Overexposed snapshots: The potential of shallow marine death assemblages for assessing ecological change
Snapshots capture the state of a system at an instant in time but without information on the rate or direction of change the system may be undergoing. In shallow marine habitats, time-averaged shelly death assemblages capture information about the past, providing a basis for comparison with living snapshots. (This approach has been used in marine settings to recognize anthropogenic alteration of benthic communities.) However, like over-exposed photographs, death assemblages overprint images of the system over time making single instants hard to discern. In general, it is not possible to un-mix temporally integrated accumulations, but studies on postmortem age distributions of shells have revealed a great deal about the duration and nature of time averaging in death assemblages that make them potentially useful tools for putting living snapshots into a long-term context.
Studies using radiocarbon and amino-acid racemization to date individual shells from a variety of marine habitats (including estuarine bays, coral reefs, and continental shelves) have revealed that shells in a single sample of present-day surface sediments can range from 0 to >10,000 years old. Postmortem age distributions of actively accumulating death assemblages are typically highly skewed and are best understood as a mixture of at least two distinct populations: a sharp peak of young shells (0 to <100 years) that transition into a long tail of very old shells (100 to >10,000 years). The young population shows a monotonic decay distribution reflecting a balance between the rate of input from mortality and the rate of shell loss by a variety of chemical, physical, and biological processes. The old population results from admixing of previously buried shells exhumed by physical or biologic reworking of sediment into surface assemblages. The result is that death assemblages have an ephemeral component that is sensitive to the short-term dynamics of the local living community and a separate enduring component that averages the long-term history of the community. Numerical simulations demonstrate that death assemblages are more consistent and accurate recorders of long-term conditions when loss in the ephemeral component is slow and reworking in the enduring component is continuous. Understanding the magnitude and variation in rates of loss and reworking is critical when using live-dead comparisons to put living snapshots into a long-term ecological context.