Chapter 7 What Controls the Composition of Riverwater and Seawater: Equilibrium versus Kinetic Ocean(4/15/01) James W. MurrayUniversity of WashingtonWe have covered how to calculate the equilibrium chemical composition of natural water systems.You have learned how to set up simple box models to learn about controls on ocean chemistry. Letsnow tie this up with the final question in this section: What controls the chemical composition ofriverwater and seawater? Some of this materail will be your responsibility in the group studyproject #1.In Chapter 2 we learned that to a first approximation the input to the ocean from rivers is balancedby removal to the sediments with adjustments for inputs and removal from hydrothermalcirculation at mid-ocean ridges. In the early days of chemical oceanography it was thought that,because of the long residence times, the major ion composition of seawater had been approximatelyconstant over geological time and that chemical equilibrium might explain the composition. Withthe growth of paleoceanography it has become clear that there have probably been significantexcursions in the composition of the ocean-atmosphere system (Berner et al., 1983; Berner, 1991)and that a kinetic or dynamic flux balance model is more appropriate.I. The Chemical Inflow from RiversIons transported by rivers are the most important source of most elements to the ocean. Thecomposition of river water is significantly different from seawater. The concentrations arecompared in Table 8-1. Some characteristic ratios are also compared. To a first approximationseawater is mainly a Na+ and Cl- solution while river water is a Ca2+ and HCO3- solution. It is prettyclear that we can not make seawater simply by evaporation of river water. Other factors must beinvolved and significant chemical reactions and modifications must take place.Table 8-1 The composition of average seawater and river water in mmol kg-1.Element Seawater (mmol kg-1) River water (mmol kg-1)Na 468.0 0.26Mg 53.1 0.17Ca 10.3 0.38K 10.2 0.07Sr 0.09 ----Cl 546.0 0.22SO4 28.2 0.11HCO3 2.39 0.96Br 0.84 ----mainly mainlyNa+ and Cl-Ca2+ and HCO3-RATIOSNa/K Mg/Ca Na/Ca (Ca+Mg)/HCO3Oceans 45.6 5.22 45.9 26.64Rivers 6.0 0.42 0.8 0.59There is significant variability in the composition of rivers between continents. The averagecompositions for different continents are shown in Table 8-2 (from Holland,1978).Table 8-2 The mean composition of rivers on different continents (in ppm). The concentrations foranions and cations are given in meq l-1 in the bottom two rows. Data from Livingston (1963).In general the weathering reaction on continents can be written as congruent or incongruentreactions (see Table 14.1 of Libes, 1992). In congruent reactions the total mineral goes intosolution. In incongruent reactions the initial mineral is leached and modified and converted into asecondary mineral. Weathering of CaCO3 is considered a congruent reaction.CaCO3(s) + CO2(g) + H2O = Ca2+ + 2 HCO3-Weathering of alumino-silicate minerals to clay minerals are examples of incongruent reactions.silicate minerals + CO2(g) + H2O == clay minerals + HCO3- + 2 H4SiO4° + cationWe can write these weathering reactions in terms of H+, CO2(g) or H2CO3. For example theweathering of the potassium feldspar mineral called orthoclase (KAlSi3O8(s)) to the clay mineralcalled kaolinite (Al2Si2O5(OH)4(s)) is an important reaction in soils from humid climates. We canwrite the reaction in terms of H+ as follows:KAlSi3O8(s) + H+ + 9/2H2O = 1/2 Al2Si2O5(OH)4(s) + K+ + 2 H4SiO4°The same reaction written in terms of atmospheric CO2(g) would be:KAlSi3O8(s) + CO2(g) + 11/2H2O = 1/2 Al2Si2O5(OH)4(s) + K+ + HCO3- + 2H4SiO4°For a different feldspar called plagioclase which contains an equal mole fraction Na and Ca, we canwrite:4 Na0.5Ca0.5Al1.5Si2.5O8 (s) + 6CO2(g) + 5H2O = 3 Al2Si2O5(OH)4(s) + 2Na+ + 2 Ca2+ + 4 H4SiO4° + 6 HCO3-You can see that in general, during weathering, a structured aluminosilicate (feldspar) is convertedinto a cation-poor, degraded aluminosilicate (clay), cations and silicic acid go into solution, CO2(g)is consumed and HCO3- is produced. The bicarbonate concentration released is equivalent to thecations released according to the stoichiometry of the reaction.There are many different minerals in rocks and they weather with different susceptibilities. Thestability of minerals with respect to weathering (Goldrich's "mineral stability series") is shown inTable 8-3 (Goldich, 1938). Among the mafic minerals (those with Mg and Fe), olivine weathersmuch faster than biotite. Quartz and K-feldspar are more resistant to weathering than theplagioclase minerals. Such weathering susceptibilities are clear when you look at rocks in the field.Table 8-3 (from Goldich, 1938)Weathering of carbonate minerals consumes one CO2 from the atmosphere and produces one CO32-(which can be expressed as two HCO3-) from the mineral thus there should be about twice as muchHCO3- as Ca2+. In a plot of HCO3- versus Ca2+ (Fig. 8-1, from Holland, 1978) we see that most ofthe world's major rivers fall close to the line of HCO3- = 2Ca2+ which is consistent with weatheringof carbonate minerals being a major control. Most rivers that don't fall on the line are above the lineconsistent with a silicate weathering source for some of the HCO3-. The Rio Grande is the onlymajor river below the line because gypsum can be a major source of Ca2+.The average composition of rivers from different continents is plotted in Fig 8-2 (from Garrels andMackenzie, 1971). You can see that most of the variability in composition between differentcontinents is due to Ca2+ and HCO3-. This is because Europe, North America and Asia have morecarbonate rocks than South America and Africa. The products of silicate weathering are moreuniformly distributed between continents..Example: We can estimate the percent of CO2 neutralized by silicate weathering using thefollowing simple model. We assume that on average silicate minerals produce one HCO3- fromeach CO2(g) consumed while releasing 2H4SiO4(e.g. silicate + CO2 + H2O= HCO3- + 2H4SiO4°).All the Ca2+ and Mg2+ comes from carbonate minerals except that required to balance SO42- (e.g.gypsum). Each CO2(g) neutralized by carbonate minerals produces two HCO3-. The results for thiscalculation for