海洋環境における微量元素の循環と鉄-マンガン酸化物の役割
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概要
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Dissolved elements are supplied to the sea both from the continental crust as river run-off and aeolian dust and from the oceanic crust as hydrothermal solution. They are finally deposited on the sea-floor as sediments after various removal processes. The scenario of their journey from the source to the sink is as follows. Dissolved elements are first taken up by phytoplankton which are grazed by zooplankton and excreted as fecal pellets. Fine-grained suspended materials decomposed from plankton are coagulated into large amorphous aggregates (marine snow). Fecal pellets, marine snow and aeolian dust from land get together and are transported to the sea-floor as settling particles. The biogenic components of settling particles are partially decomposed during sinking through the water column, and at depth, Fe-Mn oxides are precipitated on the settling particles by microbial mediation. According to the "train-passengers" model, a train of settling particles left the surface layer detaches biogenic cars one by one and connects clay and Fe-Mn oxide cars one after another during descending through the water column, and finally reaches the sea-floor. Passengers of trace elements get on or off the train depending on their concentrations in seawater and by the changes of their chemical species in settling particles. Settling particles on the sea-floor undergo early diagenesis and elements associated with settling particles are dissolved into interstitial water. Most elements associated with settling particles are released back into bottom water as benthic fluxes from mildly reducing near-shore and continental shelf sediments, and transported laterally to deep waters. In pelagic sediments, Fe-Mn oxides precipitate and take up minor elements from interstitial water as the result of oxic and suboxic diagenesis. Fe-Mn oxides play an important role in controlling the concentrations of minor elements in river water, scavenging them from the water column of seawater and fixing them into pelagic sediments. The chemical composition of marine sediments are simulated by the [shale (near-shore or continental shelf sediments) + ferromanganese nodules (Fe-Mn oxides)] model. Even for metalliferous sediments, this model is applicable except for hydrothermal sulfidic and nontronite sediments. The exceptional elements to this model are Se, Re and U, which are enriched in mildly reducing near-shore sediments relative to pelagic sediments.
- 日本海洋学会の論文
- 1995-08-30
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