Chapter : Seawater

Fig. 6-19

Density of seawater

1.022 to 1.030 g/cm3

Ocean layered according to density Density of seawater controlled by

temperature, salinity, and pressure Most important influence is

temperature Density increases with decreasing

temperature

Salinity greatest influence on density in polar oceans

Pycnocline, rapid change of density with depth

Thermocline, rapid change of temperature with depth

Polar ocean is isothermal

Seawater

Salinity=total amount of solid material dissolved in water (g/1000g)

Typical salinity is 35 o/oo or ppt Brackish (hyposaline) < 33 ppt Hypersaline > 38 ppt

Measuring salinity

Evaporation Chemical analysis

Principle of Constant Proportions

Chlorinity (19.2‰) Electrical conductivity

(salinometer)

How salinity changes

Salinity changes by adding or removing water

Salinity decreases by Precipitation (rain/snow) River runoff Melting snow

Salinity increases by Evaporation Formation of sea ice

Hydrologic cycle describes recycling of water

Horizontal variations of salinity

Polar regions: salinity is lower, lots of rain/snow and runoff

Mid-latitudes: salinity is high, high rate of evaporation

Equator: salinity is lower, lots of rain

Thus, salinity at surface varies primarily with latitude

Vertical variations of salinity

Surface ocean salinity is variable

Deeper ocean salinity is nearly the same (polar source regions for deeper ocean water)

Halocline, rapid change of salinity with depth

Fig. 6-20

Dissolved substances Added to oceans

River input, dissolving crustal rock (primarily)

Excess volatiles (not through weathering) Circulation through mid-ocean ridges

Removed from oceans Salt spray Recycling through mid-ocean ridges Biogenic sediments (hard parts and fecal

pellets) Evaporites

Residence time Average length of time a

substance remains dissolved in seawater

Long residence time = unreactive Higher concentration in seawater

Short residence time = reactive Smaller concentration in seawater

Steady state Ocean salinity nearly constant

through time

Mixing Time

Amount of time it takes to mix constituents evenly in seawater

Estimated to be around 1600 years Constituents that have long

residence times are evenly mixed

Dissolved gases

Solubility depends on temperature, pressure, and ability of gas to escape

Gases diffuse from atmosphere to ocean Wave agitation increases amount of

gas Cooler seawater holds more gas Deeper seawater holds more gas

Conservative vs. nonconservative constituents

Conservative constituents change slowly through time Major ions in seawater

Nonconservative constituents change quickly due to biological and chemical processes Gases in seawater

Oxygen and carbon dioxide in seawater

Nonconservative O2 high in surface ocean due to

photosynthesis O2 low below photic zone

because of decomposition O2 high in deep ocean because

source is polar (very cold) ocean

CO2 low in surface ocean due to photosynthesis

CO2 higher below photic zone because of decomposition

Deeper seawater high CO2 due to source region and decomposition

Conservative & Nonconservative ConstituentsConservative Chloride Sodium Magnesium Potassium sulfate

Nonconservative Oxygen Carbon dioxide Nitrates Phosphates

Biologically important

List the following gases in order of abundance (greatest – Least):

Carbon dioxide Oxygen Nitrogen

Dissolved Gases

Nitrogen = 48% Oxygen = 36% Carbon dioxide = 15%

Page 172

Acidity and alkalinity Acid releases H+ when

dissolved in water Alkaline (or base) releases OH- pH scale measures

acidity/alkalinity Low pH value, acid High pH value, alkaline (basic) pH 7 = neutral

Carbonate buffering

Keeps ocean pH about same (8.1) pH too high, carbonic acid releases

H+ pH too low, bicarbonate combines

with H+ Precipitation/dissolution of calcium

carbonate CaCO3 buffers ocean pH Oceans can absorb CO2 from

atmosphere without much change in pH

Fig. 6-17

Hydrologic cycleFig. 6-19