Exploring the impact of diagenesis on (isotope) geochemical and microstructural alteration features in biogenic aragonite

• Published in: Sedimentology
• Main Authors: Ritter, Ann‐Christine, Mavromatis, Vasileios, Dietzel, Martin, Kwiecien, Ola, Wiethoff, Felix, Griesshaber, Erika, Casella, Laura A., Schmahl, Wolfgang W., Koelen, Jennifer, Neuser, Rolf D., Leis, Albrecht, Buhl, Dieter, Niedermayr, Andrea, Breitenbach, Sebastian F. M., Bernasconi, Stefano M., Immenhauser, Adrian
• Other Authors: Lokier, Stephen
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2017
• Subjects: Arctica islandica
• Online Access: http://dx.doi.org/10.1111/sed.12356; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fsed.12356; https://onlinelibrary.wiley.com/doi/pdf/10.1111/sed.12356

Abstract For the Quaternary and Neogene, aragonitic biogenic and abiogenic carbonates are frequently exploited as archives of their environment. Conversely, pre‐Neogene aragonite is often diagenetically altered and calcite archives are studied instead. Nevertheless, the exact sequence of diagenetic processes and products is difficult to disclose from naturally altered material. Here, experiments were performed to understand biogenic aragonite alteration processes and products. Shell subsamples of the bivalve Arctica islandica were exposed to hydrothermal alteration. Thermal boundary conditions were set at 100°C, 175°C and 200°C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different 18 O‐depleted fluids for durations between two and twenty weeks. Alteration was documented using X‐ray diffraction, cathodoluminescence, fluorescence and scanning electron microscopy, as well as conventional and clumped isotope analyses. Experiments performed at 100°C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Δ 47 which show the effects of annealing processes. At 175°C, valves undergo significant aragonite to calcite transformation and neomorphism. The δ 18 O signature supports transformation via dissolution and reprecipitation, but isotopic exchange is limited by fluid migration through the subsamples. Individual growth increments in these subsamples exhibit bright orange luminescence. At 200°C, valves are fully transformed to calcite and exhibit purple‐blue luminescence with orange bands. The δ 18 O and Δ 47 signatures reveal exchange with the aqueous fluid, whereas δ 13 C remains unaltered in all experiments, indicating a carbonate‐buffered system. Clumped isotope temperatures in high‐temperature experiments show compositions in broad agreement with the measured temperature. Experimentally induced alteration patterns are comparable with individual features present in Pleistocene shells. This study ...