Micro-Scale Physical and Chemical Heterogeneities in Biogenic Materials - A Combined Micro-Raman, Chemical Composition a
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Micro-Scale Physical and Chemical Heterogeneities in Biogenic Materials - A Combined Micro-Raman, Chemical Composition and Microhardness Investigation E. Griesshaber 1), R. Job 2), T. Pettke 3), W. W. Schmahl 1) 1)
University of Bochum, Dept. of Geology, Mineralogy and Geophysics, Bochum, Germany University of Hagen, Electrical Engineering and Information Technology, Hagen, Germany 3) ETH Zürich, Institute of Isotope Geochemistry, Zürich, Switzerland 2)
ABSTRACT The ultrastructure and chemical composition of calcitic shells of the modern brachiopod specimen Magellania flavescens (Linnaeus) – order: Terebratulida – was investigated with µRaman spectroscopy, Vickers microhardness indentation and laser-ablation-inductivelycoupled-plasma-mass-spectrometry. The shells contain a thin outer, nanocrystalline primary layer, which is followed by an inner, much softer, secondary layer composed of inorganic/organic fibre composite material. We observed significant chemical and structural inhomogeneities within the shells. The calcite A1g Raman mode was slightly reduced from 1084 cm-1 at the hinge (lock) down to 1083.5 cm-1 towards the tip. This is accompanied by a variation of some chemical impurity concentrations (e.g. Mg, Sr). A strong decrease in microhardness and distinct changes in chemical composition from the primary or the outermost part of the secondary layer towards the innermost portion of the secondary shell layer can be observed. Thus, our measurements show that chemical and structural inhomogeneities occur in modern brachiopods and not only between the primary and the secondary shell layer, but also within the secondary layer of the shell. INTRODUCTION AND AIM OF THE STUDY Magnesian calcites of biogenic origin play an important role in geochemical processes such as diagenesis, calcification, cement formation/dissolution and seawater chemistry. These hard tissues are advanced materials which can be addressed as multi-scaled (from nanoscale to macroscale) biological composites (inorganic matter and biopolymers), where inorganic processes (e.g. crystallization of calcite and thus shell formation) occur through biological meditation [1-4]. Brachiopods are one of the most intriguing phyla of marine organisms for evolutionary studies since they exist since the lower Cambrian, are still extant and have preserved their basic microstructural design until today [5-7]. They crystallize stable lowmagnesium calcite shells and thus preserve the isotopic signature of paleooceanic environments much better than most other marine carbonate components. However, the utility of invertebrate skeletons as paleoenvironmental indicators depends on their being either deposited in isotopic equilibrium with the ambient waters or deviating by some known amount. Furthermore, the isotopic variability caused by metabolic effects or other factors, such as growth rate, must be additionally known, since all these factors set limits to the resolution of paleoenvironmental conditions. Not only may diagenesis indicators fail [8] but also sys
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