Auguste Hassler

Rationale: Populations of North American caribou and Eurasian reindeer ( Rangifer tarandus sspp .), essential for Arctic ecosystems and human communities, are experiencing unprecedented declines due to climate change and anthropogenic pressures. While understanding their historical resilience and demographic dynamics is critical for conservation, long‐term population trends and the mechanisms driving these declines remain partly unresolved. Methods: This study explores if calcium (Ca) isotopic compositions (δ ^44/42 Ca) measured by MC‐ICP‐MS could help quantify the number of nursing periods experienced by female reindeer during their lifetimes, as proxy for monitoring fecundity throughout the Quaternary. This pilot study focuses on five reindeer specimens who lived in controlled environmental conditions with documented life histories (including periods spent nursing). Skeletal elements with specific mineralization timings (molars and jawbones) were targeted to help untangle the effect of nursing from confounding factors, such as specimen specific diet and physiological baselines. Results: Despite this sampling approach, no clear effect of nursing was observable in jawbones δ ^44/42 Ca values. We hypothesize that this may be the consequence of differences in experienced nursing activity and isotopic fractionation compared to other cases from the literature or of a faster than expected turnover rate of Ca in Rangifer jawbones. In parallel, insights of age‐related Ca imbalance from an undiagnosed pathology (e.g., osteoporosis) were found in one specimen, while generated data also provide complementary perspectives for investigating seasonal diet and weaning habits in Rangifer . Conclusions: While exploring past fecundity might prove difficult using this approach, these results are promising regarding the study of paleo‐diet and pathology. A path for future methodological improvements to address the shortcomings of our approach is outlined. This work notably underscores the urgent need for better quantifying elemental turnover rates in bones and for more controlled environment studies, two critical steps to develop robust geochemical proxies for wildlife conservation and paleoecology.

Stable calcium (Ca) isotopes are increasingly applied across geosciences, medical sciences, ecology, paleontology, and archaeology. However, the deployment speed of Ca isotope applications worldwide is hampered by three major challenges: 1) the necessity for complex Ca purification procedures prior to analysis; 2) expensive instrumentation (typically TIMS or ICP-MS) requiring specific configurations and fine-tuning to generate reliable data; and 3) the exhaustion of some of the most widely used reference materials for cross-laboratory comparisons. This study presents methodological advances aimed at lifting some of these barriers. First, existing chromatography methods for purifying Ca were refined by developing a branching procedure based on commercially available labware to allow faster method transfer and to minimize resin and reagent consumption for a variety of sample matrices. These adjustments drastically improved strontium (Sr) separation from Ca, including for Sr-rich samples such as seawater. Second, the potential of 10¹³Ω Faraday cup amplifiers for improving Ca isotope measurements was explored. Results show improved precision in ⁴³Ca measurements under low ionic transmission configurations with δ^43/42Ca standard deviation value reduced by half. This expands the list of ICP-MS configurations capable of producing reliable Ca isotope measurements and delineates a path for less sample-destructive methods (i.e., lower Ca analytical requirements). These amplifiers also markedly enhanced the correction of Sr²⁺ interferences typically affecting Ca ion beams. In this configuration, accurate and precise Ca isotopic measurements were obtained without prior Sr removal for Sr/Ca concentration ratios up to 10^-2. Lastly, using these technical advancements, existing and new international certified reference materials (SRM1486, SRM1400, IAPSO, CACB-1, DOLT-5, DORM-5, TORT-3) were analyzed, complementing existing and out-of-stock standards of the Ca isotope toolbox, notably for Ca carbonate and marine soft tissues. Together, these advances open the door of Ca isotope research to more laboratories and pave the way for future developments and applications.

La capacité à produire du lait (i.e. la lactation) est un trait partagé par toutes les espèces de mammifères actuelles. Cette capacité est capitale pour le développement des juvéniles et constitue un facteur déterminant de l’écologie et des stratégies de reproduction des espèces mammaliennes. Toutefois, sur la base des méthodes conventionnelles et des indices fossiles disponibles, il est difficile d’ancrer évolutivement et temporellement l’origine précise de la lactation et de ses fonctions biologiques. Dans la présente thèse, j’ai exploré les possibilités offertes par l’étude des isotopes du calcium (Ca) contenus dans la structure minérale des os et des dents. Plusieurs études publiées en amont de cette thèse suggèrent que la composition en isotopes stables du Ca des tissus minéralisés (44Ca et 42Ca notamment) est susceptible d’être modifiée par la consommation et la production de lait. Par ailleurs, d’autres publications suggèrent que la composition isotopique du Ca de l’émail dentaire est capable de se préserver dans l’émail fossilisé et de fournir des informations sur l’écologie et la physiologie d’espèces disparues. Toutefois, les effets de la consommation et de la production de lait sur ces compositions isotopiques étaient jusqu’ici documentés chez une poignée d’espèces seulement. En outre, des doutes planaient encore sur la responsabilité réelle de la lactation sur certains de ces changements isotopiques. Il était donc nécessaire d’étendre le spectre des mammifères chez qui ces changements isotopiques étaient documentés et d’identifier plus précisément les causes physiologiques derrière ces changements. Pour investiguer ces questions, j’ai réalisé un suivi temporel des compositions isotopiques du Ca organique d’une population de cochons domestiques (Sus scrofa domesticus) durant un cycle de reproduction complet. J’ai ensuite comparé ces données expérimentales avec des données de modélisations permettant d’identifier les facteurs en jeu dans les variations de compositions isotopiques observées expérimentalement. J’ai également étudié les compositions isotopiques du Ca des tissus minéralisés de différentes populations de mammifères actuels comprenant des cerfs élaphes (Cervus elaphus), des souris grises (Mus musculus), des tammar wallabys (Macropus eugenii), des échidnés à nez court (Tachyglossus aculeatus) et des ornithorynques (Ornithorhynchus anatinus), ainsi qu’une population de rennes fossiles (Rangifer tarandus). Les résultats de ces études soulignent le rôle majeur de la lactation dans l’apparition de différences inter-sexes de compositions isotopiques du Ca des tissus minéralisés, et ont permis de documenter l’hétérogénéité de ces différences isotopiques à l’échelle des mammifères placentaires, des marsupiaux et des monotrèmes. Les résultats obtenus au cours de cette thèse permettent des avancées importantes dans la compréhension du cycle isotopique du Ca chez les mammifères, et offrent de nouvelles perspectives pour explorer l’histoire évolutive de la lactation. En outre, ces résultats offrent aussi des perspectives intéressantes dans les domaines du biomédical et de la paléoécologie.

Stable calcium (Ca) isotopes are increasingly applied across geosciences, medical sciences, ecology, paleontology, and archaeology. However, the deployment speed of Ca isotope applications worldwide is hampered by three major challenges: 1) the necessity for complex Ca purification procedures prior to analysis; 2) expensive instrumentation (typically TIMS or ICP-MS) requiring specific configurations and fine-tuning to generate reliable data; and 3) the exhaustion of some of the most widely used reference materials for cross-laboratory comparisons. In this study we present methodological advances aimed at lifting some of these barriers. First, we refined existing chromatography methods for purifying Ca by developing a branching procedure based on commercially available labware to allow faster method transfer and to minimize resin and reagent consumption for a variety of sample matrices. Our adjustments drastically improved strontium (Sr) separation from Ca, including for Sr-rich samples such as seawater. Second, we explored the potential of 10¹³Ω Faraday cup amplifiers for improving Ca isotope measurements. Our results show improved precision in 43Ca measurements under low ionic transmission configurations with δ43/42Ca standard deviation value reduced by half. This expands the list of ICP-MS configurations capable of producing reliable Ca isotope measurements and delineates a path for less sample-destructive methods (i.e., lower Ca analytical requirements). These amplifiers also markedly enhanced the correction of Sr²⁺ interferences typically affecting Ca ion beams. In this configuration, accurate and precise Ca isotopic measurements were obtained without prior Sr removal for Sr/Ca concentration ratios up to 10⁻². Lastly, using these technical advancements we analyzed existing and new international certified reference materials (SRM1486, SRM1400, IAPSO, CACB-1, DOLT-5, DORM-5, TORT-3), complementing existing and out-of-stock standards of the Ca isotope toolbox, notably for Ca carbonate and marine soft tissues. Together, these advances open the door of Ca isotope research to more laboratories and pave the way for future developments and applications.

Climate affects habitat, food availability, and the movement and sustainability of all life. In this work, we apply Indigenous and Western scientific methods, including genomics and isotope profiling, on fossils from across Beringia to explore the effect of climate change on horses. We find that Late Pleistocene horses from Alaska and northern Yukon are related to populations from Eurasia and crossed the Bering land bridge multiple times during the last glacial interval. We also find deeply divergent lineages north and south of the American ice sheets that genetically influenced populations across Beringia and into Eurasia. As climate warmed and horses entered the ice-free corridor connecting Beringia and midcontinental America, restricted mobility and food availability impeded population growth. Our combined Western and Indigenous framework offers critical guidance for wildlife conservation amid ongoing climate change.

Rationale: Understanding the interactions between marine mammals and their environment is critical for ecological and conservation purposes. Odontocetes offer a continuous record of their life history from birth as recorded in annual increments of their tooth dentine. Because dentine is not remodeled and contains collagen, nitrogen stable isotope compositions (δ ¹⁵ N) reflect nursing and weaning events, life history traits that would otherwise be impossible to retrieve in such elusive marine animals. Yet, capturing the magnitude and temporal changes in these events is constrained by tooth size and sampling resolution. Moreover, historical and fossil specimens undergo collagen decay, hence the need to develop the measurements of other proxies. Methods: Here, we present a multiproxy approach to investigate the use of Ca isotope compositions (δ ^44/42 Ca) in relation to δ ¹⁵ N and laser ablation profiles for different trace metal (Ba, Mg, Sr, Zn) concentrations across the dentine of a single individual of the common bottlenose dolphin Tursiops truncatus . Results: To help interpret the dentine data, we provide milk elemental compositions and δ ^44/42 Ca values for two odontocete individuals. We discuss the observed changes in δ ^44/42 Ca across the dentine as potential markers of birth, weaning interval, incidental ingestion of seawater, trophic level and physiology. Incidental ingestion of seawater during nursing induces a positive offset in δ ^44/42 Ca values recorded in the early formed dentine. Conclusions: Life history parameters of individual marine mammals are extremely difficult to retrieve due to limitations in observing specimens in the wild and the methodology presented here offers new ecological and paleoecological perspectives.

Sharks occupy all living environments of the marine realm as well as some freshwater systems. They display varied and flexible feeding behaviours, but understanding their diet remains challenging due to their elusive ecology and the invasiveness of stomach content analyses in regard of their threatened status. As a potential alternative, we discuss the variability in δ^44/42Ca values recorded in the tooth enamel of size‐graded individuals belonging to three species of large sharks with distinct diets (Isurus oxyrinchus, Hexanchus griseus and Carcharodon carcharias). The preliminary results highlight shifts in diet linked to ontogeny (I. oxyrinchus and H. griseus) and spatial distribution (C. carcharias) characterizing feeding behaviour in these species at individual and population level. These outcomes agree with the results of traditional stomach analyses supporting that nontraditional stable isotopes thus represent new perspectives for the study of modern and extinct shark ecology. In addition, for the first time, the Sr/Ca elemental ratios measured in H. griseus reflect sexual differences that could be interpreted in terms of spatial segregation or physiological heterogeneities.

Lactation and gestation are among the physiological events that trigger the most intense changes in body calcium (Ca) fluxes. Along with the composition of the animal 2021 diet, these events are suspected to impact the Ca isotopic composition of Ca body reservoirs but their dynamics are poorly understood. In this study, we monitored a group of domestic sows across a full reproduction cycle. We collected tissues and fluids (blood, urine, milk, colostrum, umbilical blood, adult and piglet bones) at different steps of gestation and lactation, and analyzed their Ca isotopic compositions (i.e. δ^44/42Ca) by means of multi-collector inductively coupled plasma mass spectrometry. Among other results, we report the first observations of Ca isotopic fractionation between maternal and umbilical blood (Δ^44/42Ca_{umbilical blood-sow blood} = −0.18 ± 0.11‰, n = 3). Our data also highlight that gestation and lactation periods are characterized by small diet-bone Ca isotopic offsets (Δ^44/42Ca_bone-diet = −0.28 ± 0.11‰, n = 3), with ⁴⁴Ca-enriched blood compositions during nursing (Δ^44/42Ca_{nursing blood-gestation blood} = |$+ 0.42{\rm{\,\,}}_{ - 0.12}^{ + 0.11}$|‰, n = 3). Under the light of an up-to-date mammalian box model, we explored different scenarios of gestation and lactation Ca fluxes experienced by a sow-like animal. These simulations suggest that gestation changes on body δ^44/42Ca values may result from the intensification of Ca absorption by the animal, whereas the production of ⁴⁴Ca-depleted milk is the main driver for the ⁴⁴Ca enrichment in blood during lactation. In addition, our results also support that bone mineralization could be associated with a more restricted Ca isotopic fractionation than previously envisioned. Together, these results refine the framework of Ca isotope applications, notably regarding the monitoring of human bone balance and the study of species and ecosystems from the present and the past.

Large predators are overabundant in mid-Cretaceous continental dinosaur assemblages of North Africa. Such unbalanced ecosystem structure involves, among predatory dinosaurs, typical abelisaurid or carcharodontosaurid theropods co-occurring with long-snouted spinosaurids of debated ecology. Here, we report calcium (Ca) isotope values from tooth enamel (expressed as δ ^44/42 Ca) to investigate resource partitioning in mid-Cretaceous assemblages from Niger (Gadoufaoua) and Morocco (Kem Kem Beds). In both assemblages, spinosaurids display a distinct isotopic signature, the most negative in our dataset. This distinct taxonomic clustering in Ca isotope values observed between spinosaurids and other predators provides unambiguous evidence for niche partitioning at the top of the trophic chains: spinosaurids foraged on aquatic environments while abelisaurid and carcharodontosaurid theropods relied almost exclusively on terrestrial resources.