Isotropic microscale mechanical properties of coral skeletons

Abstract

Scleractinian corals are a major source of biogenic calcium carbonate, yet the relationship between their skeletal microstructure and mechanical properties has been scarcely studied. In this work, the skeletons of two coral species: solitary Balanophyllia europaea and colonial Stylophora pistillata, were investigated by nanoindentation. The hardness H IT and Young's modulus E IT were determined from the analysis of several load -depth data on two perpendicular sections of the skeletons: longitudinal ( parallel to the main growth axis) and transverse. Within the experimental and statistical uncertainty, the average values of the mechanical parameters are independent on the section's orientation. The hydration state of the skeletons did not affect the mechanical properties. The measured values, E IT in the 76-77 GPa range, and H IT in the 4.9-5.1 GPa range, are close to the ones expected for polycrystalline pure aragonite. Notably, a small difference in H IT is observed between the species. Different from corals, single-crystal aragonite and the nacreous layer of the seashell Atrina rigida exhibit clearly orientation-dependent mechanical properties. The homogeneous and isotropic mechanical behaviour of the coral skeletons at the microscale is correlated with the microstructure, observed by electron microscopy and atomic force microscopy, and with the X-ray diffraction patterns of the longitudinal and transverse sections.

Impact of ocean acidification on crystallographic vital effect of the coral skeleton

“…S4, 0.119% to 0.098% for the (111) reflection). Although the crystallite size varies in each method, the values calculated for pH 8.2 sample are consistent with the nanocrystal size observed in S. pistillata by AFM 4,41 . The variation in crystallite size is the same for the planes (111) and (021) and they have a comparable slope to Rietveld calculations (Fig.…”

“…These parameters are not prone to change due to decomposition of intraskeletal organic matter during diagenesis 53 , thus may become a new crystallographic tool for detecting OA in well preserved aragonitic fossil corals supporting other well established geochemical tools e.g., boron isotopes 54 . In the long run (colony life span) crystallo-chemical changes may scale up at higher levels of skeletal hierarchy and potentially affect biomechanical properties of the reef structure 41 .…”

Crumbling Reefs and Cold-Water Coral Habitat Loss in a Future Ocean: Evidence of “Coralporosis” as an Indicator of Habitat Integrity

“…With regard to skeletal stiffness and hardness, our data were comparable to those recorded in shallow-water zooxanthellate corals around localised volcanic acidification sites with low pH conditions (Fantazzini et al, 2015;Pasquini et al, 2015), where Fantazzini et al (2015) reported an ∼8% difference in stiffness but no difference in hardness over 0.4 pH points. While similar, results are not directly comparable to CWC investigated here due to the higher aragonite saturation ( Arag > 1) of the volcanic sites used, and the zooxanthellate nature of the shallow corals.…”

Environmental impact on the mechanical properties of Porites spp. corals

“…These studies were conducted on the upper most part of coral skeletons from pristine, oligotrophic reefs. However, as suggested by Pasquini et al (2015), corals from a wider range of environments with differing organic matrix content and porosity may have distinct mechanical properties.…”

“…However, the role of organic content on the overall strength of hard corals is complex, with no straightforward correlation between organic content and mechanical durability (Amini and Miserez 2013). In scleractinian corals, reported values for organic matrix content vary from 1 to 3% (Cuif et al 2004;Dauphin et al 2006;Falini et al 2015;Pasquini et al 2015). These studies were conducted on the upper most part of coral skeletons from pristine, oligotrophic reefs.…”