Taphonomic tiering: experimental field taphonomy of molluscs and foraminifera above and below the sediment–water interface

A review and synthesis of field-based taphonomic experiments using molluscs and foraminiferans in marine systems reveal differing patterns of hardpart accumulation and destruction for these two major groups of fossil organisms. For benthic molluscs, most destruction appears to occur at the sediment–water interface in siliciclastic settings. Mollusc preservation is suggested to be high once the shell is buried below the Taphonomically Active Zone although this observation has not been experimentally addressed. Foraminiferal preservation, however, depends on microhabitat, biogeochemical conditions within the sediments, and test composition. The Taphonomically Active Zone for foraminiferans appears to be the upper 10 cm in most depositional settings (from shallow to deep sea). The preservation of tests or shells, however, has not been examined below the sediment–water interface using field-based experiments. Pore water geochemistry and associated microbial processes may be the most important factors in which to examine subsurface taphonomy. The term, taphonomic tiering, is applied to variations in taphonomic activity within the sediments that differentially affect hard parts. In essence, the subsurface region where hardparts are presumed to be preserved, is one of the new frontiers in experimental field taphonomy.