Deep-marine brine seeps stimulate microbial nitrogen cycling: implications for the formation of sediment-hosted ore deposits

Abstract

Deep-marine brine seeps in the modern ocean are considered analogues for settings that favoured the formation of sedimentary-exhalative zinc and lead deposits in deep time. Microbial activity plays an important role in the accumulation of ore minerals, meaning that the extent of mineralization is at least indirectly dependent on nutrient fluxes. Here, we investigated the biogeochemical nitrogen cycle in shallow (15-50 cm) sediment cores from the Orca Basin brine pool and surrounding sites, as well as from an active brine seep area near Dead Crab Lake in the Gulf of Mexico, with the aim of constraining the effect of brine seepage on this bio-essential element. We find high porewater ammonium concentrations in the millimolar range, paired with elevated ratios of organic carbon to nitrogen in sediments, which confirm previous hypotheses that the brine recycles ammonium from sedimentary strata back into the water column. Within Orca Basin, we note tentative evidence of microbial ammonium utilization. At the active seep, ammonium is mixed into the overlying water column and likely undergoes oxidation. Isotopic data from sediments and dissolved ammonium, paired with previously published genomic data, suggest the presence of dissimilatory nitrate reduction to ammonium (DNRA) at the brine-seawater interface. We conclude that brine seeps can stimulate biological nitrogen metabolisms in multiple ways. Our results may help calibrate studies of biogeochemical cycles around brine seeps that are archived in the rock record.