Experimental diagenesis: insights into aragonite to calcite transformation of <i>Arctica islandica</i> shells by hydrothermal treatment

Abstract

Abstract. Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. In generating their mineralised hard parts, most marine invertebrates produce metastable aragonite rather than the stable polymorph of CaCO 3 , calcite. After death of the organism the physiological conditions, which were present during biomineralisation, are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features.In order to understand the diagenetic replacement of biogenic aragonite to non-biogenic calcite, we subjected Arctica islandica mollusc shells to hydrothermal alteration experiments. Experimental conditions were between 100 and 175 • C, with the main focus on 100 and 175 • C, reaction durations between 1 and 84 days, and alteration fluids simulating meteoric and burial waters, respectively. Detailed microstructural and geochemical data were collected for samples altered at 100 • C (and at 0.1 MPa pressure) for 28 days and for samples altered at 175 • C (and at 0.9 MPa pressure) for 7 and 84 days. During hydrothermal alteration at 100 • C for 28 days most but not the entire biopolymer matrix was destroyed, while shell aragonite and its characteristic microstructure was largely preserved. In all experiments up to 174 • C, there are no signs of a replacement reaction of shell aragonite to calcite in X-ray diffraction bulk analysis. At 175 • C the replacement reaction started after a dormant time of 4 days, and the original shell microstructure was almost completely overprinted by the aragonite to calcite replacement reaction after 10 days. Newly formed calcite nucleated at locations which were in contact with the fluid, at the shell surface, in the open pore system, and along growth lines. In the experiments with fluids simulating meteoric water, calcite crystals reached sizes up to 200 µm, while in the experiments with Mg-containing fluids the calcite crystals reached sizes up to 1 mm after 7 days of alteration. Aragonite is metastable at all applied conditions. Only a small bulk thermodynamicPublished by Copernicus Publications on behalf of the European Geosciences Union. 1462 L. A. Casella et al.: Experimental diagenesis: insights into aragonite to calcite transformation driving force exists for the transition to calcite. We attribute the sluggish replacement reaction to the inhibition of calcite nucleation in the temperature window from ca. 50 to ca. 170 • C or, additionally, to the presence of magnesium. Correspondingly, in Mg 2+ -bearing solutions the newly formed calcite crystals are larger than in Mg 2+ -free solutions. Overall, the aragonite-calcite transition occurs via ...

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

“…The shell of A. islandica is comprised of an assemblage of irregularly-shaped and micrometre sized aragonitic basic mineral units (white stars in Fig. 1A), that are larger in the outer shell layer compared to basic mineral units of the inner shell layer (this study and Casella et al, 2017). An irregular network of thin biopolymer fibrils interconnects these basic mineral units (Casella et al, 2017).…”

“…In particular, deciphering the sequence of those processes with many steps of alteration and unknown intermediate stages poses one of the major problems in understanding carbonate diagenesis (Immenhauser et al, 2015a;Swart, 2015;Ullmann and Korte, 2015). Our previous studies on the shell of the modern bivalve Arctica islandica have shown that simulated diagenetic alteration discloses microstructural and geochemical features that are comparable to those found in fossils (Casella et al, 2017;Ritter et al, 2017). However, both studies covered only the hard tissue of one taxon.…”

Oxygen Isotope Composition of Arctica islandica Aragonite in the Context of Shell Architectural Organization: Implications for Paleoclimate Reconstructions

“…The outer shell layer in A. islandica is most prone to diagenesis (Casella et al, 2017), which can affect the original oxygen isotope signature in fossil shells (Cochran et al, 2010;Prendergast & Stevens, 2014;Urey et al, 1951). Consequently, the oOSL might not be available for the analysis.…”

Biomineral Reactivity: The Kinetics of the Replacement Reaction of Biological Aragonite to Apatite

“…The microstructure of the shell of the bivalve Arctica islandica is also highly resistant to the formation of new apatite (compare images in Figure 4a [32]). Mineral unit size, porosity and density of aragonite crystal packing are unevenly distributed within the shell (Figures A7 and A8), such that, relative to inner shell portions, mineral unit and pore sizes along the shell rim pointing to seawater, are increased.…”

“…This is followed by a time span where we see a sudden increase in AP precipitation, especially along the seaward pointing rim of the shell (Figure 1a, Table 1). The shell of Arctica islandica is dense (especially shell parts next to the soft tissue of the animal) and consists of irregularly shaped mineral units that are embedded into a network of biopolymer fibrils ( Figure A7 and Casella et al, 2017 [32]). The shell is crossed by growth lines, where the organic matter content is slightly increased (Figure A8).…”