Discours de remerciement – Rome, 23.11.1998 (anglais)

États-Unis

Harmon Craig

Prix Balzan 1998 pour la géochimie

Pionnier des sciences de la terre, Harmon Craig a utilisé les différents instruments de la géochimie isotopique pour résoudre des problèmes d’une importance fondamentale et d’un intérêt immédiat pour l’étude de l’atmosphère, de l’hydrosphère et de la lithosphère.

Mr. President,
Members of the Balzan Foundation,
Ladies and Gentlemen,
It is a great honour to be the recipient of the first Balzan Prize far Geochemistry, and it is clear that this prestigious award is as much a recognition that geochemistry has become a mature science in its own right as it is of my own research, Thus all geochemists share with me this award, which recognizes the efforts of the pioneers and present day scientists who have developed the study of the earth based upon the principles of chemistry, physics, and geology.
Geochemistry is the study of the chemistry of the solid earth, the atmosphere, and the oceans. In the modem sense of being based securely on chemistry and physics, it began with the work of V.M. Goldschmidt in Norway, whose 1954 magnum opus “Geochemistry” was the first treatise to deal with the laws that determine the geochemical distribution of the elements. Goldschmidt’s determinations of the abundances of the elements, especially those with the “magic numbers” of neutrons, led to the systematic study of his results by physicists and chemists and ultimately to two Nobel Prizes far theories of the origin of the elements based on nuclear physics. In the context of this present gathering in Rame, it is pleasant to recall that Goldschmidt had the honour, at the age of forty-one, to be elected as a Foreign Member of the Accademia dei Lincei in 1929.
Isotope Geochemistry, or Nuclear Geology as it is sometimes called, includes two major disciplines: stable isotope geochemistry and the study of radioactive and radiogenic isotopes, which ore distinguished by the fact that the stable isotopes of the light elements, primarily carbon, hydrogen, oxygen, nitrogen, and sulphur, ore separated to a small extent in physical and chemical processes (“isotope fractionation”), while radioactive elements provide geologic ages by their known decay rates, and characteristic geochemical signatures of radiogenic elements when their parent isotopes ore enriched or depleted in various phases by chemical processes. These two fields originated in the laboratories of Alfred O.C. Nier, a physicist at the University of Minnesota, and Harold C. Urey, a physical chemist of the University of Chicago, in the years immediately after the second World War. Both these scientists were responsible for truly seminal innovations. Alfred Nier studied both stable and radiogenic isotopes, particularly the isotopes of lead (which led to the first precise age of the earth) and invented a simple but precise mass spectrometer in 1947 which become the machine that revolutionized geochemistry.
Harold Urey’s equally seminal contribution was his classic paper “The thermodynamic properties of isotopic substances”, also published in 1947, in which he calculated the equilibrium separation factors for isotopes of the light elements in chemical reactions and solid—liquid-vapor phase equilibria, based on quantum mechanics and spectroscopic data on isotopic molecules. Urey noted that seawater should be enriched in the heavy oxygen isotope (18 O) relative to fresh water because of the preferential evaporation of the light isotope (16O) from the oceans. Thus shells precipitated from seawater should have higher 18 O/16O ratios than shells formed in fresh water, which could provide a method for distinguishing fossil origins. When Urey calculated the temperature variation of the oxygen isotope fractionation between carbonates and water, he saw that “Suddenly I had a geological thermometer on my hands”. With characteristic élan he immediately set about a far-reaching program for determining whether a sudden climatic change to a colder regime at the end of the Cretaceous period had caused the extinction of the dinosaurs. Thus began the application of oxygen isotope geochemistry to studies of climatic change, which has become the fundamental methodology of the study of ancient climates. Through the generosity of Alfred Nier, Urey acquired the working drawings of the Nier mass spectrometer and formed a group that constructed three of these machines and set to work on Cretaceous climatology
Unfortunately, as we now know, the demise of the dinosaurs was caused by the impact of a large meteorite that happened to fall on Mexico, rather than by a sudden cooling of the ocean. That the flowering of oxygen isotopic climatology began in Urey’s Chicago laboratory was due to another, and perhaps equally improbable impact, which similarly caused the demise of the conventional scientific wisdom on the glacial and interglacial climatic cycles during the Pleistocene Epoch. This improbable event was the arrival of a young Italian scientist from Bologna, a micropaleontologist named Cesare Emiliani who had a PhD from the University of Bologna, and acquired a second PhD at Chicago in 1950, Cesare Emiliani had not the slightest knowledge of isotopes or mass spectrometers, but like a Gift of the Magi, he wandered into Urey’s laboratory following the path of a number of Chicago geology students (I was one of these) and set to work on what had by then become the study of “isotopic paleotemperatures”. Heinz Lowenstam had suggested to Urey that marine microfossils (foraminifera) would be useful for paleotemperature studies, and Emiliani was an expert on foraminifera. His great idea was to work not in the Cretaceous Period, but to concentrate on the Pleistocene glaciation history embedded in oceanic sediment cores. From 1955 to 1966 he published four classic papers which produced a revolution in climatology by showing that rather than the four classical glaciations that had been established by geologists working on continental glacial deposits, the oceanic microfossils recorded a great many glacial and interglacial stages during the past half-million years, representing an extremely complicated and rapidly fluctuating sequence of climatic events. With one bold stroke, he had established a major new field, the application of oxygen isotope variations to the detailed study of past climates, now the fundamental underpinning of our knowledge of climatic change. Isotope geochemistry had become a fundamental science, and never looked back.
I turn now to the developments in Europe. As I have described, there were two major laboratories in the United States where isotope geochemistry originated, but in those early days there also arose two very similar groups in Europe. And indeed it is very appropriate that the first Balzan Prize in Geochemistry is a combined award from Italy and Switzerland, for it was in these two countries that isotope geochemistry first developed in Europe, in the laboratories of Ezio Tongiorgi, an Italian geologist at Pisa, and Fritz Houtermans, a physicist who directed the Physikalisches Institut in Bern. Houtermans, an eminent nuclear physicist, developed a strong group of students who made enormous contributions to subjects as various as the solar wind, the composition of comets, cosmochemistry, and the application of oxygen isotopes to deciphering the chronology and climatic record contained in polar ice cores. Houtermans and Tongiorgi were close friends and remember well his visits to Pisa, when I spent a sabbatical year there, and our many swimming excursions to Marina di Pisa. Ezio Tongiorgi was an eclectic Italian geochemist who could tell by fasting a wine the type of soil that had produced it, and was an expert on Pisan pottery and many other things. Thus our seminars, swimming trips, and conversations over wine were infinitely variable in both scientific and sociological topics.
When Tongiorgi decided to develop isotopic geochemistry at Pisa, he had the same good fortune as Harold Urey, in receiving an Italian Gift of the Magi. This time it came fortuitously in the form of Giovanni Boato, a young physicist from Genoa, who, at the suggestion of Edoardo Amaldi, had arrived with a Fulbright grant at Urey’s laboratory in 1952. in Chicago, Boato made a discovery of fundamental importance by showing for the first time that the hydrogen isotope ratios in carbonaceous meteorites proved that the water in these objects was of extraterrestrial origin, and was thus indigenous to the meteorites rather than due to post-impact weathering on the earth. This remarkable finding was a major impetus in expanding research in cosmochemistry, extending even to present-day studies of the possibility of the existence of primitive life at an early stage on Mars. During this period Boato also made an excursion to the western US, collecting water from hotsprings which we showed from oxygen and hydrogen isotope ratios was derived essentially in toto from deep circulating ram water, rather than from volcanic magmas as had been generally assumed. Thus began my life-long friendship with “Gianni” who was also a “Magi Gift” to me,
Shortly after Boato returned to Italy, he and Tongiorgi began a project to build at Genoa three mass spectrometers similar to Urey’s machines, one of which went to Pisa, where Tongiorgi had formed a group of young scientists in isotope geochemistry, one went to Professor Fornaseri in Rome for similar studies, and the third remained in Genoa for Boato’s research, which by then had shifted to the physics of isotope effects in phase equilibria, such as the low-temperature vapor-liquid equilibrium in liquid argon. By this time I had moved from Chicago to La Jolla, but I was privileged to spend sabbatical leaves as a Visiting Professor in both Pisa and Bern, and ultimately Fritz Houtermans, and both Giorgio Ferrara and Antonio Longinelli from Tongiorgi’s group, spent sabbatical leaves in my laboratory, while Houtermans sent Johannes Geiss and Peter Ebenhardt to Urey’s laboratory, so the Italian-Swiss-Chicago-La Jolla connections continued. Additionally, there were magnificent conferences in Spoleto and Varenna organized by Tongiorgi, and isotope geochemistry thereby prospered both in Europe and the United States.
Today there are stable and radioactive isotope geochemistry groups in almost every major city in the developed countries, and research is proceeding at a rate that seems incredible when one looks back on those early and formative days in the 1950’s. Looking forward is more productive (though not as pleasant) as looking back, and so I will conclude by offering a few thoughts on several frontiers that I think will be among the most exciting in the near future.
Climatology, driven by concerns over anthropogenic contributions to global warming during this century, is the most rapidly expanding field in the earth sciences, and isotope geochemistry is of fundamental significance in this field. I have described above how oxygen isotopes in marine microfossils are used to delineate the timing of the many glacial and interglacial stages recorded in sediments. Similar studies of oxygen isotopes in deep ice cores from the Greenland and Antarctic icecaps not only show the amplitudes and timing of these events in much more detail, but are used to establish the ambient temperatures over the ice sheets during the last hundred thousand years or more. These temperatures have been deciphered using the present-day temperature relationships of oxygen isotope ratios in surface snow and ice to calculate past temperatures recorded by the ice cores. Recently, however, this methodology has been strongly challenged by temperature profiles measured directly in the ice-sheet drill holes (suitably adjusted for diffusion of heat in the ice and the dynamic flow regime of the glacier). In Greenland for example, where the present ambient temperature is -31°C, the isotopic temperature result for the Last Glacial Maximum Is -42° C, while the borehole value Is -52°C, a very significant and fundamental difference for climate modeling, which depends strongly on the temperature difference between the tropics and the poles.
A major new discovery from the oxygen isotope profiles in ice cores is the existence of so-called “millenialtimescale” oscillations, the rapid switching from cold to warm temperatures in times of decades to a century, which occur at intervals of the order of a thousand years. These remarkable events are thought to be related to rapid changes in ocean currents, and possibly to EI Niño oscillations. Developing an understanding of the climatic processes involved in such rapid and important temperature changes requires more accurate information on the actual oxygen isotope temperature scale in ice cores. Some progress has recently been made by research in my own laboratory, which shows that gases and their isotopes are enriched in the higher-mass components by gravitational separation in the firn (snow) layer above the level where ice is formed by compaction. We use the Kr/ Ar ratios trapped in the ice when it forms, which are related to ambient ice temperatures by correlation with firn thickness and accumulation rates, and our recent results from Greenland are in agreement with the borehole-derived temperatures. This entire field is undergoing a major expansion in research activity which is certain to accelerate.
Atmospheric chemistry is one of the most exciting and revolutionary frontiers in geochemistry today: particularly the chemistry of the ozone layer and the gases that interact with ozone (it is interesting to note that the word “geochemistry” was invented in 1838 by Schönbein, the Swiss chemist who discovered ozone). In 1983 my colleague Mark Thiemens discovered that ozone formed from molecular oxygen displayed a hitherto completely unknown type of enrichment of the heavy isotopes 17O and 18O (relative to 16O), in which the large enrichments of these two isotopes are equal, rather than in the ratio of 1 to 2 as predicted by the relative mass differences using conventional isotope fractionation theory. This totally unexpected effect, now known as “mass-independent fractionation”, was actually observed in stratospheric ozone in 1985 and 1987, and is currently the focus of much research, both observational and theoretical (it may be due to effects of symmetry differences in O3 isotopic molecules, but no satisfactory theory is yet available). Further work by Thiemens has shown that oxygen isotopes in stratospheric CO2, CO, and N2O also have massindependent isotopic compositions, presumably from interaction with activated O atoms, while the components of these gases produced in the troposphere, both by nature and anthropogenic processes do not display such effects. These isotopic effects provide fundamental clues to the stratospheric chemistry of oxygen-containing molecules (“greenhouse gases”) and at the same time, important isotopic tracers far mixing processes between the stratosphere and troposphere, and are currently the focus of a large and growing number of experimental and theoretical groups.
Mass-independent fractionation of oxygen isotopes is an equally “hot topic” in cosmochemistry, in the study of the early history of the solar system. R. Clayton at Chicago had shown that oxygen in inclusions in certain minerals in meteorites has a mass-independent isotopic composition, which until recently was thought to be due to incorporation of a pure 6O component derived from nucleosynthesis in stars. The discovery of mass-independent isotope fractionation by kinetic gas-phase chemical reactions, has forced a re-examination of this subject with emphasis on gas—phase reactions in the early nebula with subsequent incorporation of the oxygen isotopic enrichments in solid minerals accumulating in nebular dust. This research area is still in its infancy, and will expand rapidly.
In solid-earth geochemistry, perhaps the most exciting current research is on the composition of the lower mantle, below the 670 km seismic discontinuity. Thirty years ago Brian Clarke and discovered that, contrary to the then conventional wisdom, the interior of the earth is still degassing to the atmosphere, as shown by the emission of helium greatly enriched in the primordial isotope He relative to the much more abundant isotope He which is produced by radioactive decay of uranium and thorium in the earth. This emission occurs along the oceanic seafloor spreading axes where new ocean crust is forming, and very high 3He/4He ratios (some 30 times the atmospheric ratio) are found in lavas and volcanic gases of “hotspots”, which mark the loci where oceanic island
volcano chains (such as the Hawaiian Islands) are forming, and in massive eruptions on the continents (as in the Ethiopian lavas of the Afar Depression). The hotspots lie atop the sites of massive hot convective “plumes” rising vertically from the core-mantle boundary (at least in the majority opinion there are dissenters). We have found sixteen of these “High-3He hotspots” around the globe, and they appear to correlate with indications from seismic tomography of deep hot rising currents from the earth’s core, Thus we are now approaching a synthesis of geochemistry and geophysics in studies of the earth’s interior, What remains to be established is the degree to which these high helium isotope ratios correlate with the systematics of the radiogenic isotopes (Sr, Nd, and Pb), which delineate the chemical and physical effects responsible for the generation of the compositional variety of rocks in the lower and upper mantle and crust of the earth.
Geochemists all over the world ore now engaged in research on these and many other fundamental problems related to our ultimate understanding of the chemical, physical, and biological processes which shape our earth. I have been greatly privileged to have been a port of these efforts, and to share in the exciting growth of geochemistry, oceanography, and geophysics with so many good friends, students, and colleagues. As I look backward to the beginnings and forward to the future I am profoundly grateful to the Balzan Foundation for their recognition of the contributions of all of us to the maturation of the field of geochemistry.
Many of my friends and colleagues here today know Valerie Craig. and a who do are aware of her participation in my work in the field, at sea, and in the laboratory. It has been a wonderful collaboration, and she has an equal share in the recognition you have so generously awarded to us.