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.

Thus the ideal student project goes something like: Identify an interesting and challenging scientific question, preferably somewhat off the beaten path...  Find a way to do it: develop new analytical methods, streamline existing methods, create a model, go to the field, etc....  Develop conceptual / physical / numerical models for interpreting / explaining / predicting data...  Figure out what it all means and creatively package the results for meeting presentations and for publications.

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 paleotemperature reconstruction during the late Paleozoic.

What's New...

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 / describe the observed reaction progress.  We have just published the 'retrograde' (geospeedometry) aspect of this thermometer that describes how clumped isotope compositions of carbonate minerals evolve during 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' or 'circuit' part of the story:  How do clumped isotope compositions evolve given specific temperature histories during burial and exhumation?  Graduate student Haoyuan Ji is beginning to tackle solid state reordering in dolomite -- an experimentally tricky proposition given the tendency for dolomite to break down at high temperatures.

Triple oxygen isotope compositions of CO2 and carbonates  We have recently developed a method for converting oxygen in CO2 to oxygen in H2O.  We then fluorinate this H2O to O2, permitting high-precision triple oxygen isotope analysis.  Graduate student Huanting Hu is interested in using this method to study global fluxes of carbon in the short-term carbon cycle.  We are also interested in using the method to estimate past levels of atmospheric carbon dioxide.  Stay tuned!