Sea Water ChemistrySea Water Chemistry

Chapter 7Chapter 7

Major Ions in Typical SeawaterMajor Ions in Typical Seawater

Ion Parts per thousand by weight ‰ (ppt)(Table 7.1)Ion Parts per thousand by weight ‰ (ppt)(Table 7.1) Cl- Cl- 18.98 18.98 SOSO44-2 -2 2.649 2.649 HCOHCO33- - 0.140 0.140 Na+ Na+ 10.556 10.556 MgMg2+2+ 1.272 1.272 Ca+Ca+ 0.400 0.400 K+K+ 0.380 0.380

On average, concentration of dissolved salts, On average, concentration of dissolved salts, i.e., i.e., the salinity, in seawater is 3.5% or the salinity, in seawater is 3.5% or 3535‰.‰.

The relative abundances of the ions listed The relative abundances of the ions listed above does not change, even though salinity above does not change, even though salinity does; are said to be conservative.does; are said to be conservative.

Relative abundances of minor and trace Relative abundances of minor and trace constituents do varyconstituents do vary

Determining SalinityDetermining Salinity Evaporation of water to weight the salt Evaporation of water to weight the salt

is an imprecise methodis an imprecise method Because of the constancy of composition Because of the constancy of composition

if we measure one component we can if we measure one component we can get a more precise measurementget a more precise measurement

Salinity ppt = 1.80655 x Chlorinity in pptSalinity ppt = 1.80655 x Chlorinity in ppt If chlorinity is 19.2 ppt, what is the If chlorinity is 19.2 ppt, what is the

salinity of sea water?salinity of sea water? 34.7 ppt = 35 ppt34.7 ppt = 35 ppt

Density -Density - mass per unit volume mass per unit volume (grams/cm(grams/cm33))

density of water phases (ice, liquid, density of water phases (ice, liquid, vapor) due to structural changes at vapor) due to structural changes at molecular levelmolecular level

density maximum at density maximum at °C in pure water°C in pure water Major role in deep ocean circulation and Major role in deep ocean circulation and

water column structure and stabilitywater column structure and stability

Water DensityWater Density Influenced by three factorsInfluenced by three factors

– Salinity - proportionalSalinity - proportional– Pressure - proportionalPressure - proportional– Temperature – inversely proportionalTemperature – inversely proportional

Ocean StructureOcean Structure Upper 100-500 m to have uniform Upper 100-500 m to have uniform

temperature and salinity because temperature and salinity because of mixing by waves.(6.13)of mixing by waves.(6.13)

Below this, to a depth of ~1000 Below this, to a depth of ~1000 m, Temp., Salinity, and density m, Temp., Salinity, and density change ( Thermocline, Halocline, change ( Thermocline, Halocline, Pycnocline) (6.12)Pycnocline) (6.12)

In deep water, temperature, salinity In deep water, temperature, salinity and density are relatively uniformand density are relatively uniform

This structure varies latitudinally. At This structure varies latitudinally. At mid-latitudes, it also varies mid-latitudes, it also varies seasonally: upper mixed layer will seasonally: upper mixed layer will deepen in summer; deepen in summer;

thermocline might largely thermocline might largely disappear in winterdisappear in winter