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Research article 10 Jul 2018

Research article | 10 Jul 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Assessment of hydrothermal alteration on micro- and nanostructures of biocarbonates: quantitative statistical grain-area analysis of diagenetic overprint

Laura A. Casella1, Sixin He1, Erika Griesshaber1, Lourdes Fernández-Díaz2, Elizabeth M. Harper3, Daniel J. Jackson4, Andreas Ziegler5, Vasileios Mavromatis6,7, Martin Dietzel7, Anton Eisenhauer8, Uwe Brand9, and Wolfgang W. Schmahl1 Laura A. Casella et al.
  • 1Department of Earth and Environmental Sciences and GeoBioCenter, Ludwig-Maximilians-Universität München, Munich, 80333, Germany
  • 2Instituto de Geociencias, Universidad Complutense Madrid (UCM, CSIC), Madrid, 28040, Spain
  • 3Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK
  • 4Department of Geobiology, Georg-August University of Göttingen, Göttingen, 37077, Germany
  • 5Central Facility for Electron Microscopy, University of Ulm, Ulm, 89081, Germany
  • 6Géosciences Environnement Toulouse (GET), CNRS, UMR 5563, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400 Toulouse, France
  • 7Institute of Applied Geosci ences, Graz University of Technology, Rechbauerstr. 12, 8010 Graz, Austria
  • 8GEOMAR - Helmholtz Centre for Ocean Research, Marine Biogeochemistry/Marine Geosystems, Kiel, Germany
  • 9Department of Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, L2S 3A1, Canada

Abstract. The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. A correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies (Casella et al., 2017) have shown that replacement of biogenic carbonate by inorganic calcite occurs via an interface coupled dissolution–reprecipitation mechanism. Furthermore, for a comprehensive assessment of alteration, structural changes have to be assessed on the nanoscale as well, which documents the replacement of pristine nanoparticulate calcite by diagenetic nanorhombohedral calcite (Casella et al., 2018a, b).

In the present contribution we investigated six different modern biogenic carbonate microstructures for their behaviour under hydrothermal alteration in order to assess their potential to withstand diagenetic overprint and to test the integrity of their preservation in the fossil record. For each microstructure (a) the evolution of biogenic aragonite and calcite replacement by inorganic calcite was examined, (b) distinct carbonate mineral formation steps on the micrometre scale were highlighted, (c) microstructural changes at different stages of alteration were explored, and (d) statistical analysis of differences in basic mineral unit dimensions in pristine and altered skeletons was performed. The latter analysis enables an unequivocal determination of the degree of diagenetic overprint and discloses information especially about low degrees of hydrothermal alteration.

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Laura A. Casella et al.
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Short summary
Biogenic carbonates record past environmental conditions. Fossil shell chemistry and microstructure change as metastable biogenic carbonates are replaced by inorganic calcite. Simulated diagenetic alteration at 175 °C of different shell microstructures showed that (nacreous) shell aragonite, and calcite were partially replaced by coarse inorganic calcite crystals due to dissolution-reprecipitation reactions. EBSD maps allowed for qualitative assessment of the degree of diagenetic overprint.
Biogenic carbonates record past environmental conditions. Fossil shell chemistry and...