THE INFLUENCE OF DEPOSITIONAL SETTING ON TIME AVERAGING IN A MODERN ESTUARINE EMBAYMENT (COPANO BAY, TX)

To investigate time averaging (the result of mixing dead remains of organisms that were not alive contemporaneously) under different conditions in a shallow marine system, shells of the bivalve Mulinia lateralis were collected from the top 10-15 cm of sediment at six locations in Copano Bay, Texas in June 2007. Copano Bay is an estuarine embayment characterized by three main habitats that differ with respect to sedimentary substrate and living benthic fauna: Fine sand is characteristic of the margins of the bay, mud is characteristic the interior, and shell hash is associated with actively growing oyster banks. Two samples from each habitat consisting of 50 shells each were dated using amino acid racemization calibrated to radiocarbon ages. Postmortem age estimates range from 1 month to 11,217 years with a median value of 4.22 years; 95% of all shells were <56 years old (i.e., died since 1950). Three distinct populations of postmortem shell ages were recognized: a young population (1-2 years) interpreted as the result of mortality in currently living communities, an intermediate population (10-70 years) that appears to be sequestered primarily in muddy habitats, and an old population (100-11,000 years) interpreted to come from actively eroding Holocene sediments around the margins of the bay. These three distinct populations are differentially represented among the three habitats. Sandy habitats have a very high proportion of particularly young shells and a long, sparse tail of very old shells. Muddy habitats are dominated by the intermediate age population, with very few shells from the young population and none >70 years. Shelly habitats associated with oyster banks are dominated by young shells, have very few shells >10 yrs, and none >50 years. The presence and nature of distinct postmortem age populations helps constrain how different depositional settings act as sources, reservoirs, and sinks of dead shell material.