Daniel Sigman received the Macelwane Medal at the 2004 Fall Meeting Honors Ceremony on 15 December, in San Francisco, California. The medal is given for significant contributions to the geophysical sciences by a young scientist of outstanding ability.
Danny Sigman is a talented and imaginative geochemist with extraordinarily broad interests. The focus of his research has been on nitrogen, which is commonly the nutrient whose availability limits the fertility of ecosystems on land and in the sea.
In his graduate work with Mark Altabet at Woods Hole, and in his time since at Princeton University, Danny has developed ingenious new techniques for analyzing the nitrogen isotope composition of environmental materials. He and his collaborators have applied these methods to atmospheric chemistry, biogeochemistry, and paleoclimatology. Meredith Galanter, working with Danny and Chip Levy, characterized the source of nitrate in precipitation at Bermuda, distinguishing natural and pollutant nitrate, and suggesting that lightning is an important nitrate source.
Ben Houlton, working with Danny and Lars Hedin, compared the isotope composition of fixed nitrogen entering and leaving Hawaiian ecosystems. They found compelling evidence that a large fraction of nitrogen passing through was lost by denitrification. Karen Caciotti, Bess Ward, and Danny collaborated on a pioneering study of phylogenetic variations in N isotope fractionation.
Angie Knapp and Peter DiFiore, along with Danny, have made extensive studies of nitrate isotopes in seawater to delineate physical and biological sources of N in the euphotic zone. This work is enhanced by their analyses of the oxygen isotope composition of nitrate, never before measured in large numbers of samples. Combining the two properties allows one to determine rates of the four major processes affecting the nitrogen balance of the oceans: uptake and remineralization of nitrate, nitrogen fixation, and denitrification.
Danny and his collaborators have also dabbled successfully in ice core research, archeology, and bacterial evolution in the Precambrian. In a paper written as a graduate student, Danny (along with Dan McCorkle and Bill Martin, Woods Hole) made a compelling argument that one could not invoke changes in low-latitude biological productivity to explain glacial-interglacial variations in the CO2 concentration of air. This conclusion stimulated renewed interest in the Southern Ocean as the key region regulating atmospheric CO2 on glacial-interglacial timescales, and Danny has helped lead the effort to evaluate this possibility.
He and his longtime collaborator Gerald Haug have demonstrated that the polar oceans were far more stratified during glacial times than today. They have argued that this stratification suppressed the ventilation of the deep ocean and contributed to lower atmospheric CO2 concentrations during the ice ages, an idea Danny is currently investigating with ocean general circulation models.
He and his collaborators are also studying the isotopic composition of organic nitrogen in sediments as a way of understanding the role of Southern Ocean mixing and biology on the CO2 concentration of the ice age atmosphere. His integration of modern biogeochemistry and paleoceanography in the Southern Ocean is important and distinctive. Danny’s wonderful research efforts are matched by his terrific teaching and his contributions to his department, university, and the broader scientific community. He is admired by his undergraduate classes, devoted to his students, and appreciated by his colleagues at Princeton and elsewhere.
—MICHAEL L. BENDER, Princeton University, N. J.
Michael, I cannot match your penetrating intelligence, but I have still strived to follow your example. So I am greatly honored by your citation. Please bear with me as I acknowledge those who have made this day possible for me.
I am proud to owe my fundamental worldview to my parents, who have approached every aspect of their lives with humanity and contemplation. Given their example, it is easy to understand why my sister and I followed their footsteps into academics.
Since my introduction to the Earth sciences at Stanford, I have never looked back. Still, the early appeal of the field was greatly enhanced by the energy and generosity of the Stanford faculty. My most vivid memories are from that time of discovery.
When I applied for a summer student fellowship at the Woods Hole Oceanographic Institution, Dan McCorkle began our relationship by sending me a copy of Tracers in the Sea. Rather than putting me to good use in the lab that summer, Dan sat me in front of a computer and told me to build a model of the ocean carbon cycle. In a matter of hours, I was hooked. As a graduate student under Dan’s wing, I learned the skills on which I rely today. I brought enthusiasm and some measure of creativity to Dan’s door, but he taught me the essentials of scientific rigor.
I remember sitting on a dirty couch in the MIT student center, jittery with excitement over an NSF proposal by Mark Altabet and Roger Francois to use the nitrogen isotopes to reconstruct past changes in surface ocean nutrient conditions. Since that time, Mark has worked to engage me in the inherent richness of the nitrogen cycle, a topic that reaches broadly across disciplines. Mark also instilled in me the importance of method development, and I have tried to inherit some of his characteristic resourcefulness.
I am indebted to Ed Boyle, John Edmond, and Francois Morel, whose courses were the basis of my graduate education, and to WHOI scientists such as Scott Lehman, Lloyd Keigwin, and Bill Martin, who provided a less formal education. I owe close friends to that time, including my colleague Jess Adkins, who continues to inspire me with his independent vision and his intense and tireless intellect.
One of my great pleasures is my long-term collaboration with Gerald Haug, a cherished friend. Gerald, endowed with inexplicable intuition and a uniquely broad perspective, values collaboration above all else. It has come to the point that neither of us can recall who thought of which idea.
I have been educated and aided in innumerable ways by my colleagues at Princeton, including Michael Bender, Anand Gnanadesikan, Lars Hedin, Simon Levin, Francois Morel, Satish Myneni, Steve Pacala, George Philander, Jorge Sarmiento, Robbie Toggweiler, and Bess Ward. I must also thank Dan Schrag, a brilliant and dynamic scientist who nevertheless puts enormous energy into the development of his younger and less interesting colleagues. I thank Brigitte Brunelle, Greg Cane, Karen Casciotti, Peter DiFiore, Julie Granger, Meredith Hastings, Ruby Ho, Ben Houlton, Angie Knapp, Moritz Lehmann, and Becky Robinson, who, as graduate students, postdocs, and technicians, have been my closest scientific partners over the past five years. Finally, I am inexpressibly grateful to my wife, Ellyn, and my daughter, Hana, who keep me in shape by questioning my wisdom on virtually every subject. Thanks.
—DANIEL SIGMAN, Princeton University, N. J.