Littérature scientifique sur le sujet « Biocarbonates »

A method is developed to precisely determine Mg/Ca and Sr/Ca ratios in biocarbonates by inductively coupled plasma atomic emission spectrometry (ICP-AES). This method precision (RSD%) is 0.52% for Mg/Ca and 0.28% for Sr/Ca, respectively. The precision suggests that ICP-AES is satisfactory for supplying good quality Mg/Ca and Sr/Ca data of biocarbonates for paleo-reconstruction. This ICP-AES technique was applied to 51 continuous coral sub-samples, and the results show annually periodical variations in coral Mg/Ca and Sr/Ca ratios, which are consistent with previous findings.

Abstract. Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completely lost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite often transforms into calcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diagenetic alteration processes that they undergo. Several works have recently investigated the hydrothermal alteration of aragonitic hard tissues during short-term experiments at high temperatures (T > 160 ∘C). In this study, we conduct long-term (4 and 6 months) hydrothermal alteration experiments at 80 ∘C using burial-like fluids. We document and evaluate the changes undergone by the outer and inner layers of the shell of the bivalve Arctica islandica, the prismatic and nacreous layers of the hard tissue of the gastropod Haliotis ovina, and the skeleton of the coral Porites sp. combining a variety of analytical tools (X-ray diffraction, thermogravimetry analysis, laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic-alteration-related changes in aragonitic biocarbonate structural hard materials. Furthermore, we unveil that the diagenetic alteration of aragonitic biological hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study, well before any aragonite into calcite transformation takes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed by abiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only subsequently does the transformation of the aragonite into calcite occur. The kinetics of the alteration process is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered for the entire duration of the conducted experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation. The outer and inner shell layers of Arctica islandica are significantly affected by aragonite transformation into calcite. This transformation is extensive for the prismatic shell layer of Haliotis ovina. Our results suggest that the majority of aragonitic fossil archives are overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies.

Abstract. The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale.In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175 °C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons.We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product.At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80 mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.

Les biocarbonates produits par les organismes marins tels que les foraminifères ou les coraux sont des archives importantes pour étudier les environnements et climats du passé. Depuis les travaux de H. Urey en 1947, il est établi que la composition isotopique en oxygène-18 (δ18O) des carbonates reflète à la fois la température de formation et le rapport isotopique (18O/16O) de l'eau de mer. Cette relation, supposément fondée sur l'équilibre thermodynamique entre l'eau et le carbonate, peut cependant être perturbée soit par des paramètres environnementaux, soit par des mécanismes biologiques . Chez certains organismes, comme les coraux, ces « effets vitaux » se manifestent par des déséquilibres isotopiques flagrants, qui compliquent l'interprétation d'une partie non négligeable du registre sédimentaire fossile. En réponse à ces problèmes, cette thèse adopte une approche novatrice en tirant partie de nouveaux traceurs isotopiques, les anomalies d'oxygèen-17 (Δ17O) et les « clumped isotopes » (Δ47, Δ48), en complément des mesures traditionnelles de δ13C et δ18O. En travaillant à partir d'échantillons modernes dont les conditions de croissance sont bien documentées, les contraintes supplémentaires fournies par ces traceurs permettent de mieux caractériser les facteurs qui influençent la composition isotopique des biocarbonates.Ce travail a commencé par une première étape exploratoire, visant d'une part (1) à identifier les organismes / cas d'étude à cibler en priorité, en associant des mesures isotopiques à faible résolution spatiale avec des observations in situ fournissant des informations minéralogiques, élémentaires et isotopiques à bien plus haute résolution, et d'autre part (2) à établir des protocoles expérimentaux optimisés pour les techniques instrumentales de pointe utilisées pour les mesures de Δ47/Δ48 (par spectrométrie de masse à très haute sensibilité) et de Δ17O (par VCOF-CRDS, une technologie spectroscopique innovante). Le premier volet de ce travail a donné lieu à la première l'étude publiée sur les clumped isotopes dans les squelettes de bryozoaires. Les résultats mettent en évidence une forte influence de la minéralogie sur le signal Δ47, qui semble résulter d'un simple effet thermodynamique. Par contre, il apparaît que les organismes de certains sites présentent des déséquilibres isotopiques spécifiques, potentiellement associés à la salinité locale. Ces résultats suscitent de nouvelles interrogations sur l'influence de certains paramètres environnementaux sur l'activité de certaines enzymes, en particulier l'anhydrase carbonique qui joue un rôle majeur dans le maintien de l'équilibre isotopique entre l'eau et le carbone inorganique dissous (CID). Le second volet porte sur la caractérisation, chez les coraux d'eau froide, des écarts à l'équilibre dans cinq dimensions isotopique (δ13C, δ18O, Δ17O, Δ47, Δ48), observées ici conjointement pour la première fois. Les observations sur Δ47 et Δ48 sont en accord avec une publication indépendante récente, et cohérentes avec un modèle théorique du CID, suggérant que la signature isotopique des coraux profonds est principalement contrôlée par des effets cinétiques liés à l'absorption du CO₂ métabolique. Cependant, les prédictions de ce même modèle sont en désaccord avec nos mesures de Δ17O (qui ont été depuis confirmées par des observations indépendantes), soulignant la nécessité de réviser certains paramètres du modèle. La richesse des questions nouvelles suscitées par ce travail de thèse illustre l'intérêt de combiner les observations dans un espace multi-isotopique à 5 dimensions avec des modèles théoriques quantitatifs, tout en confrontant ces modèles aux spécificités biologiques de chaque organisme marin, permettant tout à la fois de mieux comprendre les mécanismes de biominéralisation et de quantifier l'influence des effets vitaux dans les biocarbonates sur les paléo-reconstructions
Biocarbonates produced by marine organisms such as foraminifera or corals serve as important archives for studying past environments and climates. Since the work of H. Urey in 1947, it has been established that the oxygen-18 isotopic composition (δ18O) of carbonates reflects both the formation temperature and the isotopic ratio (18O/16O) of seawater. However, this relationship, supposedly based on the thermodynamic equilibrium between carbonate and water, can be disturbed by either environmental parameters or biological mechanisms. In certain organisms, such as corals, these "vital effects" manifest as significant isotopic disequilibrium, complicating the interpretation of a substantial portion of the fossil sedimentary record.In response to these challenges, this thesis adopts an innovative approach by leveraging new isotopic tracers, namely oxygen-17 anomalies (Δ17O) and "clumped isotopes" (Δ47, Δ48), in addition to traditional δ13C and δ18O measurements. By working with modern samples for which growth conditions are well documented, the additional constraints provided by these tracers allow for a better characterization of the factors influencing the isotopic composition of biocarbonates.This work began with an exploratory phase, aimed at (1) identifying the organisms/case studies by combining low spatial resolution isotopic measurements with in situ observations that provide mineralogical, elemental, and isotopic information at much higher resolution, and (2) establishing optimized experimental protocols for cutting-edge instrumental techniques used for Δ47/Δ48 measurements (via ultra-high sensitivity mass spectrometry) and Δ17O (via VCOF-CRDS, an innovative spectroscopic technology).The first part of this work resulted in the first published study on clumped isotopes in bryozoan skeletons. The results highlight a strong influence of mineralogy on the Δ47 signal, which seems to result from a simple thermodynamic effect. However, it appears that organisms from certain sites exhibit specific isotopic disequilibrium, potentially linked to local salinity. These findings raise new questions about the influence of certain environmental parameters on the activity of specific enzymes, particularly carbonic anhydrase, which plays a key role in maintaining isotopic equilibrium between water and dissolved inorganic carbon (DIC).The second part focuses on characterizing isotopic disequilibria in five isotopic dimensions (δ13C, δ18O, Δ17O, Δ47, Δ48) in cold-water corals, observed together here for the first time. The observations on Δ47 and Δ48 are consistent with a recent independent publication and align with a theoretical DIC model, suggesting that the isotopic signature of deep-sea corals is primarily controlled by kinetic effects related to the absorption of metabolic CO₂. However, the predictions of this same model are at odds with our Δ17O measurements (which have since been confirmed by independent observations), underscoring the need to revise certain model parameters.The richness of new questions raised by this thesis highlights the value of combining observations in a five-dimensional multi-isotopic space with quantitative theoretical models, while confronting these models with the biological specifics of each marine organism. This approach enables a deeper understanding of biomineralization mechanisms and allows for a more precise quantification of the influence of vital effects in biocarbonates on paleo-reconstructions

Cette étude a pour but de documenter le climat à deux périodes clés de l’Holocène à partir d’archives biocarbonatées. Le potentiel du bénitier Tridacna maxima (géochimie et croissance) comme archive paléoclimatique a été montré. Des spécimens fossiles de cette espèce ont ensuite été utilisés pour documenter la variabilité climatique du Pacifique Sud-Ouest durant la transition Holocène inférieur-Holocène moyen (7. 0-6. 0 ka BP) et lors de la migration Lapita en Océanie Lointaine (3. 5-2. 5 ka BP) en complément d’enregistrements issus d’autres bénitiers et d’un corail massif Porites sp. Des enregistrements Sr/Ca de Porites sp. Ont aussi été utilisés comme paléothermomètre. A 7. 0-6. 0 ka BP, la phase de réchauffement postglaciaire était terminée et la marge sud-est de la Warm Pool était dans sa position actuelle. L’influence saisonnière de la SPCZ était plus faible qu’actuellement dans le Pacifique S. O. , montrant que la SPCZ était située au nord de sa position actuelle. La variabilité El Niño Southern Oscillation (ENSO) était réduite de 25-30% par rapport à la variabilité actuelle. La période coincidant avec l’émergence de la culture Lapita, 3. 6-3. 4 ka BP, est caractérisée par un état moyen de type La Niña sur le Pacifique S. O avec une forte variabilité ENSO. Le climat a donc pu agir comme déclencheur et/ou favoriser la migration Lapita à 3. 2-2. 7 ka BP. En effet, des conditions climatiques instables peuvent favoriser le développement et l’expansion de populations nomades dépendantes des ressources marines, comme l’était la population Lapita
This study aims to document the climate for two key periods of the Holocene from biocarbonated archives. The giant clam Tridacna maxima was validated as a paleoclimatic archive. Fossil specimens of this species were used to document the climatic variability in the Southwest Pacific during the early mid-Holocene – 7. 0-6. 0 ka BP – and during the Lapita migration over Remote Oceania – 3. 5-2. 5 ka BP – in addition to d18O records from fossils specimens of giant clams T. Gigas, Hippopus hippopus and fossils massive coral Porites sp. Records of Sr/Ca from Porites sp. Corals also provided paleo-SST records. The post glacial SST rise was completed at 7. 0-6. 0 ka BP and the southeastern edge of the WPWP was located in its current location. At that time the seasonal influence of the SPCZ over the SW Pacific was weaker than it is today as a result of the more northerly location of this structure. The El Niño Southern Oscillation (ENSO) variability was 25-30% weaker than today. The period when the Lapita culture at 3. 6-3. 4 ka BP, was characterized by a La Niña-like climatic mean state in the SW Pacific. The Lapita migration-phase at 3. 2-2. 7 ka BP is coincidental with a strong ENSO variability supporting that the climate had a significant role in the Lapita migration. Indeed, unstable climate may have triggered and/or promoted the development and the expansion of nomad foraging population based on marine resources as the Lapita were