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Abstract. Phosphorus is often invoked as the ultimate limiting nutrient,modulating primary productivity on geological timescales. Consequently, alongwith nitrogen, phosphorus bioavailability exerts a fundamental control onorganic carbon production, linking all the biogeochemical cycles across theEarth system. Unlike nitrogen that can be microbially fixed from anessentially infinite atmospheric reservoir, phosphorus availability isdictated by the interplay between its sources and sinks. While authigenicapatite formation has received considerable attention as the dominantsedimentary phosphorus sink, the quantitative importance of reducediron-phosphate minerals, such as vivianite, has only recently beenacknowledged, and their importance remains underexplored. Combiningmicroscopic and spectroscopic analyses of handpicked mineral aggregates withsediment geochemical profiles, we characterize the distribution andmineralogy of iron-phosphate minerals present in methane-rich sedimentsrecovered from the northern South China Sea. Here, we demonstrate thatvivianite authigenesis is pervasive in the iron-oxide-rich sediments belowthe sulfate–methane transition zone (SMTZ). We hypothesize that the downwardmigration of the SMTZ concentrated vivianite formation below the currentSMTZ. Our observations support recent findings from non-steady-statepost-glacial sedimentary successions, suggesting that iron reduction belowthe SMTZ, probably driven by iron-mediated anaerobic oxidation of methane(Fe-AOM), is coupled to phosphorus cycling on a much greater spatial scalethan previously assumed. Calculations reveal that vivianite acts as animportant burial phase for both iron and phosphorus below the SMTZ,sequestering approximately half of the total reactive iron pool. Byextension, sedimentary vivianite formation could serve as a mineralogicalmarker of Fe-AOM, signalling low-sulfate availability against methanogenicand ferruginous backdrop. Given that similar conditions were likely presentthroughout vast swathes of Earth's history, it is possible that Fe-AOM andvivianite authigenesis may have modulated methane and phosphorus availabilityon the early Earth, as well as during later periods of expanded marine oxygendeficiency. A better understanding of vivianite authigenesis, therefore, isfundamental to test long-standing hypotheses linking climate, atmosphericchemistry and the evolution of the biosphere.

Vivianite formation in methane-rich deep-sea sediments from the South China Sea