April 2019 Letter from the Director
April 22 was the official celebration of “Earth Day” but at DTM, we celebrate the Earth pretty much every day. Work in the department spans from questions of how planets, like Earth, form in the first place, to the interior processes that drive the geologic activity that creates a surface environment supportive of life. Our work on this wide range of subjects results in visits to spectacular places on Earth, some of which are captured in the photo gallery we put together for our web page and social media.
In April, we reported a couple of exciting discoveries relating to how our planet obtained and processed the volatile compounds essential for life; water and carbon. DTM Staff Scientists Larry Nittler and Conel Alexander working with former DTM Postdoc Jemma Davidson and colleagues, found and carried out extensive analyses of a small fragment of material contained within a primitive meteorite that may be the stuff from which comets are made. This carbon-rich material provides evidence for substantial transport of material from the icy regions in the outer Solar System into the inner Solar System where this particular meteorite formed. Traditional wisdom suggests that the inner Solar System was too hot to retain volatile compounds during planet formation. In contrast, along with evidence for high-temperature minerals found in material returned from the Comet Wild 2 by the Stardust mission, the DTM-led discovery shows that exchange of material between the inner and outer Solar System likely was common during the planet-forming stage. This kind of evidence derived from microscopic examination of the materials left over from the early stages of formation of the Solar System, coupled with the astronomical observations and theoretical modeling of circumstellar disk evolution done at DTM provides a dramatic new view of the process of planet formation and its role in determining the basic compositional characteristics of a planet.A diamond from Sierra Leone with sulfur-containing mineral inclusions courtesy of the Gemological Institute of America.
Once Earth obtained its carbon, what did it do with it? Venus put most of its surficial carbon into its atmosphere as CO2, resulting in a greenhouse that maintains surface temperatures hot enough to melt lead. On Earth, some of the carbon went into life, a good part of which we are putting back into the atmosphere as CO2 through the burning of fossil fuels. Another large amount of carbon ended up in Earth’s interior. The two-way cycling of material between Earth’s interior and surface was shown clearly by the recent results reported in Science magazine by DTM visiting investigator Karen Smit working with DTM Staff Scientist Steve Shirey and our departed colleague Erik Hauri. In a long-running program investigating the origin of the mineral cargo contained within diamonds, this study found that diamonds from deep within the mantle beneath Sierra Leone contain sulfide minerals whose sulfur was at one time in the upper atmosphere. This conclusion arises from application of a discovery made by former Geophysical Laboratory postdoc James Farquhar, who found that sedimentary sulfides older than 2.5 billion years old show an isotope fractionation that is best explained by interaction of volcanically emitted sulfur-containing gases with ultraviolet light in the upper atmosphere. The rise of oxygen in the atmosphere at about 2.5 billion years put an end to this effect by effectively blocking this form of photochemistry. The analyses of the sulfide inclusions within the Sierra Leone diamonds show them to contain the same isotopically fractionated sulfur that is present in ancient surficial sedimentary sulfides. A similar result was obtained in earlier work on sulfide inclusions in diamonds from Botswana.Lara Wagner (left) and the delegation from the Botswana Seismological Research Center at DTM on April 4, 2019. Photo by Jan Dunlap, Carnegie DTM.
As an aside, we were pleased to be able to host a visit in April by four members of the Botswana Geoscience Institute and Botswana International University looking for our advice on their plan to establish a Botswana Seismological Research Center. The visit allowed us to reconnect with a former DTM predoctoral fellow, Motsamai Tarzan Kwadiba, who worked with us during the Kaapvaal seismic project to study the origin of the ancient crust and upper mantle of southern Africa that was run out of DTM at the end of the last millennium. The implication of the new diamond results is that surficial sulfur, and likely carbon, were transported to sufficient depths beneath western Africa to form the diamonds that were brought back to the surface by relatively recent kimberlite volcanism. The result is clear evidence that materials from Earth surface can be transported to great depth in Earth’s interior as a result of plate tectonics. The deep Earth responds to this contribution by returning to the surface a variety of materials, including fresh rock (lava) and a number of atmospheric gases via volcanism.
Studying the processes that lead to volcanic outgassing and eruption was the theme behind the Neighborhood Lecture presented by DTM Staff Scientist Hélène Le Mével. Once again packing the Greenewalt Auditorium, our friends and neighbors followed Hélène in a tour of historical measurements of the strength of gravity at Earth’s surface used to determine basic features of the planet, for example that Earth’s diameter is bigger at the equator than the poles. She showed how modern, high-precision measurements of gravity can be used to investigate the mass flux of molten rock beneath a volcano and thereby gain information on magmatic volumes and the amount of gas it contains. Her presentation finished with a preview of an upcoming study of the active Ambrym volcano in Vanuatu that will combine gravity measurements with various geodetic, seismic, and gas measurements. The project also involves DTM Staff Scientist Diana Roman and postdoc Elodie Brothelande and will study how the active lava lake at Ambrym is connected to the underground magmatic plumbing system that drives the eruptions. One of their goals will be to better understand what factors control whether the magmatic gasses are released passively or instead stay dissolved in the magma until they bubble out violently, causing an explosive eruption.Le Mével (left) mingled with the audience after the lecture and showed DTM’s gravimeter, an instrument that she uses to gather gravity data in the field. Photo by Roberto Molar Candanosa, Carnegie DTM.
In closing, DTM would like to offer its congratulations to two former colleagues at the Geophysical Laboratory who were just elected to the National Academy of Sciences. The honorees are former GL Staff Scientist Marilyn Fogel, currently Director of the EDGE Institute and Wilbur W. Mayhew Endowed Professor of Geoecology at the University of California at Riverside, and former GL Postdoctoral Fellow, James Farquhar, now Professor in the Department of Geology and Earth Systems Science at the University of Maryland. We congratulate them on this recognition of their many important contributions to understanding the Earth around us, and take pride in the role that Carnegie played in their careers.