Awarded to: Daniel Stolper
Abstract: An Experimental and Theoretical Calibration of CH₄-H₂-H₂O Hydrogen Isotopic Equilibrium from 3-200℃
Medal lecture in:
Session 06l in Room 2, Goldschmidt2020 - Virtual Venue on Thursday 25th June 00:51 - 00:54

Citation: Daniel Stolper was born and raised in Pasadena, California. He earned an A. B. degree from Harvard, was Fulbright Scholar for a year at the University of Southern Denmark, and did his Ph. D. at Caltech. After a postdoc at Princeton, Daniel moved to the University of California, where he is Assistant Professor at the time of writing. Daniel is a geochemist whose research has 3 broad themes. The first is the isotope geochemistry of methane. Daniel did pioneering work using a reimagined isotope ratio mass spectrometer conceived and developed by his Ph. D. advisor, John Eiler. This instrument enabled the user to determine the abundance of methane isotopologues with 2 heavy atoms, ¹³CH₃D or ¹²CH₂D2. The property of merit is the clumped isotope anomaly, the departure in the abundance of an isotopologue with 2 heavy atoms from its stochastic abundance. Daniel and his collaborators used experiments and theory to determine the temperature dependence of the clumped isotope anomalies in methane. They then showed that many methane sources had a clumped isotope composition at isotopic equilibrium, given their temperature of formation or alteration. They also showed that kinetics induced disequilibrium in biogenic methane sources, especially when methane production was rapid. They developed a concept for classifying methane according to its C and H isotope composition, its clumped isotope composition, and the abundance of methane relative to that of other light hydrocarbons. They also explored the application of clumped isotopes to studies of methane geochemistry in particular deposits. The second theme of Daniel’s research is an investigation of clumped isotope diagenesis in carbonates. With colleagues, Daniel made models of diagenetic changes to clumped isotope abundances, and challenged the models’ simulations with data. Daniel had the deep insight that apparently large changes observed when calcites were first heated could be attributed to isotope exchanges with neighboring calcite molecules. He and his colleagues worked out the roles of diagenesis in the presence and absence of water. They also showed that one could reconstruct environmental information with diagenetic models of calcite recrystallization in deep-sea sediments. The third theme of Daniel’s research is centered around O₂, in both geological and biogeochemical contexts. Daniel reasoned that he could use the oxidation state of ocean crust, accessed via ophiolites and island arc volcanics, to determine the oxidation state of the deep ocean. The connection comes from the fact that ocean crust is altered by reaction with seawater. Seawater-basalt exchange then imprints the ocean crust, and eventually arc volcanics, with the oxidation state of the deep ocean. This approach then gave a time of about 400-500 Ma for elevating the deep ocean O₂ concentration. Since gases in the deep ocean mix with air over a timescale of ~ 1 kyr, the history of deep ocean oxygenation may be similar for the surface ocean and atmosphere. Daniel has also worked on the more recent history of atmospheric O₂. He repurposed a large database of O₂ concentration in ice core trapped gases to determine changes over the past 800 kyr. This work showed that the O₂ concentration is decreasing at a rate of about 1200 ppm/Myr (out of 210,000 ppm O₂ in air). The imbalance is very small, pointing to the role of strong feedbacks in the carbon cycle that remain to be identified. On the biogeochemical side, Daniel characterized oxygen isotope fractionation associated with respiration with exceptional depth. He showed that the fractionation patterns he observed implied a 2-step process for O₂ consumption at cytochrome oxidase: a reversible step in which O₂ is bound, followed by a kinetic step in which O₂ is reduced. He also showed that O₂ isotope fractionation at cold temperatures was much smaller than heretofore estimated. This observation accounts for weak O₂ isotope fractionation associated with O₂ consumption in the deep ocean, solving a mystery going back nearly 50 years. Daniel is a brilliant scholar who takes great joy in the doing of science. He is a wonderful colleague: interactive, stimulating, and deeply knowledgeable about a very broad range of topics. He is hardworking and efficient. He is generous intellectually and personally. Daniel will contribute to the community of earth scientists, and provide leadership in the research, for many years to come.

Response: Thank you Michael and thank you to the Geochemical Society for this honor. Although I am unable to deliver this in person due to the COVID-19 pandemic, I still take pleasure in being able to commit to words my thanks to the many people who have been instrumental in getting me to this point today both personally and professionally. Receiving the Clarke Medal means a lot to me as many of my heroes, mentors, and friends have received it. My father received this award exactly 35 years ago, and so I feel a personal connection to it. The citation covers various things I have done as student, postdoc, and professor, and I would like to speak briefly about these experiences so as to acknowledge the large roles my mentors and friends played in bringing me here and to suggest a few lessons I have learned along the way.
One of the topics cited is my work on methane clumped isotopes conducted as a graduate student at Caltech with John Eiler and Alex Sessions. I began my PhD at Caltech in 2009 having just completed a Fulbright in Don Canfield’s lab in Denmark and before that, a degree in Earth and Planetary Sciences at Harvard — not a day goes by where I don’t think about something I learned at all three universities. At Caltech, I began working with John Eiler and, in large part because of John, fell in love with isotope geochemistry. In my early years as a student, there was a palpable feeling of excitement in the air as a special mass spectrometer John had commissioned was arriving and it was expected to be able be able to measure things previously unmeasurable. A primary target was to be the clumped isotopic composition of hydrocarbons, and John was looking for volunteers. I remember thinking about it one day, and I decided that working on this was a risk worth taking. In my opinion, methane was the simplest hydrocarbon to work on and was environmentally important, and, on that limited basis, I decided that was what I should go after. In retrospect, I find it difficult to believe that the whole thing worked out. First, the machine had to work; second, we had to get the measurements working and calibrated; and third, we had to find something interesting in nature. But it all worked out (more or less). What did I learn from this? First, never underestimate the importance of luck and good timing in a person’s career. But it is also important to take on reasonable risks so as to put yourself in a position to get lucky. Second, following on this and using a baseball analogy, sometimes it is OK to walk up to the plate and take your swing without overthinking the problem. Otherwise, you might just convince yourself the problem is impossible or already solved. I thank John for teaching me these lessons. John gave me unbelievable freedom, including the freedom to fail, and thus to learn how to persevere through challenges. At the same time, he was always there when I needed him. More importantly, thank you John being a mensch.
The second part of the citation is for work I did with Michael Bender and John Higgins as a postdoc at Princeton associated with reconstructing past atmospheric O₂ levels. When I arrived at Princeton, I went to John and Michael and told them I wanted to try to using previously measured ratios of O₂/N₂ from ancient air trapped in ice for this purpose. This idea was based on a few papers I had read as a graduate student. I knew next to nothing about ice cores, but John and Michael patiently tutored me on the subject. Both Michael and John were wonderful, kind, and supportive mentors. We had the data set largely assembled in a few months, and it told a coherent story: O₂ levels appeared to be declining (slightly) over the past 800,000 years. The next year was spent trying to convince ourselves this was a robust observation. Every time I would go into Michael’s office to look at the data, he would suggest an issue we had to consider. I would go back to my office, think about Michael’s comments over a few weeks, and we would all design a test to evaluate Michael’s challenge. This went on for months, even requiring us to dig up unpublished data from an undergraduate Princeton thesis stored on microfilm or data archived for 20-30 years on Michael’s computer before we finally wrote the paper up. What did I learn from this? Michael taught me that we are in the business of the search for the truth and that a key and often challenging part of this is not only to question and challenge your own work, but to receive, listen, and respond to the questioning from others. Put another way, Michael ingrained in me the importance of rigor. And for that I am immensely grateful.
The final work cited was done at Berkeley on the history of ocean oxygen levels conducted partly with Brenhin Keller now at Dartmouth and partly with Claire Bucholz at Caltech. The idea for this work originated as I was leaving Princeton and I wondered whether or not changes in deep ocean O₂ concentrations through time would be reflected in the oxidation state of iron preserved in ancient, hydrothermally altered seafloor basalts and in ancient island arc rocks. When I arrived at Berkeley, it quickly became clear that my lab was going to take a long time to build and that I needed to find something to occupy myself for at least a year. I settled on following up on the iron oxidation state idea. After a few months, I had assembled an initial data set that looked promising. I was down visiting Caltech and mentioned the work to John Eiler. John was reasonably skeptical for a variety of reasons based on his training as metamorphic petrologist. As I had never taken metamorphic petrology (blame Harvard), I was only vaguely aware of the issues he raised. In John’s mind, the question was settled. I should find something else to work on. However, after I showed him the preliminary data set, John recognized that though his criticisms still stood, there was something there worth pursuing. What did I learn from this? First, when you work on problems at the edge of your expertise, do talk to colleagues, but perhaps wait until after you have done a little digging. Otherwise, based on their vast knowledge and your lack thereof, they might just convince you it isn’t worth doing. They might be right, but they might be wrong. Regardless, their thoughts and criticisms will always make the work better. Second, it is critical when moving into new areas to work with generous and thoughtful experts like Brenhin and Claire and I thank them for their exceptional patience and willingness to work with me.
Finally, I wish to comment on my path to geochemistry, as it explains, I think, my somewhat random walk from methane clumped isotopes, to ice cores, to ancient basalts. I went to Caltech largely because of the strength of its program in the area of geobiology. My conversion to geochemistry came later during the final months of my PhD. At this time, I was invited to give a few seminars at Harvard. I remember sitting in Dan Schrag’s office, and him stating that though it didn’t matter to him, he had overheard a discussion about whether what I was doing was really geobiology. At the end of the visit, I made a pilgrimage to Mr. Bartley’s Gourmet Burgers in Harvard Square to get a coffee ‘frappe’ and mulled over Dan’s comment. My thoughts drifted to my intellectual heroes, people like Sam Epstein. I realized that one reason why I admired them is their breadth. They worked on problems from the mantle to the moon. And then it hit me. If you call yourself a geochemist, you have clearance to work on whatever you want. At that moment and on this basis, I started describing myself as geochemist and I haven’t looked back. In my final words, I want to first thank my colleagues at UC Berkeley and Lawrence Berkeley National Laboratory who have made a wonderful and supportive home for me and my research. From the day I arrived at Berkeley, I have been treated as an intellectual equal and supported by friends and mentors like Don DePaolo, David Shuster, Nick Swanson-Hysell, and Seth Finnegan. I also wish to thank my nascent group at UC Berkeley including Andrew Turner, Daniel Eldridge, Daniel Ibarra, and Max Lloyd.
Finally, I want to thank my family. I want to thank parents for their unending support throughout my life. Last of all, and because this is the most important position, I wish to thank my wife Leslie for her love, friendship, and support over the years from those early days of weekend lunches next to the mass specs in the basement of North Mudd at Caltech to starting a family together in the Bay Area.