INFLUENCE OF ORGANIC MATRIX ON THE POSTMORTEM DESTRUCTION OF MOLLUSCAN SHELLS

To examine the role of organic constituents in the destruction of calc ium carbonate skeletons, we aged fresh shells of the bivalves Nucula s ulcata (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 wa s tracked by SEM and weight-loss, and compared to damage produced by r eagents of specific effect. The same qualitative sequence of damage wa s observed in all tanks for both species, but rates of deterioration w ere greater than or equal to 2 x higher in microbial than in sterile t anks at a given saturation state, and were as high or higher in the mi crobial saturated and supersaturated tanks than in the sterile undersa turated tank. Damage to shell surfaces was limited almost entirely to loss of organic matrix, which eventually exposed and loosened surficia l crystallites. Mineral dissolution in undersaturated tanks was appare ntly limited to crystallites occurring as loose particulate matter, as direct pitting of shell surfaces was rare. Shells of organic-rich ara gonites did not suffer greater weight loss than those with organic-poo r aragonites, but in microbial tanks they did suffer more rapid and in tense microboring. The only macroscopic evidence of microstructural de terioration was a loss of surface sheen. The experiments show that int raskeletal matrix plays a more complex role in the persistence of calc ium 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 mo st laboratory studies. Organics initially protect crystallites (eviden ced by slow shell deterioration in sterile tanks): this may counterbal ance 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 crystall ite-by-crystallite disintegration and, as a microbial substrate, appea r to fuel dissolution and microboring. Organic-rich microstructures th us may ultimately have lower preservation potential than organic-poor types. Only after intercrystalline organics have been lost should shel l 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 diss olution rates than expected from laboratory measurements. The experime nts also suggest that no aerobic environment should be considered as t aphonomically or diagenetically neutral, since matrix decomposes in su persaturated waters and even under sterile conditions, albeit slowly. This overall vulnerability of organic-rich microstructures suggests th e potential for systematic biases in the taxonomic and age-class compo sition of fossil datasets, since ecological groups and evolutionary li neages differ in their shells' microstructures, and since the proporti on of organics within carbonate skeletons may vary both with latitude and through individual ontogeny.