The spatial separation of suspended Fe and Mn in plumes, recorded at Mid-Atlantic Ridge (MAR) sites, some regions on the East Pacific Rise (EPR) and in the western Pacific, is reflected in the Fe and Mn distribution in metalliferous sediments of hydrothermal origin. Investigations at 21.5–23.5°S EPR show that at a downstream distance of 10 km from the vents the total Fe and Mn contents are lower than those on the flanks. Primary suspended Mn precipitates more than 20 km from the high-temperature vents, whereas the main suspended Fe precipitates within 10 km of the active vent area. In the vicinity of the vents a zone of ‘geochemical minimum’ was observed. Bacterial masses were observed in plumes and sediments in hydrothermal sites of the Pacific. Microbial coenoses were studied to show the increased activity of heterotrophic and Fe-Mn bacteria at several horizons of MAR hydrothermal plumes. Near-surface plumes studied in Matupi Harbour (New Britain) were characterized by both the separation of suspended Fe and Mn horizontally and a corresponding increased presence of micro-organisms. The activity of micro-organisms accompanied differentiation of the suspended and dissolved Mn at the periphery of the plume. Evidence suggests a significant role for bacteria in removing Mn from plumes in different regions of the ocean.

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