ESTIMATING TIME-AVERAGING IN THE CHESAPEAKE BAY VIA AMINO ACID RACEMIZATION OF HOLOCENE MOLLUSKS

Time-averaging is the mixing of non-concurrent organisms into the same deposit. This results in a single stratigraphic layer that records an extended period of time, thereby limiting the temporal resolution of the fossil assemblage. The goal of this study is to quantify time-averaging for Chesapeake Bay mollusks and explore the environmental and taphonomic factors that are influencing time-averaging in the bay in order to construct a baseline for paleoecological research.

Time-averaging in Chesapeake Bay mollusks is quantified here using USGS piston cores sampling twenty meters of Holocene sediment from the bay floor. Shell ages were obtained by applying amino acid racemization to Mulinia lateralis, the most abundant and well preserved bivalve recorded in bay sediments. A minimum of twelve specimens were collected from at least three core depths for three cores drilled in the axial channel of the bay near Kent Island, the Potomac River, and the Patuxent River. Amino acid racemization was performed at the Northern Arizona University aminochronology lab and calibration was accomplished using the Simonson et al. 2013 geochronology for Chesapeake Mulinia. Time-averaging was quantified using several metrics, including interquartile range, standard deviation, coefficient of variation, and age-frequency distributions. Results were evaluated for trends in age mixing relative to sediment type, core depth, shell preservation, shell size, and time interval.

Results indicate that time-averaging in Chesapeake Bay Mulinia ranges from centennial to millennial scales. Sandy sediments record higher magnitudes of time-averaging than clay-rich sediments. Shell preservation is better in clay-rich sediments and is thus indirectly connected to time-averaging. Shell size is not an important control on time-averaging of Mulinia in the bay. Age-frequency distributions transition from right-skewed in the Late Holocene to normally distributed in the Early and Middle Holocene. Time-averaging is applied here to expound both the Holocene record of the Chesapeake Bay and how that record should or should not be utilized for paleoecological reconstruction.