May 2009 Geology media highlights

Boulder, CO, USA - Hot topics include (1) opposition to the idea that chevron-shaped dunes are indicative of mega-tsunamis; (2) discovery of a complex microbial community that extends the fossil record of cavity-dwelling life by more than 1.5 billion years; (3) documentation of nanoscale, respirable cristobalite fibers in volcanic ash from Chaiten volcano and the likely adverse health effects; (4) closing the gap between Earth's first animals and fossil and geochemical evidence; and (5) the largest trilobites ever found.

New evidence from seismic imaging for subduction during assembly of the North China craton
Tianyu Zheng et al., Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Pages 395-398.

The "frozen-in" information in the crust plays an important role in improving our understanding of cratonic formation and evolution, and of plate tectonics in the Precambrian. The Trans-North China orogen (TNCO) is a continental-to-continental collision belt generated by the assembly of the North China craton (NCC). The mechanism and modality of the collision are disputed. Here, Zheng et al. present a seismic image of the Western Block and the TNCO of the NCC-derived using receiver function analysis of the teleseismic records from a dense array. A low-velocity zone extending from the middle crust to the Moho is interpreted as a remnant of upper-middle crustal material associated with westward-dipping subduction beneath the Western Block of the NCC. Crustal uplifting and magmatic underplating resulting from subsequent tectonic events were responsible for modifying the remaining subduction architecture. The western boundary of the TNCO is located west of the boundary earlier identified by surface investigation. The results, combined with previous seismic imaging in the eastern NCC, provide insight into the amalgamation of the Eastern and Western Blocks and the subsequent tectonic deformation of the NCC.

Reconstructing Earth's surface oxidation across the Archean-Proterozoic transition
Qingjun Guo et al., State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China. Pages 399-402.

Earth's atmosphere experienced a substantial increase in free oxygen during the Archean-Proterozoic transition (2650-2100 million years ago), termed the Great Oxidation Event. Although the atmospheric oxygen content was still far less than today, surface environments were affected by oxidation. This change in redox conditions was associated with changes in ocean biogeochemistry. Interestingly, observed geochemical changes appear, in part, to be related also to Earth's earliest ice age. Guo et al. pursued a multi-geochemical study of a respective rock succession, in order to reconstruct the history of atmospheric oxygenation. Time-series data for multiple sulfur isotopes from carbonate-associated sulfate, as well as sulfides in sediments of the Transvaal Supergroup, South Africa, capture the significant rise of atmospheric oxygen (as well as the protective ozone layer), its subsequent consequences for ocean chemistry and biology, and the loss of atmospheric mass-independent sulfur isotope fractionation. In phase with sulfur is the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event.