Stanford University

Geological Sciences Seminar: Dr. François Tissot, Caltech

Braun Corner Building 320 room 220 and Zoom
Alumni, Faculty, Staff, Students
Department of Geological Sciences

The U paleoredox proxy: it’s not just for deep time anymore

In less than 15 years, the 238U/25U ratio has seen its status completely transformed: from a long-cherished constant value onto which U-Pb ages could reliably be anchored, to a powerful tracer of seafloor anoxia in Earth’s past. Indeed, improvements in mass spectrometry have revealed variability in 238U/235U at the permil to sub-permil level, which has opened new avenues of research in cosmochemistry, high-T geochemistry, and paleoredox reconstruction. This latter has received unparalleled attention in recent years (see figure), and the 238U/235U is now arguably the most widely used proxy for global ocean anoxia reconstructions in Earth’s past. In this talk, I will begin by discussing how the U system became such a prevalent tool, and highlighting key milestones that established the strengths and limitations of the system. I will then present some recent and ongoing work in my group that aims to tackle the question "how robust and quantitative is the U paleoredox proxy?”. In particular, we will look at (i) how U isotope signatures are modified during carbonate diagenesis, (ii) the potential for inverse modeling approaches to overcome the pitfalls of illustrative forward modeling when estimating the extent of oceanic anoxia from the U data, and (iii) the promise of biogenic carbonates as recorders of subtle redox variations in the recent past (i.e., Quarternary). 

Dr. François Tissot is an isotope chemist and broad-based Earth scientist. He trained as an engineer in Geology at the ENSG (France) and received his Ph.D. in Geochemistry and Cosmochemistry from the University of Chicago in 2015. François was also the W.O. Crosby Postdoctoral Fellow at the Massachusetts Institute of Technology from 2016 to 2017. He joined Caltech in 2018 where he created the Isotoparium: a state-of-the-art facility for high-precision isotope studies. His research focuses on both cosmochemical and geochemical problematics such as (i) the evolution of the Solar System through the isotopic characterization of meteorites and their inclusions, (ii) the reconstruction of paleo-redox conditions in the ocean/atmosphere system, and (iii) the potential of non-traditional stable isotopes for studying magmatic processes. When science allows, he enjoys climbing, answering the many questions of his two young daughters, photography, and Mel Brooks’ musicals. 


For the Zoom information, please contact Rey Garduño (

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