TIME-AVERAGING AND POSTMORTEM SKELETAL SURVIVAL IN BENTHIC FOSSIL ASSEMBLAGES - QUANTITATIVE COMPARISONS AMONG HOLOCENE ENVIRONMENTS

We used radiocarbon ages on dead Holocene shells of the venerid bivalv e Chione spp. to investigate how time-averaging and taphonomy in shall ow marine benthic assemblages vary with sedimentary and tectonic setti ng. We compared shells collected from the sediment surface in five dep ositional environments from two regions of the Gulf of California, Mex ico: Bahia Concepcion, a young faulted rift basin with high rates of t errigenous and carbonate sedimentation; and Bahia la Choya, an interti dal system along a sediment-starved shelf. Frequency distributions of shell ages in all environments form a hollow curve, with a mode at you ng ages and a long tail toward older ages. This pattern suggests that shells are added to the taphonomically active zone (TAZ) at roughly co nstant rates (via continuous shell deaths), and removed from the TAZ a t random,either through destruction or by achieving final burial. Shel l half-lives (the amount of time to remove half the shells from the TA Z) provide a comparative measure of time-averaging. Time-averaging var ies with sedimentary and tectonic setting. The lowest amounts of time- averaging (shell half-lives of 90 to 165 years) occur in Bahia Concepc ion, where rapid rates of terrigenous sedimentation (on fan-deltas) an d carbonate sedimentation (in pocket bays) bury shells rapidly. Time-a veraging is higher in the sediment-starved environments of Bahia la Ch oya (shell half-lives of 285 to 550 years). The highest amounts of tim e-averaging occur the inner tidal flats of Bahia la Choya (shell half- life of 550 years). Here the conjunction of low sedimentation rates wi th low rates of shell destruction (due to periodic tidal emergence) pe rmits shells to persist in the TAZ for very long time spans. There is no systematic relationship between a shell's age and its taphonomic co ndition (taphonomic grade) in any environment, probably because of the complex and random nature of burial-exhumation in the TAZ. Age varian ce tends to increase with increasing taphonomic alteration: highly alt ered shells range in age from young to several thousand years old, whi le less altered shells are mostly young. The correspondence between ti me-averaging and the taphonomic condition of entire shell assemblages is also weak, but might be resolved with further study. These results provide quantitative data on time-averaging in benthic assemblages as a function of sedimentary and tectonic setting, and suggest some guide lines for facies appropriate for particular studies. Shallow marine ri ft basins like Bahia Concepcion can potentially contain within-horizon fossil assemblages representing time spans of only a few hundred year s-time resolution of ten beyond reach in paleontology. In contrast, se diment-starved shelf habitats like Bahia la Choya are unlikely to yiel d assemblages with time resolution finer than several thousands of yea rs.