By Yigal Erel, presented at the 2013 Goldschmidt Conference in Florence, Italy
Mr. President, Colleagues, Ladies and Gentlemen: It gives me a great pleasure to introduce Joel D. Blum, the 2013 recipient of the Patterson Medal. Joel and I were classmates at Caltech in the late 1980s, where he did his Ph.D. with Jerry Wasserburg, and I with Clair Patterson. Towards our graduation in 1990 we discussed the possible efficacy of radiogenic isotopes as tracers of rock weathering. These discussions led to a rather fruitful scientific venture. During the past 23 years we have carried on a very close collaborative research effort that has included a series of projects related to various aspects of chemical weathering in natural soil systems and in laboratory experiments. Throughout this collaborative research Joel's merits as a scientist were distinctive. Joel is a very analytical, methodical, and visionary scientist. He has been always exceptionally insightful in pursuing new and clever approaches to research, and was always the one who pointed to the "big-picture" implications of our research. Joel went on and studied the weathering of rocks in tectonically active regions of the World such as the Himalayas. He was able to establish a strong case for the interactions between tectonics, climate change, and rock weathering (Blum et al., 1998; Jacobson and Blum,2000; 2003; Jacobson et al., 2002a, b). In the past 20+ years Joel pursued additional research paths related to the cycling of base cations in soils after their release by rock weathering. These papers are highly cited and are used in university courses that deal with nutrient cycles in soils (Miller et al., 1993; Blum et al., 2002).
However, the most outstanding contribution of Joel to the field of geochemistry is his work on Hg isotopes and their use in the study of the fate of Hg in the environment. Similarly to Pb, humans have perturbed the global cycle of Hg. It is estimated that human activities have increased the amount of actively cycling Hg by approximately a factor of three. Like Patterson and his co-workers, Joel and his students were among the first to apply isotopic measurements in order to decipher the fate of the metal-under investigation in the environment. Joel and his students were the first to report on mass-independent isotopic fractionation (in short – MIF; enriched in the odd isotopes – 199Hg, 201Hg) of Hg in natural systems (Bergquist and Blum, 2007). They showed that positive MIF are produced by photo-reduction of Hg species and went on to propose a working hypothesis that has served to guide future research. They postulated that the existence of a positive MIF pool in fish (and presumably in other organisms in aquatic food webs), implies a complementary reservoir with negative MIF Hg. They went on and said that "if positive MIF Hg is retained in the aqueous system after photo-reduction of Hg species, then negative MIF Hg may be released to the atmosphere as Hg0. Because Hg cycles between several active pools near Earth's surface, and Hg0 has an atmospheric residence time of about one year, there should be the opportunity for isotopically distinct Hg0 to be transported away from the aquatic systems in which it was produced and be deposited elsewhere." They concluded that "The preservation of both MDF and MIF in environmental reservoirs of Hg will depend on the mass balance of Hg sources, but the observation of even a small degree of MIF in soils or other atmospherically derived pools would suggest that photochemical cycling of Hg species plays a substantial role in the global cycle of Hg." Thus, they suggested that MDF and MIF measured in environmental samples "will contribute to the understanding and quantification of important processes in the global cycle of Hg" (Bergquist and Blum, 2007). Indeed in a recently accepted paper to Nature Geoscience Joel and his co-workers observed that microbial production of mono-methyl-mercury below the surface mixed layer contributes up to 80% of the mercury contamination of marine food webs (Blum et al., 2013). They also pointed out that the problem of Hg pollution is a global issue, and only an international collaboration which includes China and India might solve the problem of Hg pollution of the marine food web.
These exciting and groundbreaking discoveries are (as was said in the Geochemical Society announcement) "in some ways parallel Patterson's seminal work tracing Pb contamination in the environment." Hence, Mr. President, I have the privilege to present to you Joel D. Blum for the 2013 Patterson Award for "his innovative breakthrough studies on Hg isotopes."
References
Bergquist B. A. and Blum J. D. (2007) Mass-dependent and -independent fractionation of Hg isotopes by photo-reduction in aquatic systems. Science 318, 417–420.
Blum J. D., Gazis C. A., Jacobson A. D., and Chamberlain C. P. (1998) Carbonate versus silicate weathering in the Raikhot Watershed within the High Himalayan Crystalline Series. Geology 26, 411-414.
Blum J. D., Klaue A., Nezat C. A., Driscoll C. T., Johnson C. E., Siccama T. G., Eagar C., Fahey T. J., and Likens G. E. (2002) Mycorrhizal weathering of apatite as an important Ca source in base-poor forest ecosystems. Nature 417, 729-731.
Blum J. D., Popp B. N., Drazen J. C., Choy C. A., and Johnson M. W. (2013) Methylmercury production below the mixed layer in the North Pacific Ocean. Nature Geoscience.
Jacobson A. D. and Blum J. D. (2000) Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influnce on river-water chemistry. Geology 28, 463-466.
Jacobson A. D. and Blum J. D. (2003) Relationship between mechanical erosion and CO2 consumption in the New Zealand Southern Alps. Geology 31, 865-868.
Jacobson A. D., Blum J. D., Chamberlain C. P., Poage M. A., and Sloan V. F. (2002) The Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: Carbonate versus silicate weathering rates as a function of landscape surface age. Geochim. Cosmochim. Acta 66, 13-27.
Jacobson A. D., Blum J. D., and Walter L. (2002) Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: Insight from the carbonate geochemistry of stream waters. Geochim. Cosmochim. Acta 66, in press.
Miller E. K., Blum J. D., and Friedland A. J. (1993) Determination of soil exchangeable-cation loss and weathering rates using Sr isotopes. Nature 362, 438-441.