OXYGEN IS A FUNDAMENTAL REQUIREMENT FOR THE DEVELOPMENT OF COMPLEX LIFE AND ITS BUILDUP IN THE EARTH SYSTEM HAS BEEN A MAJOR FOCUS OF GEOBIOLOGICAL RESEARCH. ALTHOUGH MANY STUDIES HAVE FOCUSED ON THE MAJOR OXYGEN PULSES THAT BOOKEND THE PROTEROZOIC EON FUNDAMENTAL QUESTIONS PERSIST ABOUT GLOBAL REDOX DURING THE MIDDLE CHAPTERS OF EARTH HISTORY. THIS IS PROBLEMATIC BECAUSE THIS MID- PROTEROZOIC INTERVAL (~1.8 TO 0.8 BILLION YEARS AGO) WAS CHARACTERIZED BY A MAJOR DIVERSIFICATION OF EARLY EUKARYOTES THAT SET THE STAGE FOR SUBSEQUENT ANIMAL EVOLUTION. EXISTING CONSTRAINTS ON ATMOSPHERIC PO2 RANGE MORE THAN TWO ORDERS OF MAGNITUDE HINDERING OUR ABILITY TO DECIPHER THE ROLE THAT OXYGEN PLAYED IN DRIVING KEY MILESTONES IN EUKARYOTIC EVOLUTION. URANIUM (U) ISOTOPES CAN POTENTIALLY PROVIDE CRITICAL NEW CONSTRAINTS BECAUSE THE U ISOTOPE COMPOSITION OF SEAWATER WHICH CAN BE RECORDED IN MARINE CARBONATES IS CONTROLLED BY THE GLOBAL EXTENT OF SEAFLOOR ANOXIA. DEVELOPMENT AND VALIDATION OF THE U ISOTOPE PROXY IN CARBONATES WAS THE SUBJECT OF PRIOR SUPPORT TO OUR GROUP FROM NASA EXOBIOLOGY AND THE CURRENT PROPOSAL IS A LOGICAL EXTENSION OF THE PIONEERING WORK THAT RESULTED FROM THAT SUPPORT. OVER THE PAST YEAR OUR TEAM HAS AMASSED A LARGE U ISOTOPE DATASET FOR CARBONATES OF MID-PROTEROZOIC AGE. SURPRISINGLY U ISOTOPE VALUES (0.40 0.28 [1?]) ARE SIMILAR TO MODERN SEAWATER SUGGESTING THAT LARGE PORTIONS OF THE MID-PROTEROZOIC OCEANS MIGHT HAVE BEEN OXYGENATED. HOWEVER THERE ARE STILL MAJOR GAPS IN OUR UNDERSTANDING OF THE U ISOTOPE SYSTEM THAT PRECLUDE DEFINITIVE INTERPRETATION. U ISOTOPE VALUES IN THIS RANGE COULD ALSO BE CONSISTENT WITH A LARGELY SUBOXIC OR ANOXIC IRON-RICH (FERRUGINOUS) OCEAN. UNFORTUNATELY EXISTING CONSTRAINTS ON U ISOTOPE BEHAVIOR UNDER SUBOXIC AND FERRUGINOUS CONDITIONS ARE LIMITED TO JUST A FEW DATA POINTS. HERE WE PROPOSE TO SYSTEMATICALLY INVESTIGATE U ISOTOPE BEHAVIOR IN SUBOXIC AND FERRUGINOUS SETTINGS TO REFINE OUR ABILITY TO INTERPRET OUR U ISOTOPE DATA. OUR PROPOSED WORK INVOLVES: 1) BIOTIC AND ABIOTIC U-REDUCTION EXPERIMENTS TO CONSTRAIN FRACTIONATION FACTORS ASSOCIATED WITH A VARIETY OF U-REDUCTION PATHWAYS IN THE PRESENCE OF CA; 2) COMPARISON OF U ISOTOPE SEDIMENT PROFILES FROM TWO MODERN SUBOXIC ANALOGUES; AND 3) WATER AND SEDIMENT U ISOTOPE MEASUREMENTS FROM TWO MODERN FERRUGINOUS ENVIRONMENTS. SPECIFICALLY WE WILL INVESTIGATE BOTH MICROBIALLY-MEDIATED AND ABIOTIC U-REDUCTION UNDER A VARIETY OF CONDITIONS FOCUSING ON HOW CA CONCENTRATIONS (WHICH AFFECTS U-SPECIATION) AND THE REDUCTANT (H2S FE(II)AQ MICROBIAL FE-REDUCERS) INFLUENCE U ISOTOPE FRACTIONATION. WE WILL CONSTRAIN U ISOTOPE FRACTION UNDER SUBOXIC AND ANOXIC CONDITIONS USING PUBLICLY-AVAILABLE ODP CORES FROM TWO PROMINENT UPWELLING ZONES: THE OMAN AND PERU MARGINS. SEDIMENT PROFILES AT THESE SITES RECORD EXPANSION AND CONTRACTION OF THE OXYGEN MINIMUM ZONE ON MILLENNIAL TIMESCALES ALLOWING US TO INVESTIGATE U ISOTOPE BEHAVIOR UNDER A RANGE OF LOW-OXYGEN LOW-SULFIDE CONDITIONS THAT EXISTED IN THESE BASINS OVER THE PAST 30 000 YEARS. FINALLY WE WILL EXPLORE U ISOTOPE FRACTIONATION UNDER FERRUGINOUS CONDITIONS IN TWO FERRUGINOUS LAKE ANALOGS CANYON LAKE (MICHIGAN) AND KABUNO BAY (EAST AFRICA). THESE SITES PROVIDE AN IDEAL ANALOGUE FOR STUDYING U ISOTOPE BEHAVIOR UNDER FERRUGINOUS CONDITIONS WHICH MAY HAVE BEEN PREVALENT IN THE PROTEROZOIC OCEANS. THIS EXPERIMENTAL AND MODERN ANALOGUE APPROACH WILL SUBSTANTIALLY REFINE OUR ABILITY TO INTERPRET THE U ISOTOPE RECORD ELIMINATING CRITICAL BLIND SPOTS IN OUR UNDERSTANDING OF U ISOTOPE GEOCHEMISTRY. MOREOVER WHEN COMBINED WITH OUR EXISTING MID-PROTEROZOIC DATASET THIS STUDY WILL PROVIDE ROBUST NEW CONSTRAINTS ON THE REDOX LANDSCAPE OF THE MID-PROTEROZOIC OCEANS THUS ALLOWING A FUNDAMENTAL RE-EVALUATION OF THE RELATIONSHIP BETWEEN OXYGENATION AND EVOLUTION ON THE EARLY EARTH.