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
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