5.4 Why does seawater salinity vary?

brackish water

saltish water, with only 1% salinity (1 part per thousand) because physical conditions

• produced in areas where freshwater and seawater mix

hypersaline

extremely saline water

• typical of seas and inland bodies of water that experience high evaporation rates and limited open-ocean circulation

• much easier to float bc they’re super dense

processes affecting seawater salinity

change either the amount of water or the amount of dissolved substances in the water

• adding more water dilutes the dissolved component and lowers the salinity

• removing water increases salinity

  • changing salinity in these ways doesn’t affect the amount or composition of dissolved components, which remain in constant proportions

processes that decrease seawater salinity

precipitation, runoff, melting iecbergs, melting sea ice

• decrease seawater salinity by adding more freshwater to the ocean

proc

esses that increase seawater salinity

formation of sea ice, evaporation, increase seawater salinity by removing water from the ocean. sea ice forms when seawater freezes

• depending on salinity of seawater and rate of ice formation, about 30% dissolved components in seawater are retained in sea ice

hydrologic cycle

continual movement of water on, above, and below the surface of Earth. the movement of water through various components of the hydrologic cycle involves processes that recycle water among the ocean, atmosphere, and continents

• rain cycle, water evporates from ocean, condenses in clouds, falls onto land, runs off into ocean —> keeps going

dissolved components added to and removed from seawater

dissolved substances don’t stay in the ocean forever —> they’re cycled into and out of seawater by processes:

• stream runoff (streams dissolve ions from continental rocks and carry them to the sea) **PRIMARY METHOD

• volcanic eruptions

• atmosphere (contributes gases)

• biological interactions

if stream water is the main source of dissolved substances in seawater, why do the components of stream water and sea water not match each other more closely?

(1) some dissolved substances stay in the ocean and accumulate over time

residence time

average length of time that a substance resides in the ocean

• long residence times = higher concentrations of the dissolved substance

are the oceans becoming saltier through time?

analysis of ancient marine organisms and sea floor sediments suggests that the oceans have not increased in salinity over time, nor have the proportions of dissolved components changed.

• bc rate at which an element is added = rate at which it is removed (steady state condition)

  • average amounts of various elements remain constant

materials added to oceans are affected by several processes that cycle dissolved substances out of seawater

(1) when waves break at sea, sea spray releases tiny salt particles into the atmosphere, where they may be blown over land before being washed back to Earth

(2) infiltration of seawater along MOR near hydrothermal vents, which incorporates magnesium and sulfate ions into sea floor mineral deposits

**the entire volume of ocean water is recylced through this hydrothermal circulation system at the MOR approx. every 3 million years —> chemical exchange between ocean water and basaltic crust has major influence on comp. of water

dissolved substances are removed from water in other ways as well:

(1) Calcium, carbonate, sulfate, sodium, and silicon are deposited in ocean sediments w/in shells of dead micrscopic organisms and animal feces

(2) vast amounts of dissolved substances can be removed when inland arms of seas dry up, leabing salt deposits called evaporites

(3) ions dissolved in ocean water are removed by adsorption (physical attachment) to the surfaces of sinking clay and biological particles

What physical conditions create brackish water in the Baltic Sea and hypersaline water in the Red Sea?

(1) brackish baltic = freshwater from rivers and rainfall mix w/ seawater

(2) hypersaline red = seas and inland bodies of water that experience high evaporation rates and low/limited open ocean circulation

Describe the ways in which dissolved components are added and removed from seawater.

Added

(1) runoff from streams

(2) volcanic activity

Removed

(1) water circulation/infiltration at hydrothermal vents

(2) sea spray

List the components (reservoirs) of the hydrologic cycle that hold water on Earth and the percentage of Earth’s water in each one. Describe the processes by which water moves among these reservoirs.

(1) world ocean 97.2%

(2) ice caps, glaciers, and snow 2.15%

(3) groundwater and soil moisture 0.62%

(4) streams and lakes 0.02%

(5) atmospheric water vapor 0.001%