Seawater chemical composition

Input of river water to seawater is the most important process controlling chemical composition of seawater. Chemical composition of river water varies widely (Table 4.4). Thus, it is difficult to estimate average chemical composition of river water in the world. However, it can be regarded as average chemical composition of major river water in continent (Missippi, Naile, Yangtze river etc.). Chemical composition of major river water is determined by water-rock interaction, mixing... 

MacKenzie and Carrels (1966) approached this problem by constructing a model based on a river balance. They first calculated the mass of ions added to the ocean by rivers over 10 years. This time period was chosen because geologic evidence suggests that the chemical composition of seawater has remained constant over that period. They assumed that the river input is balanced only by sediment removal. The results of this balance are shown in Table 10-13. 

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

Several workers have intended to estimate the chemical compositions of Kuroko ore fluids based on the chemical equilibrium model (Sato, 1973 Kajiwara, 1973 Ichikuni, 1975 Shikazono, 1976 Ohmoto et al., 1983) and computer simulation of the changes in mineralogy and chemical composition of hydrothermal solution during seawater-rock interaction. Although the calculated results (Tables 1.5 and 1.6) are different, they all show that the Kuroko ore fluids have the chemical features (1 )-(4) mentioned above. 

Origin of ore fluids is constrained by (1) chemical compositions of ore fluids estimated by thermochemical calculations (section 1.3.2) and by fluid inclusion analyses, (2) isotopic compositions of ore fluids estimated by the analyses of minerals and fluid inclusions (section 1.3.3), (3) seawater-rock interaction experiments, (4) computer calculations on the seawater-rock interaction, and (5) comparison of chemical features of Kuroko ore fluids with those of present-day hydrothermal solutions venting from seafloor (section 2.3). 

During the last two decades, many experimental studies on the seawater-rock interaction at elevated temperatures (100-400°C) have been conducted. Particularly, detailed seawater-basalt interaction experiments have been done. Several experimental studies on seawater-rhyolite interaction and seawater-sedimentary rock interaction are also available (Bischoff et al., 1981). Examples of chemical compositions of modified seawater experimentally interacted with various kinds of rocks are shown in Table 1.9. 

The above argument on the calculation of chemical composition of ore fluids, seawater-rock interaction experiments, and isotopic compositions of ore fluids clearly demonstrates that Kuroko ore fluids were generated by seawater-rock interaction at elevated temperatures. The chemistry of present-day hydrothermal solution venting from back-arc basins and midoceanic ridges (sections 2.3 and 2.4) also support this view. 

These differences are considered to be attributed to the dilferences in compositions of rocks and alteration minerals interacted with circulating seawater or modified seawater at elevated temperatures. For example, high K and Li concentrations in the hydrothermal solution in the Mid-Okinawa Trough baek-arc basin (Jade site) are due to the interaction of hydrothermal solution with acidic volcanic rocks (Sakai et al., 1990). It is evident that the chemical compositions of hydrothermal solution are largely alfected by water-rock interaction at elevated temperatures. 

These differences are caused by the influences of /02, /s2> pH, temperature of ore fluids and chemical compositions of rocks interacted with seawater in hydrothermal system. 

These deposits are characterized by polymetallic (Cu, Pb, Zn, Au, Ag, etc.) mineralization and formation in extensional stress fields. Ore fluids responsible for these ore deposits are dominated by seawater origin, considering isotopic and chemical composition of ore fluids. 

Figure 2.5. Elemental enrichment factors in seawater, related to the ionic potential of the elements (after Banin and Navrot, 1975. Reprinted from Science, 189, Banin A. and Navrot J., Origin of Life Clues from relations between chemical compositions of living organisms and natural environments, pp 550-551, Copyright (1975), with permission from AAAS)...

The ion proportions in most river water is significantly different from that in seawater. As a result, river runoff can have a local impact on the ion ratios of coastal waters. This effect is most pronounced in marginal seas and estuaries where mixing with the open ocean is restricted and river input is relatively large. The variable composition of river water and its impact on the chemical composition of seawater are discussed further in Chapter 21. 

Hydrothermal vents are another source of water entering the ocean. These vents are submarine hot-water geysers that are part of seafloor spreading centers. The hydrothermal fluids contain some major ions, such as magnesium and sulfete, in significantly different ratios than foimd in seawater. The importance of hydrothermal venting in determining the chemical composition of seawater is described in Chapters 19 and 21. 

A significant amount of seawater is trapped in the open spaces that exist between the particles in marine sediments. This fluid is termed pore water or interstitial water. Marine sediments are the site of many chemical reactions, such as sulfate reduction, as well as mineral precipitation and dissolution. These sedimentary reactions can alter the major ion ratios. As a result, the chemical composition of pore water is usually quite different from that of seawater. The chemistry of marine sediments is the subject of Part 111. 

The chemical composition of seawater is largely regulated by biogeochemical processes that cause dissolved materials to be converted into solid forms. These solids are then deposited on the seafloor, making the sediments a very important reservoir in the crustal-ocean-atmosphere factory. Marine sediments are also important because they contain our only record of past conditions in the ocean. 

Clay minerals are important to the crustal-ocean-atmosphere fectory, not just for their abundance, but because they participate in several biogeochemical processes. For example, the chemical weathering reactions responsible for their formation are accompanied by the uptake and release of cations and, thus, have a large impact on the chemical composition of river and seawater. This includes acid/base buffering reactions, making clay minerals responsible for the long-term control of the pH of seawater and, hence, of importance in regulating atmospheric CO2 levels. 

After delivery to the ocean, clay minerals react with seawater. The processes that alter the chemical composition of the terrigenous clay minerals during the first few months of exposure are termed halmyrolysis. These include (1) cation exchange, (2) fixation of ions into inaccessible sites, and (3) some isomorphic substitutions. Another important transfiarmation is flocculation of very small (colloidal-size) clay particles into larger ones. 

If the chemical composition of seawater has remained constant over time, a steady-state balance should exist in which the total supply rate of a particular ion is matched by its... 

Unfortunately, most of the DOM in seawater is LMW (75 to 80%) and its chemical composition has not been as well studied as that of the HMW fraction. LMW DOM is thought to be composed primarily of biopolymers containing 10 or fewer monomers. Radiocarbon measurements indicate LMW is older than HMW DOM, suggesting that LMW is fer less reactive than HMW DOM. 

Halmyrolysis The processes that alter the chemical composition of terrestrial clay minerals during their first few months of exposure to seawater. 

A third relevant factor is the chemical composition of the membranes, with cellulose acetate, polyamide and a number of composite membranes sharing the seawater installed capacity. It is my estimate that polyamide membranes have at least 90% of the market. 

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