7 Recent Advances in Paleontology

Thursday, 24 March 2011
3:10 PM - 5:00 PM
Salon B

A MULTI-MILLENNIAL RECORD PROVIDED BY AMINO-ACID DATED BIVALVE SHELLS FROM THE SOUTHERN BRAZILIAN SHELF

Paper number: 7-5
Program: Technical Sessions
Time: 4:30 PM – 4:50 PM

Authors

Abstract

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.