Seawater chemistry

Magnesium is the third most abundant element in seawater, behind sodium and chorine, and has an average concentration of approximately 1300 ppm. Table 3.2 displays the major and some minor elemental constituents of seawater. Eleven major constituent ions account for 99.5% of the total solutes present in seawater. These 11 are chloride, sulfate, bicarbonate, bromide, fluoride, sodium, magnesium, calcium, potassium, strontium, and boron, and they largely determine the chemistry of seawater. 

Seawater is slightly alkaline with a pH between 7.8 and 8.3 and is buffered primarily by the carbonate system. The equilibrium reactions between CO2 gas in the atmosphere and seawater are shown in reactions (3.1)—(3.5)  

When equilibrium with the atmosphere is reached, approximately 87% of ionic carbonate is present as bicarbonate ion, the remainder being carbonate. In many places, especially close to the surface, seawater is saturated with respect to calcium carbonate, which will precipitate slowly from solution, thus regulating the amount of carbonate in solution. This process is perhaps the most important of all the geological systems since it regulates the amount of carbon dioxide in the atmosphere. 

Seawater also contains a wide variety of dissolved organic compounds, the total amount being about 2 ppm. More than 100 different compounds have been identified in solution in seawater. These include a wide variety of organic acids, carbohydrates, amino acids, polypeptides, as well as various vitamins. 

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Bischoff, J.L. and Dickson, F.W. (1975) Seawater-basalt interaction at 200°C and 500 bars Implications for origin of seafloor heavy-metal deposits and regulation of seawater chemistry. Earth Planet. Sci. Lett., 25, 385-397. 

It was shown in previous chapters that intense hydrothermal activities occurred in the Neogene age in and around the Japanese Islands under the submarine and subaerial environments. In this chapter the influence of these hydrothermal activities on the seawater chemistry, and the global geochemical cycle are considered. 

The studies on the hydrothermal systems at midoceanic ridges during the last three decades clearly revealed that the seawater-basalt interaction at elevated temperatmes (ca. 100-400°C) affects the present-day seawater chemistry (Wolery and Sleep, 1976 Edmond et al., 1979 Humphris and Thompson, 1978). For example, a large quantity of Mg in seawater is taken from seawater interacting with midoceanic ridge basalt, whereas Ca, K, Rb, Li, Ba and Si are leached from basalt and are removed to seawater (Edmond et al., 1979 Von Dammet al., 1985a,b). 

Many drillings have been made for the purpose of the exploration of Kuroko deposits in the Green tuff region. A large number of petrographical and analytical data are available. Thus, it is possible to (1) assess the proportion of volcanic rocks that have suffered hydrothermal alteration, and (2) attempt a quantitative evaluation of the influence of hydrothermal alterations at back-arc basins on middle Miocene seawater chemistry. 

Shikazono, N. (1994) Hydrothermal alteration of green tuff belt, Japan Implications for the influence of seawater/volcanic rock interaction on the seawater chemistry at a back arc basin. The Island Arc, 3, 59-65. 

Hardie LA (1996) Secular variation in seawater chemistry An explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y. Geology 24 279-283... 

Stanley SM, Hardie LA (1998) Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeography Palaeoclimatology Palaeoecology 144 3-19... 

Manceau A, Schlegel ML, Musso M, Sole VA, Gauthier C, Petit PE, Trolard F (2000) Crystal chemistry of trace elements in natural and synthetic goethite. Geochim Cosmochim Acta 64 3643-3661 Marchitto Jr. TM, Curry WB, Oppo DW (2000) Zinc concentrations in benthic foraminifera reflect seawater chemistry. Paleoceanogr 15 299-306. 

Hydrothermal venting injects fluids into seawater as buoyant, jetlike pliunes. These turbulent flows mix rapidly with seawater becoming diluted by factors of lO" to 10. This mixing eventually makes the plumes neutrally buoyant, after which they are transported laterally through the ocean basins as part of the intermediate and deepwater currents. Hydrothermal plumes have the potential to greatly affect seawater chemistry. From global estimates of hydrothermal fluid emissions and dilution ratios, a volume of seawater equivalent to the entire ocean can be entrained in the hydrothermal plumes every few thousand years. 

The overall effect of the terrestrial weathering reactions has been the addition of the major ions, DSi, and alkalinity to river water and the removal of O2, and CO2 from the atmosphere. Because the major ions are present in high concentrations in crustal rocks and are relatively soluble, they have become the most abimdant solutes in seawater. Mass-wise, the annual flux of solids from river runoff (1.55 x 10 g/y) in the pre-Anthropocene was about three times greater than that of the solutes (0.42 x 10 g/y). The aeolian dust flux (0.045 X 10 g/y) to the ocean is about 30 times less than the river solids input. Although most of the riverine solids are deposited on the continental margin, their input has a significant impact on seawater chemistry because most of these particles are clay minerals that have cations adsorbed to their surfaces. Some of these cations are desorbed... 

Because Olivella biplicata snails grow in tidal environments, it was hypothesized that their shell chemistry might be influenced by nearshore seawater chemistry, which would include dissolved minerals from local shoreline deposits. If geology varied enough along the California coast, certain regions might be bathed by seawater with a chemically distinct composition that would be incorporated into the shells. 

In Chapter 1 it was noted that speciation may be defined as a description of species types (forms/phases) and concentrations. The descriptions of speciation found in this chapter will largely be viewed from this perspective and described in terms of comparative chemistries. The framework for this discussion will be the associations and groupings of the Periodic Table. For further descriptions of seawater chemistry, including sampling and analytical considerations, the reader is directed to the excellent reviews of Bruland (1983) and Donat and Bruland (1995). 

The seawater chemistries of Mn, Fe, Co, Ni, Cu and Zn are, in many respects, quite diverse. One characteristic that these elements have in common, however, is an accessible +11 oxidation state. Except in the case of iron, which exists dominantly as Fem in seawater, the solution speciation of these elements is dominated by the +11 oxidation state. The aspect of these elements seawater speciation which most distinguishes them from other cations in the Periodic Table is their substantial involvement in organic complexation. 

Figure 4. Concentrations in a model of seawater chemistry including the copper sulfide reactions, a) n = 1 with Kj = 1010 and a value R - 10s for the ratio of organic copper complexes to Cu2+. b) n = 2 with K2 = 1020, R = 105.

More recent calculations such as those in this book indicate substantially lower saturation depths. Those calculated here are plotted in Figure 4.21. The SD is generally about 1 km deeper than that presented by Berger (1977). Clearly the new SD is much deeper than the R0 and appears only loosely related to the FL. Indeed, in the equatorial eastern Atlantic Ocean, the FL is about 600 m shallower than the SD. If these new calculations are even close to correct, the long cherished idea of a "tight" relation between seawater chemistry and carbonate depositional facies must be reconsidered. However, the major control of calcium carbonate accumulation in deep sea sediments, with the exceptions of high latitude and continental slope sediments, generally remains the chemistry of the water. This fact is clearly shown by the differences between the accumulation of calcium carbonate in Atlantic and Pacific ocean sediments, and the major differences in the saturation states of their deep waters. 

Figure 5.11. Results of macrocosm experiments on Porolithon gardineri, a coralline alga grown under controlled conditions of temperature, light and seawater chemistry. Different symbols simply represent different experiments. (After Agegian, 1985.)...

Maynard, J. B., Kinetics of silica sorption by kaolinite with application to seawater chemistry. Am. J. Sci., 275, 1028-1048 (1975). 

Calcium carbonate is accumulating in deep ocean sediments, in which the overlying water is undersaturated with respect to both aragonite and calcite, and sediment marker levels closely correspond to unique saturation states. This indicates that dissolution kinetics play an important role in determining the relation between seawater chemistry and calcium carbonate accumulation in deep ocean basins. It is, therefore, necessary to have knowledge of the dissolution kinetics of calcium carbonate in seawater if the accumulation of calcium carbonate is to be understood. 

Hales, B., van Geen, A., and Takahashi, T. (2004). High-frequency measurement of seawater chemistry Flow injection analysis of macronutrients. Limnol. Oceanogr. Methods 2, 91—101. 

Th and Th, e.g., would tend to cancel each other out. Because of variations in seawater chemistry over time, it is not likely that the assumption of constant Nq, or of the initial ratios, is strictly true, but the linearity generally observed in In N versus z plots indicates that it is a sufficiently good approximation that valid estimates of average accretion rates over long time intervals can be obtained. 

Marchitto T. M., Curry W. B., and Oppo D. W. (2000) Zinc concentrations in benthic foraminifera reflect seawater chemistry. Paleoceanography 15, 299—306. 

Lowenstein T. K., Timofeeff M. N., Brennan S. T., Hardie L. A., and Demicco R. V. (2001) Oscillations in Phanerozoic seawater chemistry evidence from fluid inclusions. Science 294(5544), 1086-1088. 

Zimmermann H. (2000) Tertiary seawater chemistry impheations from primary fluid inclusions in marine hahte. Am. J. Set 300(10), 723-767. 

Steuber T. and Veizer J. (2002) Phanerozoic record of plate tectonic control of seawater chemistry and carbonate sedimentation. Geology 1123—1126. 

Strauss H. (2002) The isotopic composition of Precambrian sulphide—Seawater chemistry and biological evolution. In Precambrian Sedimentary Environments A Modern Approach to Ancient Depositional Systems. International Association of Sedimentologists Special Publication no. 33 (eds. W. Altermann and P. L. Corocoran). Blackwell, Oxford, pp. 67-105. 

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