POSTMORTEM AGE DISTRIBUTION OF THE BIVALVE MULINIA LATERALIS FROM COPANO BAY, TX: THE RELATIONSHIP BETWEEN TIME AVERAGING, RATE OF SHELL LOSS, AND SEDIMENT REWORKING

The postmortem age distribution of shelly material in marine sediments reflects a dynamic balance between the rates of influx (i.e., mortality in living populations) and loss (i.e., destruction and burial). Using death assemblages, either modern or fossil, to understand the state of past ecological communities requires understanding the form and range of the preserved age distribution. In order to better understand time averaging in shallow marine systems, shells of the bivalve Mulinia lateralis were collected from the top 10-15 cm of the sediment column at six locations in Copano Bay, Texas in June 2007. Copano Bay is an actively aggrading estuarine embayment characterized by three main sedimentary settings: fine sand along the margins of the bay, mud in the interior, and shell hash associated with oyster banks. Two samples from each habitat consisting of 50 shells each were dated using amino acid racemization calibrated to radiocarbon ages. The median postmortem age of M. lateralis shells in Copano Bay was 3.89 years and all but seven were younger than 60 years. The seven oldest shells ranged in age from 294 to 11,911 years and occurred only in the two sandy sites; they are interpreted to have been exhumed from actively eroding Holocene exposures along the bay margin. Shells <60 years old are interpreted to reflect dead material that has never been removed from the current sediment column in the bay. These shells show a bimodal distribution of postmortem ages consisting of a young population between 0-10 years (mode <1 year) and an older population between 4-60 years (mode ~13 years). Both populations are present at all six sites sampled in the bay. The age distribution of the young population resembles a decay function consistent with rapid loss in the taphonomically active zone. The age distribution of the older population suggests that it represents an accumulation of shells in the sediment column below the taphonomically active zone. These results suggest that the age distribution of shells in an aggradational setting is determined by the frequency and intensity of sediment reworking – i.e., the transition from the taphonomically active zone to the preservable sediment column – rather than the inherent destructability of shelly material.