Dissolved molybdenum asymptotes in sulfidic waters

Molybdenum enrichment in organic-rich sediments is well known and is an accepted proxy for sulfidic conditions during sedimentation, but debate persists about how Mo enrichment arises. Organic scavenging and sulfide precipitation are the processes most often invoked. When dissolved Mo (Mo-aq) precipitates solely within sulfidic pore waters of sediments, contiguous particulate organic carbon concentrations (POC) are orders of magnitude higher than when it precipitates in euxinic water columns. Nevertheless, in both cases, Mo-aq concentrations decline until reaching quite similar asymptotes centred around 7.8 nM. That Mo-aq asymptotes are independent of organic carbon availability is inconsistent with organic scavenging as the dominant Mo fixing mechanism. In contrast, the FeMoS4 precipitation mechanism predicts both the existence of asymptotes and their POC independence. Because asymptotes block quantitative Mo-aq precipitation, final delta Mo-98 values in euxinic marine sediments are apt to be offset from those in seawater, but the difference is small in modern environments because (Mo-aq)(asymptote) << (Mo-aq)(seawater). Asymptotes lie above the Mo-aq concentration believed to limit Mo nitrogenase biosynthesis, suggesting that without substantial acidification, global euxinia cannot deplete Mo-aq sufficiently to create marine nitrogen crises.