Ocean water, manganese nodule

The present study was initiated in order to obtain quantitative data on the relative adsorption potentials of metal ions in the region of the z.p.c. of hydrous manganese oxide. This information is of considerable importance in a variety of practical phenomena ranging from the mechanism of trace metal inclusion in ocean-floor manganese nodules and pisolitic manganese ores to the sorption behavior of manganese precipitates in natural water and waste systems. 

Ocean Basins. Ocean basins are primarily formed from oceanic basalts and maybe interspersed with continental remnants, ridges, seamounts, or volcanic islands rising from the depths. Average water depth is around 4000 m but the most significant mineralization is generally found at 5000 m for manganese nodules, 4000 m for biogenic oozes, and 3000 m for hydrothermal metalliferous sulfides. The area is poorly explored, however. 

The ready accumulation of plutonium by seaweeds, concentration factors of 10s have been observed (173), is presumably the result of plutonium uptake by the sul-phated polysaccharides which compose part of the cell wall. It is possible that plutonium could concentrate on manganese nodules in the deep ocean since it has been shown that manganese dioxide adsorbs plutonium from water (174). 

Schematic representation of manganese nodule end-member morphologies. The size of the arrows Indicates the proportion and direction of metal supply, (a) Typical situation In the open ocean with the nodules lying on an oxidized sediment substrate dominant mode of formation Is hydrogenous, (b) Typical situation In nearshore and freshwater environments with nodules lying on a sediment substrate that Is partly reducing In character. Dominant supply of metals Is via Interstitial waters from below the substrate surface. Source From Chester, R. (2003). Marine Geochemistry, 2nd ed. Blackwell, p. 425.

The nodules are formed by the oxidation and precipitation of iron and manganese. The oxidation of Mn24 is catalyzed by a reaction surface io a tetravalent state that absorbs additional Fe2+ or Mn2+ which, in turn, becomes oxidized. A surface is required and the initial deposition may be of iron oxide, possibly from volcanic or geothermal sources. Proper conditions of pH, redox potential, and metal ion concentration are found in deep ocean waters. The rate of accumulation appears to be very slow. The growth also may be discontinuous, and is estimated at a faster rater rate near the continental margins. 

As mentioned earlier, sea water seems to be in equilibrium with CuO. An apparent undersaturation is seen for Zn and Cd. Nevertheless, assuming that CuO, ZnO, and CdC03 are minor constituents of solid solutions—such as occur in manganese nodules—it is possible that the ocean is slightly oversaturated with respect to these three elements. A small oversaturation would indeed agree with the residence times given here. 

Marine processes. Sea water reacts with ocean-bed sediments containing volcanic ash, or biogenic silica from Radiolarians, to form zeolites. This process is common in deep-sea sediments and phillipsite makes up 80% of the sediment in the Indian and Pacific Oceans. Clinoptilolite is abundant in Atlantic sediments and both phillipsite and clinoptilolite are found in manganese nodules on the ocean beds. 

Zinc occurs to the extent of between 5 and 10 x lO mgL" in seawater, with a residence time of about 2 X 10" years there are significant amounts of zinc in the manganese nodules on the ocean floor. In river waters, there is between lOOmgL (in the NUe) and 900mgL (in the Garonne), mainly in particulate form rather than dissolved. The principal species present in natural waters are Zn +aq, [ZnOH]+, ZnCOs, and, in the oceans, [ZnCl]+. Kinetic processes in natural waters have been reviewed, as have methods for analysing for zinc in natural, potable, and effluent waters. ... 

At about 4000 m water depth, carbonate tests are dissolved, because their solubility depends on pressure (this depth in the ocean is called calcite compensation depth, abbreviated CCD). With the dissolution of tests the metals collected before are again liberated and can contribute to the growth of manganese nodules. 

Deep-sea manganese nodules can widely be observed in all oceans. The majority of them is located on the sediment surface so that the upper side is in contact with the sea-water and the lower side in the nearly liquid mud of the sediment/water interface. Only a small part of nodules gets buried during their growth. 

Of the trace elements that are of most interest in the formation of manganese nodules, Co is present in deep ocean water at concentrations of <0.1 mnol kg, mainly as Co ", CoCO ° and CoCT (Bmland 1983 Burton and Statham 1988 Nozaki 1997). Its extremely low concentration suggests that it is rapidly removed from seawater, probably scavenged by manganese oxides. Ni is present in deep ocean water at concentrations of about 10 nmol kg", mainly as Ni, NiCOs ... 

Biotuibation is an essential requirement for benthic lifting of manganese nodules and maintaining the nodules at the sediment surface. In both the C-C F.Z. and the Pern Basin, the biological productivity of the oceanic surface waters is high enough that the amount... 

S.I., 1998. Rare earth elements in manganese nodules from the South Atlantic Ocean as indicators of oceanic bottom water flow. Marine Geology, 146 33-52. 

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