Speaker: DIstinguished Professor Mark Thiemens University of Calfornia San Diego

 

Title: "A Five Sulfur Isotop Approach to Interpreting the Early Earth Atmosphere: from lab to rock record to Mount Everest"

Abstract: It is now widely accepted that the observation of the Mass Independent Isotopic character of early earth sulfur samples may trace the origin and evolution of oxygen and life. The basis of the interpretation evolving out of the UCSD lab uses the presence or absence mass independent isotopic sulfur of SO2 as a measure of oxygen and ozone. At significantly lower oxygen and ozone levels shorter UV light penetrates to the earth’s surface where SO2 may be photolyzed, which otherwise only occurs in the stratosphere in the present day. The photolysis of SO2 at this wavelength, for quantum mechanical reasons, creates an anomalous, or MIF fractionation in the isotopes. Laboratory photochemical measurements and extensive measurements of the rock record have greatly amplified our understanding of the record.

The limit at present is that the record is difficult to explain by one process, such as photolysis. A second issue is that it is now known that nearly all sulfate in the earth’s atmosphere is mass independent and lies within the Archean isotopic world. This is not explainable by SO2 photolysis and consequently a major part of the sulfur system is unknown.

Using measurements of all four stable isotopes in sulfur and radiogenic 35S (87 day half-life) in atmospheric sulfate from present to the past 150 years at locations from La Jolla, to the South Pole and Mt Everest new information on sulfur processes have been obtained that address these issues.

Bio: Mark Thiemens is a distinguished Professor of Chemistry and Biochemistry and Chancellors Associates Chair in the Department of Chemistry and Biochemistry at UCSD. He began his career as an Assistant Professor at UCSD, arriving following a Post doc at the Enrico Fermi Institute for Nuclear Studies at the University of Chicago. He was hired as a replacement for Hans Suess and took over the laboratory of Harold Urey.

Thiemens is the discoverer of mass independent isotope effect. His work involves experiments and development of the basic quantum chemistry theory of these processes, including synchrotron and laboratory photochemical processes. His measurements focus upon solar system origin and evolution of the oxygen, sulfur, and nitrogen records in meteorites, the moon, Mars, and the early earth. He works on Archean and other old records of the earth to understand its atmospheric origin. He examines paleo atmospheric composition and has collected ice samples at the South Pole, Greenland Summit, the Himalayas, and Mt Everest. He studies present day atmospheric chemistry using isotopes and has measured most oxygen bearing species for source identification and aerosol sulfate and nitrates for their record of the oxidizing capacity of the earth over time.

Thiemens is a member of the National Academy of Sciences and the American Academy of Arts and Sciences. His awards include the Goldschmidt medal (Geochemical Society), Leonard medal (Meteoritical Society), and the E.O Lawrence medal from the Department of Energy. He has an asteroid named after him: Asteroid 7004markthiemens.