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Alex Sessions
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Email: als@gps.caltech.edu
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Emi Y. Vasquez
Phone: 626-395-8372
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Home  /  People  /  Alex L. Sessions

Alex L. Sessions

Professor of Geobiology
Option Representative: Geobiology; 
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-.
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Research Summary
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.

Research Options

Environmental Science and Engineering;  Geobiology; 

Research Areas

Isotope Biogeochemistry;  Stable Isotope Geochemistry; 

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.

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

Ge/ESE 143. Organic Geochemistry. 9 units (3-2-4); third term, 2020-21. Prerequisites: Ch 41 a or equivalent. Main topics include the analysis, properties, sources, and cycling of natural organic materials in the environment, from their production in living organisms to burial and decomposition in sediments and preservation in the rock record. Specific topics include analytical methods for organic geochemistry, lipid structure and biochemistry, composition of organic matter, factors controlling organic preservation, organic climate and CO2 proxies, diagenesis and catagenesis, and biomarkers for ancient life. A laboratory component (three evening labs) teaches the extraction and analysis of modern and ancient organic biomarkers by GC/MS. Class includes a mandatory one-day (weekend) field trip to observe the Monterey Formation. Offered 2020-21.
Instructor: Sessions
Ge/ESE 140 c. Stable Isotope Biogeochemistry. 9 units (3-0-6); third term, 2019-20. Prerequisites: Prerequisites: Ge 140 a or equivalent. An introduction to the use of stable isotopes in biogeochemistry, intended to give interested students the necessary background to understand applications in a variety of fields, from modern carbon cycling to microbial ecology to records of Ancient Earth. Topics include the principles of isotope distribution in reaction networks; isotope effects in enzymemediated reactions, and in metabolism and biosynthesis; characteristic fractionations accompanying carbon, nitrogen, and sulfur cycling; and applications of stable isotopes in the biogeosciences. Offered 2019–20.
Instructor: Sessions
Ge 145. Isotope-Ratio Mass Spectrometry. 9 units (1-4-4); first term, 2019-20. This class provides a hands-on introduction to the construction and operating principles of instrumentation used for isotope-ratio mass spectrometry. The class is structured as a 1-hour lecture plus 4-hour lab each week examining the major subsystems of an IRMS, including vacuum systems, ionization source, mass analyzer, and detector. Laboratories involve hands-on deconstruction and re-assembly of a retired IRMS instrument to examine its components. Course is limited to 6 students at the discretion of the instructor, with preference given to graduate students using this instrumentation in their research. Taught in odd-numbered years; offered 2019-20.
Instructor: Sessions
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