Scientific Objectives and Lab Approach
I lead a research group at Johns Hopkins University that uses isotope geochemistry to study different aspects of the Earth system, focusing mainly on the junction of geology, biology, and climate. Our basic objectives are 1) to learn more about the Earth system; 2) to develop methods for studying the Earth system; and 3) to provide students with solid scientific training by carrying out objectives 1 and 2.
Topics we have worked on include the history of the East Asian monsoons, the global expansion of C4 vegetation during the late Neogene, paleoenvironments of human evolution, and seawater paleotemperature and isotopic reconstruction during the late Paleozoic and Mesozoic.
Solid-state clumped isotope reordering We are working on developing a geothermometer / geospeedometer based on solid-state diffusion of C and O in carbonate minerals. This has involved lab experiments where we heat calcite mineral specimens at controlled temperatures and pressures to drive solid state reordering of 13C-18O bonds. We characterize the clumped isotope compositions using our mass spectrometer, and then develop kinetic models to explain the observed reaction progress. We recently published the 'retrograde' (geospeedometry) aspect of this thermometer, describing how clumped isotope compositions of carbonate minerals evolve during the cooling of rocks, and how the final 'locked-in' clumped isotope compositions relate to the rate of cooling (Passey and Henkes, 2012, EPSL). Graduate student Greg Henkes is now working on writing up the prograde and circuit part of the story: How do clumped isotope compositions evolve given specific temperature histories during burial and exhumation, and how 'hot can a sample get' before it loses it's primary clumped isotope signature?
Triple oxygen isotope compositions of CO2 and carbonates We have recently developed methods for converting oxygen in CO2 to oxygen in H2O. Using a water CoF3 fluorination system built by Naomi Levin and her student Shuning Li, we then convert the H2O oxygen to O2, permitting high-precision triple oxygen isotope analysis. Grad student Huanting Hu is studying the triple oxygen isotope compositions of dinosaurian eggshells as a basis for reconstructing past levels of atmospheric CO2, and grad student Haoyuan Ji has initiated a study of soil carbonates, seeking to find out whether evidence of evaporation and aridity can be retrieved with this measurement.
Benjamin H. Passey ** Johns Hopkins University ** Department of Earth and Planetary Sciences
120 Olin Hall ** 3400 N Charles St, Baltimore MD 21218