Lecture 6: Determinants of Seawater Composition

12.742 - Marine Chemistry Lecture 6 - Determinants of Seawater Composition Prof. Scott Doney • What is seawater? – Water – Dissolved inorganic salts (major ions) – Trace species, organics, colloids, particles, life, . . . • Water: H2 O – A very odd molecule + H 105o − O− Sets up electric dipole because O is more electronegative and bond angle is asymmetric H+ Figure 1. H dipole O H Figure 2. Hydrogen bond - 4.5 kcal/mol, while covalent bond is ∼ 100 kcal/mol. H H 2.76 A O o O o 0.96 A H H Figure 3. Hydrogen bonding has a large impact on physical properties of water, ion-pairing solvent solubility; anomalous relative to similar H2 S, surface tension 1 – Flickering cluster model Water does not like to be broken up (unless adding ionic species) This causes a high heat capacity, latent heat of evaporation, and latent heat of fusion Figure 4. T vaporization 100 0 540 cal fusion 80 cal heat Figure 5. 1.2 m kg cal J g 1 yr W · 1025 3 · 590 · 4.184 · 1000 · ∼ 96 2 y m g cal kg 3.15 · 107 s m W compared to solar heating of ∼ 170 m 2 2 • Water as a solvent – Very good at dissolving ionic species and polar compounds N aCl ! N a+ + Cl− Cl − Na+ primary region − well oriented secondary region − less structured Figure 6. • Water more dense than ice – Profound impacts for climate, life, etc. • Seawater versus freshwater – Comparison increase with salinity density electrical conductivity osmotic pressure decrease with salinity freezing point temperature of maximum density – Salinity g · Total mass of salt in water ∼ 35 kg , that is, 35 parts per thousand (ppt), and there is a relatively constant composition for major ions – Methods of salinity measurement · Simply drying and weighing (±0.01 ppt ) · Lose some species with heat (HCl) · Oxide formation · Reproducibility · Tedious 3 4 max density 2 0 freezing point −2 35 0 S Figure 7. · Titration of Cl− (and Br− ions with AgN O3 (or electrochemical) S = 1.806 · Cl parts per thousand Chlorinity in g , ±0.002 kg · Conductivity (±0.001) · Compare against KCl standard at fixed pressure, 15◦ C sample KCl solution 1/2 3/2 5/2 2 s = a1 − a2 K15 + a3 K15 + a4 K15 − a5 K15 + a6 K15 K15 = · in practice use “secondary standards” (Wormely water) Standard Sea Water UNESCO, 1978. · Practical salinity scale (PSS) · isotopes - empirically derived from lab work (s = f (K15 ), ρ, etc) – Major constituents in seawater, charge balance N a+ Cl− 2+ Mg SO42− Ca2+ HCO3− K+ others · “law of constant proportions” · Except for comments below, major ions act essentially conservatively in ocean change due to net E-P input 4 Ion Residence time (in units of 106 y) N a+ 70 14 M g 2+ 0.7 Ca2+ 100 Cl− 10 SO42− input Inventory τ (residence time) = inventory input at steady state loss Figure 8. Compare with mixing time of the ocean ∼ 103 years – Where does this fall apart? · Estuaries · Average river composition differs · Much lower inventories - ∼ 0.2 parts per thousand · Increased ratios of Ca2+ , M g 2+ , K + , SO42− , HCO3− to Cl− , s = f (K15 ) and ρ do not work as accurately · Evaporative basins · As salinity increases, start to deposit salts (Brine can also differ from seawater) CaCO3 (M g) CaSO4 · 2H2 O (gypsum) N aCl halite M gSO4 , KCl, N aBr, M gCl2 bitterns (K, M g salts) · Internal biological cycling of CaCO3 Ca2+ + CO32− ! CaCO3 (s) 5 Ca/Salinity 1000 Figure 9. Depletion of SO42− in anoxic basins or sediments; microbes using SO42− as an electron acceptor • Geochemical cycles - what controls the major ion composition ? – Two models - “equilibrium” (Sillen), “kinetic” – Equilibrium model says that solution chemistry of seawater in thermodynamic equilibrium with atmosphere and minerals on ocean floor; has not held up well but deviations from thermodynamics equilibrium are quite interesting rivers − CR input = CR FR 1 output = C0 V0 k FR 1 C0 = V0 k CR k = residence time C0 input > output → C0 increases input < output → C0 decreases Figure 10. Examine ratio of seawater concentration/river water concentration versus residence time. • Major ion budgets – Rivers and terrestrial weathering are primary source for most major ions. – Loss mechanisms include: – Cl− · Cyclical salts - salt spray transported to land; makes river Cl− source look larger than what is really coming from weathering. · Pore water trapping - burial in interstitial waters 6 − Cl Figure 11. · Evaporite production · Many of the land masses underlain by evaporite deposits · Not occurring to great extent right now · Lots of shallow seas - episodic in geological history Figure 12. – N a+ · On long time-scale evaporite formation must balance sources - exchangeable cations on clays B-clay + A+ ! B + + A-clay XA-clay [A+ ] = KAB + XB-clay [B ] X: mole fraction at exchange site Concentration of N a+ to other cations is higher as one goes from fresh water to salty water A A B A B B Figure 13. 7 Freshwater → saltwater, act as removal mechanism for N a+ also 2B-clay + C 2+ ! C-clay + 2B + XC-clay [C 2+ ] = KBC + 2 XB-clay [B ] which shows how the charge balance for divalent ions and concentration affect the equi10 librium: 10× concentration → 11 → 100 As total value of [B + ] increases, it forces more B onto clay and release more C into solution. – Was once thought that clays were heavily altered in interstitial waters/sediments forming source/sinks for ions (reverse weathering). seawater z sink source Figure 14. Does not appear to be a major process for most elements • Mid-ocean ridges – hot 300-350◦ C reactions of rock with seawater - variability from vent to vent; some generalizations – sinks for M gSO4 (though sulfate may have source from low temperature hydrothermal off the ridge axis) – Biogenic Ca2+ , CaCO3 sediments SO42− → H2 S some lost as elemental sulfur/pyrite DMS (?), cont. (?) margin sediments – Complications in closing ocean budgets for major ions · River concentrations reflect natural levels plus pollution and human perturbations. · Long term imbalance because of episodic evaporites. – Ion-ion pairing or chemical speciation CaCO3 (s) ! Ca2+ + CO32− Ksp = [Ca2+ ][CO32− ] · Conditional or apparent solubility product 8 · Ionic strength; activities rather than concentrations · Affected by presence of other ions in seawater mix Similar equations M g 2+ + SO42− ↔ M gSO4 (aq) [M gSO4 ] β= [M g 2+ ][SO42− ] N a+ , K+, Cl− M g 2+ , Ca2+ SO42− CO32− , Ca2+ mostly/all in free ion form 90% free ∼ 50% free ∼ 10% free 9