Influence of Organic Matrix on the Post-Mortem Destruction of Molluscan Shells

To examine the role of organic constituents in the destruction of calcium carbonate skeletons, we aged fresh shells of the bivalves Nucula sulcata (organic-rich nacreous aragonites with low crystallite surface areas) and Cerastoderma edule (organic-poor porcellaneous aragonites, high crystallite surface areas) under both sterile and non-sterile ("microbial") conditions in aragonite-undersaturated, -saturated, and -supersaturated seawaters for periods up to 11 months. Deterioration was tracked by SEM and weight-loss, and compared to damage produced by reagents of specific effect. The same qualitative sequence of damage was observed in all tanks for both species, but rates of deterioration were ≥ 2 × higher in microbial than in sterile tanks at a given saturation state, and were as high or higher in the microbial saturated and supersaturated tanks than in the sterile undersaturated tank. Damage to shell surfaces was limited almost entirely to loss of organic matrix, which eventually exposed and loosened surficial crystallites. Mineral dissolution in undersaturated tanks was apparently limited to crystallites occurring as loose particulate matter, as direct pitting of shell surfaces was rare. Shells of organic-rich aragonites did not suffer greater weight loss than those with organic-poor aragonites, but in microbial tanks they did suffer more rapid and intense microboring. The only macroscopic evidence of microstructural deterioration was a loss of surface sheen. The experiments show that intraskeletal matrix plays a more complex role in the persistence of calcium carbonate shells than generally appreciated, and that the dynamics of dissolution for fresh biogenic carbonates may differ significantly from the behavior of aged or organic-free carbonate grains used in most laboratory studies. Organics initially protect crystallites (evidenced by slow shell deterioration in sterile tanks): this may counterbalance the effects of undersaturated water and high crystallite surface areas for at least the first several months of aging. With progressive breakdown, however, organics increase shell vulnerability to crystallite-by-crystallite disintegration and, as a microbial substrate, appear to fuel dissolution and microboring. Organic-rich microstructures thus may ultimately have lower preservation potential than organic-poor types. Only after intercrystalline organics have been lost should shell destruction be dominated by mineralogy, microstructural surface area, and ion adsorption. The initial period of low mineral reactivity in fresh shells may help to explain why in situ sediments show lower dissolution rates than expected from laboratory measurements. The experiments also suggest that no aerobic environment should be considered as taphonomically or diagenetically neutral, since matrix decomposes in supersaturated waters and even under sterile conditions, albeit slowly. This overall vulnerability of organic-rich microstructures suggests the potential for systematic biases in the taxonomic and age-class composition of fossil datasets, since ecological groups and evolutionary lineages differ in their shells' microstructures, and since the proportion of organics within carbonate skeletons may vary both with latitude and through individual ontogeny.