Sediments dissolution

The predictable flux of °Thxs to the seafloor means that the flux of other components into marine sediments can be assessed by simply measuring their concentration relative to that of °Th (Fig. 5). This approach, termed °Thxs profiling, has seen widespread use in the last decade and has become a standard technique for measuring accumulation rates of many chemical species and sedimentary components. °Thxs provides possibly the best constraint on such accumulation rates for late Pleistocene sediments and is therefore an important tool. It is the best constrained of the constant flux proxies which include ofher chemical species such as Ti (Murray et al. 2000) and He (Marcantonio ef al. 1995). As wifh these other proxies, °Thxs is not mobilized during sediment dissolution because of its extreme insolubility so that °Thxs profiling assesses the final sedimentary burial flux, rather than the flux that initially arrives at the seafloor. 

Table 12 the characteristics of the papers dealing with edl at the oxide-electrolyte solutions interface were presented. There are some that have considerable solubility (CdO and ZnO). These oxides are sensitive to the CO2 adsorption but also eagerly form the hydroxy salts type compounds [73,255]. The latter may form also during the potentiometric titrations because of the sediment dissolution. 

The effect of organic-matter-driven CaCOs dissolution is to raise the CaCOs depth transition in sediments relative to the saturation horizon in the water column (Fig. 12.12). Because organic matter degradation promotes CaCOs dissolution even in saturated and supersaturated waters, the water column saturation horizon should be below the depth where sediment dissolution begins. The organic matter degradation effect on CaCOs dissolution should have little effect on the ACOs,iys-ccD necessary to create the transition in percent CaCOs, so it remains mainly controlled by the kinetics of dissolution. 

Fossil salts can also be dissolved when water-storage or water-transmission structures are placed over saline sediments. The Lake Mead reservoir behind Hoover Dam in southern Nevada overlies deposits of gypsiferous sediments. Dissolution of this gypsum substantially increases the salinity of the Colorado River during its passage through the reservoir. 

It is also important to consider where in the sediment dissolution occurs. Metabolically produced COj released immediately at the sediment-water interface is probably much less effective for carbonate dissolution than in deeper sediment strata, because neutralization with bottom water COj might occur instead of dissolution. If the particulate organic matter is more rapidly mixed down, i.e. by bioturbation, and oxidized in deeper sediment strata, the CO released into the pore waters can probably more effectively dissolve carbonates (Martin and Sayles 1996). 

The biogeochemical processes that generally describe the interaction of elements with particles are quite well known dissolution, flocculation, ion exchange, sorption, (co)precipitation, electron transfer, and biological uptake. In aquatic environments these reactions often occur simultaneously and competitively. In order to utilize marine tracers effectively, we must understand how elements are associated with particles and sediments. 

The formation and dissolution of CaCOa in the ocean plays a significant role in all of these effects (34)- CaCOa is produced by marine organisms at a rate several times the supply rate of CaCOa to the sea from rivers. Thus, for the loss of CaCOa to sediments to match the supply from rivers, most of the CaCOa formed must be redissolved. The balance is maintained through changes in the [COa] content of the deep sea. A lowering of the CO2 concentration of the atmosphere and ocean, for example by increased new production, raises the [COa] ion content of sea water. This in turn creates a mismatch between CaCOa burial and CaCOa supply. CaCOa accumulates faster than it is supplied to the sea. This burial of excess CaCOa in marine sediments draws down the [COa] - concentration of sea water toward the value required for balance between CaCOa loss and gain. In this way, the ocean compensates for organic removal. As a consequence of this compensation process, the CO2 content of the atmosphere would rise back toward its initial value. 

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Nelson, M. B., Davis, J. A., Benjamin, M. M. and Leckie, J. O. (1977). The Role of Iron Sulfides in Controlling Trace Heavy Metals in Anaerobic Sediments Oxidative Dissolution of Ferrous Monosulfides and the Behavior of Associated Trace Metals." Air Force Weapons Laboratory, Technical Report 425. 

Gamo (1995) showed that mFe/ Mn of hydrothermal solution from sediment-hosted hydrothermal site is high (Fig. 2.40). This suggests that Mn concentration of hydrothermal solution increased by the interaction with sediments (probably dissolution of Mn-oxides and hydroxides). 

The CO2 concentrations of hydrothermal solutions at Guaymas Basin vary widely and some data show high CO2 concentrations. These high CO2 concentrations and low of fluids (—10.5%o) are considered to be caused by the effect of decomposition and dissolution of organic matters and carbonates in the sediments overlying basalt (Simoneit et al., 1984). 

Francois R, Bacon MP, Suman DO (1990) Thorium 230 profiling in deep-sea sediments high resolntion records of flux and dissolution of carbonate in the equatorial Atlantic during the last 24,000 years. Paleoceanography 5(5) 761-787... 

Special attention must be paid to the interpretation of particle size data presented in terms of either weight or number of particles. Particle weight data may be more useful in sedimentation studies, whereas number data are of particular value in surface-related phenomena such as dissolution. Values on the basis of number can be collected by a counting technique such as microscopy, while values based on weight are usually obtained by sedimentation or sieving methods. Conversion of the estimates from a number distribution to a weight distribution, or vice versa, is also possible using adequate mathematical approaches, e.g., the Hatch-Choate equations. 

The protocol involving NaOAc-HOAc at pH 5 was first proposed and used by Jackson (1958) to remove carbonates from calcareous soils to analyze soil cation exchange characteristics (Grossman and Millet, 1961). Other researchers used HOAc for the extraction of metals from sediments and soils (Nissenbaum, 1972 Mclaren and Crawford, 1973). Tessier et al. (1979) first used the NaOAc-HOAc solution at pH 5 to dissolve the carbonate fraction from sediments. Since then, the NaOAc-HOAc buffer has been widely used as a specific extractant for the carbonate phase in various media (Tessier et al., 1979 Hickey and Kittrick, 1984 Rapin et al., 1986 Mahan et al., 1987 Han et al., 1992 Clevenger, 1990 Banin et al., 1990). Despite its widespread use, this step is not free from difficulties, and further optimization is required in its application. Questions arise with regard to this step in the elemental extraction from noncalcareous soils, the dissolution capacity and dissolution rates imposed by the buffer at various pHs, and the possibility that different carbonate minerals may require different extraction protocols (Grossman and Millet, 1961 Tessier et al., 1979). 

The kinetics of dissolution of pure CaC03 and soil CaC03, as indicated by the volume of C02 released and Ca dissolved during extraction, are presented in Fig. 4.3. It shows that dissolution of both pure and soil CaC03 by the NaOAc-HOAc solutions at various pHs reached a plateau after two hours. This indicates that a certain acid dose reacts completely with the proper content of soil carbonate within two hours. Tessier et al. (1979) reported that after five hours of leaching sediments, there was no increase in the calcium concentration, thus indicating that it is unnecessary to allow 16 hours for extraction of the CARB fraction, as was originally done in this sequentially selective dissolution procedure. 

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