Weathering profiles provide a record of chemical and physical processes occurring at the surface of terrestrial planets. 

The profiles reflect the balance between chemical-physical weathering and chemical-physical erosion, providing insight into landscape evolution and global geochemical cycles.  A unique challenge is to place the information preserved by weathering profiles into a temporal context, i.e., when did the weathering reactions take place?

In collaboration with Paulo M. Vasconcelos and Jon Heim (U. Queensland) and Ken Farley (CIT), I developed a technique based on the (U-Th)/He system to reliably determine absolute precipitation ages of supergene polycrystalline goethite (see SEM image at right).  Although ⁴He diffuses from polycrystalline goethite at Earth's surface temperatures, we can determine absolute precipitation ages using the ⁴He/³He methodology to quantify a correction for diffusive loss.

We are currently studying suites of samples from the Brazilian Amazon and Australia to constrain rates of weathering front propagation.  Temporal knowledge of weathering reactions also gives information on the potential periodicity of chemical weathering relating to past climatic conditions.