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Analysis of growth and form in Aerothyris kerguelenensis (rhynchonelliform brachiopod) - Shell spiral deviations, microstructure, trace element contents and stable isotope ratios

Abstract : Brachiopods are usually attached benthic marine invertebrates with a shell (exoskeleton) that archives modifications of the environment via chemical proxies. Growth lines regularly occurring (i.e. excluding random ones) at the shell's surface reflect phenotypically controlled interruptions or changes in secretion (Williams et al., 1997). Modifications in the secretion rate and reorientation of shell elements within the thickness of the shell induce marked changes in shell morphology providing elements for estimating age throughout ontogeny for specimens of Aerothyris kerguelenensis (Davidson 1880, formerly known as Waldheimia kerguelensis Davidson, 1978), from the Southern Indian Ocean. Maxima and minima of deviations from underlying shell spiral growth (SSD), plus maps (SEM-EDS) and Electron Microprobe (WDS) of Mg, S, Ca, Mn, Fe that are components of shells along with Scanning Electron Microscopy (SEM) all show evidence of rhythms at different time scales, including annual ones. Growth rates differ in young and adult forms as well as between dorsal and ventral valves; additionally the amplitudes of Mg peaks are higher early in ontogeny and in the primary shell layer, progressively decreasing with sporadic peaks. Anterior regions of shells are more informative regarding external growth (i.e. major growth marks) and mainly record later events in the life of the individual. Posterior regions are informative for internal morphology (cumulative growth) and reveal Mg bands and related concentrations that are consistent with the main extrema (maxima and minima) observed when using SSD and WDS analyses. Mg/Ca ratios indicate fluctuations in temperature and food supply during the growth period. Seawater temperatures calculated from the oxygen isotope compositions of the secondary layer of calcite in shells of A. kerguelenensis are close to those measured in situ in the environment in which they live. However, the difference in carbon isotope composition between the two studied shells reveal that these specimens record isotopic compositions most likely resulting from a combination of kinetic `vital' isotope effects that may, sometimes, mimic equilibrium values.
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https://hal-univ-lyon1.archives-ouvertes.fr/hal-02327153
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Submitted on : Tuesday, October 22, 2019 - 5:08:32 PM
Last modification on : Tuesday, February 18, 2020 - 3:54:02 PM

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Daniele Gaspard, Anthony E. Aldridge, Omar Boudouma, Michel Fialin, Nicolas Rividi, et al.. Analysis of growth and form in Aerothyris kerguelenensis (rhynchonelliform brachiopod) - Shell spiral deviations, microstructure, trace element contents and stable isotope ratios. Chemical Geology, 2018, 483, pp.474--490. ⟨10.1016/j.chemgeo.2018.03.018⟩. ⟨hal-02327153⟩

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