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 degrees C, 175 degrees C and 200 degrees C. These comparably high temperatures were chosen to shorten experimental durations. Subsamples were exposed to different O-18-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 degrees C show redistribution and darkening of organic matter, but lack evidence for diagenetic alteration, except in Delta(47) which show the effects of annealing processes. At 175 degrees C, valves undergo significant aragonite to calcite transformation and neomorphism. The delta O-18 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 degrees C, valves are fully transformed to calcite and exhibit purple-blue luminescence with orange bands. The delta O-18 and Delta(47) signatures reveal exchange with the aqueous fluid, whereas delta C-13 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 represents a significant step towards sequential analysis of diagenetic features in biogenic aragonites and sheds light on reaction times and threshold limits. The limitations of a study restricted to a single test organism are acknowledged and call for refined follow-up experiments.
| Dokumententyp: | Zeitschriftenartikel |
|---|---|
| Fakultät: | Geowissenschaften > Department für Geo- und Umweltwissenschaften |
| Themengebiete: | 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften, Geologie |
| ISSN: | 0037-0746 |
| Sprache: | Englisch |
| Dokumenten ID: | 54808 |
| Datum der Veröffentlichung auf Open Access LMU: | 14. Jun. 2018, 09:57 |
| Letzte Änderungen: | 04. Nov. 2020, 13:34 |
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