What's in a shell? - interactions of chemistry and structure at submicron levels in bivalve shells

The wide geographical distribution of bivalve shells makes them much favoured paleoclimate proxy archives. However, they are amongst the materials most affected by physiological effects, making the correct deciphering of these archives a challenging task. Shell building plans are usually hierarchic, thus optimizing mechanical properties. However, different structures common to certain bivalve families, such as the prism-nacre or the crossed-lamellar structures, are assembled from very different building blocks. These structural differences coincide with chemical and crystallographic differences suggesting critically different formation mechanisms within the bivalve class. Most importantly some bivalves form their shells from amorphous calcium carbonate that crystallizes in situ once assembled into the shell. We present new correlated multi-scale structural and compositional data for different shell bivalve shell structures such as nacre-prism, cross-lamellar intermediate structures. Data are obtained using EBSD, FIB-assisted TEM and Transmission Kikuchi Diffraction combined with Nano-SIMS and Raman Microspectrometry and suggest that formation from amorphous phases is widespread and results in different calcium carbonate polymorphs to be present in the shell with distinct chemical compositions. The results highlight the complex nature of the biomaterials, which has consequences for the precision and accuracy of paleotemperature calculations