Evidence for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran theropod

The dinosaur–bird transition involved several anatomical, biomechanical, and physiological modifications of the theropod bauplan. Non-avian maniraptoran theropods, such as Troodon, are key to better understand changes in thermophysiology and reproduction occurring during this transition. Here, we applied dual clumped isotope (Δ47 and Δ48) thermometry, a technique that resolves mineralization temperature and other nonthermal information recorded in carbonates, to eggshells from Troodon, modern reptiles, and modern birds. Troodon eggshells show variable temperatures, namely 42 and 29 ± 2 °C, supporting the hypothesis of an endothermic thermophysiology with a heterothermic strategy for this extinct taxon. Dual clumped isotope data also reveal physiological differences in the reproductive systems between Troodon, reptiles, and birds. Troodon and modern reptiles mineralize their eggshells indistinguishable from dual clumped isotope equilibrium, while birds precipitate eggshells characterized by a positive disequilibrium offset in Δ48. Analyses of inorganic calcites suggest that the observed disequilibrium pattern in birds is linked to an amorphous calcium carbonate (ACC) precursor, a carbonate phase known to accelerate eggshell formation in birds. Lack of disequilibrium patterns in reptile and Troodon eggshells implies these vertebrates had not acquired the fast, ACC-based eggshell calcification process characteristic of birds. Observation that Troodon retained a slow reptile-like calcification suggests that it possessed two functional ovaries and was limited in the number of eggs it could produce; thus its large clutches would have been laid by several females. Dual clumped isotope analysis of eggshells of extinct vertebrates sheds light on physiological information otherwise inaccessible in the fossil record

An Isotopologue-Enabled Model (∆₄₇, ∆₄₈) for Describing Thermal Fluid-Carbonate Interaction in Open and Closed Diagenetic Systems

Abstract The geochemical and ultrastructural properties of thermally altered skeletal carbonate are expected to be compromised to varying degrees by disequilibrium processes between solids and the ambient aqueous fluids. When assessing the alteration history of carbonates, it is important to apply models that quantitatively describe these diagenetic processes on multiple geochemical systems, such that they can be identified in natural samples. Carbonate clumped isotope analysis provides a unique tool for validating such models and can be combined with other geochemical tools/proxies to more comprehensively describe the processes and products. Here, we have analyzed bivalve shells that have undergone hydrothermal alteration (experimental diagenesis) in high water/rock ratio experiments at 130 and 160°C, demonstrating that non‐linear changes in ∆47 and ∆48 values can be attributed to heterogeneous replacement of precursor carbonates. Importantly, this model predicts decoupled ∆47 and ∆48 values, despite all reactions occurring at clumped isotope equilibrium with respect to the experimental temperature. We demonstrate that the rapid, thermally induced re‐equilibration occurs in a “closed system” with minimal exchange with the ambient fluid, similar to the results of heating experiments conducted without an extraneous fluid. Later, stages of alteration occur in an “open system” wherein internal fluid is exchanged with the external fluid at a similar rate to recrystallization and neomorphism. In these experiments, some oxygen from the original inorganic‐organic composite‐biomineral is inherited, indicating restrictions on the availability of fluid oxygen. Our experiments and models validate a novel application for dual‐clumped isotopes for reconstructing hydrothermal temperatures and fluid δ18O compositions

Calibration of the dual clumped isotope thermometer for carbonates

International audienceTheD47(paleo)thermometer has opened a new avenue to determine carbonate formation temperatures independent of theoxygen isotopic composition of the fluid from which the carbonate crystallized. A major limitation of this thermometer isrelated to kinetic effects if homogeneous isotopic equilibrium is not attained during carbonate precipitation. Dual clumpedisotope thermometry – the high-precision analysis ofD48along withD47in CO2evolved from phosphoric acid digestion ofcarbonates – makes it possible to resolve temperature from the kinetic information recorded in an individual carbonate phase.Therefore, it provides a new opportunity to identify (bio)mineralization pathways and to determine carbonate formation tem-peratures devoid of a kinetic bias, based solely on isotopic analysis of a single carbonate phase.Identification of the nature and extent of kinetic effects as well as the reconstruction of accurate formation temperaturesrequires knowledge of the position of equilibrium inD47vsD48space. Here, we presentD47andD48data of carbonates thatwere previously considered as having crystallized closest to equilibrium in a temperature range of 8 to 1100°C. Across thisrange, the temperature dependences ofD47andD48are best expressed by the following fourth order polynomials of 1/T:D47(CDES 90) (‰) = 1.038 (5.897 1/T3.521 1031/T2+ 2.391 1071/T33.541 1091/T4) + 0.1856D48(CDES 90) (‰) = 1.028 (6.002 1/T1.299 1041/T2+ 8.996 1061/T37.423 1081/T4) + 0.1245with CDES 90 representing the Carbon Dioxide Equilibrium Scale at a reaction temperature of 90°C. In its entire tem-perature range, ourD47(CDES 90) - T - relationship agrees within 2 ppm with two previousD47(I-CDES) - T - relationshipsreported by Jautzy et al. (2020) and Anderson et al. (2021). Accuracy of our proposedD47(CDES 90)D48(CDES 90)equilibrium relationship is independently confirmed by additional dual clumped isotope data of experimental and geothermalcarbonates which precipitated from potentially equilibrated dissolved inorganic carbon pools at a temperature range of25–100°C. Furthermore, we reprocessed original dual clumped isotope data of natural carbonates (Bajnai et al., 2020)and compared their composition to the position of equilibrium inD47vsD48space. These results corroborate preliminaryevidence that the hydration/hydroxylation reactions became rate-limiting during the calcification of a speleothem-like sample,a warm water coral, a cold water coral and a brachiopod, finally evoking significant departures of carbonate-D47and -D48from dual clumped isotope equilibrium

Dual clumped isotopes from Mid-Eocene bivalve shell reveal a hot and summer wet climate of the Paris Basin

Accurate reconstruction of seasonal atmospheric patterns of the past is essential for reliable prediction of how climate will evolve due to anthropogenic CO2 forcing. The Eocene ‘hot house’ climate, as the warmest epoch during the Cenozoic, is considered as a potential analogue for ‘high-CO2’ future climate scenarios. In this context, the reconstruction of variations in seasonality are as important as changes in mean annual conditions. Here we combine stable oxygen (δ18O) and dual clumped isotope (Δ47 + Δ48) measurements of a bivalve shell to determine sub-annual variations in sea surface temperatures and oceanic freshening in the Paris Basin during the Mid-Eocene Climate Optimum, 40 million years ago. Our reconstruction indicates to high mean annual temperatures with a small seasonal amplitude (33.3 °C ± 4.4 °C) and an enhanced fresh water input during the summer period. Our results implying a substantially warmer climate state with different hydrological conditions for Western Europe during the Eocene than previously suggested by proxy data or climate modelling

Fingerprinting Kinetic Isotope Effects and Diagenetic Exchange Reactions Using Fluid Inclusion and Dual-Clumped Isotope Analysis

Geochemical analyses of carbonate minerals yield multiple parameters which can be used to estimate the temperature and water composition at which they formed. Analysis of fluid trapped in minerals is a potentially powerful tool to reconstruct paleotemperatures as well as diagenetic and hydrothermal processes, as these could represent the parent fluid. Internal fluids play important roles during the alteration of carbonate fossils, lowering energetic barriers associated with resetting of clumped isotopes, as well as mediating the transport of elements during diagenesis. Here, we explore the behavior of the ∆47–∆48 “dual-clumped” isotope thermometer during fluid-carbonate interaction and demonstrate that it is highly sensitive to the water/carbonate ratio, behaving as a linear system in “rock buffered” alteration, and as a decoupled system in water-dominated systems due to non-linear mixing effects in ∆48. Dry heating experiments show that the extrapolated “heated” end-member is indistinguishable from the predicted ∆47 and ∆48 value expected for the experimental temperature. Furthermore, we evaluate two common laboratory sampling methods for their ability to thermally alter samples. We find that the temperature of the commonly used crushing cells used to vapourize water for fluid inclusion δ18O analyses is insufficient to cause fluid-carbonate oxygen isotope exchange, demonstrating its suitability for analyses of fluid inclusions in carbonates. We also find that belemnites sampled with a hand-drill yield significantly warmer paleotemperatures than those sampled with mortar and pestle. We conclude that thermally-driven internal fluid-carbonate exchange occurs indistinguishably from isotopic equilibrium, limited by the extent to which internal water and carbonate can react.ISSN:1525-202

La construcción de la Hacienda hispánica en el largo siglo XVIII: una investigación en curso

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