Chapter : Seawater Fig. 6-19

Density of seawater to g/cm 3 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 O 2 high in surface ocean due to photosynthesis O 2 low below photic zone because of decomposition O 2 high in deep ocean because source is polar (very cold) ocean

CO 2 low in surface ocean due to photosynthesis CO 2 higher below photic zone because of decomposition Deeper seawater high CO 2 due to source region and decomposition

Conservative & Nonconservative Constituents Conservative 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 CaCO 3 buffers ocean pH Oceans can absorb CO 2 from atmosphere without much change in pH

Fig. 6-17

Hydrologic cycle Fig. 6-19