Abstract Introduction Acknowledgements Material and Methods - Core Sites and Collection - Faunal/Floral Analyses > Ostracode Shell Chem. Analyses - Prelim Age Model Results Discussion Significant Findings, Implications And Future Work References Appendices PDF version

Ostracode Shell Chemistry Analyses

In addition to faunal analyses, geochemical studies of metal/calcium ratios also were carried out on ostracode shells from the cores at Featherbed Bank (SEI297 FB-1) and No Name Bank (GLW402 NNB) to reconstruct patterns of salinity variability. Metal/Ca ratios (especially magnesium/calcium (Mg/Ca)) are useful proxies for estimating past salinity changes in Florida and Biscayne Bays (Dwyer and Cronin, 2001). For efficiency and to allow for direct comparison between ostracode shell chemistry data and faunal assemblage data, ostracodes selected for analyses were picked directly from slides or sample residues used in the faunal assemblage analyses.

Four to five adult valves of ostracode Loxoconcha matagordensis were selected from each sample interval. These valves were processed and analyzed individually for Mg/Ca, Sr/Ca, and Na/Ca ratios by direct current plasma atomic emission spectrophotometry (DCP-AES) following procedures in Dwyer and Cronin (2001, and references therein). Precision on Mg/Ca, Sr/Ca, and Na/Ca measurements is around 2%, 4%, and 10%. Some of the disparity in precision results is because the instrument is optimized for Mg/Ca ratio analysis since Mg/Ca has proven to be the most useful paleoenvironmental indicator. While their usefulness is unclear, Sr/Ca and Na/Ca ratios are collected because it requires no extra effort and with further study these data may provide additional paleoenvironmental information.

Prior to DCP-AES analysis, each valve was examined and assigned a preservation index (optical clarity-based dissolution index). In core SEI297-FB1, valve lengths were measured using a binocular microscope at 50X magnification in order to evaluate the possible use of this measure as a proxy indicator of water temperature. Preliminary studies by Japanese and US ostracode workers suggest that valve length in Loxoconcha is temperature dependent. Gender and valve (left, right) were also recorded. An effort was made to limit the possibility of analyzing valves from the same carapace, by selecting female left valves from each sample interval. Two sample intervals from the SEI297- FB1 core were replicated to evaluate intra-sample variability.