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CUNY GEOL 180 - Ocean Chemistry

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111Chapter 7 Chapter 7 Ocean ChemistryOcean ChemistryAbout solutions and mixturesA solution is made of two components, with uniform(meaning ‘the same everywhere’) molecular properties:The solvent, which is usually a liquid, and is the more abundant component.The solute, often a solid or gas, is the less abundant component.A mixture is different from a solution. In a mixture the components retain separate identities, so it is NOT uniform throughout.22Water is a powerful solvent and we have it everywhere – the hydrological cycle33Ocean Salinity• Salinity is the total quantity of dissolved inorganic solids in water.• 3.5% salt on average• measured in g/kg (ppt = parts per thousand)Ocean salinities vary in spaceProcesses that affect salinity: evaporation, precipitation, runoff, freezing, and thawingAnd recall that: The heat capacity of water decreases with increasing salinityAs salinity increases, freezing point decreasesAs salinity increases, evaporation slows (boiling point increases)44Tropic of CancerTropic of CancerEquatorEquatorTropic of CapricornTropic of CapricornSalinitySalinityTemperatureTemperatureLatitudeNorthSouthOcean-Surface Conditions Depend on Latitude, Temperature, and Salinity255Mid Ocean Average Surface Salinity66Surface Salinity Northern Hemisphere SummerHigh = high evaporationLow = coastal regions and high precipitation regions77Fig 73 p 189One kilogram of seawaterWater 965.6 gMost abundant ions producing salinityOther components (salinity) 34.4 gSodium (Na+) 10.556 gChloride (Cl–) 18.980 gMagnesium (Mg2+) 1.272 gBicarbonate (HCO3−) 0.140 gOtherCalcium (Ca2+) 0.400 gPotassium (K+) 0.380 gSulfate (SO42−) 2.649 g88• Dissolved salts Major constituents and trace elements Conservative/nonconservative constituents• Major Constituents = [] > 1 part per million Na+Sodium Cl-Chloride SO4-Sulfate Mg2+Magnesium Ca2+Calcium K+Potassium99 %86 %• Trace Elements = [] < 1 part per million399A few ions (charged particles) account for most of the salinity of the oceans. See Table 7.2 for minor and trace elements in seawater1010Regulating the major constituents in seawaterSources of salt:• Positive ions: weathering and erosion• Negative ions: gases from volcanic eruptions• Hydrothermal activity supply and remove salt from the deep oceanBalance of salt:Input: rivers, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms.Output: sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.1111The ratio of dissolved solids in the ocean is constant: Well-mixed solution Principle of Constant Proportions: the ratiosbetween the concentrations of major conservative ions in open-ocean water are constant 1212• Salt composition constant for last 1.5 billion years 5*1022g= total dissolved material  2.5*1015g=total run off per year salinity is not increasing!therefore: rate of addition ~ rate of removalfor a 360/00 salinity waterThe ocean is in chemical equilibrium41313salinometerDetermining salinity: Electrical conductivity is a function of salinity More Ions = greater transmission of electricity1414Seawater’s constituents may be conservative or nonconservative Conservative = concentration changes only as a result of mixing, diffusion, and advection Non-conservative = concentrationchanges as a result of biological or chemicalprocesses as well as mixing, diffusion, and advection1515• Distribution with depthPhotosynthesis removes CO2and produces O2at the surfaceRespiration produces CO2and removes O2at all depthsCompensation depth (Photosynthesis = Respiration)CO2O2Gasesphotosynthesisrespiration1616Oxygen and CO2profilesCO2ConcentrationsDirect solution of gas from the atmosphereRespiration of marine organismsOxidation (decomposition) of organic matterO2ConcentrationsPhotosynthesis Bottom water enrichmentcompensation depthoxygen minimum51717metric tons C (106)The Carbon/Carbon Dioxide Cycle -numbers in black = rates of exchangenumbers in green = total amounts stored in reservoirsnumbers in parenthesis = net annual changes Ocean uptake from atmosphere Depends on: pH, temperature, salinity, chemistry Biological pump1818Some words to keep in mind:Ion – charged atomcat-ion (+) – positively charged ionan-ion (-) – negatively charged ionDissociation = to break apart into ionsOH-H2O (all 3 will be in water solutions)H+Pure Water; [] = concentration[H+]=10-7[OH-]=10-7Neutral solution: [H+] = [OH-]1919Non-pure water solutions• [OH-] and [H+] are inversely proportional• Imbalance between the relative concentration of H+and OH-produces an acidic or basic solution• pH scale-measures acidity/alkalinity 0-14 Logarithmic scale: pH=-log10[10-5]=5 pH of rainwater ~ 5-6 (on the acidic side)202062121Seawater• Alkaline, pH from 7.5-8.5• Average pH=7.8• pH relatively constant due to buffering action of CO2• Buffer = substance that prevents sudden or large changes in the acidity or alkalinity of a solution• Important for biological processes• pH inversely proportional to the concentration of CO22222CO2combines readily with seawater to form carbonic acid (H2CO3). Carbonic acid can then lose a H+ ion to become a bicarbonate ion (HCO3-), or two H+ ions to become a carbonate ion (CO32-). Some bicarbonate ions dissociate to form carbonate ions, which combine with calcium ions in seawater to form calcium carbonate (CaCO3), used by some organisms to form hard shells and skeletons. When their builders die, these structures may fall to the seabed as carbonate sediments, eventually to be redissolved. As the double arrows indicate, all these reactions may move in either direction.CO2Buffer2323CO2 + H2O H2CO3 HCO3-+H+ or CO32-+2H+CO2Buffercarbonic acid bicarbonate carbonateCO2ConcentrationsDirect solution of gas from the atmosphereRespiration of marine organismsOxidation (decomposition) of organic matter2424• pH: acidity of seawater 7.8 - 8.5• Carbon dioxide acts as a buffer that prevents large variations in pH• Major salt ions are in constant proportions except in coastal areas• Biological processes pump CO2 into the deep ocean Chapter 7 Chapter 7 --Summary Summary72525The polar nature of the water molecule is responsible for water's remarkable ability to dissolve more substances than any other natural solvent.The most abundant ions dissolved in seawater are chloride, sodium,


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