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Redox renaissance

¹ School of Earth & Space Exploration, and Department of Chemistry & Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
² School of Earth & Space Exploration, Arizona State University, Tempe, Arizona 85287, USA

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The oxygen content of the oceans is intimately connected with the geochemical cycles of carbon and other elements in a complex network of feedbacks. Hence, biogeochemists are interested in reconstructing how redox conditions in the oceans—and in the broader environment—have changed with time. In the wake of recent analytical advances, this decade has seen the invention of several new paleoredox proxies, as well as the refinement of existing ones, catalyzing a renaissance of research on this topic. Much of this effort has focused on the Precambrian, an era that apparently witnessed at least two very large, stepwise increases in environmental oxygenation (Canfield, 2005). In this issue, Pearce et al. (2008, p. 231) demonstrate the promise of a new tool, molybdenum (Mo) stable isotopes, to understand the smaller and more rapid redox perturbations that have occurred in Phanerozoic oceans.

Pearce et al. (2008) aim to quantify the extent of seafloor anoxia during the Early Jurassic (Toarcian) Ocean Anoxic Event (OAE), a time when carbon release rates from methane hydrate dissociation were similar to those from modern fossil fuel burning (Hesselbo et al., 2000; Cohen et al., 2007). Mo stable isotopes are useful in this effort because there is a marked contrast in the extent of Mo isotope fractionation during removal of Mo from the oceans to at least two sedimentary sinks. Specifically, light Mo isotopes adsorb preferentially to ferro-manganese oxyhydroxides with a fractionation factor of 1 /amu (Barling and Anbar, 2004), while there is little . . . [Full Text of this Article]