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Alex L. Sessions

Professor of Geobiology
Contact information for Alex L. Sessions
Contact Method Value
Email: als@​
Administrative Assistant:
B.A., Williams College, 1991; M.Sc., Dartmouth College, 1996; Ph.D., Indiana University (Bloomington), 2001. Assistant Professor, Caltech, 2003-10; Associate Professor, 2010; Professor, 2010-.

Research Interests

Isotopic biogeochemistry and organic geochemistry, including: controls on the distribution of hydrogen and sulfur isotopes in lipids, sedimentary organic matter and petroleum; biomarker and isotopic records of the Neoproterozoic; and the marine organic carbon cycle. Also, the development and application of novel isotopic analytical methods for biogeochemistry, such as compound-specific analysis of 34S in volatile organics, and the analysis of 13C in whole cells sorted by flow cytometry.
My research can be broadly described as isotopic biogeochemistry. Put another way, I am interested in the interaction of the biological and geological worlds, particularly in the organic materials that are so crucial to that interaction. Since the time of Charles Darwin, we have known that our physical environment helps to shape life through the process of evolution. More recently, we have come to understand that the guidance is reciprocal, and that biology also influences the evolution of the Earth (the oxygen we breath today is present only because of photosynthesis by plants). One of the goals of my research is to understand how this happens. Several specific projects are described following the links on your left.

I study the waxy, organic molecules that are produced by living organisms and eventually buried and preserved in sediments. Known generically as lipids, these molecules are the raw materials that eventually produce petroleum and natural gas, as well as record a rich history of life over the past ~3 billion years. Because they are so well preserved, lipids are often called "molecular fossils" and can be used to study the origins and evolution of life. As an example, we have used these molecular fossils to understand environmental conditions that prevailed during one of the Proterozic low-latitude glaciations, which have become known as "Snowball Earth" events.

The tools of my research are those of the organic chemist, primarily gas chromatography and mass spectrometry. These instruments allow us to separate individual organic compounds from complex samples, to identify the structures of those compounds, and to measure their abundance and stable-isotopic composition. We use the isotopes of carbon, hydrogen, and nitrogen as a sort of chemical fingerprint to follow organic molecules through the geologic environment.

Selected Publications

For a list of my most recent publications please see my research website.