Abstract
Shells of oysters (Ostreidae) are predominantly composed of foliated and chalky calcite microstructures. The formation process of the more porous chalky structure is subject to debate, with some studies suggesting that it is not formed directly by the oyster but rather through microbial mineralization within the shell. Here, this hypothesis is tested in modern shells of the Pacific oyster (Crassostrea gigas) from coastal regions in France and the Netherlands. We combine measurements of stable carbon, oxygen, nitrogen, sulfur, and clumped isotope ratios with high-resolution spatially resolved element (Na, Mg, Cl, S, Mn and Sr) data and microscopic observations of chalky and foliated microstructures in the oyster shells. Our results show no isotopic differences between the different microstructures, arguing against formation of the chalky calcite by microorganisms. However, we observe a small difference in the oxygen isotope ratio (0.32‰) and clumped isotope composition (0.017‰) between the microstructures, which is likely caused by sampling biases due to seasonal differences in growth rate and the short timespan over which the chalky microstructure forms. We therefore recommend sampling profiles through the foliated microstructure to control for strong seasonal variability recorded in the shell which can bias environmental reconstructions. High-resolution (25–50 µm) Na, Mg, Cl, S, Mn and Sr profiles yield empirical distribution coefficients between seawater and shell calcite for these elements. Significant differences in element concentrations and distribution coefficients were confirmed between the two microstructures, likely reflecting differences in mineralization rates or inclusion of non-lattice-bound elements. Only Mg/Ca ratios in the foliated microstructure vary predictably with growth seasonality, and we show that these can be used to establish accurate oyster shell chronologies. The observed effect of mineralization rate on element incorporation into oyster shells should be considered while developing potential element proxies for paleoclimate reconstructions. Trace element proxies in oyster shells should be interpreted with caution, especially when element chemical properties were measured in different microstructures.
Highlights
Oysters (Ostreidae) are a highly diverse and specialized group of bivalves that live cemented to hard substrates, predominantly in shallow marine environments (Yonge, 1960)
Under higher magnification using Scanning Electron Microscope (SEM), it becomes clear that these differences stem from the microscopic organization of both microstructures: Chalky structures are composed of loosely organized blades of calcite with ample interconnected porosity (Fig. 3E and G), while the foliated structure consists of densely packed calcite laths organized in semi-parallel bands (Fig. 3D, F and H), as observed by Carriker et al (1980)
A combination of microscopy, stable isotope analyses and elemental analysis on 18 specimens of Crassostrea gigas from coastal waters in the Netherlands and France reveals that the chalky microstructures in oysters are not formed via microbially assisted carbonate mineralization, as previously proposed
Summary
Oysters (Ostreidae) are a highly diverse and specialized group of bivalves that live cemented to hard substrates, predominantly in shallow marine environments (Yonge, 1960). The presence of the chalky structure in the form of lenses between the foliated calcite is typical of the Ostreidae family and its process of formation is highly debated This has recently spurred researchers to investigate the chemical (Surge et al, 2001; Ullmann et al, 2010; 2013; Mouchi et al, 2016), microstructural (Lee et al, 2011; Checa et al, 2018; Banker and Sumner, 2020) and physiological (Higuera-Ruiz and Elorza, 2009) differences between chalky and foliated structures and their organic matrices. Others have challenged this hypothesis by suggesting the structural difference results from local detachment of the mantle from the forming shell This would serve as a mechanism to accommodate the typical plasticity of shell shape allowing oysters to attach to rough substrates and adapt to space limitations during growth (Checa et al, 2018; Banker and Sumner, 2020). This distinction has important implications both for understanding the formation pathway of these biomineralized structures and for the interpretation of the chemistry of oyster shell calcite for environmental monitoring and paleoclimate reconstruction
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