LAUSR

Persistently low atmospheric oxygen requires that net organic carbon burial was muted through much of Earth’s middle age. In order to achieve global mass balance with respect to O2, recent models have suggested that redox-dependent mechanisms, such as Fe(II)-phosphate precipitation, limited phosphate availability in dominantly anoxic and ferruginous oceans, in turn limiting net primary production, and therefore organic carbon burial. Nevertheless, observational constraints on phosphorus cycling in ferruginous Proterozoic systems are rare, leaving these models largely untested. Here, we present high-resolution petrographic and mineralogical data showing that the 1.3 Ga Sherwin Ironstone (Roper Group, Australia) was dominated by syndepositional precipitation of the Fe(II)-silicate minerals greenalite and berthierine, interlaminated with abundant authigenic calcium fluorapatite (CFA). Set in a quantitative geochemical framework, these data reveal that elevated marine SiO2(aq) concentrations facilitated extensive Fe(II)-silicate production, leaving CFA, rather than Fe(II)-phosphate, as the principal inorganic phosphorous sink in shallow-water Roper Group sediments. More broadly, the physical and chemical factors that triggered Fe(II)-silicate and CFA burial in the Roper Seaway highlight semi-restricted basins as important loci of phosphorus removal from the mid-Proterozoic ocean.