Geocheminar spring-2012

Extending the temperature record beyond the modern instrumental record is critical to understanding past climate variability and ecosystem response. Lake Tanganyika, Africa is one of the world’s largest rift lakes and is a hotspot of endemism. Our ability to predict the lake’s ecological response to climate change is of the utmost importance for fishery economy in eastern Africa. Here, we use ostracode trace metal geochemistry as a paleoenvironmental indicator in Lake Tanganyika. The trace element compositions of ostracode valves reflect discriminatory element uptake. This uptake reflects ambient environmental conditions and has previously shown promise for quantitative paleotemperature determination. In this study, HR-ICP-MS geochemical data are compared to an existing TEX-86 temperature record from Lake Tanganyika. Two ostracode species were chosen for analyses from core LT-98-58 (1759 +/- 133 AD-modern). Mecynocypria opaca is a near bottom swimming species whereas the lifestyle of Romecytheridea ampla is uncertain. Molar Mg/Ca ratios for M. opaca range from .04 to .16, and a trend towards increased Mg/Ca begins ~1880 AD. Molar Mg/Ca ratios for R. ampla range from .05 to .2, increase beginning at 1880 AD and decrease within the most modern sediments. Sr/Ca ratios in both species range from .003-.006 and remain relatively stable, indicating that changes in Mg/Ca are likely the result of temperature rather than salinity. The M. opaca Mg/Ca record closely resembles the existing TEX-86 paleotemperature record of Tierney et al. (2010) for the past ~240 yr. The variations within R. ampla Mg/Ca may be due to pore water geochemistry or some other unknown parameters. These preliminary results demonstrate that future research is necessary if the use of ostracode trace metal geochemistry is to be used as a paleotemperature proxy in lakes dominated by endemic species. Finally, preliminary data from an initial calibration set for the use of carbonate clumped isotope thermometry in lakes are shown.