Crystallography, stable isotope and trace element analysis of Mytilus edulis shells in the context of ontogeny

Dalbeck, Paul C. (2008) Crystallography, stable isotope and trace element analysis of Mytilus edulis shells in the context of ontogeny. PhD thesis, University of Glasgow.

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Living systems exert exquisite control over biomineral production determining mineral type, polymorph, morphology and ultrastructure and ultimately producing biomineral structures that perform a range of functions. In addition to this biological control, the environment also influences aspects of biomineralisation. In marine invertebrates, biominerals record seawater chemistry and temperature at deposition. Past chemistry and temperature information is interpreted via proxies such as oxygen isotope and Mg/Ca ratios in calcium carbonate biominerals and used to help predict future climatic trends. Proxies must be robust and accurate yet the presence of biological or biologically-induced kinetic effects, many of which are a consequence of ontogeny, can make it difficult to isolate the environmental signal. In many cases, proxies are applied without detailed knowledge or assessment of shell microstructures and how this is altered as the animal ages. Interpretation of detailed crystallographic, chemical and isotopic changes in the context of ontogeny is therefore essential.
This study considers crystallography, trace element chemistry and stable isotopic composition of six ontogenetic stages of farmed Mytilus edulis collected from a single location at the same time. The shell of Mytilus edulis is comprised of two calcium carbonate polymorphs: an outer layer of prismatic calcite and an inner layer of nacreous aragonite, both with very different morphologies.
The crystallographic ultrastructure is analysed through Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscopy (SEM). No significant changes in crystallography are evident between ontogenetic stages. Crystallographic orientation of calcite is more strictly constrained towards the polymorph interface and from anterior to posterior in all stages of ontogeny. Further evidence of ‘mineral bridging’ is observed in nacre.
The minor element distribution varies between the polymorphs, through the shell thickness and between ontogenetic stages. Younger specimens have higher Mg, S and Sr concentrations while older specimens have increasing Na concentrations. Mg and S are present in the calcite layer but are virtually absent in aragonite. While Na and Sr occur in both polymorphs, higher concentrations occur in aragonite. Changes in Na and Sr concentrations in the aragonite layer can be linked to prismatic bands of myostracal aragonite in the nacre. A decrease in Na concentration through the calcite layer toward the polymorph interface is observed in all specimens. Increases in Mg and S concentration in the calcite layer near the interface are often observed. Greater trace element concentrations are also observed in the umbo region of the shell.
Stable oxygen isotope signatures show small amounts of variation between regions of the shell and between ontogenetic stages. Decreasing δ18O values are observed in older animals and in the posterior edge of the shell. Large variations of δ18O are also observed through the shell thickness which cannot be accounted for by environmental changes. The majority of δ18O data falls within the expected range for equilibrium of calcite and aragonite with ambient seawater. No significant change is observed in δ13C values between ontogenetic stages or through the shell. Values of δ13C and δ18O co-vary in aragonite but not calcite.
Refinement in crystallography occurs in conjunction with changes in trace element chemistry and stable oxygen isotope composition moving towards the shell posterior. This indicates a change as the animal grows, resulting in differences in chemical information retained in different parts of the shell. The differences in trace element chemistry between polymorphs and isotopic and chemistry changes across the shell thickness indicate changes that can be attributed to ultrastructure as crystal habit, polymorph and orientation change occur across the shell thickness.
These results suggest that greater understanding of factors involved in biomineralising systems is required before undertaking work involving application of climate proxies.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QH Natural history > QH301 Biology
Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Supervisor's Name: Maggie, Dr. Cusack
Date of Award: 2008
Depositing User: Mrs Marie Cairney
Unique ID: glathesis:2008-1870
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 01 Jun 2010
Last Modified: 10 Dec 2012 13:47

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