Document Type
Thesis
Abstract
Time-averaging is the mixing of non-contemporaneous organisms into a single deposit, resulting in a stratigraphic layer that records an extended period of time. Time-averaging directly limits the temporal resolution of the fossil record. If multiple generations of species that lived thousands of years apart are preserved within the same bed, the temporal resolution of any study sampling that bed must exceed a millennial scale, and valid observations over centennial, decadal, or seasonal scales are impossible. The goal of this study is to quantify time-averaging for Chesapeake Bay mollusks and identify any environmental or taphonomic factors which may influence time-averaging in order to construct a baseline for paleoecological research. Time-averaging is quantified here for Chesapeake Bay mollusks using USGS piston cores drilled in Holocene sediments on the bay floor. Changes in Holocene time-averaging can be tracked observing at strata of different depths within each core. Shell ages were obtained using amino acid racemization on Mulinia lateralis, the most common bivalve recorded during the Holocene in the bay. Samples were collected from three unique cores located in the main channel of the bay near Kent Island, Potomac River, and Patuxent River. Amino acid racemization was accomplished by the Northern Arizona University aminochronology lab for aspartic acid D/L ratios. Amino geochronology has previously been calibrated for Mulinia in the Chesapeake Bay using radiocarbon dating by Amy Simonson. Time averaging was quantified using a variety of metrics, including interquartile range, standard deviation, coefficient of variation, and age-frequency distributions. Results were examined for trends in age mixing relative to core depth, sediment type, 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 indirectly linked to timeaveraging. Shell size is not an important control on time-averaging in the bay. Age-frequency distributions indicate Mulinia populations transition from right-skewed to normal distributions over time. This study informs our understanding of the Holocene record of the Chesapeake Bay and how that record should or should not be used for paleoecological reconstruction.
Date Awarded
2014
Department
Geology
Advisor 1
Rowan Lockwood
Recommended Citation
Martinez, Aaron M., "Time-averaging in Chesapeake Bay Mollusks: Estimates based on Amino Acid Racemization of Holocene Mulinia" (2014). Geology Senior Theses. William & Mary. Paper 321.
https://scholarworks.wm.edu/geologyseniors/321
