Sedimentary exhalative venting of bioavailable nitrogen into the early ocean

Ore deposits found in Proterozoic marine sedimentary basins supply much of the world's zinc. Many formed contemporaneously with their host sediments when saline brines circulating from deeper basin reached sea floor. Textural, geochemical and isotopic features these SEDEX (‘sedimentary-exhalative’) indicate that biologically active seeps, vents brine pools were a feature many ore-forming systems. In mineralised pockets mid-Proterozoic basins, ‘microbial oases’ productive areas an otherwise low productivity, anoxic, deep realm. Here we hypothesize metal-rich which circulated through organic matter-rich substrate also carried high levels fixed nitrogen stimulated distinct ecosystems at sites mineralisation, or enhanced productivity more broadly basin. We tested this hypothesis carbon analyses samples carbonaceous siltstone shale 1.64 Ga Barney Creek Formation northern Australia. The hosts several Zn systems, including one largest McArthur River Mine (the HYC deposit). Samples come edge correlated strata drill cores varying distances (1-60 km) deposit. data reveal lower ratios total (TOC) to (TN) closer ore body. Strong correlations (r2>0.7) between TOC TN absence excess N suggest most was buried as bound matter. Bioavailable thus probably abundant HYC, consistent input by hydrothermal fluids. If correct, our may such point source enabled microbes develop C:N biomass. A is supported gradient δ15N values = +4‰ proximal vent +7.5‰ distal sites, towards recycling ammonium underlying Wollogorang (1.73 Ga). This unit has previously been identified over-mature hydrocarbons fluid. speculate that, during mid-Proterozoic, have locally offset lack aerobic nutrient remineralization characterized anoxic Precambrian ocean biological where floor, and, possibly, spent mixed into water column.