Southeastern Section - 60th Annual Meeting (23–25 March 2011)

Paper No. 5
Presentation Time: 4:30 PM


KOWALEWSKI, Michal, Florida Museum of Natural History, University of Florida, 1659 Museum Road, PO Box 117800, Gainesville, FL 32611, DEXTER, Troy A., Department of Geosciences, University of Arizona, Tucson, AZ 85716, KAUFMAN, Darrell S., School of Earth & Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011-4099, KRAUSE Jr, Richard A., Department of Geology & Geophysics, Yale University, New Haven, CT 06520, BARBOUR WOOD, Susan L., Geosciences and Natural Resources, Western Carolina University, 331 Stillwell Building, Cullowhee, NC 28723, SIMOES, Marcello G., Department of Zoology, Sao Paulo State University, Institute of Biosciences, District of Rubiao Junior, s/n, Botucatu, 18618000, Brazil, HUNTLEY, John W., GeoZentrum Nordbayern Fachgruppe Paläoumwelt, Friedrich-Alexander-Universität, Loewenichstr. 28, Erlangen, 91054, Germany, YANES, Yurena, Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, Profesor Albareda, 1, Granada, 18008, Spain and ROMANEK, Chris S., Department of Earth and Environmental Sciences, University of Kentucky, 101 Slone Research Building, Lexington, KY 40506,

In order to explore time-averaging (age mixing) as well as assemble a time-series of dated shells for the southern Brazilian Shelf, numerous shells of the infaunal bivalve Semele casali have been dated using amino acid racemization rates calibrated against radiocarbon. The shells were collected from Ubatuba Bay, Sao Paulo, Brazil; a shallow sub-tropical marine environment. A total of 275 Semele casali shells, collected from multiple sites varying in depth from 10 to 57 meters, were analyzed using amino acid racemization methods. Also, 35 of the shells were analyzed using carbon-14 radiometric dating. Major axis regression was applied to calibrate aspartic acid D/L ratios against radiocarbon (r-square = 0.8477).

The results represent one of the largest datasets of individually dated shells ever compiled. The calibrated ages of the shells range in age from modern to a maximum age of ~10,500 years. The distribution of the ages is right skewed with a long tail that slowly tapers off in frequency moving backwards in time indicating that the shells in this area have long residence times around the sediment surface. Radiocarbon calibrations of shells from different collection sites suggested comparable racemization rates. This, in turn, suggests that water depth had a limited effect on the targeted amino acids, which implies similar thermal histories of the sampled sites. Previous studies used multiple calibrations whenever sites with variable depth were included. In contrast, in this study, a single calibration formula could be used to calibrate the amino acid racemization rate, despite a substantial bathymetric range of the samples.

The results reveal that bivalve shells, even when restricted to surficially collected samples of one species only, can provide a continuous record spanning back thousands of years. Multi-millennial time series assembled by dating individual shells represent a potentially important, high-resolution proxy for studying environmental, oceanographic, and climatic changes in the Holocene.