RECONSTRUCTING PALEO-ENVIRONMENTAL CONDITIONS USING THE BIVALVE SEMELE CASALI FROM UBATUBA BAY, BRAZIL

This project was performed to detect environmental changes recorded in the shells of the bivalve Semele casali from a mixed carbonate-siliciclastic shelf off the coast of off the coast of the state of São Paulo, Brazil, in Ubatuba Bay. The Semele casali shells were collected from numerous localities within the Ubatuba Bay, and the shells range in water depth from 10 to 45 meters. Carbon and oxygen stable isotope profiles were collected through the ontogeny of a number of the bivalve shells as an environmental proxy to create a high resolution time series of seasonal fluctuations in the sea water temperature. Trace element analyses (measuring barium and manganese concentrations) were also conducted on the bivalve shells in an effort to indentify freshwater runoff events and upwelling events from deeper, nutrient-rich waters. Upwelling events should be recognized by an increase in the Ba/Ca ratio, along with a wider range of δ¹⁸O values and a notable decline in δ¹³C values. Freshwater runoff events, on the other hand, should exhibit increases in the Mn/Ca and Ba/Ca ratios, as well as decreases in δ¹⁸O and δ¹³C values. Our current results suggest that the majority of the Semele casali bivalves grew primarily in marine background conditions, but a few were occasionally exposed to freshwater runoff or deep water upwelling events.

In addition, amino acid racemization was used to determine the ages of all of the collected Semele casali shells, with radiocarbon dating methods used for amino acid racemization calibration on a number of pre-analyzed shells. The shells range in age from modern–up to 10,000 years old. Radiocarbon dating of the shells from different collection sites that varied in depth from 10 to 45 meters suggests that water depth has a limited affect on the racemization rate of the amino acids. A single calibration formula has been generated that allows for all Semele casali shells at any depth to be age-dated using amino acid racemization techniques.