AMINOSTRATIGRAPHY OF THE MARINE PLIO-PLEISTOCENE OF FLORIDA – HISTORY AND PROGRESS REPORT

One of the earliest applications of amino acid racemization (AAR) in Quaternary stratigraphy was the work of Mitterer and Hare in the late 1960s and early 1970s. Using fossil mollusks obtained primarily from museum collections, their work demonstrated that the degree of racemization increased with stratigraphic age and that kinetic modeling could be used to assign numerical age estimates to the observed aminozones. Although the kinetic models and resulting ages are uncertain, the basic premise of using AAR to map stratigraphic units remains valid. More recent field and laboratory study has been conducted by several groups, involving collaboration among researchers at the universities of Waterloo, Maine, Florida (Florida Museum of Natural History), South Florida, and Delaware, along with parallel studies by USGS colleagues. This effort has focused on our own field collections from active borrow pits to avoid ambiguities about stratigraphic or locality terminology associated with museum collections, with corresponding efforts to match field collections with independent biochronology and U-series or strontium-isotope isotopic data on associated fossils. Excavated exposures have been available at numerous sites since the mid-1980s. Analytical methods employed by different investigators have varied and include ion-exchange liquid chromatography (IEX), gas chromatography (GC), and reverse-phase liquid chromatography (RPLC). In order to compile all available data in a common format, a large database of AAR data for nearly 180 Late Cenozoic sites (and associated independent geochronologic information) is now being compiled in an MS-Access database with specific information on localities, collectors, and AAR analyses. This database is being interfaced with Google Earth in order to create a single archive for AAR results. A regional synthesis of AAR data, and their implications for stratigraphic analysis, will result from this effort. Additionally, insights into both diagenetic issues and potential calibration of racemization kinetics are pending outcomes of this study. Reliable age-calibrated results are now available for the Leisey and Caloosa Shell pits in southern Hillsborough County, where units representing both MIS 5 and MIS 9 or 11 sea-level high-stands are found in superposition.