Physical and Chemical Oceanography

SECTION 7 PART I: CHEMICAL COMPOSITION

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Salinity• salt concentration in parts per thousand (ppt)•ocean average: 35 ppt (35%)• Variance:•Sea diluted with freshwater by:•River•Melting glaciers

Chemical Composition• fairly constant for millions of years• Salinity•Mainly: sodium, chloride ions•Smaller: sulphate, magnesium, hydrogencarbonate, potassium

From Section 4 Nutrient Cycles

Chemical Composition• local changes can occur

1. Volcanic activity2. Runoff3. Atmospheric dissolution

1. Volcanic Activity•Gases: CO2, Sulphur dioxide, hydrogen sulphide, hydrogen chloride•dissolve in atmospheric H2O• enter sea by precipitation

•Submerged volcanoes at plate boundaries emit gases (chlorine)•Major source of chloride ions in sea

2. Runoff•Flow of water from land•Rain, melted snow & ice

•Drains to oceans directly or from rivers•Passes through soil•Urban runoff into drains

2. Runoff•Picks up pollutants – pesticides, fertilizers and oil-derived substances• food chains and webs increasing concentration at each trophic level

•Example: industrial wastewater with mercury•Minamata Bay 1932-1968•Shellfish & organisms•Human consumption – neurological disorders, paralysis and death

3. Atmospheric Dissolution•Gases dissolved are at equilibrium with atmosphere•Concentration of it depends on relative solubility, temperature and salinity of sea

•Dissolved gases:• Nitrogen (N)• nitrogen-fixing microorganisms make products for other organisms

• Carbon Dioxide (CO2)• Photosynthesis• Oxygen (O)• Respiration

Salinity• Evaporation – salt stays behind•Hypersaline (increased amount of salt)• Ex: lagoon – high temp. Evaporation• Ex: Dead Sea – extreme• Accumulation of solutes• 10x saltier than ocean

hydrometer

Precipitation• rain, snow•Dilutes sea water, decreases salinity• Estuaries•Melting glaciers

Density• temperature• salinity

•Warm on top of cold, dense water•Temperature gradient

temperature

density

Density• If temp. abruptly as depth = thermocline•Shallow layer of warm on deep layer of cold

•Ocean surface 25 ⁰ C

•2000 m deep 1⁰C

depth

temperature

Density• halocline – abrupt change in salinity as depth

•Lower salinity (lower density) on top of higher salinity (higher density)

•Mixing occurs by wind blowing at the surface down to ~200 m•Turbulence and currents•Temperature changes

salinitydensity

Dissolved Oxygen (DO)• General rule: temperature

Oxygen solubility

Dissolved Oxygen• O2 slightly less soluble in salt than fresh

Temp of H2O Dissolved Oxygen Concentration Freshwater

0⁰C 14.6 mg/dm3

5⁰C 12.8 mg/dm3

10⁰C 11.3 mg/dm3

15⁰C 10.2 mg/dm3

20⁰C 9.2 mg/dm3

25⁰C 8.4 mg/dm3

What is the pattern?

D.O.• surface layer = high d.o.•Can be supersaturated by 2 processes:• Turbulence & mixing by waves causing atmospheric O2 to dissolve• Photosynthesis by algae•O2 as byproduct

•D.O. removed by respiration of marine organisms

D.O.• decreases to minimum as depth increases• Increases again as depth increases•Oxygen minimum layer – depth that concentration of DO is lowest•Between 100m and 1000m