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Evaluating the biomineralization and chemical differentiation of modern brachiopod archives

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(1)Evaluating the biomineralization and chemical differentiation of modern brachiopod archives Sara Milner*, Claire Rollion-Bard Institut de Physique du Globe de Paris, CNRS, Université Paris Diderot, Sorbonne Paris-Cité, Paris, France *corresponding author: milner@ipgp.fr. This PhD project focuses on the determination of biomineralization processes related to brachiopod shell formation and their impact on geochemical proxies, in order to evaluate the potential use of brachiopods as chemical proxies of paleo-seawater. We aim at deciphering the influence of the vital effects superimposing the original proxy record thereby improving proxy calibration and paleo-environmental reconstructions.  . Brachiopods as potential paleoenvironmental proxies. Fossil brachiopods have been extensively used to reconstruct physicochemical conditions and secular chemical variations of ancient oceans (Veizer et al., 1986), because their low-magnesium calcite shells is the diagenetically more stable polymorph of calcium carbonate and resistant to all except the most aggressive diagenetic processes. Most of the studies assume that brachiopods incorporate stable isotopes and trace elements into their calcitic into shell calcite into or near equilibrium. However, the use of brachiopod shells as paleoenvironmental archives is problematic due to the biological processes of the organism (the so called “vital effect”), which cause their chemical and isotopic compositions to be significantly different from inorganic calcite precipitated in isotopic equilibrium with ambient seawater .. Goal of the study. Equilibrium?. δ180, Mg/Ca, Sr/Ca (temperature). To unravel which brachiopod taxa and/or portions of the shells and brachiopod taxa are most. δ13C δ11B δ180 Mg/Ca 7Li δ δ44Ca Sr/Ca δ26Mg. δ44Ca, δ26Mg, δ7Li. reliable as paleoenvironmental proxies. We will determine the transport, the elemental. (weathering). discrimination and isotope fractioning processes of trace elements and their isotopes from seawater to the site of calcification.. δ11B. δ13C. We will measure in situ, at least, Oxygen, Carbon, Boron, Lithium, Magnesium and Calcium. (pH). (DIC). isotope compositions using the ion microprobe technique and trace element contents by LAICPMS of shells from modern environment and from culture specimens. Latter measurements will be combined with scanning electron microscopy to study the brachiopod shell structure.. What do we know so far. Brachiopods. Shell structure. P  . PL  . P  . values in brachiopod shells.. P  . Cusack et al. (2008d). Fig.  3.    . Penman et al. (2013). Terebratalia transversa. Terebratalia transversa SL   δ11B (‰) NBS951. 100  µm  . Shaded area: range of equilibrium values. 0,5. 18 17. -2,5. -1,5. Outer SL. 14. PL  . b.  Notosaria  nigricans  . Primary layer: out of equilibrium. Inner SL. -1,5. δ180 (‰) VPDB 0,5. SL  . -2. Outer SL. PL  . Inner SL. Figure  3.  δ180-­‐δ13C  cross  plot   of  Terebratalia  transversa,   determied  by  electron   microprobe  analyses.  . SL  . Figure  4.  δ18O  composi<on   from  outer  to  innermost   secondary  layer  of  N.   nigricans  and  T.  transversa,   determined  by  SIMS.    . b. Terebratalia transversa 4. -4,5.    . 0. Secondary layer: equilibrium?. -3,5. transversa  by  NTIMS  analysis.  . 2. -4. 1,5. -2,5. Figure  2.  Depth  transects  of  δ11B  through  T.  . PL  . -0,5 -0,5. 16 15. 1cm  . 4. 1,5. 19. P  . a. Notosaria nigricans. δ13C (‰)   VPDB  . 20. Cusack et al. (2012). Fig.  4.    . δ180 (‰) VPDB. a.  Terebratalia  transversa  . δ13C and δ18O values in brachiopod shells.. δ180 (‰) VPDB. δ11B. 2 0 -2 -4 -6. SL  . 100  µm  . 1cm  . Primary layer: Isotopic disequilibrium. δ13C. and Portion of the shell in equilibrium. Vital fractionation effects. δ18O:. c.  Liothyrella  neozelanica  . Disequilibrium between brachiopod shells and seawater. Why?. δ13C. CL  . P  . PL   Outer   SL  . PL  . Inner   SL  . Perez-Huerta (2008). SL   100  µm  . δ18O. 1cm  . Figure  1.  Brachiopod  shell  structure  analyzed  by  SEM     PL:  Outer  primary  layer,  made  of  acicular  calcite.   SL:  Inner  secondary  layer,  made  of  calcite  fibers.   CL:  Columnar  layer,  made  of  pillar-­‐shaped  calcite  crystals.     P:  Punctae.  Characteris<c  perfora<ons  of  brachiopod  shells.  . Bibliography. Mg/Ca. δ11B δ11B:Portion of the shell in equilibrium. Mg/Ca: Portion of the shell in equilibrium. δ26Mg? High metabolic activity (Metabolic effects) High growth rates (Kinetic fractionation effects) Change the environmental parameters at the site of calcification (pH). δ7Li? δ44Ca? Li/Ca? Sr/Ca?.  . Kine<c  effect?  . Kine<c   effects   have   been   observed   for   these   elements   i n   i n o r g a n i c   c a l c i t e   experiments.       S'll   need   to   be   tested   the   poten<al   use   of   these   isotopic   and   elemental   ra<os   in   brachiopods   as   reliable   paleoenvironmental  proxies  . Auclair A., M.M. Joachimski, and C. Lécuyer. 2003. Deciphering kinetic, metabolic and environmental controls on stable isotope fractionations between seawater and the shell of Terebratalia transversa (Brachiopoda). Chem. Geol. 202: 59–78. Brand U., A. Logan, N. Hiller, and J. Richardson. 2003. Geochemistry of modern brachiopods: applications and implications for oceanography and paleoceanography. Chem. Geol. 198: 305–334. Cusack M., D. Parkinson, A. Perez-Huerta, J. England, G.B. Curry, and A.E. Fallick. 2008d. Relationship between d18O and minor element composition of Terebratalia transversa. Trans. R. Soc. Edinburgh. 98: 443-449. Cusack M. and A.P. Huerta. 2012. Brachiopods recording seawater temperature—A matter of class or maturation?. Chem. Geol. 334: 139–143. Penman, D.E., B. Hönisch, E.T. Rasbury, N.G. Hemming, and H.J. Spero. 2013. Boron, carbon, and oxygen isotopic composition of brachiopod shells: Intra-shell variability, controls, and potential as a paleo-pH recorder. Chem. Geol. 340: 32-39. Perez-Huerta, A., M. Cusack, T.E. Jeffries, C.T. Williams. 2008. High resolution distribution of magnesium and strontium and the evaluation of Mg/Ca thermometry in Recent brachiopod shells. Chem. Geol. 247 (1–2): 229–241. Veizer, J., P. Fritz and B. Jones. 1986. Geochemistry of brachiopods: oxygen and carbon isotopic records of Paleozoic oceans. Geochim. Cosmochim. Acta. 50: 1679–1696.      .

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