Seawater models

Fig. 22.5. Concentrations of components (sulfate, sulfide, carbonate, methane, and acetate) and species (O2 and H2) that make up redox couples, plotted against temperature, during a model of the mixing of fluid from a hot subsea hydrothermal vent with cold seawater. Model assumes redox couples remain in chemical disequilibrium, except between 02(aq) and H2(aq). As the mixture cools past about 38 °C, the last of the dihydrogen from the vent fluid is consumed by reaction with dioxygen in the seawater. At this point the anoxic mixture becomes oxic as dioxygen begins to accumulate.

Plavsic, M., Kozar, S., Krznaric, D., Bilinski, H. and Branica, M., 1980. The influence of organics on the adsorption of copper (II) on yAl203 in seawater, model studies with EDTA. Mar. Chem., 9 175-182. 

With the help of stability constants valid for seawater, a model for distribution of the most important dissolved species can be calculated. Garrels and Thompson (1962) were the first to establish such a seawater model. Their calculations were based on stability constants (determined in simple electrolyte solutions and corrected or extrapolated to / = 0) and estimated activity coefficients of the individual ionic species in seawater. The mean ionic activity coefficients were assumed to be the same as those that would apply to a pure solution of the salt at the same ionic strength as seawater. This assumption is supported phenomenologically. 

Example 7.8. Calcite in Seawater Compare the composition of a CaC03(s) (calcite)-C02-H20 seawater model system, made by adding calcite to pure H2O containing the seawater electrolytes (but incipiently no Ca and no carbonates and, for simplicity, no borate) and by equilibrating this solution at 25°C and 1 atm total pressure with the atmosphere (pcoi = 3.55 X 10 atm), with the composition of a real surface seawater whose carbonate alkalinity, Ca(II) concentration, and pH have been determined as 2.4 x 10 eq liter", 1.06 X 10 M, and 8.2, respectively. Estimate the extent of oversaturation of this seawater with respect to calcite. The solubility of calcite at 25 °C is taken as "K q = [Ca/-] [CO3 ] = 5.94 X 10 , where [Ca ] and [CO37] are the concentration of total soluble Ca(II) ([Ca ] plus concentration of Ca complexes with medium ions) and of total soluble carbonate ([CO "] and concentration of carbonate complexes with medium ions), respectively. The other constants needed, Henry s law constant and the acidity constant of H2CO, are taken as ... 

Arguments against brine formation by basin ward recharging meteoric water (model 1), or modification of connate Cretaceous meteoric water or Cretaceous seawater (model 2) ... 

Sillen constructed his models in a stepwise fashion starting with a simplified ocean model of five components [HCl, H2O, KOH, Al(OH)3, and Si02] and five phases (gas, liquid, quartz, kaolinite, and potassium mica) (Sillen, 1967). His complete (almost) seawater model was composed of nine components HCl, H2O, and CO2 are acids that correspond to the volatiles from the Earth KOH, CaO, Si02, NaOH, MgO, and Al(OH)3 correspond to the bases of the rocks. If there was an equilibrium assemblage of nine phases, the system would have only two independent variables. Sillen argued that a plausible set could include a gas phase and a solution phase and the following seven solid phases ... 

Mineral water, seawater (model SS) commercial table salts (with iodide or iodate)... 

Essentially all organic matter in the ocean is ultimately derived from inorganic starting materials (nutrients) converted by photosynthetic algae into biomass. A generalized model for the production of plankton biomass from nutrients in seawater was presented by Redfield, Ketchum and Richards (1963). The schematic "RKR" equation is given below ... 

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."... 

There is some debate about what controls the magnesium concentration in seawater. The main input is rivers. The main removal is by hydrothermal processes (the concentration of Mg in hot vent solutions is essentially zero). First, calculate the residence time of water in the ocean due to (1) river input and (2) hydro-thermal circulation. Second, calculate the residence time of magnesium in seawater with respect to these two processes. Third, draw a sketch to show this box model calculation schematically. You can assume that uncertainties in river input and hydrothermal circulation are 5% and 10%, respectively. What does this tell you about controls on the magnesium concentration Do these calculations support the input/removal balance proposed above Do any questions come to mind Volume of ocean = 1.4 x 10 L River input = 3.2 x lO L/yr Hydrothermal circulation = 1.0 x 10 L/yr Mg concentration in river water = 1.7 X 10 M Mg concentration in seawater = 0.053 M. 

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. 

Ohmoto et al. (1983) and Kusakabe and Chiba (1983) also reached the conclusion that the vs. Sr/ Sr relationship and S S vs. temperature relationship of barite from the Fukazawa deposit in the Hokuroku district may be explained by a mixing model with a seawater contribution of less than 20% at temperatures around 200°C. 

The behavior of silica and barite precipitation from the hydrothermal solution which mixes with cold seawater above and below the seafloor based on the thermochemical equilibrium model and coupled fluid flow-precipitation kinetics model is described below. 

Janecky, D.R. and Seyfried, W.E. Jr. (1984) Formation of massive sulfide deposits on oceanic ridge crests incremental reaction models for mixing between hydrothermal solutions and seawater. Geochint. Cosmochim. Acta, 48, 2723-2738. 

Farrell and Holland (1983) cited ba,sed on Sr isotope study on anhydrite and barite in Kuroko deposits that the most appealing model for the formation of Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately below above the vent exit in the overlying seawater (Eldridge et al., 1983). The study on the chimney ores from Kuroko deposits support this model which is discussed below. 

Fig. 2.47. Model predicting mineral assemblages and proportions produced when basalt reacts with seawater in different water/rock mass ratios. The model is based on experimental data but is close to actual observed assemblages in recovered greenschist facies metabasalts (Mottl, 1983).

J. P. Salanitro, M. P. Williams, and G. C. Langston. Growth and control of sulfidogenic bacteria in a laboratory model seawater flood thermal gradient. In Proceedings Volume, pages 457-467. SPE Oilfield Chem Int Symp (New Orleans, LA, 3/2-3/5), 1993. 

Which of these models best illustrates the composition of seawater ... 

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