River dissolved silicate

Distribution and seasonal variations of the silicate content in the Aral Sea waters were subject to the effect of two key factors a river flow that imported dissolved silicic acid and a biological silicate cycle in the sea proper. In general, the content of silicic acid in the Aral Sea is not high compared to other seas. 

Silicate is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical only for certain biota such as diatoms, radiolarian, sihcoflageUates, and siliceous sponges. However, this biology is one of the most important producers in marine. The estimation shows that diatoms contribute more than 40% of the entire primary production. Therefore, silicate cycling has received significant scientific attention in recent years and many scientists have studied silicate behavior in marine environments. Biogenic silicate is the amorphous content extracted by chemical methods, which is named as biogenic opal or opal in brief. The concentration of dissolved silicate in the world ocean is about 70.6 pmol/L and the net input of dissolved silicate from land to ocean is (6.1 2.0)x 10 mol (calculated by Si) every year, and the primary contribution (about 80%) comes from river. 

The spatial and diurnal variations of dissolved silicate in the Changjiang River Estuary were investigated from May 19 to 26, 2003. The results showed... 

Shiller AM, Boyle EA (1991) Trace elements in the Mississippi River Delta outflow region behavior at high discharge. Geochim Cosmochim Acta 55 3241-3251 Shiller AM, Mao L (2000) Dissolved vanadium in rivers Effects of silicate weathering. Chem Geol 165 13-22... 

Roy S, Gaillardet J, Allegre CJ (1999) Geochemistry of dissolved and suspended loads of the Seine River, France anthropogenic impact, carbonate and silicate weathering. Geochim Cosmochim Acta 63 1277-1292... 

The cations become a component of river water and are eventually transported to the sea. About 45% of the dissolved solids entering the ocean are derived from the weathering of detrital silicates. Feldspars are the most important somce rock for terrigenous clays as illustrated by the following reaction... 

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

To test the practical applicability of the quinoline solvent systems, samples of raw, crushed (< 125 p) Green River oil shale were extracted in the previously described manner (Table I). The samples lost 37% of their weight, suggesting that a large portion of the quartz and other silicates had been dissolved from the rock. 

Along the same lines, it was reported very recently from China [12] and India [13] that silicate and carbonate rock weathering in rivers gives rise to increased concentrations of dissolved ions such a Mg2+, Ca2+, Na2+, HCOJ, and CT, and that the C02 consumption of this should be taken into consideration as a sink for greenhouse gases. Further details on C02 reactions in aqueous solutions can be found elsewhere [6, 14]. 

Figure 11.14 Based on an average global concentration of dissolved SiC>2 (DSi) in rivers of 150 pm L-1 the relative abundance of dissolved S species (DSi) in rivers is in the form of silicic acid (H2Si03), which typically has a pH in the range 7.3-8.0. (Modified from Conley, 1997.)...

In the dissolved load, the preferential dissolution of the different types of lithology or mineral can cause large variations in trace-element abundances in rivers. A classical tracer of rock dissolution is strontium. The isotopic ratio of strontium in river waters is, to a first approximation, explained by mixing between different sources. Rain, evaporite dissolution, and carbonate and silicate weathering are the predominant sources of strontium in rivers (see Chapter 5.12). Examples of the use of Ca/Sr, Na/Sr, and Sr isotopic ratios to quantify the proportion of strontium derived from carbonate weathering in large basins can be found in Negrel et al. (1993)... 

Their contribution to the total dissolved load in rivers can be estimated by considering the mean composition of river water and the relative importance of various rocks to weathering. Estimates (18) indicate that evaporites and carbonates contribute approximately 17% and 38%, respectively, of the total dissolved load in the wodd s rivers. The remaining 45% is the result of the weathering of silicates, underlining the significant role of these minerals in the overall chemical denudation of the earth s surface. 

The chemical composition of a newly formed groundwater is initially determined by rainwater (sometimes also by river water) that becomes exposed to increased partial pressure of CO2 (from the microbially mediated oxidation of organic matter in the soil horizon) after infiltration into the soil. The CO2-enriched water dissolves minerals such as aluminum silicates, CaC03, and CaMg(C03)2. 

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