IV. Circulation of the Liquid Earth: The Oceans and the Hydrologic Cycle. A. Origin of the Oceans:...

IV. Circulation of the Liquid Earth: The Oceans and the Hydrologic Cycle.

A. Origin of the Oceans: 1. Where did the water come from?

Outgassing from the Earth’s interior (volcanos), the same as the atmosphere.

There have been oceans as far back as we can find rocks, almost 4 billion years. And there is still another ocean inside the Earth.

V. Circulation of the Liquid EarthA. Origin of the Oceans

1. Where did the water come from?2. Where did all the salt come from?

Early geologists thought they could calculate the age of the Earth from the salt in the ocean.

Ocean holds 5 x 1019 kg of salt.Rivers deliver 4 x 1012 kg of salt/year.Age of Earth is 13 x 106 years old.But, Earth is 4.6 x 109 years old…… what went wrong?

?

V. Circulation of the Liquid EarthA. Origin of the Oceans

1. Where did the water come from?2. Where did all the salt come from?

Two key assumptions:• Rivers are the only source of salt.• All salt that enters the ocean stays there.

Both are violated:• Salt is removed from the ocean.• New salt is also added to the ocean from mid-ocean spreading centers.

V. Circulation of the Liquid EarthA. Origin of the Oceans

Our early geologists did not yet know about• Plate tectonics• Mid-ocean spreading centers• Evaporite basins• Sea critters that precipitate sea salts in their shells.• Sea-spray to the land.

V. Circulation of the Liquid EarthA. Origin of the Oceans

1. Where did the water come from?2. Where did all the salt come from?

Calculating the age of the Earth this way adds three terms to our view of the Earth as a system:

Reservoir, Flux and Residence Time

V. Circulation of the Liquid EarthA. Origin of the Oceans

Reservoir: Reservoir: (a noun; it’s a “thing”) A volume or mass of something.

Ocean (water, salt, etc.)Atmosphere (Oxygen, water vapor, etc.)

Flux: (a verb; it’s the “action”) The rate at which energy or matter is transferred between reservoirs. The rate at which energy or matter is transferred between reservoirs. Expressed in amount per unit time.

V. Circulation of the Liquid EarthA. Origin of the Oceans

Reservoir: Flux: Residence time: The average time a substance stays in a reservoir. (Reservoir/Flux).In our salty ocean example, the calculated “age of the Earth”, 13 million years, is the residence time of salt in the ocean.

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthReservoirs

Reservoir Mass (1015 kg) Residence Time

Ocean 1,400,000

Snow and Ice 43,400

Groundwater 15,300

Freshwater 360

Atmosphere 16

Biosphere 2

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthReservoirs Fluxes (1015 kg/year)

Evaporation from the ocean: 434Evapo-transpiration: 71(biota and lakes)Precipitation: 505

Reservoir Mass (1015 kg) Residence Time

Ocean 1,400,000

Snow and Ice 43,400

Groundwater 15,300

Freshwater 360

Atmosphere 16

Biosphere 2

MRTocean = Mass of the ocean

flux of water from atm to ocean

Reservoir Mass (1015 kg) Residence Time

Ocean 1,400,000 3000 years

Snow and Ice 43,400

Groundwater 15,300

Freshwater 360

Atmosphere 16

Biosphere 2

MRTocean = 1,400,000 = 3000 years

505

Reservoir Mass (1015 kg) Residence Time

Ocean 1,400,000 3000 years

Snow and Ice 43,400

Groundwater 15,300

Freshwater 360

Atmosphere 16 12 days

Biosphere 2

MRTatmosphere = 16 = 12 days

505

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthC. Surface currents

Troposphere unstable, ocean stable

Unlike the troposphere, which is heated from the bottom, hence is fundamentally unstable and circulates in response to this instability, the oceans are heated from the top, hence the warmest (least dense) water is mostly at the surface. Stable.

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthC. Surface currents

1. Surface currents are driven by winds through the frictional coupling between atmosphere and sea surface.

Surface currents generally restricted to uppermost 100 m.

Average depth of the deep ocean = 4000 m

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthC. Surface currents

1. Surface currents - winds2. Coriolis: surface currents

deflected to right (NH) or left (SH) of the prevailing winds.

Simplified Surface Currents

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthC. Surface currents

1. Surface currents - winds2. Coriolis: surface currents

deflected to right (NH) or left (SH) of the prevailing winds.

We expect simple gyres. But simple gyres are complicated by continents.

Generalized ocean surface currents

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthC. Surface currents

1. Surface currents - winds2. Coriolis -- Actual net motion of

water is further complicated by Ekman Spiral

V. Circulation of the Liquid EarthA. Origin of the Oceans

B. Distribution of water on EarthC. Surface currents

1. Surface currents - winds2. Coriolis -- Ekman Spiral:

transfer of Coriolis Effect down through the water column. Net effect is that surface water moves at right angles to the wind. Ekman Transport

V. Circulation of the Liquid EarthC. Surface currents

1. Surface currents - winds2. Coriolis -- Ekman Spiral3. Convergence: surface water

tends to pile up at the center of gyres, where downwelling occurs.

4. Divergence: Winds on both sides of the equator are easterly, so net motion of water is N in NH and S in SH, creating divergence, and upwelling.

V. Circulation of the Liquid EarthC. Surface currents

1. Surface currents - winds2. Coriolis -- Ekman Spiral3. Convergence: downwelling4. Divergence: upwelling5. Coastal Upwelling: Winds

moving parallel to the continental coast can result in strong upwelling.

Southern Hemisphere

Strong NW windsEkman Transport to the SW.Upwelling

V. Circulation of the Liquid EarthC. Surface currents

1. Surface currents - winds2. Coriolis -- Ekman Spiral3. Convergence: downwelling4. Divergence: upwelling5. Coastal Upwelling: Why might

we care about areas of upwelling? Upwelling brings nutrients to the surface…50% of all fisheries occur in upwelling areas, even though they represent only 2% of the ocean surface.

NorthernHemisphere

NorthernHemisphere

V. Circulation of the Liquid EarthC. Surface currents D. Deep ocean currents

1. Vertical circulation is controlled by

differences in density.Density is controlled by temperature, salinity

Measured as the proportion of dissolved salt to pure water. Measured in parts per thousand (‰).

V. Circulation of the Liquid EarthC. Surface currents D. Deep ocean currents

1. Vertical circulation is controlled by

differences in density.Density is controlled by temperature, salinity

Measured as the proportion of dissolved salt to pure water. Typical ocean salinity is 35‰, which means dissolved salts make up 3.5% of the mass of a volume of water.

V. Circulation of the Liquid EarthD. Deep ocean currents

1. Vertical circulation is controlled by

differences in density.Density is controlled by temperature, salinity

What makes up salinity?

Mostly Sodium (Na) and Chloride (Cl)55% 31%

With smaller amounts of sulfate, magnesium, calcium, potassium, bicarbonate

How is density controlled by temperature and salinity?

More dense Less dense

Colder WarmerSaltier Less salty(higher salinity) (lower salinity)

V. Circulation of the Liquid EarthD. Deep ocean currents

1. Vertical circulation is controlled by

differences in density.2. Thermohaline circulation: Density driven vertical circulation3. Vertical structure of the ocean

V. Circulation of the Liquid EarthD. Deep ocean currents

1. Vertical circulation is controlled by

differences in density.2. Thermohaline circulation: Density driven vertical circulation3. Vertical structure of the ocean

Example: Mediterranean Sea outflow

Salinity of the Atlantic Ocean at 1100 meters depth.

What does the salinity tell us about the temperature?

V. Circulation of the Liquid EarthD. Deep ocean currents

4. Formation of Bottom Water and deep ocean circulation

For surface water to sink it needs to be denser than the water underneath it. This requires:• Cold• SaltyWhere might these conditions be met?

V. Circulation of the Liquid EarthD. Deep ocean currents

4. Formation of Bottom Water and deep ocean circulation

Occurs in the northern North Atlantic (very salty and pretty cold) andAround Antarctica (not quite so salty, but very cold)

We can use tracers to actually follow deep ocean circulation.

In the movie that follows, we take advantage of 30 years of sampling vertical transects through the Atlantic, and analyzing that water. The movie shows the concentration of CFCs, man-made chemicals that were not present before about 1950 to trace how the deep ocean moves.

V. Circulation of the Liquid EarthD. Deep ocean currents

4. Formation of Bottom Water and deep ocean circulation.5. The thermohaline conveyor

belt

The large-scale motion of the deep ocean based on density differences.

V. Circulation of the Liquid EarthD. Deep ocean currents

6. Life in the ocean: how do we measure?

Primary productivity: The amount of organic matter produced by photosynthesis. Measured in amount per unit are per unit time (for example: kg/m2/year).

What is going to control primary production?• Sunlight: we know what controls sunlight• Nutrients: what controls nutrients?

V. Circulation of the Liquid EarthD. Deep ocean currents

6. Life in the ocean: how do we measure?

Primary productivity: The amount of organic matter produced by photosynthesis. Primary production occurs in the photic (sunlight) zone, where primary producers are eaten by larger things, that sink when they die, removing nutrients from the surface water.

V. Circulation of the Liquid EarthD. Deep ocean currents

6. Life in the ocean: how do we measure?

Primary productivity: The amount of organic matter produced by photosynthesis. Sinking dead things decompose in the deep ocean and most of their nutrients are returned to the water column. But below the photic zone where they cannot be easily used.

Typical “open ocean” distribution of nutrients vertically in the water column.

Much of the surface ocean is nutrient limited.

V. Circulation of the Liquid EarthD. Deep ocean currents

6. Life in the ocean: how do we measure?

Primary productivity: The amount of organic matter produced by photosynthesis.

We expect most primary productivity where deep water returns to the surface = upwelling areas. Fig. 5-13 in the color section of your text shows the global pattern of primary productivity.

V. Circulation of the Liquid EarthD. Deep ocean currents

6. Life in the ocean: how do we measure?

Primary productivity: The amount of organic matter produced by photosynthesis.

Why is primary productivity important?• Where we catch all those tasty fish we like to eat….• Exerts strong control on the global carbon cycle

V. Circulation of the Liquid EarthD. Deep ocean currentsE. Climate impacts of changing ocean

circulation…. What might happen if we shut down Thermohaline circulation in the northern North Atlantic? Why might that happen?