PRESERVATIONAL BIAS AND TIME AVERAGING IN BENTHIC MOLLUSKS IN THE CHESAPEAKE BAY: IMPLICATIONS FOR ESTABLISHING BASELINES FOR RESTORATION

As the sedimentary record of estuaries is increasingly used to provide baselines for ecological restoration, the drawbacks of these records need to be explicitly assessed. Recent work on Chesapeake Bay mollusks has quantified the extent to which the record is influenced by preservational and time averaging processes.

Compositional fidelity, which focuses on the reliability of species composition, richness, and abundance metrics preserved in the fossil record, can be assessed using live-dead comparisons, in which live communities are sampled and compared with death assemblages. A live-dead comparison on molluscan assemblages from the main channel of the upper Chesapeake Bay yielded a total of 3911 dead and 23,466 live mollusk specimens. Live-dead metrics indicate strong fidelity-- 99% of the individuals of species found in the death assemblage were found in the live community. This study indicates that Holocene data can be used, albeit with caution, to reconstruct changes in molluscan species composition and community structure through time.

Time averaging is defined as the mixing of non-contemporaneous fossil material such that specimens from successive generations or ecologically unrelated communities occur within a single sedimentary stratum. Recent work has established a radiocarbon-calibrated amino acid racemization (AAR) chronology for the Chesapeake Bay, and quantified time averaging within core-collected samples of Mulinia lateralis. An age calibration curve for 14C and AAR was created using both direct and indirect calibration methods after testing samples for internal consistency, or the coherent relationship between D/L aspartic acid (Asx) and D/L glutamic acid (Glx). Each shell was assigned an age based on direct calibration, then standard deviation and range of Asx-ages were calculated to estimate time averaging at each core depth. The two metrics yielded estimates of 720 years (standard deviation) and 2600 years (range) when averaged across multiple depths in each core. Practically speaking, this means that mollusks can provide useful baseline data for ecological restoration and paleoenvironmental reconstruction, but not necessarily at decadal or centennial timescales.