Ocean floor/marine

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Wiggins et al. [456] used neutrons from the thermal column of a 10 kW pool-type research reactor and from a 120 pg Cf source to study the prompt photon emission resulting from neutron capture in magnesium nodules (ter-romanganese oxides) from the ocean floor. Spectra were recorded with a Ce(Ii) detector and a 1024-channel analyser. Complex spectra were obtained by irradiation of seawater, but it was possible to detect and estimate manganese in nodules in a simulated marine environment by means of the peaks at 7.00, 6.55, 6.22, and 6.04 pV. 

For a simple model of Ca cycling, where the Ca sources for the ocean are weathering of continental rocks, pore fluids in the marine environment, and ocean floor basalt (Gieskes and Lawrence 1981 Berner etal. 1983 Elderheld etal. 1999), and the primary sink is the biological fixation of Ca into sediments, the rate of change of 5 Ca (= 5sw ) of the oceans is given by ... 

Subsurface depth of the irorn redox boundary versus organic carbon flux to the ocean floor. Source From Bleil, U. (2000). Marine Geochemistry, Springer Verlag, p. 80. See Bliel (2000) for data sources. 

Magnetite-rich iron sands have been dredged from the ocean floor just off Kagoshima Bay (Japan) in water averaging from 15 to 40 meters in depth. Iron sand concentrates were produced in Japan as recently as 1976, although a major marine iron sand operation in Kyushu ceased operation in 1966. 

Chert is another organic marine sediment, less common than carbonate rocks, but found in huge deposits in some parts of the world. It initially consists of the skeletons of billions of tiny, single-celled animals called radiolaria. These skeletons are composed of microcrystalline quartz or chalcedony (Si02). Dense layers of this material accumulate on the ocean floor, where they are buried and compressed over time. The term chert is sometimes also applied to any compact, very fine-grained siliceous sediment that has resulted from precipitation or consolidation of silica gel. There may be chert lenses or very thin layers within other types of sediments, such as limestone. 

Marine sediments cover the ocean floor to a thickness averaging 500 m. The deposition rates vary with topography. The rate may be several millimetres per year in nearshore shelf regions, but is only from 0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal material is formed along spreading ridges and moves outwards eventually to be lost in subduction zones, the major trenches in the ocean. Because of this continual movement, the sediments on the seafloor are no older than Jurassic in age, about 166 million years. 

The formation of marine sediments depends upon chemical, biological, geological and physical influences. There are four distinct processes that can be readily identified. Firstly, the source of the material obviously is important. This is usually the basis for classifying sediment components and will be considered below in more detail. Secondly, the material and its distribution on the ocean floor are influenced by its transportation history, both to and within the ocean. Thirdly, there is the deposition process that must include particle formation and alteration in the water column. Finally, the sediments may be altered after deposition, a process known as diagenesis. Of particular importance are reactions leading to changes in the redox state of the sediments. 

Along with the silicate debris carried to the sea by rivers and wind, the calcitic hard parts manufactured by marine organisms constimte the most prominent constituent of deep-sea sediments. On high-standing open-ocean ridges and plateaus, these calcitic remains dominate. Only in the deepest portions of the ocean floor, where dissolution takes its toll, are sediments calcite-free. The foraminifera shells preserved in marine sediments are the primary carriers of paleoceano-graphic information. Mg/Ca ratios in these shells record past surface water temperatures temperature corrected 0/ 0 ratios record the volume of continental ice ratios yield information... 

Figure 3 Marine organisms produce calcite at 4 times the rate at which the ingredients for this mineral are supplied to the sea by continental weathering and planetary outgassing. A transition zone separates the mid-depth ocean floor where calcite is largely preserved from the abyssal ocean floor where calcite is largely dissolved.

The benthic realm is an especially rich environment for living organisms. Scientists now believe that up to 98% of all marine species (not individuals, but species) are found in or near the ocean floor. Some of these are fish or shellfish swimming just above the ocean floor, but most are organisms that burrow in the sand or mud, bore into or are attached to rocks, live in shells, or simply move about on the ocean floor. 

The temperatures of ocean environments vary with geographic location, water depth, and the seasons. On average, the ocean s temperature is only a few degrees above freezing. The warmest marine waters are found at the surface and in coastal areas. The coolest are found near the poles, in the open ocean, and near the ocean floor. 

In order to fully appreciate the reasons for carrying out the conservation method selected, it is important to understand in the first instance how the metal or alloy was manufactured. From modern theories of corrosion of metals in marine environments, it is possible to predict the mode of corrosive attack that the artefact may have experienced while being buried or laying on the bottom of the ocean floor. Any adverse effect on the rate of corrosion on exposure to the atmosphere can possibly be predicted. From this knowledge, the most efficient methods of field treatments, storage conditions and conservation can be recommended. 

We start with another set of isotope signatures. The rate of erosion in the distant past can be estimated by measuring the ratio of strontium isotopes in marine carbonates. Two stable isotopes of strontium — strontium-86 and strontium-87 — differ in their distribution between the Earth s crust and the mantle underneath it. The mantle is rich in strontium-86, whereas the crust is more richly endowed with strontium-87. The major source of strontium-86 in the oceans is the igneous rock basalt. This rock is extruded continuously from the mantle at the mid-ocean ridges, from where it spreads slowly across the ocean floor before diving back into the mantle beneath the ocean trenches. A little strontium dissolves from the basalt into seawater. The speed of dissolution is more or less constant. The gradual build-up of dissolved strontium-86 in the oceans is balanced by a steady uptake of strontium by marine carbonates, such as limestone (calcium carbonate). This is because strontium can displace its sister element, calcium, in the crystalline structure of limestone. As each of these processes takes place at a steady rate, we would not expect the relative amount of strontium-86 in limestone to fluctuate a great deal. In fact it varies quite a lot. Strontium-87 is to blame. 

The origin of authigenic minerals on the ocean floor has been extensively discussed in the past with emphasis on two major processes precipitation from solutions originating from submarine eruptions, and slow precipitation from sea water of dissolved elements, originating from weathering of continental rocks. It is concluded that in several marine authigenic mineral systems these processes overlap. ... 

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