TRACING EARTH’S O2 EVOLUTION USING ZN/FE SYSTEMATICS IN MARINE CARBONATES
We analyzed the major and trace element concentrations in micro-drilled, well-characterized carbonate rocks from around the world throughout Mesozoic, Paleozoic and Precambrian. In particular, we explored the use of Zn/Fe ratios as redox proxies to trace redox-state of seawater evolution. We observed a two step increase of mean Zn/Fe ratios in carbonates through Earth’s history, which correponds with two major oxdiation events (the Great Oxidiation Event and Neoproterozic Oxidation Event, respectively). We have carefully screened these carboantes for possible late digenetic effects and hydrothermal alterations. In addition, it is very unlikely that change of depositional environments, secular evolution of mantle and continental inputs greatly influenced the observation trace element behavior. Therefore, Zn/Fe ratios in these shallow marine carbonates have the potential to be a useful tracer for atmosphereic O2 evolution, which potentially provides a tool in quantatively evaluting secular evolution of Earth’s atmospheric O2.
 Sahoo et al. (2012) Ocean oxygenation in the wake of the Marinoan glaciation. Nature 489:546-549.  Konhauser et al. (2009) Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event. Nature 458:750-753.