Professor Brenhin Keller is an exceptionally innovative scientist who has made key contributions to understanding the evolution of continental crust under biologic, marine and atmospheric influences. A pioneer in geoinformatics, Brenhin’s ability to bring large and varied datasets to bear on the question of the origin and evolution of terrestrial lithosphere, coupled with his recognition that mantle secular cooling would radically change geochemical expectations of ancient magmatic behavior, challenged long-standing views of Earth history. In particular, his work questioned the long-standing view that terrestrial crust had remained dominantly mafic until the late Archean by showing that the numerous lines of evidence thought to support that model are either deeply flawed or outright incorrect. Instead, he found that widespread early continental crust was not only equally plausible to the problematic paradigm that had stood for decades but also provides a mechanism to maintain terrestrial habitability during the eon in which life is thought to have emerged. This magnum opus demonstrated both Brenhin’s iconoclastic way of looking at Earth history and his tremendous intellectual courage in challenging geochemical orthodoxy. If this were his sole scientific contribution, his presence on this stage would be justified. But he and his co-workers also addressed and similarly challenged other big ideas, including conceiving of a way to directly assess deep-ocean oxygen concentrations by examining the redox state of hydrothermally altered seafloor, finding that the rise to present levels was forestalled until the Paleozoic, extending ultraprecise dating to the Deccan Traps, permitting selection between competing models for the Cretaceous–Paleogene (K-T) extinction, and radically reinterpreting the Great Unconformity as reflecting Cryogenian glacial denudation. These remarkable contributions have all come in a spirit of cooperation and openness that is the epitome of AGU ideals. His computational tools have been downloaded tens of thousands of times, and his public outreach efforts have reached millions of viewers. For these outstanding contributions, we are delighted to introduce Brenhin Keller as a recipient of AGU’s 2024 James B. Macelwane Medal.—T. Mark HarrisonUniversity of California, Los AngelesLos Angeles, California—Xiahong FengDartmouth CollegeHanover, New Hampshire

Prof. Brenhin Keller is an exceptionally innovative scientist with an extraordinary record of contributions to understanding Earth evolution. He is the leading figure of his generation in understanding the coevolution of continental crust with biologic, marine, and atmospheric systems. I first encountered Brenhin when he was a graduate student and was immediately struck by his pioneering work in geochemical informatics. All disciplines attract stereotypes, and the cynical take on geochemists takes two forms. The first is that we think every problem is a nail to be hit with our particular analytical hammer. The second is that we produce massive amounts of data but don’t think very deeply about them. There is some truth to both, and Brenhin responded by creating an original path along which he exploits massive data sets made available by an international army of us crank turners. The danger, of course, is that the modeler may be insensitive to underlying analytical quirks, but Brenhin transcended this pitfall by simultaneously mastering the most sophisticated geochronological methods and, frankly, by being really smart. Brenhin’s research seriously challenged the view that terrestrial crust remained mafic until the post-Archean by showing that evidence for this model is deeply flawed, in part by not accounting for mantle cooling. He and his coworkers showed that an early and possibly widespread felsic crust is at least as plausible as the teetering paradigm that had stood for 50 years. This demonstrated not only Brenhin’s iconoclastic way of looking at Earth history, but his tremendous intellectual courage as well. If this were his sole scientific contribution, his presence on this stage would be justified. But he has addressed and similarly challenged other big ideas. He conceived of a way to directly assess deep-ocean oxygen concentrations by examining the redox state of hydrothermally altered seafloor, finding that the rise to present levels was forestalled until the Paleozoic. He made important contributions to precision dating of the Deccan Traps to permit selection between competing models for the K-T extinction. His take on the origin of the Great Unconformity as reflecting Cryogenian glacial denudation moved the goalposts of the debate on this global feature. A signature of Brenhin’s research style is the surprising links he makes between the growth of continental crust and its coevolution with the biosphere and surface environments. For these outstanding contributions, I am delighted to introduce Brenhin Keller as the 2023 Hisashi Kuno awardee.

—T. Mark Harrison, University of California, Los Angeles

I am tremendously honored to receive this citation and the 2023 Hisashi Kuno Award from the AGU Volcanology, Geochemistry, and Petrology section. I feel that Earth science, and certainly geochemistry, is often characterized by exceeding complexity—and in so many cases our job as Earth scientists with data sets both large and small is to try to find the key underlying processes within all this complexity. Sometimes I like to think of this from a signal-and-noise perspective, wherein some trend may emerge if only enough data are applied and enough nonsystematic errors made to cancel out. Another way, however, is to try to go closer to the source. As we all know, any time you melt Earth’s mantle to an appreciable degree, you get, roughly speaking, a basalt. So basalt is often a great place to start. This may be perhaps one reason why Hisashi Kuno’s work—on, for example, along-arc variations of, and fractionation trends within, basaltic magmas—has been of such lasting influence in our field. At this time, I am reminded to look to basalts as the most direct geochemical record of Earth’s mantle and its evolution over geologic time. It is, after all, this primary magma that both records and mediates the mantle’s influence on the crust, the hydrosphere, the atmosphere, and the biosphere—sometimes causing mass extinctions, but without which there would be no crust (continental or oceanic), no tectonics, and no nutrients for life on our planet. As to my own origin and path in this field, I owe innumerable thanks to my graduate adviser, Blair Schoene, my undergraduate adviser, Sue Kay, and many, many other friends and mentors in the geochemistry and geochronology communities and beyond. I am grateful to the Volcanology, Geochemistry, and Petrology section for the 2023 Hisashi Kuno Award and humbled to be placed alongside the previous awardees. —C. Brenhin Keller, Dartmouth College, Hanover, N.H.