Alex L. Sessions

Alex L. Sessions is a Professor of Geobiology in the Division of Geological and Planetary Sciences at the California Institute of Technology (Caltech). He serves as the option representative for geobiology and maintains affiliations with both the geobiology and geochemistry academic options. Dr. Sessions teaches three specialized courses: Organic Geochemistry (Ge 143), Isotope Ratio Mass Spectrometry (Ge 145), and Stable Isotope Biogeochemistry (Ge 140c), with Ge-145 offered in fall and the others in spring during alternate years. He also co-directs the International Geobiology Course, a five-week summer graduate training program hosted at Caltech and the Wrigley Marine Institute.

Dr. Sessions' educational background includes:

• Ph.D. in geochemistry from Indiana University, Bloomington (2001)
• M.Sc. in geochemistry from Dartmouth College, Hanover (1996)
• B.A. in geology from Williams College, Williamstown (1991)

His research focuses on stable isotope biogeochemistry and organic geochemistry, with particular emphasis on the biogeochemistry of marine sediments and the deep subsurface, controls on hydrogen and sulfur isotope distribution in lipids and sedimentary organic matter, and biomarker records of Precambrian environments. A significant portion of his work involves developing novel isotopic analytical methods, including compound-specific analysis of 34S in volatile organics and isotopic clumping in organic molecules. His laboratory employs GC/P/IRMS techniques to analyze hydrogen isotopes in individual organic compounds rather than bulk materials, providing greater specificity for understanding biogeochemical processes.

Analysis of Dr. Sessions' publication record reveals a consistent research trajectory centered on hydrogen isotope fractionation across biological systems. His work spans from modern environments including oceans, soils, plants, and bacteria to ancient 3 billion-year-old rocks. A distinctive pattern in his research involves manipulating microbial growth conditions to understand how metabolic pathways affect lipid D/H fractionation, with potential applications for studying uncultured organisms and ancient life. His publications demonstrate increasing technical sophistication in isotopic measurement techniques while maintaining focus on fundamental biogeochemical questions.

Dr. Sessions leads an active research group comprising a lab manager, 4 postdocs, and 5 PhD students. He co-founded Isotomics with John Eiler to commercialize clumped-isotope measurements in organic molecules. His extensive field experience includes research in Yellowstone National Park, Santa Barbara Basin, Owens Valley, Mono Lake, South China, and the northeastern Pacific. Professional affiliations include the Geochemical Society, European Association of Organic Geochemists, American Geophysical Union, and Geological Society of America.