Experimental diagenesis: insights into aragonite to calcite transformation of <i>Arctica islandica</i> shells by hydrothermal treatment
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 biomineralis...
| Main Authors: | , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-03-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/14/1461/2017/bg-14-1461-2017.pdf |
| Summary: | 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<sub>3</sub>, 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.
<br><br>
In order to understand the diagenetic replacement of biogenic aragonite to
non-biogenic calcite, we subjected <i>Arctica islandica</i> 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 thermodynamic 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<sup>2+</sup>-bearing solutions the newly formed calcite
crystals are larger than in Mg<sup>2+</sup>-free solutions. Overall, the
aragonite–calcite transition occurs via an interface-coupled
dissolution–reprecipitation mechanism, which preserves morphologies down to
the sub-micrometre scale and induces porosity in the newly formed phase. The
absence of aragonite replacement by calcite at temperatures lower than
175 °C contributes to explaining why aragonitic or bimineralic
shells and skeletons have a good potential of preservation and a complete
fossil record. |
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| ISSN: | 1726-4170 1726-4189 |