Articles | Volume 15, issue 24
Biogeosciences, 15, 7451–7484, 2018
Biogeosciences, 15, 7451–7484, 2018

Research article 21 Dec 2018

Research article | 21 Dec 2018

Hydrothermal alteration of aragonitic biocarbonates: assessment of micro- and nanostructural dissolution–reprecipitation and constraints of diagenetic overprint from quantitative statistical grain-area analysis

Laura A. Casella1, Sixin He1, Erika Griesshaber1, Lourdes Fernández-Díaz2, Martina Greiner1, Elizabeth M. Harper3, Daniel J. Jackson4, Andreas Ziegler5, Vasileios Mavromatis6, Martin Dietzel6, Anton Eisenhauer7, Sabino Veintemillas-Verdaguer8, 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
  • 6Institute of Applied Geosciences, Graz University of Technology, Rechbauerstr. 12, 8010 Graz, Austria
  • 7GEOMAR-Helmholtz Centre for Ocean Research, Marine Biogeochemistry/Marine Geosystems, Kiel, 24148, Germany
  • 8Instituto de Ciencia de Materiales de Madrid (ICMM, CSIC), 28049 Madrid, Spain
  • 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. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale.

In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175 °C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons.

We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product.

At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80 mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.

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.
Final-revised paper