CALCITE COATS ON ARAGONITIC BIVALVE SHELLS IN SHALLOW MARINE SEABEDS: FERRETING OUT THE EARLY DIAGENETIC PROCESSES THAT PERMIT PROLONGED TIME AVERAGING

Shell input to the seabed generally results in a time-averaged assemblage. Radiocarbon and other dates reveal that shells from the top 5-10 cm typically range in age up to a few decades or centuries in settings with high sedimentation rates (e.g., estuaries, lagoons) and up to a few to tens of millennia where those rates are slow (e.g., open shelves). Time averaging is further promoted by bioturbation, which moves young shells down and older shells up. However, the normoxic conditions that support benthic carbonate production and bioturbation also support microbially active and carbonate-undersaturated porewaters, leading to high rates of shell loss. Previous analyses of shell age-frequency distributions indicate that the taphonomic half-lives of shells can be only decadal in scale immediately after death, with only ≤1% of individuals surviving to permanent burial. The existence of fossil assemblages, permanently buried below the surface mixed layer (SML), thus depends upon the prolonged persistence of a subset of shell input. One possible pathway to long-term survival is “luck”: the shell moved immediately to the base of the SML and stayed there, or otherwise encountered temporary refuges within the biogeochemically aggressive SML. Another long-suspected pathway, however, is some form of diagenetic stabilization that reduces the inherent reactivity of individual shells. Bivalve shells from tropical (Gulf of Eilat), warm temperate (southern California), and boreal (Bering Sea) siliciclastic shelves examined with SEM and Raman spectroscopy have uncovered a common post-mortem sequence of changes to the shell surface, most notably the overgrowth of the original aragonitic microstructure by syntaxial prismatic calcite, forming a skim-coat of interlocking amoeboid plates up to a few microns thick. This coating is almost certainly microbial in origin. The rate and extent of overgrowth declines with temperature – it is rare in boreal shells but starts within a few decades postmortem elsewhere, with additional overgrowths of diverse but non-biogenic calcite rhombs following. The calcitic skim-coat presents a coarser and mineralogically more stable front to ambient waters than the molluscan aragonitic microstructure it encases that should decrease the likelihood of shell loss.