Biomineralization of modern and fossil calcitic and phosphatic brachiopod shells: Significance of shell microstructural organization and shell chemistry with respect to growth, preservation and paleo environment

This study investigates the ultrastructure and chemistry of the shells of modern calcitic (Liothyrella neozelanica, Tegulorhynchia sublaevis), and fossil (Uncinulus sp., Plagiorhynchia sp., Gypidula sp.) brachiopod species along with modern phosphatic (Lingula anatina, Discinisca tenuis) brachiopod species. I use a multi-analytical approach which involves structural characterization of shells with optical microscopy, scanning electron microscopy (SEM), high spatial resolution electron backscatter diffraction (EBSD) and shell geochemical analyses with electron probe micro analyser (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Modern articulated and inarticulated brachiopod shells have different organic-inorganic composite bio-structural design units that form a hierarchical architecture. The outer compact primary layer in calcitic shells present an interlocking texture of micro and nano crystallites while in the phosphatic shells, this layer is dominantly organic. The inner secondary layer in calcitic shells have a fibrous morphology wherein each calcite fibre behaves as a single crystal. The organic matrix is a biopolymer sheath lining the fibre margins. In organophosphatic shells, the secondary layer microscale structure is laminated and characterized by alternating laminae of strongly and partially mineralized layers. Here, the organic component is a chitin fibre which incorporates calcium phosphate nano particles. Varying degrees of diagenetic changes to the original fibre microstructure are observed in fossil brachiopods. Geochemical investigations in different regions of the shell provided clues to the link between shell ontogeny and chemical signatures. Mg, Na and S contents varied intrashell owing to seasonal growth characteristics and environmental parameters, and were considerably higher in modern brachiopods as compared to fossil brachiopods, owing to preservation state. Mn and Fe contents were higher in diagenetically altered fossil brachiopods and were in agreement with microstructural observations. Such key observations along with significant correlation trends between element/Ca ratios improved our understanding of using brachiopod shell chemistry as paleoenvironment and diagenetic indicators.