Pore-water supersaturation

The pore-water in the near-surface tailings is supersaturated with respect to stibnite oxidation products sernamontite, Sb205, and Sb(OH)3 (Fig. 3). 

The most complete study of the inhibition of calcium carbonate precipitation by organic matter was carried out by Berner et al. (1978), where primary concern was the lack of carbonate precipitation from supersaturated seawater. Both synthetic organic compounds and organic-rich pore waters from Long Island Sound were used to measure the inhibition of aragonite precipitation. Natural marine humic substances and certain aromatic acids were found to be the strongest inhibitors. The rate of precipitation in pore waters was also found to be strongly inhibited. 

In Chapter 6, we discussed observations of marine pore water compositions in carbonate sediments that indicated that these waters usually come to a dynamic equilibrium with the most soluble carbonate phase present. Even though these waters are supersaturated with respect to other carbonate minerals, it appears that precipitation reactions are usually sufficiently inhibited that major diagenetic... 

Three major factors can alter these predicted rates by orders of magnitude. The first is that reaction inhibitors, such as organic matter, phosphate, and trace metals, are ubiquitous in pore waters of carbonate sediments. Even in highly supersaturated pore waters from modern sediments (see Chapter 6), these inhibitors are capable of effectively blocking precipitation. Because their inhibitory influence increases with decreasing supersaturation, and only small... 

White (1995) found that the apparent thermodynamic supersaturation of silicate minerals in most soil pore waters resulted from excessive values for total dissolved aluminum. In reality, much of this aluminum is complexed with dissolved organics in shallow soils and does not contribute to the thermodynamic saturation state of silicate minerals. Solubility calculations involving low dissolved organic concentrations in deeper soil horizons and in groundwater appear to produce much clearer equilibrium relationships (Paces, 1972 Stefansson and Amorsson, 2000 Stefansson, 2001). 

The numerator of the right side is the product of measured total concentrations of calcium and carbonate in the water—the ion concentration product (ICP). If n = 1 then the system is in equilibrium and should be stable. If O > 1, the waters are supersaturated, and the laws of thermodynamics would predict that the mineral should precipitate removing ions from solution until n returned to one. If O < 1, the waters are undersaturated and the solid CaCOa should dissolve until the solution concentrations increase to the point where 0=1. In practice it has been observed that CaCOa precipitation from supersaturated waters is rare probably because of the presence of the high concentrations of magnesium in seawater blocks nucleation sites on the surface of the mineral (e.g., Morse and Arvidson, 2002). Supersaturated conditions thus tend to persist. Dissolution of CaCOa, however, does occur when O < 1 and the rate is readily measurable in laboratory experiments and inferred from pore-water studies of marine sediments. Since calcium concentrations are nearly conservative in the ocean, varying by only a few percent, it is the apparent solubility product, and the carbonate ion concentration that largely determine the saturation state of the carbonate minerals. 

If either the biomass or the respiration CO2 mechanisms are called upon, the magnitude of the Early Holocene maximum must have been uniform throughout the deep ocean. The most straightforward explanation for the up-water column reduction in the magnitude of the preservation event is that at mid-depths the sediment pore waters are presently close to saturation with respect to calcite. Hence, the Early Holocene maximum in deep-sea CO3 ion concentration pushed them into the realm of supersaturation. If so, there is no need to call on a depth dependence for the magnitude of the preservation event. 

Van Cappellen and Berner (1991) smdied the growth kinetics of FAP in seeded precipitation experiments using carbonate-free solutions. The growth of FAP was inhibited by Mg " " at concentrations typical of marine pore waters and enhanced by H" " ions in the pH range of 7-8.5. At moderate degrees of supersaturation, a precursor phase similar in composition to octacalcium phosphate formed on the FAP... 

FeS formation can play an important role in regulating pore-water chemistry. Systems with more intense FeS formation have a tendency to become more supersaturated with respect to aragonite. N-poor organic matter appeared to result in more corrosive conditions. 

Precipitation of cement in all environments is controlled by a number of factors, including the presence of saturated solutions, the degree of supersaturation of pore fluids (which affects the amount of cement precipitated as it controls both the crystal fabric developed and the amount of CaO available for precipitation), the composition of the solution, the rate of pore-water movement and the chemistry of the substrate. 

Shown in Fig. 9.9 are water-composition ranges for some humid-climate streams (in New Jersey), a dilute, freshwater lake (Lake Huron) and lake-bottom muds from the Great Lakes (Sutherland 1970), and deep-soil moisture from Pennsylvania (Sears 1976 Sears and Langmuir 1982). Lake Huron and the Delaware River are dilute, humid-climate waters. They both plot near the kaolinite-gibbsite boundary. Their composition can be described as water dominated. In other words, their chemistries are controlled chiefly by dilution with fresh rainfall and runoff, not by reactions with geological materials. In a study of acid rain (water-dominated) control of soil moisture and ground-water chemistry of a sandy aquifer in Denmark, Hansen and Postma (1995) found that pore waters were close to equilibrium with gibbsite and supersaturated with kaolinite (Fig. 9.9). Precipitation pH = 4.34 at the site, and log([K+]/lH+]) = -0.95. 

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