Cr and U isotope perspectives on Earth's oxygenation history informed by studies of their modern cycles

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Marine sedimentary rocks are valuable archives of global-change in the geological past, recording information on climate, sea level, nutrient cycling, and ocean oxygenation. These phenomena are believed to underpin many biological extinctions and originations in the history of life on earth. My research centers around development and testing of novel geochemical proxies that can be used to track global-change in the geological past. ***Theme 1: Oceanic Cr cycle ***Development and testing of geochemical proxies is undertaken in modern environments to build the interpretive frameworks that will guide their geological applications. The emerging Cr isotope proxy requires additional study to test the hypothesis that redox-sensitive Cr(VI) exhibits nutrient-like behavior in the oceans, and to determine Cr isotope fractionation factors accompanying reduction of seawater Cr(VI) and oxidation of Cr(III). Using an inverse modeling toolbox with realistic ocean circulation, we will explore the origin and significance of the oceanic global Cr array' and quantify input/output Cr-fluxes to and from the oceans. We will then use Cr isotopes in a multiproxy study of global-change associated with the second largest extinction of the past 540 million-years.***Theme 2: Causes and consequences of global-change related to the Hirnantian mass extinction ***Recently published evidence utilizing the Hg proxy suggests that the Hirnantian mass extinction occurred during a period of increased volcanism, despite the lack of geological evidence for increased volcanic deposits. Volcanic outgassing of carbon dioxide warms the climate, while emissions of volcanic sulfur gases, and accelerated continental weathering, cools the climate. Indeed, two pulses of Hirnantian extinction appear to correlate with two advances and retreats of continental ice sheets, thus indicating a link with major climate change. Using a selection of traditional and novel geochemical proxies, my students, colleagues and I will reconstruct the detailed history of environmental changes surrounding the two extinctions in an exceptional sequence of sedimentary deposits in the Canadian Arctic. To avoid imprecisions in global correlation of sedimentary strata, all geochemical proxies will be measured on the same stratigraphic sections in order to accurately determine the relative timing (stratigraphic position) of global-change indicators. We will test the hypothesis that the extinctions are linked to global-change events triggered by volcanism. ***Impact: The proposed research will fill important gaps in knowledge regarding the behaviour of redox-sensitive Cr and its isotopes in the modern oceans, while the knowledge gained will be used to build a more robust interpretive framework guiding geological applications. A multiproxy study of the environmental effects of increased volcanism during in the Hirnantian mass extinction event will help to predict future global-change from present-day burning of fossil fuels.********