Geology & Geophysics AGU News

Cassak, Ehlmann, Heald, Jackson, and Maher Receive 2015 James B. Macelwane Medals

Paul Cassak, Bethany List Ehlmann, Colette L. Heald, Matt Jackson, and Kate Maher were awarded the 2015 James B. Macelwane Medal at the American Geophysical Union Fall Meeting Honors Ceremony, held on 16 December 2015 in San Francisco, Calif. The medal is for "significant contributions to the geophysical sciences by an outstanding early career scientist."


Citation for Paul Cassak

Paul Cassak
Paul Cassak

Paul Cassak has made groundbreaking contributions to our understanding of magnetic reconnection, which operates in the boundary regions of planetary magnetospheres and the solar corona as well as having great significance in laboratory plasma physics and plasma astrophysics. Good examples of Paul’s groundbreaking research are his work on asymmetric reconnection and reconnection with a flow shear. Early models of reconnection, where magnetic fields effectively break and release their stored energy, treated symmetric systems for simplicity. However, most realistic systems, especially planetary magnetospheres, have different magnetic fields, densities, and/or bulk flow speeds across the boundary region. Just as simulation studies on the topic were beginning in earnest, Paul performed a first-principles calculation predicting the properties of reconnection in asymmetric systems for generic conditions and has since included the effects of plasma flow. These results have facilitated the analysis of satellite data and may be important for predicting solar wind–magnetosphere coupling, a key aspect of space weather phenomena.

In the solar context, his work on the initiation of reconnection through which built-up magnetic energy is released explosively has been influential. It was generally assumed that reconnection, when it happens, is always fast (matching solar flare and magnetospheric substorm time scales), but if it’s always fast, how can it be explosive? Paul’s research answered this question, which had lingered for decades. In a series of papers, Paul showed that when a current sheet separating opposing magnetic fields forms and has large width, resistive effects dominate, and the fields reconnect relatively slowly. When the sheet thins to kinetic scales, collisionless effects abruptly become dominant, and reconnection becomes much faster. Paul’s research showed that there exists a vast parameter regime in which both types of reconnection are stable. He developed several innovative tests of this hypothesis and successfully validated it; the results may be crucial for understanding solar flares.

These and other unique new results in Paul’s impressive body of work have helped revitalize the field of magnetic reconnection and have significantly changed its course.

As an associate professor at West Virginia University, Paul is an active mentor in the fields of space and plasma physics, publishing important papers with his students. As a teacher, he developed novel active-learning materials for graduate courses in plasma physics. His service activities for the American Geophysical Union (AGU) include chairing the Scarf Award Committee, being an associate editor for Journal of Geophysical Research, and serving on the Space Physics and Aeronomy Policy Committee.

—James L. Burch, Southwest Research Institute, San Antonio, Texas


Thank you, Jim, for the kind citation. I guess all those years of not being popular in high school really paid off! My sincere thanks to the Macelwane Medal Committee, my nominators, and AGU for their efforts for the community.

An honor like this is truly humbling and makes me reflect on the people who contributed to my career, especially four people I’ve never even written papers with. Jim Burch is the principal investigator of the Magnetospheric Multiscale (MMS) mission, which successfully launched in March 2015. In his “spare” time, he was the lead on my nomination. As long as our community has science-driven and civic-minded people like Jim, we’ll be in good shape. Joe Borovsky shared my work with many people, Jim Klimchuk opened doors for me, and Kile Baker believed in me. All of you give the community something to aspire to.

Words cannot express my gratitude to my mentors, especially my doctoral and postdoctoral advisers Jim Drake (Maryland) and Mike Shay (Delaware). I met you accidentally in 2002 and feel extremely lucky to be able to call you mentors, colleagues, and friends.

I have learned much from my colleagues in solar and space physics; an incomplete list includes Dr. Dorelli, Dr. Eriksson, Dr. Fuselier, Dr. Glocer, Dr. Gosling, Dr. Matthaeus, Dr. Mullan, Dr. Murphy, Dr. Phan, Dr. Servidio, Dr. Swisdak, and Dr. Wilder.

I am grateful to my supportive colleagues at West Virginia University, especially Earl Scime. I am forever indebted to you for your guidance and support.

I have been fortunate to have a supportive family throughout my life. My mother, Kit, my father, Barry, and my brother, Todd, have been there for me through thick and thin.

To my love, Julie Bryan, I’ll never know how I got such a great wife. You are funny and serious, patient and encouraging, thoughtful, sweet, smart, and, most of all, supportive. Did I mention smart? And funny? It’s been a pleasure and privilege to go through time with you. Thank you for making me a better person.

Finally, James Macelwane treasured his students. I too have collaborated with outstanding students (Dr. Malakit, Dr. Parashar, Dr. Shepherd, Dr. Komar, Dr. Beidler, and the future Dr. Haggerty, Dr. Doss, and Dr. French), who have enriched my scientific pursuits immeasurably. To all the students reading this—know that you can make important contributions to science and be successful with a lot of hard work and a little luck. Remember that devoting a career to the pursuit of knowledge is an honor.

—Paul Cassak, West Virginia University, Morgantown


Citation for Bethany List Ehlmann

Bethany List Ehlmann
Bethany List Ehlmann

Bethany Ehlmann has made exceptional contributions to the identification and understanding of the alteration mineralogy of Mars, linking surface composition to its geologic context and the implications for the planet’s habitable past. She has made major new mineralogical discoveries on Mars, including carbonates, clay minerals, and other aqueous alteration phases. She has demonstrated that geologic associations of these minerals suggest their production through hydrothermal and groundwater processes occurring in the shallow or middle crust. This is among the most important unanticipated discoveries of the last decade of Mars exploration, as it reveals the nature of the crustal reservoir of mineral-bound water and associated alteration phases and points us toward new terrestrial analog models of the Mars water cycle.

While her dissertation and postdoctoral work has emphasized orbital spectral data analyses from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA), Bethany’s contributions to Mars rover missions began when she was an undergraduate at Washington University. She served in many indispensable mission operations roles on the Mars Exploration Rovers, Spirit and Opportunity, and honed her already formidable scientific skills by contributing to team publications in Science and Nature. Her rover involvement continues to this day with mission-critical leadership roles on the Mars Science Laboratory (MSL) rover Curiosity, including the fact that the results of her orbital data analysis influenced the landing site selection for MSL.

As evidence of Bethany’s far-ranging intellectual interests, she began in an undergraduate program for students with interests in scientific, political, cultural, and ethical issues associated with the environment. She was selected as a Rhodes Scholar, completing two master’s degrees in environmental change and management and geography at the University of Oxford before her Ph.D. in geological sciences at Brown University. Following her Ph.D., she completed a Marie Curie Fellowship at Université Paris-Sud in Orsay. Her extensive publication list (more than 65 peer-reviewed papers) includes topics in astrobiology, polar processes, terrestrial analogs, and advanced instrumentation for future landed missions.

In a field that attracts highly capable scientists, Bethany is clearly a standout, with the technical sophistication, intellectual rigor, and leadership abilities to be at the vanguard of the scientific and technical challenges that await the future of Mars and planetary exploration.

—Wendy Calvin, University of Nevada, Reno


It was a surprise and honor to receive this award. Many thanks to my nominator, Wendy Calvin, whom I’ve had the privilege of working with and learning from on two mission teams.

About 15 years ago—it does not seem so long ago—I attended my first fall AGU meeting as an undergrad. Surfing between remote sensing, biogeochemisty, mineralogy, geomorphology, and planetary sciences, I remember thinking “This is where the fun science is at.”

It remains both fun and a privilege to be part of this community today. Mars science has been an exciting scientific ride, fueled in no small part by the superb data generated by two dedicated Mars imaging spectrometer teams, OMEGA and CRISM, led by Jean-Pierre Bibring and Scott Murchie, along with numerous other amazing friends and colleagues on the Mars Exploration Rover (MER), Mars Reconnaissance Orbiter (MRO), and MSL missions. One of the lessons I’ve learned is that doing great science takes technical skills, commitment, and intuition but also surrounding yourself with a supportive community of mentors. Special thanks go to Ray Arvidson at Washington University for setting me off on this path of exploration as an undergrad, as well as to my mentors on the MER mission—I wouldn’t have found my way to planetary science without you. My thanks also go to John Boardman and Heather Viles at Oxford for honing my skills as an independent researcher and to Jack Mustard, my awesomely supportive Ph.D. adviser at Brown, who encouraged me to dive deeply into the data. Thanks also go to the cohort at Institut d’Astrophysique Spatiale–Orsay for support during my second foray over the Atlantic and to many supportive faculty, postdoc, graduate student, and staff colleagues at the California Institute of Technology and Jet Propulsion Laboratory. Special thanks go to John Grotzinger, Ray, and Scott for writing letters in support of this citation.

Looking to the future, it is an exciting time to be in planetary science. Having at least flown by all the major bodies in the solar system, we now have to test our understanding of how planets work and evolve against data from the telescopic discoveries of exoplanets. MSL is climbing a mountain on Mars, we’re preparing to examine and sample the most primitive asteroids, and we’re gearing up to explore icy worlds with subsurface oceans. Armed with ever-expanding tools for doing science in situ, there is a revolution in understanding each time we send robotic landers and rovers to unexplored locales. I’m looking forward to the next decades of discoveries. Thank you.

—Bethany List Ehlmann, California Institute of Technology, Pasadena


Citation for Colette L. Heald

Colette L. Heald
Colette L. Heald

Colette Heald is a greatly respected and influential young leader in atmospheric chemistry. Her work has broken new ground in a number of areas, including the use of satellite data to quantify emissions and track intercontinental transport of pollution; methods for investigating aerosol aging processes; and insights into the factors controlling the abundances of organic aerosols, dust, and primary biological particles.

One of Colette’s great strengths is her ability to effectively interact with experimentalists, synthesizing observations and modeling into new provocative concepts. In one of her most cited papers, she used aircraft data and modeling to demonstrate the woeful inadequacy of current understanding of organic aerosol sources in the remote atmosphere. This paper triggered a decade of research to improve models and observations. Colette’s own subsequent excellent work on the topic established her as an authority on secondary organic aerosols. She documented in particular the unique interactions between natural and anthropogenic emissions in the formation of organic aerosol, and she applied her knowledge of satellite remote sensing to place new constraints on the global budget of organic aerosol, dramatically reducing the previous range of uncertainty.

Colette has a talent for innovative thinking that leads to new ways of approaching problems. An outstanding example is her proposal of the van Krevelen diagram to quantitatively track the evolution of the composition of organic aerosol during its aging in the atmosphere. The van Krevelen diagram has its origin in the petrochemical processing field. Colette’s idea to apply it to field data to track the progress of atmospheric oxidation was simply brilliant!

Colette distinguishes herself also by her service to the research community. She has chaired the Aerosols Working Group of the GEOS-Chem atmospheric model for many years, leading a group of over 50 aerosol scientists worldwide in identifying and implementing priorities in model development. She has convened five AGU Fall Meeting sessions.

Colette’s vision for the importance of aerosols in the Earth system, combined with her strong grounding in aerosol chemistry and physics, puts her in a powerful position to lead the development of new understanding on the connected roles of aerosols in affecting climate, air quality, and biogeochemical cycles.

—Sonia Kreidenweis, Colorado State University, Fort Collins


My deepest thanks to my colleagues for supporting this nomination and AGU for the tremendous honor. In particular, I’d like to thank Sonia Kreidenweis for her kind nomination and her support as a colleague and mentor.

The Macelwane Medal represents the highest honor of my career. It is especially humbling to receive this as a recognition of contributions to geosciences. Atmospheric chemistry is a fairly young discipline in the geosciences; it brings together scientists with a range of backgrounds to study what I consider to be the science behind the Earth’s most pressing environmental issues. This makes for a magical combination of multidisciplinary problem solving within a friendly and collaborative community. I count myself lucky to be a part of it. And I am grateful for AGU’s recognition of our field.

It has been my privilege to work with inspiring people within outstanding institutions. My Ph.D. adviser Daniel Jacob taught me how to put together a compelling scientific argument; he also taught me that boring talks and papers are the scourge of academia! Harvard showed me how motivating it is to be surrounded by smart people. As a postdoc at Berkeley, I learned to appreciate the diversity of scientific perspectives and approaches. Allen Goldstein taught me how to believe in my own scientific vision, and I’ve benefited tremendously from his generosity ever since. I can’t say enough about the support and encouragement I received from my colleagues at the Department of Atmospheric Science at Colorado State University; they truly helped launch my faculty career. Finally, it’s a privilege and inspiration to be at the Massachusetts Institute of Technology (MIT). I’d like to thank my colleagues in the Department of Civil and Environmental Engineering and the Department of Earth, Atmospheric and Planetary Sciences for their support. Living up to this institution is a daily challenge!

I would also like to thank all of the women in science who have made it possible for me to receive this award and whose own accomplishments are far too rarely recognized. I hope that AGU continues to support and honor the work of women in the geosciences. For myself, I cannot overstate the importance of having a support network of women colleagues whose advice, commiseration, and “gold stars” are invaluable to me: Arlene Fiore, Allison Steiner, Julie Fry, Delphine Farmer, Annmarie Carlton, and Jen Murphy.

Finally, I’d like to thank my research group, past and present, for making it such a wonderful experience to come to the office and work with you every day.

—Colette L. Heald, Massachusetts Institute of Technology, Cambridge


Citation for Matt Jackson

Matt Jackson
Matt Jackson

Matt Jackson reinvigorated the type of mantle geochemistry studies pioneered by his Ph.D. adviser Stan Hart through a combination of analytical advances and a focus on global-scale issues as revealed by extraordinarily fine details of the chemistry of ocean island volcanic rocks.

For his Ph.D., Matt perfected in situ analyses of the strontium isotopic composition of melt inclusions protected within early-crystallizing minerals in oceanic basalts. His results showed beyond doubt that the mantle source of Samoan lavas contains a component of recycled continental crust carried by the deep mantle plume that feeds Samoan hot spot volcanism. Matt, with student Rita Cabral, provided definitive evidence for the presence of recycled sediment in the mantle through their discovery of mass independently fractionated sulfur isotopic composition in basalts from the island of Mangaia. These data confirm a role for recycled sediment but also show that the sediment involved was at Earth’s surface over 2.4 billion years ago. Placing time constraints on the transit time of subducted material through the mantle has been an elusive goal for decades. Matt, working with colleague Rajdeep Dasgupta, showed that mantle compositional variation as reflected in basalt composition is not just expressed in a few obscure trace elements but instead reflects general major element compositional variation in the mantle. As such, the compositional variability has consequences for mantle dynamics because of the contribution of composition to rheology, density, and radiogenic heat production. Matt’s discoveries thus provide a major step forward in the information needed to better understand the forces driving the dynamics of Earth’s interior.

Another of his contributions is a series of papers that attempt to define the characteristics of the hypothetical primitive mantle. Matt first suggested that the high helium-3 mantle source, which most associate with primitive undegassed mantle, has some chemical and isotope characteristics inconsistent with traditional models that invoke chondritic relative abundances of the refractory lithophile elements in the bulk Earth. Matt then matched his model for the “not-so-primitive” primitive mantle to the compositional characteristics of major flood basalt provinces to suggest that the largest volcanic events on Earth sample a mantle reservoir created by differentiation events that accompanied Earth formation.

Matt’s work has dramatically impacted our understanding of the composition of Earth’s interior, the processes accompanying Earth formation that drove initial differentiation, and the longer-term consequences of continent formation and crustal recycling through plate tectonics in creating the Earth we know today.

—Richard Carlson, Carnegie Institution for Science, Washington, D. C.


Thank you, Rick, for your generous citation. And thanks to the cadre of supporters who contributed to my nomination. This really provides an opportunity to thank some of the people who have inspired me over the years.

In my high school days in Montana, Dave Mogk gave me the opportunity to pack his rocks around in the Beartooth Mountains, and I was hooked on geology. Thus primed, I took an introductory geology course from Jeff Park during my freshman year in college. I loved it. Interactions with other folks at Yale—Jay Ague, Karl Turekian, and Brian Skinner—convinced me that I had chosen the right major. Phil Ihinger introduced me to research, and I will forever be thankful for his enthusiasm and the time he invested in shaping my thinking about hot spot volcanoes.

The Woods Hole Oceanographic Institution–Massachusetts Institute of Technology Joint Program was a terrific place to explore hot spot volcanism, so I signed up for 5 years with Stan Hart. I couldn’t have chosen a better adviser and mentor, and he set a wonderful example for how one should mentor students. Nobu Shimizu and Mark Kurz were unofficial thesis advisers and geochemical coconspirators: many ideas were born during conversations in their labs.

A postdoc at Carnegie can only be described as “geochemical paradise.” Rick Carlson was supportive of exploring a lot of neat ideas, and I am lucky to have his mentorship. Rick, Steve Shirey, and Erik Hauri opened up Pandora’s box—unlimited geochemical resources and facilities—and I will always be grateful.

Al Hofmann has been omnipresent in my short career: He’s a gentleman and keeps me on the straight and narrow. Janne Blichert-Toft, Jurek Blusztajn, and Josh Curtice were generous with time and resources when I had no lab, and I am forever indebted.

My graduate and undergraduate students—my academic family—have inspired me to be a better teacher and mentor. In particular, I thank Rita Cabral, Ellie Price, and Drew Reinhard. Your ideas and hard work are the reason I am here today.

My grandfather, a bricklayer and a cowboy, taught me the value of a hard day’s work. I owe a lot to the example he set for me and to the support that I received from my parents, brother, and sister. This medal should really be presented to my wife, Anna, who is infinitely patient and has been my closest friend and my strongest supporter. Thank you, Anna.

—Matt Jackson, University of California, Santa Barbara


Citation for Kate Maher

Katharine Maher
Katharine Maher

Kate Maher has made extraordinary contributions to our understanding of the geochemistry of critical zone processes. Her achievements have impacted our understanding of silicate weathering, soil formation, groundwater flow and transport, and the global carbon dioxide cycle.

Kate’s research specifically focuses on the rates of chemical reactions that occur at Earth’s surface and down to shallow depths. She has used her expertise in isotope geochemistry and reactive transport modeling to understand how water moves through rock materials, transforming it chemically. Her insightful analysis of how to interpret the disequilibrium of uranium isotopes is informing the interpretation of erosion and soil production.

Her impactful work began during her graduate work at Berkeley and has developed into a more generalized use of isotope systematics in hydrologic systems during her tenure at Stanford. She and her students are now bridging the divide between hydrological and geochemical modeling to push forward the understanding of flow and transport in soils, aquifers, and deeper reservoirs. Geochemical measurements hold the promise of constraining hydrologic modeling at a variety of spatial and temporal scales, and Kate’s work is pushing forward this frontier both from a theoretical point of view and in application to real systems.

Her models and data have elucidated several long-standing puzzles. She clarified one of the main reasons why the kinetics of reactions are observed to be slower in the laboratory than the field. She presented quantitative models explaining paradoxes related to solutes and stream flow in catchments. Her latest work is elucidating the thermostat for the global carbon cycle. At the same time, she is contributing to more applied problems related to the geological sequestration of carbon dioxide and radionuclides in the environment.

Kate has the quantitative skills, geological insights, and leadership talent needed to tackle the biggest problems in Earth surface processes. We currently know more about modeling the movements of air masses and ocean waters globally than we know about modeling the movements of water, solutes, and particles in the highly heterogeneous critical zone. Kate will be at the forefront as we evolve in our understanding of this frontier.

—Susan L. Brantley, Pennsylvania State University, University Park


Thank you, Sue, for your generous citation and for your support and encouragement over the years. You have been a role model for so many young scientists, and on behalf of all of us, I thank you for the myriad roles that you have played in our careers.

It is a tremendous honor to receive the James B. Macelwane
medal, and I thank AGU, the nominations committee, and my nominators for creating this special moment that I will cherish for the remainder of my career.

I have had the good fortune to stand on the shoulders of several giants in my field. First, I would like to acknowledge my Ph.D. adviser, Don DePaolo of the University of California, Berkeley, whose infinite understanding of Earth processes and unique ability to envision even the most complex as simple “chemical reactors” have always challenged me to evaluate the most simple case first as it is often where the central challenges become apparent. I would also like to thank Carl Steefel at Lawrence Berkeley National Laboratory for his ceaseless patience in converting an engineer into a geochemical modeler and for leading me into the field of reactive transport. I would also like to thank the U.S. Geological Survey Mendenhall program and Jennifer Harden, Art White, and David Miller for introducing me to the fascinating world of soils.

At Stanford University, I encountered another cast of giants who have offered new shoulders and new views. I chose Professor Gordon Brown as my faculty mentor, hoping he would be honest and fair in providing feedback. This turned out to be quite an underestimate. Gordon not only introduced me to the beautiful world of surface chemistry but provided boundless advice, as well a few prescient nudges. I could not imagine a better mentor. Dennis Bird, Page Chamberlain, and Scott Fendorf at Stanford have also been outstanding mentors and teachers, as have the many postdocs, students, and staff who have crossed paths with our research group. The latter are too numerous to name; however, I am sincerely grateful to all of them for their intelligence, spirited natures, and hard work.

Finally, I wish to thank my family for their patience, encouragement, and support. My mother, Celia Kathleen (CK), has always been my anchor, and without her I could not have become a geoscientist. My husband, Matthew, is a true giant upon whose shoulders I stand every day.

—Katharine Maher, Stanford University, Stanford, Calif.

Citation: AGU (2015), Cassak, Ehlmann, Heald, Jackson, and Maher receive 2015 James B. Macelwane Medals, Eos, 96, doi:10.1029/2015EO041063. Published on 18 December 2015.

© 2015. The authors. CC BY-NC 3.0