VARIABILITY IN OSTRACODE GEOCHEMISTRY: A CALIBRATION STUDY

Ostracode geochemistry is a valuable tool in reconstructing lacustrine environments because ratios of Mg/Ca and Sr/Ca within ostracode valves can be used as indicators of paleotemperature and paleosalinity. A clear understanding of ostracode ecology, life cycle, and intra-species variability is necessary prior to any attempt at interpreting paleorecords. Furthermore, any geochemical study must analyze an appropriate number of valves from each stratigraphic layer in order to capture the full range of intra-geochemical variability introduced from differences in temperature and water chemistry during the time of valve secretion. Understanding this variability will aid in separating low frequency climate shifts from high frequency noise associated with seasonality and stochastic storm events when analyzing a core record. This study aims to determine the trace element (Mg, Sr) and ¹⁸O isotope variability within living populations of Cyprideis americana and Hemicyprideis setipunctata from three lakes on the island of San Salvador, Bahamas. These lakes, North Storr’s Lake, Reckley Hill Pond, and Blue Hole #5, range from hypersaline to oligohaline, capturing the vast range of island surface water chemistry. C. americana and H. setipunctata are present in the majority of San Salvadorian lakes and are also frequently preserved within lake cores, making each an ideal candidate for this calibration study. Surface sediment samples were collected and dyed with a 1% solution of Rose Bengal to determine the organisms’ mortality at time of collection. Dyed articulated valves were further inspected by ESEM to verify the presence of soft parts. Only those ostracodes that were articulated, dyed and had soft parts were considered to be living at the time of collection. For each living ostracode selected, a modern dead ostracode was also selected for geochemical analysis. ICPMS was used to determine the ¹⁸O/¹⁶O, Mg/Ca, and Sr/Ca ratios of individual valves. These data will be used to determine: 1) the geochemical variability within the living and modern dead populations, and 2) if there is a significant difference in the geochemistry of living and modern dead ostracodes. This study will be key to determining the usefulness of C. americana and H. setipunctata as geochemical indicators of climate change on San Salvador.