Expert on geology and geochemistry of the Earth.
- Expert on geological conditions of the early Earth: the formation of oceans, growth of continents, and surface conditions hosting the first life on Earth.
- Expert on stable isotope geochemistry as applied to genesis of rocks and minerals, astrobiology, paleoclimatology, and meteorites.
- Founded WiscSIMS, the Wisconsin Secondary Ion Mass Spectrometer Lab at UW-Madison for in situ microanalysis of stable isotope ratios.
- Fellow: AAAS, American Geophysical Union, Geochemical Society, European Association of Geochemistry, Geological Society of America, and Mineralogical Society of America. Past-president: Mineralogical Society America.
- Discovered the oldest samples of Earth, 4,400 million year old zircons, showing that oceans and clement conditions existed on the surface of earliest Earth.