Assessing the biomineralization processes in the shell microstructure of modern brachiopods: variations in the oxygen isotope composition and minor element ratios

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(1)Assessing the biomineralization processes in the shell microstructure of modern brachiopods: variations in the oxygen isotope composition and minor element ratios Sara Milner1*, Claire Rollion-Bard1, Pierre Burckel1, Adam Tomašových2, Lucia Angiolini3 and Daniela Henkel4 (1)Institut de Physique du Globe de Paris, Paris, France, (2) Earth Science Institute, Slovak Academy of Sciences, Bratislava, Slovakia, (3) Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Milano, Italy, (4) GEOMAR Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany *corresponding author: milner@ipgp.fr. Brachiopod geochemistry as potential paleoenvironmental proxies δ180,. In this study, we analysed the variability of values and trace element ratios in the shell microstructures of modern brachiopods, in order to assess which brachiopod shell portions or taxa are the most reliable for reconstructing paleoenvironmental conditions.. 1cm . b. Shells with PL and SL Terebratalia transversa. layer (TL) (Liothyrella neozelanica and Gryphus vitreus). Oxygen isotope compositions were measured in situ using the ion microprobe technique and trace element contents by Laser ablation coupled to an ICP-MS.. PL: depleted in 18O relative to equilibrium in the outermost part. Towards equilibrium in the innermost part.. SL: Towards equilibrium δ18O values.. TL: Closest to equilibrium δ18O values.. 2 1. 0. 3. -1. 2. 0 -1 -2 -3. -3 -4 -5. 0. outer layer. 25. 50. Relative distance (%). 75 inner layer. Expected δ18O equilibrium values (Brand et al., 2013). 0 -1 -2 -3 -4. -6. -5 -6. -7. -4. 1.The. 1. -2. δ18O (‰)PDB. This isotopic variations within the same shell microstructure is likely due to kinetic effects, with δ18O equilibrium achieved as the shell becomes mature and precipitation rate slows.. Conclusions. c) Liothyrella neozelanica. 0. 100. 25. 50. 75. Relative distance (%). 0. 100. 25. 50. Relative distance (%). 75. 100. PL SL TL . Trace Element ratios (TE) in modern brachiopod shells a) Pajaudina atlantica. 1cm . c. Shells with PL, SL and TL Liothyrella neozelanica. a) Abrupt decrease from outer to inner “Steady state” zone in the innermost PL. Enriched in trace elements relative to equilibrium. b) Decrease from outer to inner “Steady state” zone in the innermost SL.. TL SL . c) Depleted in trace elements relative to equilibrium.. 100 µm . Li. Na. Mg. Sr. 3. Li. Na. Mg. Sr. 2. 2. 1. 1. 3. DTEbraq/DTEin.calcite. 100 µm . DTEbraq/DTEin.calcite. 3. 0. 0 0. 25. 50. 75. 0. 100. 25. 50. 75. Li. Na. Mg. Sr. 100. primary layer has to be avoided for both, δ18O and trace element studies.. 1. 0. 25. 50. 75. 100. Relative distance (%). SL: Inner secondary layer, made of calcite fibers.. TL: Tertiary layer, made of columnar. inner part, where a constant value is reached. This is likely due to kinetic effects within the shell microstructure. The PL, SL and TL have different trace element incorporation signatures. This is likely due to chemical modifications of the internal fluid from which calcite precipitates the different shell microstructures (e.g. the biological discrimination against Mg and Na in the internal fluid, in which the SL precipitates).. Li Mg. 1.0. Na Sr. 0.8 0.6 0.4 0.2 0.0 0. 25. 50. 75. Relative distance (%). 100. best shell portion to use for trace element studies is the innermost secondary layer.. 0. Proportion of TE relative to PL. acicular calcite.. In shells made of PL or PL and SL, there is always an abrupt decrease from outer to. Proportion of TE relative to PL. PL: Outer primary layer, made of. 1.2. 2.The. 4.The. Best parts to use as proxies. DTEbraq, DTEin.calcite: partition coefficient of trace elements in brachiopod calcite and inorganic calcite, respectively 1cm . The Tertiary layer is in δ18O equilibrium w i t h s e a w a t e r. T h e i n n e r m o s t secondary layer is in or near equilibrium.. tertiary layer is depleted in trace elements relative to equilibrium. This part is not suitable for isotopic studies of trace elements (e.g. δ7Li, δ11B) due to its very low content. 2. Relative distance (%). Relative distance (%). best shell portion to use for δ18O studies, when present, is the tertiary layer, and if not, the innermost secondary layer.. 3.The. c) Liothyrella neozelanica . b) Terebratalia transversa. DTEbraq/DTEin.calcite. SL . calcite crystals.. nigricans and Magellania venosa) and c) primary, secondary and tertiary columnar. b) Terebratalia transversa. PL . PL . (pH). (Terebratalia transversa, Magasella sanguinea, Calloria inconspicua, Notosaria. a) Pajaudina atlantica. δ18O (‰)PDB. 100 µm . There is a general trend towards equilibrium values from outer to inner part of the shell, as in Cusack et al., 2012.. (‰) PDB. (all the shell) . δ11B. primary layer (PL) (Pajaudina atlantica); b) primary and secondary fibrous layer (SL). δ18O values in modern brachiopod shells. δ18O. PL . Mg/Ca, Sr/Ca (temperature). (weathering) δ13C (DIC). microscope, three main shell microstructures were identified: shells made of a) only. δ13C δ11B δ180 δ44Ca 7Li δ Mg/Ca Sr/Ca δ26Mg. δ44Ca, δ26Mg, δ7Li. δ18O. a. Shells with only PL Pajaudina atlantica. Eight modern brachiopod species were selected. Using the scanning electron. Equilibrium?. Fossil brachiopods have been extensively used to reconstruct physicochemical conditions of ancient oceans due to their extensive fossil record and shells made of stable low-Mg calcite. In this context, it is important to assess the impact of brachiopod shell biomineralization processes on geochemical proxies.. The shell microstructure. Material & Methods. 1.2 1.0. Li. Na. Mg. Sr. PL. 0.8. SL. TE ratios. 0.6 0.4 0.2 0.0 0. 25. 50. 75. Relative distance (%). 100. PL SL TL . TL. δ18O values.

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