GEOCHEMICAL DATA FROM ANCIENT MARINE SEDIMENTARY ROCKS INDICATE THAT MANY MAJOR STEPS IN THE EVOLUTION OF LIFE ARE TIED TO CHANGES IN THE OXYGENATION OF THE OCEANS AND ATMOSPHERE (E.G. LYONS ET AL. 2014). THERE ARE GEOCHEMICAL INDICATORS OF OCEAN-BASIN-SCALE OXYGENATED AND ANOXIC (0 UM O2) CONDITIONS BUT CURRENTLY THERE IS NOT A GLOBAL GEOCHEMICAL TOOL TO DETECT WHEN LARGE PARTS OF THE OCEANS ARE IN AN INTERMEDIATE-LEANING-TOWARD-ANOXIC CONDITION TERMED SUBOXIC (<10 UM O2). DETECTING SUBOXIC CONDITIONS IS PARTICULARL IMPORTANT BECAUSE SOME AEROBIC ORGANISMS CAN LIVE IN EXTREMELY LOW-O2 WATERS (DOWN TO ~10 NM O2 STOLPER ET AL. 2010) AND SO IT IS OF IMPORT TO KNOW WHEN LARGE PARTS OF THE OCEAN FIRST CROSSED FROM ANOXIC TO SUBOXIC. RHENIUM (RE) ISOTOPES (D187RE) MEASURED IN MARINE SEDIMENTARY ROCKS ARE A NOVEL GLOBAL PALEOREDOX TOOL THAT HAS THE POTENTIAL TO BE THE FIRST PROXY THAT CAN DETECT SUBOXIC CONDITIONS. THE ABILITY OF RE ISOTOPES TO TRACK SUBOXIC CONDITIONS COMES FROM THE UNIQUE GEOCHEMICAL BEHAVIOR OF THIS ELEMENT. UNDER OXIC CONDITIONS RE EXISTS PRIMARILY AS PERRHENATE (REVIIO4-) AN ION THAT IS HIGHLY UNREACTIVE AND THEREFORE ACCUMULATES IN SEAWATER (E.G. KOIDE ET AL. 1986 COLODNER ET AL. 1993). IN LOW-O2 CONDITIONS REVII IS REDUCED TO REIV LEADING TO EFFICIENT REMOVAL COUPLED TO ORGANIC CARBON BURIAL (COLODNER ET AL. 1993 1995 KENDALL ET AL. 2010 MORFORD ET AL. 2012). RHENIUM HAS TWO STABLE ISOTOPES 187RE AND 185RE. PRELIMINARY DATA INDICATE THAT THE LIGHT ISOTOPE IS PREFERENTIALLY REDUCED (MILLER ET AL. 2015) CAUSING A SHIFT IN THE ISOTOPIC COMPOSITION OF RESIDUAL RE REMAINING IN SEAWATER TO HEAVIER VALUES. AS A RESULT D187RE VARIATIONS IN ANCIENT SEDIMENTS THAT SAMPLE THE ISOTOPE COMPOSITION OF SEAWATER COULD PROVIDE A SENSITIVE TOOL TO TRACK THE GLOBAL EXTENT OF SUBOXIC SEAFLOOR THROUGH TIME. IN THIS PROPOSAL I OUTLINE THE DEVELOPMENT OF THE RE ISOTOPE PROXY TO TRACK CHANGES IN THE EXTENT OF SUBOXIC CONDITIONS IN THE OCEAN. THIS PROXY CAN BE APPLIED TO ANY TIME IN EARTH HISTORY DURING WHICH MAJOR CHANGES IN OCEANIC OXYGENATION OCCURRED.

Arizona State University, Scottsdale AZ

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