99-2343, 40° 29′N, 4° 01′E), documentant la réponse de la végétation du nord-ouest de la Méditerranée pendant les stades isotopiques marins (MIS) 4 à 2 (~73 – 14 kyrs).

Les premiers résultats révèlent que l'expansion forestière, colonisant les vallées du Rhône et de l'Èbre, en réponse aux cycles de D-Os, est modulée par la précession, comme précédemment observé dans les enregistrements polliniques situés au sud de 40 °N sur la marge de l'Europe occidentale. Dans ces régions, les HEs sont généralement associés à une expansion des steppes. Cependant, l'enregistrement de la carotte MD99-2343 met en évidence un scénario différent, où les couvertures forestières ouvertes dominent pendant les HE 6 et 2. Nous proposons que des processus atmosphériques et marins locaux spécifiques, corrélés à la quantité d'eau douce dans le golfe du Lion, ont favorisé le développement d'une forêt dans cette région pendant les événements HE 6 et 2. Ces résultats soulèvent de nouvelles questions quant à la complexité des interactions climatiques et environnementales dans le nord-ouest de la Méditerranée durant les périodes glaciaires.

<footnote n="1">1 Archéosciences-Bordeaux : Matériaux, temps, Images et Sociétés (Archéosciences Bordeaux) — CNRS, Université Bordeaux Montaigne — Maison de l'Archéologie, 33607 Pessac, France</footnote>

<footnote n="2">2 Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>
# Dynamique d'accumulation des tourbes et quantification des stocks de carbone dans les tourbières alluviales holocènes du bassin de la Somme

Chloé Garcia<fnref n="1" />, Pierre Antoine<fnref n="2" />, Emmanuel Kramarczyk<fnref n="1" />, Boris Brasseur<fnref n="1" />

A l'échelle mondiale les écosystèmes tourbeux constituent d'importants stocks de carbone (1/3 du C des sols pour 3% de la surface continentale). Formée principalement de débris végétaux fossiles, la tourbe possède un important potentiel de reconstruction paléoenvironnementale (pollens, macrofossiles) et de stockage de C organique. La formation et la conservation de la tourbe sont dépendantes de la disponibilité en eau et de l'évolution de la dynamique fluviale : périodes d'incision ou de crues, variabilité des apports détritiques. Dans le contexte actuel du changement global, les tourbières font face à différents changements susceptibles de perturber leur fonctionnement et donc la dynamique de stockage du C : drainage, extraction de la tourbe, pollution par les engrais et pesticides. Ces perturbations peuvent alors en faire des sources émettrices de carbone sous l'effet de la dégradation de la matière organique. Les tourbières " actives " sont de plus en plus citées comme des espaces naturels à protéger et restaurer grâce à leur capacité à séquestrer du C. Les enjeux de conservation de la tourbe dite " fossile " (ne se formant plus) accumulée depuis plusieurs millénaires sur des épaisseurs parfois importantes sont également de forts : baisse des niveaux de nappe, artificialisation du sol, etc. En Europe de l'Ouest, le bassin de la Somme recèle une des tourbières alluviales des plus étendues (20 à 30 000 ha), au sein de laquelle une grande quantité de C a été capté depuis le début de l'Holocène sous la forme d'un épais colmatage tourbeux (4 m en moyenne). La dynamique de recherche mise en place depuis 2020 nous permet actuellement de disposer d'une reconstitution précise des variations de la dynamique des tourbières et du système fluviatile du bassin la Somme en relation avec les modifications climatiques et l'impact anthropique depuis plus de 11 ka. L'enjeu des nouvelles recherches en cours (FenSom), est donc pour la première fois de quantifier le volume de tourbe fossile et de C stockés, sur la base de données morphostratigraphiques. Les contours de ce système tourbeux n'étaient jusqu'à maintenant pas cartographiés et les épaisseurs précises reconnues seulement ponctuellement. Le premier objectif est de délimiter le système fossile à l'échelle du bassin en utilisant une base de données SIG composée de 750 points de sondages dont 310 localisés sur 10 transects stratigraphiques détaillés. Le volume et la teneur en C des grands ensembles stratigraphiques identifiés sera ensuite modélisée sur la base des caractéristiques sédimentaires et géochimiques précédent collectées. Le volume de tourbe extraite pour son utilisation en tant que combustible sera évalué à partir des surfaces en eau, cicatrices d'extraction, et des profondeurs d'extraction moyenne. Ce volume sera alors retiré afin d'estimer le volume de tourbe et de C perdu. Les résultats seront livrés aux gestionnaires des espaces naturels et des politiques publiques territoriales afin de les sensibiliser aux questions liées au stockage de carbone dans la tourbe, et à la nécessité de favoriser leur conservation.

<footnote n="1">1 Écologie et dynamique des systèmes anthropisés (EDYSAN) — Université Picardie Jules-Verne Amiens, CNRS — 80037 Amiens Cedex 1, France</footnote>

<footnote n="2">2 Laboratoire de géographie physique : Environnements Quaternaires et Actuels (LGP) – CNRS, Université Panthéon-Sorbonne, Université Paris-Est Créteil Val-de-Marne – 94320 Thiais, France</footnote>
# The return of permafrost in Última Esperanza (Patagonia, Chile) during the Antarctic Cold Reversal (12-15 ka)

Igor Girault ¹, Dominique Todisco ², Joël Rodet ³, Carole Nehme ², Isabelle Couchoud ⁴, Fabiana Martin ⁵, Luis Borrero ⁶, Damase Mouralis ²

1 Histoire Naturelle de l'Homme Préhistorique (HNHP) — CNRS, Université de Perpignan via Domitia, MNHN, IRD — 75013 Paris, France

2 Identité et Différenciation de l'Espace, de l'Environnement et des Sociétés (IDEES) — Université Caen Normandie, Université Rouen, Université Le Havre, CNRS — 76821 Mont-Saint-Aignan Cedex, France

3 Morphodynamique Continentale et Côtière (M2C) — Université de Caen Normandie, Université Rouen, CNRS — 14000 Caen, France

4 Environnements, Dynamiques et Territoires de la Montagne (EDYTEM) — CNRS, Université Savoie Chambéry, Ministère de la Culture — 73376 Le Bourget du Lac Cedex, France

5 Centro de Estudios del Hombre Austral — Universidad Magallanes —Instituto de la Patagonia, Punta Arenas, Chili

6 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) – Avda. Rivadavia, Buenos Aires, Argentine

We document evidence of periglacial activity during the Antarctic Cold Reversal (ACR) in several caves located in Última Esperanza (Patagonia, Chile) based on new sedimentary records and geochronological data. The ACR is an episode of climate cooling that took place in the Southern Hemisphere between ca. 15 and 12 ka BP. It is marked in Patagonia by paleovegetation records dominated by steppe pollen assemblages, glacier readvances, as well as ice-wedge casts indicative of permafrost activity, previously reported in Santa Cruz, Argentina. Recent archaeological and paleontological excavations in Cueva del Milodón (Última Esperanza, Chile) have brought to light a one-metre-deep wedge penetrating a layer of ground sloth dung. This structure is interpreted as an ice-wedge cast based on morphological evidence such as downward arcuate cast strata indicative of slumping following ice thawing. Previous radiocarbon dating of bones and new radiocarbon dating of coprolites from both host and cast strata bracket the age of ice wedge development between ca. 11 and 13 ka cal. BP. In the neighbouring Cueva del Medio and Cueva Chica, recent excavations and new analyses of sediment thin sections also document macroscopic and microscopic periglacial features, including soliflucted deposits, indicative of seasonal ground freezing before and during the ACR. In Cueva Chica, calcite flowstone yields U-Th ages of 11.1 1 and 14.3 2 ka. Considering that the bedrock was unfrozen at that time, these results suggest a brief return of permafrost during the ACR rather than a continuous presence through the last glacial termination. The return of permafrost in Última Esperanza seemingly coincides with the arrival of the first human population in Última Esperanza, considering that the oldest evidence of cave occupation dates back 12.5-13.5 ka cal. BP in Cueva del Medio, and slightly precedes the extinction of most of megafauna taxa, occurring between 11 and 13 ka cal. BP.
Organic Carbon Fluxes on Seasonal to Decennial
Timescales in Patagonia's largest Fjord-River System

Sophie Hage <fnref n="1" />, Sébastien Bertrand <fnref n="1" />,<fnref n="2" />, Benjamin Amann <fnref n="3" />, Brian Reid <fnref n="4" />,
Elke Vandekerkhove <fnref n="2" />

Fjord sediments are recognized as globally significant sinks of organic carbon (OC), contributing to the long-term storage of atmospheric CO2. Despite this recognition, the transfer and burial of OC in fjord sediments are still poorly quantified and suffer from a sampling bias towards fjord distal environments where marine OC is dominant. Here we present organic geochemical data obtained on a series of sediment samples (river sediments, sediment trap, sediment core) collected in the Baker fjord-river system, with a particular focus on the submarine Baker River delta, which is fed by Chile's largest river. We measured total OC contents and OC stable isotope composition to quantify the amount and type of OC (marine or terrestrial) stored into the fjord submarine delta. We find that OC fluxes are twice higher in summer (106 g OC/m<fnref n="2" />/yr) than in winter (53 g OC/m<fnref n="2" />/yr) due to higher sediment discharge from meltwater. Sediment trap OC fluxes are on the same order of magnitude than those in the nearby sediment core (103 g OC/m<fnref n="2" />/yr) during the last 35 years, suggesting no OC degradation in fjord sediments. In addition, carbon isotope data suggest that the OC stored into the fjord submarine delta is entirely of terrestrial origin. We estimate that the Baker submarine delta buries 3.8 kt OC/yr, which corresponds to 35% of the estimated OC annual flux delivered by the Baker River (~10.8 ± 1.8 kt OC/yr). Fjord head deltas should thus be taken into account in fjord OC budgets as they could raise global estimates of terrestrial OC burial in marine sediments.

<footnote n="1">¹ Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">² Renard Centre of Marine Geology (RCMG) – Gent, Belgique</footnote>

<footnote n="3">³ Littoral, Environnement et Sociétés (LIENSs) — CNRS, La Rochelle Université – Technoforum, 17031 La Rochelle, France</footnote>

<footnote n="4">⁴ Centro de Investigación en Ecosistemas de la Patagonia (CIEP) – Coyhaique, Chili</footnote>
Evolution du réseau fluviatile et de la sédimentation
alluviale de la Haute Seine à l'Holocène : forçages
climatiques et anthropiques

Grimaud Jean-Louis <fnref n="1" />, Patrick Gouge <fnref n="2" />, Damien Huyghe <fnref n="1" />, Joseph Fournier <fnref n="1" />, Inès Gabert <fnref n="1" />, Christophe Petit <fnref n="4" />

Les plaines alluviales constituent des zones humides qui permettent le stockage du carbone. Cependant, le potentiel séquestration de carbone dépend de l'espace de stockage disponible, qui résulte de la sédimentation terrigène et de la dynamique fluviatile. Ces dynamiques sont en partie liées à l'évolution du climat. En se basant sur l'analyse des chenaux abandonnés mais préservés, l'évolution fluviatile dans la plaine alluviale de la Bassée, située entre la confluence de la Sein avec l'Aube et celle avec l'Yonne, est reconstituées durant l'Holocène Une cinquantaine de transects carottés a été réalisée et complétée par environ 70 analyses du radiocarbone afin de dater l'abandon des chenaux et calculer les taux de sédimentation dans les bras morts de la Seine. La réalisation de coupes géologiques à partir de ces carottages dans les chenaux a permis d'analyser la dynamique de leur comblement et de proposer une restitution de la taille de ces bras morts lorsqu'ils étaient encore actifs. En complément, les mesures des largeurs de plein bord et de rayon de courbures de ces bras ont été effectuées en s'appuyant sur des données topographiques de haute résolution. Elles permettent de proposer des estimations de paléo-débits de la Seine. À ces analyses, on a ajouté des données provenant de la plaine alluviale voisine du Loing et des diagrammes polliniques issus de la littérature, afin de contraindre plus en détail l'évolution paléoenvironnementale régionale. Les résultats obtenus confortent, mais complètent également, les travaux existants. Ainsi après le Tardiglacaire, période durant laquelle le système fluviatile est interprété comme étant en tresse, on observe le développement vers le milieu de l'Holocène de grands chenaux méandriformes, dont les débits pouvaient être jusqu'à 10 à 20 fois supérieurs à l'actuel. Les débits estimés ont ensuite diminué en allant vers le présent. On observe également la concomitance vers 4500 BP de l'essor de l'Aulne dans les diagrammes polliniques et de la transition vers un système d'anabranche. Ce changement, synchrone de la mise en place de marais en vallée du Loing, correspondrait à un épisode climatique plus humide qui aurait provoqué une remontée du niveau des nappes phréatiques. Le fonctionnement de la Seine d'alors aurait donc été proche de celui d'une plaine deltaïque. Cette période semble avoir été favorable au stockage de carbone dans les fonds alluviaux de la région. Enfin, on constate l'augmentation des taux de sédimentation depuis la fin du Mésolithique avec la multiplication des indices d'activités agro-pastorales, ce qui suppose un impact anthropique sur l'érosion qui aurait provoqué une hausse de la charge sédimentaire terrigène vers les fonds de vallée.

<footnote n="1">1 Centre de Géosciences (GEOSCIENCE) — ENSM Paris, Université Paris sciences et lettres — 77300 Fontainebleau</footnote>

<footnote n="2">2 Service départemental d'archéologie de Seine-et-Marne - Centre Départemental Archéologie de la Bassée - 77118 Bazoches-lès-Bray, France</footnote>

<footnote n="4">4 Archéologies et Sciences de l'Antiquité (ArScAn) — CNRS, Ministère de la Culture, Université Paris 1 Panthéon-Sorbonne, Université Paris Nanterre, Université Vincennes-Saint Denis, INRAP — Maison Archéologie & Ethnologie, 92000 Nanterre, France</footnote>
# Integrating High-Resolution Ice Core and Marine Sediment Data to Resolve Chronological Incoherence Across Termination VIII

Anna Klüssendorf<fnref n="1" />, Ellyn Auriol<fnref n="1" />, Marie Bouchet<fnref n="1" />, Mathieu Casado<fnref n="1" />,
Héloïse Guilluy<fnref n="2" />, Frédéric Parrenin<fnref n="2" />, Émilie Capron<fnref n="2" />, Elisabeth Michel<fnref n="1" />,
Frédéric Prié<fnref n="1" />, Grégory Teste<fnref n="2" />, Soizic Salaun<fnref n="1" />, Amaelle Landais<fnref n="1" />

Past climate and environmental changes can be reconstructed from palaeoclimate archives including marine sediment and polar ice cores. Understanding mechanisms associated with major climate changes requires an accurate and precise chronology of each archive and the synchronisation of these individual chronologies to a common multi-proxy timescale. However, discrepancies between the individual chronologies can lead to misinterpretation of the phase relationships and the climate dynamics. Across Termination VIII (~720,000 years before present), the increase in atmospheric CO₂ concentration detected in ice cores is found to lag behind the sea level rise recorded in marine sediment cores according to their respective age scales. This strongly contradicts the established understanding of the climate mechanisms during deglaciations based on observations over the seven most recent terminations, suggesting a mismatch between the site-specific chronologies.

In this study, we explore the incoherence within and between the AICC2023 ice core chronology and modelled LR04 age scale for the marine sediment cores for the period between 600,000 to 800,000 years ago utilising new high-resolution data (~700 years on average instead of > 1000 years on average in the previous chronology reconstruction) from the deepest 200 meters of the EPICA Dome C (EDC) ice core. Modified by solar insolation, the elemental and isotopic compositions of atmospheric O₂ and N₂ measured in the air bubbles enclosed in the ice matrix are essential for orbital dating of deep ice cores. By analysing the new records of δ¹⁸O of O₂ and δ(O₂/N₂) we revise the existing depth-age relationship for the EDC ice core on its deepest section. Further, using the statistical dating tool Paleochrono-1.1 we integrate chronological information from the ice core and a continuous high-resolution stable oxygen isotope record of planktic and benthic foraminifera over this period to link the ice chronology to marine sediment cores. This enabled us to present an improved and coherent timescale and to reconceive the CO₂ and sea level scenario over Termination VIII.

<footnote n="1">¹ Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="2">² Institut des Géosciences de l'Environnement (IGE) — Institut polytechnique de Grenoble, CNRS, IRD, Grenoble Alpes, INRAE — 38400 Saint-Martin d'Hères, France</footnote>
Unexpected wet and warm conditions during glacial
periods in the Central North Atlantic as recorded by
paleosols from Flores Island (Azores)

Laura Loïodice <fnref n="1" />, Francisco Hevia-Cruz <fnref n="2" />, Anthony Hildenbrand <fnref n="1" />,
Nathan D. Sheldon <fnref n="3" />

Major climatic variations (e.g., during glacial-interglacial transitions) induce local effects such as
temperature and precipitation changes. In turn, these local changes can significantly impact the chemical
and physical degradation of volcanic islands, affecting the evolution of volcanic edifices (1). The Azores
Archipelago, located in the Central North Atlantic region, is particularly sensitive to climate changes due
to its position influenced by regional climatic drivers such as the North Atlantic Oscillation atmospheric
system and the oceanic North Atlantic Gyre. Paleosols (PSs) are key targets for studying the paleoclimate
of this region, as they record the atmospheric conditions during their formation. Recent work (2) on PSs
spanning the past 1 Myr in the Central and Eastern Azores showed pulses of soil formation during wet
and warm interglacial stages. Flores Island, located in the westernmost part of the archipelago, is the
perfect target to further document paleoclimate reconstructions at the Regional scale (3) (~600 km
separation between Western and Eastern Azores).

In this work, K-Ar geochronology of volcanic units under and overlying PSs was used to precisely constrain their mean ages and formation times. This was complemented with paleoclimatic proxies based on the PSs' whole-rock geochemistry, which allowed us to achieve reconstructions of Mean Annual Precipitation (MAP) and Mean Annual Air Temperature (MAAT) (4) during the PS formation time. Our results show two groups of PS ages (550 ka and 633-679 ka), that were formed during glacial periods, in contrast with PSs in other Azores islands dominantly formed in interglacial stages (2). PS profiles in Flores are depleted in alkali (K, Na) and alkaline earth (Ca, Mg) elements due to leaching by weathering, and enriched in Al and Fe, showing a gradual increase of weathering indexes for most PSs. Our reconstructions of mean atmospheric conditions show that mild and wet conditions prevailed in Flores, with MAAT and MAP reaching 21.5°C and 1339 mm yr⁻<fnref n="1" />, respectively. These conditions are relatively hotter and drier than current mean annual conditions (17 C and 1716 mm yr⁻<fnref n="1" />) (5). However, they show wetter/warmer conditions than those reached around interglacial peaks (MIS20, MIS19, MIS11, MIS09, and MIS05) in the Central and the Eastern Azores.

These data are consistent with current paleoclimate conditions in the whole Archipelago. As PS ages
between Flores and other Azores islands are not overlapped, these new data could indicate (1) persistent
wet and warm local paleo-conditions in Flores due to its position farther to the north compared to the
Central and Eastern Azores, and consequently less impacted by the North Atlantic Oscillation (Azores
High) and westerlies' direction; or (2) a regional warm and wet climate around 550 ka and 650 ka that
remains to be investigated in the other parts of the Archipelago and in the Atlantic region at a broader
scale (e.g., the Canary volcanic archipelago). Further investigations of PSs could improve the temporal

<footnote n="1">1 Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">2 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="3">³ Department of Earth and Environmental Sciences — University of Michigan— USA</footnote>
and spatial resolutions of paleoclimatic data in the Azores, and consequently our understanding of the
local terrestrial response to global climatic forcing.

(1) Boulesteix, et al. (2012). *Geomorphology*, **138**, 61–73 (2) Hevia-Cruz, et al. (2024). *Paleoceanogr. Paleoclimatol., 39* (5), e2023PA004810 (3) Hildenbrand, et al. (2018). *Sci. rep.*, **8*, 13898 (4) Sheldon, N. D. & Tabor, N. (2009). *Earth-Sci. Rev.*, **95**(1-2), 1-52 (5) *Observatório do Ambiente dos Açores, OAA*.
A new insight of the MIS 3 Dansgaard-Oeschger
climate oscillations in western Europe from the study
of a Belgium isotopically equilibrated speleothem

Marion Peral <fnref n="1" />, Marta Marchegiano <fnref n="2" />, Sophie Verheyden <fnref n="3" />, Steven Goderis <fnref n="4" />,
Tiffanie Fourcade <fnref n="5" />,<fnref n="1" />, Van Helden Tom <fnref n="6" />, Frank Vanhaecke <fnref n="6" />,
Thibaut Van Acker <fnref n="6" />, Hai Cheng <fnref n="7" />, Jens Fiebig <fnref n="8" />, Philippe Claeys <fnref n="4" />

The Marine Isotope Stage (MIS) 3 records abrupt transitions from cold stadial to temperate interstadial climate conditions, termed Dansgaard-Oeschger (DO) events. Reconstructing these rapid climate changes is crucial for documenting the prevailing climatic conditions in Europe during the extinction of the Neanderthals. However, only few continental records are available to define the continental climatic responses to DO changes. Here, the elemental and stable isotope compositions of a well-dated speleothem in Belgium covering the MIS 3 are documented. This speleothem precipitated under equilibrium conditions based on $\Delta_{48}$ thermometry, allowing the use of $\Delta_{47}$ thermometry with confidence. The acquired unique thermometry paleoclimatic dataset enables the reconstruction of temperature and the hydrological information (oxygen-18 of drip water; $\delta^{18}$Ow) to shed new light on the DO climate variations. A temperature differential of ~7°C is associated with alternating temperate warm and wet Interstadials to cold and dry stadials. The DO-12 is the most pronounced MIS 3 interstadial in the record and appears to be marked by a delay of 1000 years between climate enhancement (warmer temperature) and water availability (moisture increase). By combining our speleothem record with other continental archives, the spatial variability of DO changes in Europe during the MIS 3 is defined. A gradual climate deterioration with colder and drier conditions, associated with the Heinrich 4 event, progressed southwards through Europe. This spatial climatic degradation, during the last phase of Neanderthal populations occupation in Europe, provides better environmental constraints for human mobility models.

<footnote n="1">¹ Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>

<footnote n="2">² Department of Earth Sciences, University of Geneva – Rue des Maraîchers 13, 1205 Geneva, Suisse</footnote>

<footnote n="3">³ Royal Belgian Institute of Natural Sciences (RBINS) – Rue Vautier 29, 1000 Bruxelles, Belgique</footnote>

<footnote n="4">⁴ Vrije Universiteit [Brussel] - Belgique</footnote>

<footnote n="5">⁵ Archéosciences-Bordeaux : Matériaux, temps, Images et Sociétés (Archéosciences Bordeaux) — CNRS, Université Bordeaux Montaigne — Maison de l'Archéologie, 33607 Pessac, France</footnote>

<footnote n="6">⁶ Department of Chemistry, Atomic and Mass Spectrometry – Ghent University – Campus Sterre, Krijgslaan 281-S12, BE-9000 Ghent, Belgique</footnote>

<footnote n="7">⁷ Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an, China – Chine</footnote>

<footnote n="8">⁸ Goethe-Universität – Allemagne</footnote>
Carbonate counter pump dynamic over the past
800 000 ka in the Southern Ocean and its impact on
atmospheric pCO₂

Nicolas Pige<fnref n="1" />, Margaux Brandon <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Yu Wang<fnref n="1" />, Stephanie Duchamp-Alphonse <fnref n="1" />, Amaëlle Landais <fnref n="2" />, Sophie Sépulcre <fnref n="1" />, Elisabeth Michel <fnref n="2" />

During the glacial-interglacial cycles of the past 800 000 years (800 ka), atmospheric CO₂ concentrations (pCO₂) changed with amplitudes of 50-100 ppm, the fastest increases being recorded during glacial terminations. Biological productivity in the Southern Ocean (SO) plays an important role in such pCO₂ variations, impacting the ocean-atmosphere exchanges of CO₂ through changes in the Biological Carbon Pump strength that includes the Soft Tissue Pump (i.e., the net downward flux of phytoplanktonic carbon that draws down pCO₂) and the Carbonate Counter Pump (CCP i.e., the export of planktonic carbonate that influences the surface-to-depth alkalinity gradient and increases pCO₂).

Existing paleoclimate studies usually focus on the Soft Tissue Pump and demonstrate that its weakening in the Southern Ocean (Subantarctic Zone) may contribute to the increase in pCO₂ during glacial terminations. Not much is known about the CCP due to the lack of high-resolution studies in the SO based on both, micropaleontological and geochemical approaches.

Recently, using new data and previously published records from site MD04-2718° (48°53,31S, 65°57,42E), retrieved in the Indian sector of the Subantarctic Zone, Brandon et al., (2022) demonstrated that the exceptional 30 ka-plateau of pCO₂ that defines MIS 11 might be partly attributed to a reinforced CCP. Indeed, the calcite production and export by coccolithophores and planktonic foraminifera drastically increased. Such pattern might be the result of the elevated sea surface temperature that characterize MIS 11 together with the increased nutrient availability that is associated to the reinvigoration of the Southern Ocean upwelling at that time. The low eccentricity signal recorded during MIS 11 could have additionally strengthened the CCP, exerting a specific control on *Gephyrocapsa* morphotypes.

Consequently, we aim to reconstruct changes in CCP strength over the past 800 ka and understand their impact on pCO₂. This study is part of the BIOCOD ANR project (2023-2026). Following the strategy used in Brandon et al., (2022), we will perform micropaleontological (coccolith, foraminifera) and geochemical (TOC, Ca XRF, δ¹³C *N. pachyderma*) analyses on the sediment core MD97-2115 (43°10,84S, 171°48,55W), which was retrieved in the Pacific sector of the Subantarctic Zone and records the past 800 ka. The study of coccoliths will be performed using the SYstème de Reconnaissance Automatique de COccolithes (SYRACO) that allows for precise morphometric measurements and reconstruction of coccolith abundances.

<footnote n="1">1 Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">2 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="3">³ Royal Netherlands Institute for Sea Research (NIOZ) – 1790 AB Den Burg, Texel, Pays-Bas</footnote>
# A new sea-surface temperature synthesis over the MIS 9 (300 - 350 ka): temperature changes under high natural CO₂ concentrations

Nathan Stevenard<fnref n="1" />, Émilie Capron<fnref n="1" />, Etienne Legrain <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Claire Coutelle<fnref n="1" />

Les périodes chaudes (interglaciaires) du Quaternaire offrent une série de laboratoires naturels qui permettent d'étudier les processus au sein du système Terre pour un large éventail de conditions chaudes, comparables à aujourd'hui ou légèrement plus chaudes (de l'ordre de ~1°C). Dans cette optique, la communauté scientifique s'est d'abord focalisée sur l'étude du dernier Interglaciaire (MIS 5e), présentant des températures et un niveau marin supérieurs à la période préindustrielle (PI). Ces études ont permis une meilleure compréhension des changements climatiques se déroulant dans un environnement " chaud " et une évaluation des modèles du climat. Un autre interglaciaire, le MIS 9e (320 - 334 ka), montre les valeurs de Gaz à Effet de Serre (GES) les plus élevées des derniers 800 ka (excluant la période impactée par les rejets d'origine anthropiques). Cependant, les enregistrements de températures à la surface du globe disponibles pour cette période présentent un manque d'homogénéisation dues principalement aux différences entre les traceurs, calibrations et chronologies utilisées. Ces difficultés ne permettent ni d'avoir une vision globale des changements climatiques ayant lieu durant cette période, ni de comparer ces données aux simulations climatiques réalisées sous un forçage naturel de fortes concentrations en GES.

Dans cette étude, nous avons compilé 94 enregistrements de températures de surface de la mer (sea-surface temperature, SST) distribués dans les divers océans du globe et couvrant la période allant de 300 à 350 ka. Nous avons d'abord produit une chronologie commune pour l'ensemble des enregistrements sélectionnés. Ce cadre temporel commun repose sur la dernière chronologie de référence AICC2023 des carottes de glace antarctiques et tient compte des différentes sources d'incertitudes (alignement, erreur de la référence, erreur absolue AICC2023) à partir d'une approche bayésienne. Les quatre différents traceurs sélectionnés pour cette étude (UK'37, MAT, δ¹⁸O et Mg/Ca) sont convertis en SST en utilisant les calibrations bayésiennes les plus récentes, permettant une meilleure valorisation des incertitudes associées à chaque traceur. Ces approches ont permis de produire des courbes d'évolution des températures océaniques de surface (globales et régionales) de façon plus précise, exploitant l'ensemble des erreurs induites par ce type de synthèse. Les résultats montrent des températures océaniques de surface globales inférieures de ~0.5°C au PI, alors que les taux de CO₂ atmosphérique sont supérieurs. Cependant, une grande hétérogénéité est observée entre les différents bassins océaniques, aussi bien d'un point de vue des valeurs de températures que des phases de variations observées. Par exemple : le pic interglaciaire est atteint vers 338 ka dans l'Atlantique Sud, alors que le bassin Nord-Pacifique montre ses températures les plus faibles de la période étudiée.

Cette synthèse représente une nouvelle base de données de référence pour des comparaisons modèles-données et ainsi permet d'estimer la capacité des modèles à simuler un climat caractérisé par un fort forçage de GES. Elle permettra également une comparaison avec les autres interglaciaires et ainsi améliorer notre compréhension du système climatique dans des environnements " chauds " naturels.

<footnote n="1">¹ Institut des Géosciences de l'Environnement (IGE) – Institut polytechnique de Grenoble, CNRS, IRD, Grenoble Alpes, INRAE – 38400 Saint-Martin d'Hères, France</footnote>

<footnote n="2">2 Laboratoire de Glaciologie – Université Libre de Bruxelles — 50 Avenue F.D. Roosevelt, Bruxelles, Belgique</footnote>

<footnote n="3">³ Vrije Universiteit Brussel (VUB) – Pleinlaan 2, Brussel, Belgique</footnote>
# Sans limite mais en transition : caractérisation
# chronostratigraphique du passage Pléniglaciaire
# moyen – Pléniglaciaire supérieur dans les læss
# Weichselien du Nord de la France

Salomé Vercelot <fnref n="1" />, Pierre Antoine <fnref n="1" />, David Hérisson <fnref n="2" />, Olivier Moine <fnref n="1" />

Les séquences læss-paléosols constituent un support fondamental pour la reconstitution des environnements paléoclimatiques, essentiellement durant le Dernier glaciaire (Weichselien). Dans le nord de la France, ces archives sédimentaires enregistrent des alternances de phases d'accumulation læssique (sous un climat froid et sec) et de développement de sols (intervalles plus doux). Cette alternance répond aux oscillations climatiques cycliques stade-interstade de la dernière période glaciaire et aux changements environnementaux qu'elles engendrent.

Dans ce contexte, la limite entre le Pléniglaciaire moyen (PGM) et le Pléniglaciaire supérieur (PGS) représente un moment clé dans les séquences læss-paléosols. Elle marque en effet un changement radical dans la sédimentation, avec une augmentation importante et rapide du taux de sédimentation des dépôts læssiques au début puis au cours du PGS, qui recouvrent les sols bruns arctiques du PGM. Dans les séquences læssiques régionales, la distinction entre ces deux périodes est généralement définie par la présence d'un horizon de gley de toundra, associé à un vaste réseau de grandes fentes de gel associées au développement du pergélisol dans la majorité des profils à l'échelle régionale.

Sur la base des études des sites de référence de Havrincourt (Somme, France) et Nussloch (haute vallée du Rhin, Allemagne), le dernier horizon du PGM peut être associé aux interstades GI-7 ou GI-6 (~34 cal. kBP) de NGRIP. Cependant les connaissances actuelles sur le moment exact de cette transition sont contraintes par de nombreux phénomènes d'érosion et de cryoturbations, induits par le gel/dégel qui affectent la préservation des dépôts sédimentaires. Ainsi, malgré une augmentation du taux de sédimentation à cette période contemporaine de pics de poussière distincts dans les carottes de glace du Groenland, la résolution des enregistrements sédimentaires reste trop faible et limite la précision des reconstitutions de l'évolution paléoenvironnementale de cette période charnière.

Récemment, l'étude du site d'Haynecourt (Pas-de-Calais) a apporté un nouvel éclairage sur cette transition en permettant l'observation de nouveaux horizons entre le dernier sol du PGM et le premier gley de toundra du PGS. Ces nouvelles unités sont constituées d'un gley de toundra et d'un læss brun-gris clair massif à structure lamellaire à polyédrique fine avec des revêtements orange et présentant des terriers (crotovines) d'une dizaine de centimètres de diamètre. Les descriptions pédostratigraphiques indiquent des conditions plus froides que pendant le PGM, mais plus douces (présence d'activité biologique et faunique) que pendant les phases stadiaires caractérisée un milieu de type désert polaire.

Huit datations ¹⁴C sur granules de calcite de vers de terre ont été réalisées sur ces unités au Laboratoire des Sciences du Climat et de l'Environnement (LSCE). Les résultats montrent un taux de sédimentation

<footnote n="1">1 Laboratoire de géographie physique : Environnements Quaternaires et Actuels (LGP) – CNRS, Université Panthéon-Sorbonne, Université Paris-Est Créteil Val-de-Marne – 94320 Thiais, France</footnote>

<footnote n="2">2 Archéologies et Sciences de l'Antiquité (ArScAn) — CNRS, Ministère de la Culture, Université Paris 1 Panthéon-Sorbonne, Université Paris Nanterre, Université Vincennes-Saint Denis, INRAP — Maison Archéologie & Ethnologie, 92000 Nanterre, France</footnote>
exceptionnellement faible et continu pour la période étudiée. Ces nouvelles données
chronostratigraphiques permettent d'aborder le passage PGM/PGS non plus comme une limite abrupte
mais comme une période de transition allant des interstades GI-10 à GI-5.1 de NGRIP (~41 à 31 cal. kBP).
Impact of the biological carbon pump on
atmospheric pCO₂ over the past 800 000 years

Yu Wang<sup>1</sup>, Stéphanie Duchamp-Alphonse<sup>1</sup>, Sophie Sepulcre<sup>1</sup>, Margaux Brandon<sup>2,1</sup>, Elisabeth Michel<sup>3</sup>, Vikki Lowe<sup>4</sup>, Johan Etourneau<sup>4</sup>, Xavier Crosta<sup>4</sup>, Julius Nouet<sup>1</sup>, Annachiara Bartolini<sup>5</sup>, Amaelle Landais<sup>5</sup>

Atmospheric CO₂ concentration (pCO₂) varied in a cyclic Glacial-Interglacial pattern over the past 800,000 years (800 ka), a phenomenon that is mainly related to ocean carbon cycle changes. One of the major hypotheses invokes marine productivity's role, namely the Biological Carbon Pump (BCP), as the production and downward export of planktonic organic and inorganic carbon act as a sink and source of CO₂ for the atmosphere, respectively. Many evidences hint towards the BCP's role in modulating pCO₂ over the last glacial cycle, but its evolution and impacts on the climate system beyond that period remain elusive. Here, based on micropaleontological, geochemical and Scanning Electron Microscope analyses of the sediment core MD04-2718 (48°53,31S, 65°57,42E), retrieved in the Indian sector of the Subantarctic Zone, together with a compilation of published data, we are able to reconstruct millennial changes in BCP strength over the last 800 ka, and integrate them within a coherent Glacial-Interglacial climate scenario, including pCO₂ changes. We show that the carbonate (CaCO₃%) and organic (TOC%, δ¹³Corg, C/N) fractions of the studied core reflect coccolith and planktonic foraminifera productivity as well as organic matter export productivity respectively, and that the sedimentary TOC/CaCO₃ ratio may be used as a powerful tool to reconstruct changes in BCP strength. We find that increased BCP during glacial periods is related to increased organic carbon export production due to the iron fertilization of the photic zone. Conversely, decreased BCP during interglacial periods reflects enhanced coccolith and planktonic foraminifera export production, probably due to sea-surface temperatures rise and the reinvigoration of the Southern Ocean upwelling that brings nutrient-rich water to the surface. Overall, our results show that strengthened BCP during glacial intervals enhanced the CO₂ sequestration into the deep ocean, while weakened BCP during interglacial periods contributed to the ocean CO₂ leakage.

<footnote n="1">1 Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">2 Royal Netherlands Institute for Sea Research (NIOZ) – Royal Netherlands Institute for Sea Research, Landsdiep 4, 1797 SZ 't Horntje, Texel, The Netherlands, Pays-Bas</footnote>

<footnote n="3">3 Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines – Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="4">4 Environnement, et Paléoenvironnement Océaniques et Continentaux (EPOC) – CNRS, Université de Bordeaux, Bordeaux INP, EPHE Paris, Université Paris sciences et lettres – 33615 Pessac Cedex, France</footnote>

<footnote n="5">5 Centre de recherche en paléontologie Paris (CR2P) — CNRS, MNHN, Sorbonne Université — 75005 Paris, France</footnote>