March 1, 2020

Bruce Forbes Bohor, the 2011 Barringer Medalist of the Meteoritical Society (Glass, 2011), passed away at his home in Green Valley, Arizona, on November 17, 2019. Bruce is best known in our community for his discovery of shocked quartz in layers marking the Cretaceous-Tertiary (K-T, now called the Cretaceous-Paleogene, K-Pg) boundary in the central United States in the early 1980s, following the famous paper by Alvarez and co-authors in Science in 1980, in which they report geochemical evidence for an asteroid impact from K-Pg layers in Italy. The so-called asteroid impact hypothesis to explain the mass extinction at the K-Pg boundary had created quite a controversy in the early 1980s. Bruce Bohor, working for the U.S. Geological Survey, had previously studied tonsteins (altered volcanic ash beds) in the Central U.S. and even though he was previously not involved in impact cratering or extinction studies, he wanted to test the Alvarez hypothesis against the competing volcanic theory. Thus he began to investigate the mineralogy of the various K-Pg clay layers in the area, leading to his important 1984 Science paper. In this publication, Bruce and colleagues were the first to report the presence of shock-metamorphosed quartz grains in samples from the K-Pg clay layer.

The 1984 paper by Bohor and colleagues caused quite some interest in both the mass extinction and impact/planetary (and mineralogy, petrography) communities, with the latter immediately accepting this as convincing evidence that there was a large impact event at the end of the Cretaceous, whereas the former, in their distaste for extraterrestrial explanations, now tried to discredit decades of shock metamorphic studies by claiming that other (internal) geological processes, such as volcanism, can also create shock lamellae. Bruce wouldn’t have any of this. He went on to demonstrate the presence of trace amounts of the high-pressure quartz polymorph stishovite in the K-Pg layer, and, in a series of subsequent papers, Bruce and colleagues continued to describe shocked mineral grains (quartz, feldspar, chromite, and zircon) and other evidence of an impact in K-Pg layers from many locations around the globe. The global distribution was demonstrated in 1987 in another Science paper; further work demonstrated that the geographic variation in the maximum size of shocked quartz grains and the ratio between shocked and unshocked quartz grains suggested that the source crater is on or close to the North American plate, helping to narrow down possible locations of the source crater (which ultimately resulted in the discovery of the Chicxulub impact structure). In addition, the 1987 paper contained data that indicated that the impact was into continental crust rather than oceanic crust as previously proposed, and they that the geochemical evidence for a basaltic target rock was rather the result of a vaporized mafic chondritic projectile forming a major component of the layer, especially at the more distal sites.

 Further important work, starting in the early 1990s, documented textures in shocked zircon representing different degrees of shock metamorphism. This set the stage for using zircon as an impact indicator, and, in addition, showed that the degree of resetting of the U-Pb isotopic system correlates with the degree of shock metamorphism exhibited by the zircons. For his zircon studies, Bruce developed methods for etching zircons to reveal shock textures ⁄ features that are still used in such investigations. Further work concerned the distinction between the fireball and the fallout layers, or detailed studies of K-Pg boundary impact spherules, and where he was able, with his background in clay mineralogy, to provide a better understanding of the post-impact environment and the alteration of impact glasses

 For his exceptional achievements in the study of impact processes, he was awarded the Barringer Medal at the Meteoritical Society meeting in Greenwich, UK, in 2011, among other awards and recognitions. After his retirement from the U.S. Geological Survey in 1995, he continued as an emeritus and volunteer at the USGS; during this time he returned to earlier studies of minerals in both ancient and present day Mayan potters in the Yucatan Peninsula of Mexico. He authored or coauthored more than 100 technical publications over his research career. Because of health issues, Bruce discontinued his active research and moved to Green Valley, Arizona in 2010. He is survived by his wife of 36 years, Leah. Bruce had a very dry but acute sense of humor and being with him always was an experience. We will miss him.

Christian Koeberl
University of Vienna & Natural History Museum Vienna,

Categories: In Memoriam