Establishing a new method for Cr isotopes to understand redox change of seawater in the geological pasts

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During the two years, I have established a new method to separate Cr from sedimentary rocks deposited in the geological past to measure Cr isotopes which could be a new indicator of oxic-anoxic condition of seawater in the deep time. I measured major, trace and rare earth element concentrations as well as Cr isotopic compositions of carbonate rocks of Lower Cretaceous sediments from Mid-Pacific Mountains. I compared chemical compositions of carbonate between deep pelagic chalk from DSDP Site 463, and shallow water limestone from ODP Site 866 on Resolution Guyot. My data track a stratigraphic interval representing Oceanic Anoxic Event (OAE)-1a of early Aptian time, characterised by an almost synchronous global deposition of organic carbon-rich black shale. The data suggest that seawater chemistry have changed a few hundred of thousand years before OAE-1a.