GEOL 3070 Lecture 21: 3.21.17-Seawater Chemistry

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Geological Sciences
GEOL 3070
Thomas Marchitto

3.21.17: Seawater Chemistry Tuesday, March 21, 2017 10:51 AM I. Seawater as a Solution • Solvent: H 02 • Solutes: dissolved solids, liquids, or gases ○ NaCL (Sodium Chloride) = main A. Dissolution • Solid NaCL crystal + - ○ Attraction between Na and Cl ○ Ionic Bonds • Dissolving NaCL ○ H 02reduces Na-Cl attraction by 80x ○ Na and Cl ions attracted to polar H 0 2 ○ Dissolved: 'free' ions surrounded by H 02 ○ H 02= "universal solvent" (can dissolve many solids B. Some Properties of Solutions • Molecular Diffusion: Dissolved substances move from areas of high concentration to areas of low concentration, until homogenous • Turbulent diffusion: diffusion may be aided by mixing of water (e.g. stirring) • Advection: dissolved substances being carried by circulation • Saturation: when on more of a given solute will dissolve • Supersaturated: evaporation increases the concentration further (saturate a solute, then evaporate that solution so that solids will precipitate) II. Salinity Total dissolved solids ~35 ppt (3.5%) by mass. (gases are treated separately) • Most dissolved solids occur in seawater as ions • Major ions (ppt): 7 ions account for 98% of salinity Cl- 19 Na + 10.6 2- SO 4 2.4 ○ Mg 2+ 1.3 Ca2+ 0.4 (some elements are only + found as covalent K 0.4 molecules) HCO -3 0.1 • Minor Elements (ppm): -, 2+ - ○ Br Sr , B(OH) , Si3OH) , F 4 • Trace Elements (ppb or less): every other non-gaseous elements on periodic table A. Principle of Constant Proportions • As salinity increases or decreases, the concentration of most major and minor elements increase or decrease with constant rations to each other • Such elements are called conservative, and only vary with depth as much as salinity does (i.e. very little) B. Non-conservative elements • Nutrients and most trace elements ma change independently of salinity, usually due to biological uptake (e.g. C, Si, P, N) uptake (e.g. C, Si, P, N) • Such elements are often depleted in surface waters C. Measuring Salinity • Indirect (electrical) method: electrical conductivity increases with salinity ○ Measured with a salinometer ○ Expressed as 'practical salinity units' (equivalent to ppt) D. Sampling Seawater • Niskin Bottle • CTD (
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