OSTRACOD CARAPACE GEOCHEMISTRY: LASER OPTIMIZATION AND PALEOENVIRONMENTAL RECONSTRUCTION

Ostracods are small (up to 1 mm) crustaceans that moult a calcitic carapace. The carapace records the geochemistry of the ostracod’s host water and it has been suggested that this allows for reconstruction of the water’s salinity, temperature, and other paleoenvironmental conditions. The use of the geochemistry of ostracod carapaces is a recent development in paleoceanography, however, and methods for data collection still require improvement. In this study, a random sample of Holocene ostracods was collected from a hypersaline lake on Big Darby Island in the Bahamas, and two sets of data were obtained. A sample of the collected ostracods was mounted and lasered with a laser-ablation inductively-coupled plasma mass spectrometer (LA-ICP-MS) to measure the abundance of elements in each carapace. Elements chosen to analyze for paleoenvironmental reconstruction include ¹¹B and ²³⁸U as pH indicators, and ²⁵Mg and ⁸⁸Sr as temperature indicators. In addition, a sample of ostracods was used to test the effect of changing parameters (laser energy and frequency, specifically) on the signal obtained from lasering. Combinations of 5, 10, and 15 J/cm² laser energy and 4, 5, and 10 Hz frequency were chosen for this optimization run.

The data from the sensitivity tests show that the higher combinations of frequency with laser energy of 15 J/cm² provide the most precise geochemical signal. The concentrations of ¹¹B and ²³⁸U are consistent throughout each lasered carapace, indicating that these elements have potential to act as pH and overall paleosalinity indicators in ostracod carapaces. ²⁵Mg and ⁸⁸Sr, however, display inconsistent values throughout the lasered carapaces; from this data, it is inconclusive as to whether these elements record temperature in ostracod carapaces. Additional work is necessary to further explore the environmental conditions that govern the uptake of Mg and Sr into the ostracod carapace.