Environmental and Microstructural Controls on the Short Term Degradation of Biogenic Carbonate within Temperate, Macrotidal Habitats

Carbonate skeletons are composites of varying proportions of carbonate crystallites held together by a proteinaceous organic matrix. The role that the combination of these components plays throughout the post-mortem modification of shells is a key part to understanding how shells degrade. Here we address how shells progressively degrade within the first year post-mortem in temperate macrotidal environments, and specifically the distinction between textures reflecting the intrinsic reactivity of the carbonate mineral and those related to the degradation of skeletal organic matter. Fresh Mytilus edulis shells and natural aragonite crystals were deployed for up to 1 year at and 10cm below the sediment-water interface in intertidal salt marsh and estuarine environments (Bay of Fundy, NB, Canada). Changes in net weight, macroscopic, and microstructural features on shells and aragonite crystals are discussed in the context of (micro)environmental conditions. Our results show that the rate and extent of shell degradation varies significantly among environments, despite significant variability among replicates within an environment. Throughout the deployment period, shells and aragonite crystals sitting at the sediment-water interface lose more weight overall than buried shells; buried samples are not microbored nor fragmented, and show no observable etching. Microboring dominates degradation of sediment-water interface specimens in the estuary, while dissolution dominates the degradation in the salt marsh. Etching on biogenic nacre and abiotic aragonite is selective, preferentially attacking sites along directions of higher reactivity. Abiotic aragonite crystals deployed at the sediment surface are etched preferentially along the crystallographic a-axis on the (001) face, a pattern consistent with SEM images of comb-like grooves on nacre tablets of Mytilus edulis nacre tablets. Dynamic processes within the first year post-mortem highlight both initial rates and pathways of biogenic carbonate degradation in these intertidal settings.