Mark Howard Thiemens is an American geophysicist and chemist, best known for his co-discovery of mass-independent fractionation in 1983 and research into planetary evolution and the atmospheric chemistry of climate change. He is a Distinguished Professor and the John Doves Isaacs Endowed Chair in Natural Philosophy of Physical Sciences at the University of California, San Diego in the former Department of Chemistry and Biochemistry, which split in July 2026 into the Departments of Chemistry and of Biochemistry and Molecular Biophysics. He was elected to the National Academy of Sciences in 2008 and the Royal Society in 2024. For his contributions to geochemistry he was awarded the Leonard Medal in 2017, and the Magellanic Premium in 2026. His studies have crossed a broad range of topics including basic physical and quantum chemistry, solar system development, the origin and evolution of life on early Earth, stratospheric chemistry, climate change and greenhouse gas identification, Martian atmospheric chemistry, past and future and isotope geochemistry. His work combines photochemical isotope studies, both laboratory and synchrotron based, field work in the South Pole, Greenland Summit and the Tibetan Himalayas for climate and geological sampling across China for early Earth rock records. His non-isotope work has included discovery of an unknown source of the greenhouse gas nitrous oxide that lead the global industrial elimination of all emissions, a major contribution to changing global climate change. Thiemens has worked on developing new imaging techniques for space mission return samples and detection of superconductivity in nature.
Education Thiemens earned his bachelor of Science degree from the University of Miami. His studies with isotope geochemist Cesare Emiliani, PhD student of Harold Urey and a co-discoverer of paleoclimate temperature determination stimulated his interests in isotopes. Thiemens received a MS from Old Dominion University and PhD from Florida State University for his research using stable isotopes and particle identification using the FSU Van de Graff accelerator. He moved to the University of Chicago at the Enrico Fermi Institute for Nuclear Studies (1977-1980) where he worked with Robert N. Clayton using lunar samples to track solar wind origin and evolution, meteorite cosmochemistry, and early atmospheric chemistry.
Career Thiemens moved to the department of chemistry at the University of California, San Diego in 1980, where he was hired as an assistant professor as a replacement for Hans Seuss and took over the laboratory of Nobel Laureate Harold Urey. He was promoted to full professor in 1989, and served as the chair of the department of chemistry and biochemistry from 1996 to 1999. He was the founding dean of the division of physical sciences and served from 1999 to 2016.
Research Thiemens' research at UCSD initiated after a rebuild of the Urey isotope ratio mass spectrometer to allow measurement of both oxygen isotope ratios (18O/16O, 17O/16O). His first publication as an assistant professor reported in Science the first mass independent isotope effect which occurred during ozone formation. This was the first demonstration of a chemical process that could alter isotope ratios in a manner independently of mass difference. Most strikingly was that the pattern of mass independent and the 17O/16O,18O/16O variation varied equally and reproduced the same pattern observed in primitive inclusions of the Allende carbonaceous chondritic meteorite. The underlying assumption for the inclusions anomaly deriving from a nucleosynthetic component was incorrect and new models for early Solar System formation were needed and have evolved since. Much of Thiemens research has been dedicated to experimentally exploring the relevant fractionation processes that may account for the observations; including synchrotron photodissociation effects in CO. The gas to particle formation process of the first solids in the nebula have also experimentally been shown to produce the mass independent anomaly. Meteoritic material studies of Thiemens in sulfur isotopes have shown that sulfonic acids from chondritic meteorites have shown that photochemical processes have been important contributor to their molecular synthesis as well other sulfur species. To interpret mass independent isotope effects during photodissociation, Thiemens has worked in collaboration with Raphy Levine of Hebrew University to interpret mass independent isotope effects during photodissociation and better explore the fundamental chemical physics of the processes. The understanding of the basis of the ozone effect has been extensively studied by Nobel Laureate Rudy Marcus and catalyzed deeper insight into the chemical physics. Thiemens has worked broadly on understanding the Earth system. Thiemens and Trogler identified a source of 10% of the increasing emissions of nitrous oxide, a greenhouse gas with a radiative forcing 200 times CO2 on a per molecules basis and a 100-year plus lifetime with unidentified sources. It was shown that the manufacture of adipic acid, used in nylon production is a globally important source. In the year post publication, a global inter industry consortium banded together to eliminate all N2O emissions, with far reaching climate impact.
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