Mollusk Shell Nacre Ultrastructure Correlates with Environmental Temperature and Pressure

Nacre, or mother-of-pearl, the tough, iridescent biomineral
lining
the inner side of some mollusk shells, has alternating biogenic aragonite
(calcium carbonate, CaCO3) tablet layers and organic sheets.
Nacre has been common in the shells of mollusks since the Ordovician
(450 million years ago) and is abundant and well-preserved in the
fossil record, e.g., in ammonites. Therefore, if any measurable physical
aspect of the nacre structure was correlated with environmental temperatures,
one could obtain a structural paleothermometer of ancient climates.
Using X-ray absorption near-edge structure (XANES) spectroscopy, Photoelectron
emission spectromicroscopy (PEEM), and X-ray linear dichroism we acquired
polarization-dependent imaging contrast (PIC) maps of pristine nacre
in cross-section. The new PIC-map data reveal that the nacre ultrastructure
(nacre tablet width, thickness, and angle spread) is species-specific
in at least eight mollusk species from completely different environments: Nautilus pompilius, Haliotis iris, Haliotis rufescens, Bathymodiolus azoricus, Atrina rigida, Lasmigona complanata, Pinctada margaritifera, and Mytilus californianus. Nacre species-specificity is interpreted as a result of adaptation
to diverging environments. We found strong correlation between nacre
crystal misorientations and environmental temperature, further supported
by secondary ion mass spectrometry measurements of in situ δ18O in the nacre of one shell. This has far-reaching
implications: nacre texture may be used as a paleothermometer of ancient
climate, spanning 450 million years of Earth's history.