Abstract
Redox-sensitive metallic elements, Mn and Fe, are oxidized in deep sea waters and form abundant ferromanganese crusts and nodules on the world’s ocean floors at ultraslow rates of growth. This process of oxidation and the mechanism of precipitation are yet unknown. In this paper, the results of the first successful, long-term, on-site experiment of mineral precipitation that ascertains modern, ongoing hydrogenetic deposition of oxide materials from normal seawaters at water depths of 900–4500 m of geologically active and inactive environments are presented. We succeeded in the in-situ precipitation experiment on the sea floor and characterized the precipitates using high-resolution and submicron-scale chemical, mineralogical, and structural analyses. The installed artificial plates of glass, ceramics, and plastic yielded spread-out particles of sizes varying from one to a few micrometers in diameter, of coccoid-like irregular shapes, with a maximum of 1,000–10,000 individual particles/mm2/year after 12–15 years of exposure. The results indicated a continuous substantial growth of the hydrogenetic minerals if both Mn and Fe are supplied to the bottom waters. The mineralogical, chemical, and structural properties of the precipitates are similar to those of the natural precipitates on the seabed that are made up of hydrogenetic ferromanganese crusts and nodules, together with settling sediments, suspended hydrothermal particles, or microbial precipitates from cultivated Mn-oxidizing bacteria. Our work presents new realistic insight into proposed genetic models of marine hydrogenetic ferromanganese deposits in modern diverse ocean environments.
Highlights
Redox-sensitive metallic elements, Mn and Fe, are oxidized in deep sea waters and form abundant ferromanganese crusts and nodules on the world’s ocean floors at ultraslow rates of growth
A simple model of the precipitation of ferromanganese minerals forming hydrogenetic crusts and nodules was proposed mainly based on selective chemical leaching[1,2,3,4,5], assuming dissolved forms of manganese and iron in sea waters of the oxygen minimum zone (OMZ) followed by oxidation and co-precipitation of the metals
Radiochemical, paleontological, and paleomagnetic age models have indicated an average growth rate of hydrogenetic ferromanganese crusts of several to 10 mm/Myr[4,5,6,7,8]. Monitoring of this ultraslow process of precipitation of ferromanganese minerals in modern oceans is considered unrealistic; the only attempt made was a trial on the Hawaii Island sea area[9]
Summary
The first on-site precipitation or exposure experiment was designed decades ago by mineralogists at the Massachusetts Institute of Technology; it was unsuccessful. For absorption tests, they placed some membrane-sealed synthetic Mn minerals in sea water[150] m above siliceous ooze sediments and nodules at a depth of 3000–3500 m12. A second exposure test was conducted at a depth of 800–1985 m at the Cross Seamount; the results were recovered 42 months later[9]. Small irregular-shaped amorphous aggregates of Fe and Mn oxide, Station Area Geology
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