BIOL 331

Lecture 3

Seawater Chemistry

Residence Time of Seawater Constituents

Residence Time (in years) = Amount present/Removal Rate

RT for water = 4000 m/10 cm = 40,000 years

Factors Affecting Gas Concentrations

• Increasing temperature and salinity decrease solubility.
• Most gases are at saturation point.
• Oxygen and bicarbonates often not at saturation because of life processes.

pH & Seawater

• pH of seawater around 8.1
• Dissolved CO2 buffers the pH
• Precipitation of CO2 as CaCO3 results in a net loss of CO2

Variation of Factors at the Ocean Surface

Temperature

• Varies seasonally due to changes in solar radiation.
• Equatorial and polar waters vary little.
• Greatest changes are mid-latitude.
• Note: greatest changes in land temperature are polar due to year-round ice on water.

Salinity

• Major effectors: Evaporation, precipitation, runoff, ice melt.
• Highest salinities either midocean (subtropics) or near land with dry air.
• Tropics receive high rainfall, reduces salinity.
• Monsoons in SE Asia decrease salinity greatly there.
• Highest salinities in Red Sea and Arabian Gulf.

Variation of Factors with Depth

Surface Zone

• Changes seasonally
• Contains warmest and least dense waters
• 100-500 m thick, 2% of ocean volume
• Well mixed by winds, also called mixed layer

Pycnocline Zone

• Water density changes markedly
• Top of pycnocline ~10^oC contour, bottom at 4^oC
• Usually ~1 km thick
• Stable with little exchange from above

Thermocline and Halocline

• Both salinity and temperature affect density
• Changes in temperature = thermocline (open ocean)
• Changes in salinity = halocline (coastal waters)

Deep Zone

• Below the pycnocline
• No exchange with atmosphere, very stable, ~3.5^oC
• Contains 80% of water in ocean

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