EASC 1000 - Topic 10

4 large interconnected ocean basins + 5th major ocean

1. Atlantic

2. pacific

3. Indian

4. Southern

+ 5. Arctic which is connected to the North Atlantic by the Bering Strait.

Major seas connected to the oceans

• Atlantic - Mediterranean Sea, Black Sea, Baltic Sea, Baltic Sea, Caribbean Sea, Gulf of Mexico

• Indian - Persian Gulf, Red Sea, Arabian Sea

• Pacific - many “marginal” seas (Bering sea, sea of okhotsk, Sea of Japan, east and south china seas, coral sea, Tasman sea…)

Origin of the oceans

Late bombardment of the earth by comets along with the degassing from volcanic eruptions likely contributed to most of the water on Earth.

• In the earliest stages, water would largely be in the atmosphere as a vapour

• Eventually, a stable cooler surface would exist, and condensation would start the process of forming the earliest oceans

Composition of seawater

Characterized by salinity — the total quantity of dissolved salts that it contains. In this context, salt means ionic forms of those elements dissolved in seawater. Ocean water contains an average of 3.5% dissolved salts (salinity).

• Most of the major cations → Na, K, Ca, Mg

• Most of the major anions → Cl, SO4

• These come from the chemical weathering of rocks or from volcanic gasses/hydrothermal fluids entering the oceans.

• Dissolved ions are carried in from “freshwater” in the form of groundwater and rivers, which have a much lower salinity.

Removal mechanisms of salts in the oceans

• CaCO3 precipitating into the shells of marine organisms

• Precipitate minerals such as gypsum, halite, and sylvite remove dissolved ions from seawater

Salinity variations across the globe

Although 3.5% salinity is the average of Earth’s oceans, salinity varies with location and can range from 1.0 - 4.1%.

What does sea-surface salinity reflect?

It reflects balances between the addition of freshwater by rivers or rain and removal of freshwater by evaporation. Salinity is also dependent on water temperature, which at sea-level means there’s a broad relationship between salinity and atmospheric circulation around the equator.

• High precipitation in latitudes near low pressure regions of the atmospheric cells adds freshwater to the surface of the ocean = reducing salinity to <3.5%

• Low precipitation near high pressure regions of the atmospheric cells evaporates seawater = increasing salinity to >3.5%

Most salinity variations in the oceans occur where?

Within the upper 1 km where most addition and removal of freshwater takes place and temperature is more variable.

Halocline

Salinity is more homogenous at depth; the boundary between shallow-water salinities and deep-water salinities.

Salinity influence on water density

Seawater has a higher density than freshwater because of the higher dissolved ion content without changing the volume of water. This is why objects float higher and more easily in denser liquid.

The relationship between temperature and salinity and its effects on density

Warmer water = less dense, colder water = more dense.

Therefore, the colder more saline water is the most dense.

The global average sea-surface temperature is what?

Around 17 degrees Celsius but it can range from -2 degrees Celsius near the poles to 36 degrees Celsius near the equator.

Latitudinal sea-surface temperature variations are driven by?

Intensity of solar radiation and can change slightly with the season. However, the variation is more minimal than land with seasonal changes due to the larger heat capacity of water relative to land.

• There is also variability at ant given latitude controlled largely by ocean currents.

Temperature structure of seawater

There is significant variation in water temperature with depth at mid-to-low latitudes. This structure is maintained because solar radiation only penetrates a few hundred metres and water density decreases with increasing temperature.

Thermocline

The measure of temperature of seawater and the point where the temperature changes— warmer water at the surface is separated from significantly colder water at depth by this. Below this point, temperature decreases rapidly with depth.

Thermocline circulation

Temperature and salinity both exert a control on water density, which help drive deeper water circulation.

• Density goes down as temperature goes up → warm water floats on top of col if the salinity is the same.

• Density goes up as salinity goes up → freshwater floats on top of saltwater if temperature is the same.

Although downwelling and upwelling of ocean water can be initiated by surface currents, the global “conveyor belt” of rising and sinking seawater throughout the entire ocean system is linked to density contracts and called thermocline circulation.

• Full ocean mixing takes around 1500 years.

Why is thermocline circulation important?

For distributing heat from the equator, transporting nutrients, and plays an important role in maintaining our current climate system.

Surface Currents

Affect the circulation of seawater within the upper hundredth metres of water. The study of surface currents was originally motivated by sailing expeditions.

• Some of these currents are at the oceanic-scale, travelling for 1000s of km and take their names based on their flow patterns

• Smaller scale (10s-100km) currents are called eddies and are found along coasts, at the argon’s of gyres and along equatorial currents.

Surface current flow patterns (large-scale)

• Gyres - currents that define loops around the margins of ocean basins

• Equatorial currents or equatorial counter currents (depending on the flow direction) - follow the equator

• In the Southern ocean (around Antarctica), there’s the clockwise-flowing Antarctic Circumpolar Current.

What drives surface currents and why do they flow in the directions they do?

The process starts with wind as a driver and thus the ocean currents are coupled to the atmosphere. The prevailing winds (e.g. trade winds, westerlies) generated by atmospheric cells are thus important to ocean currents. After starting the movement of water, currents are modified by the Coriolis effect, pressure gradients, and the presence of physical boundaries to water flow.

The specific paths followed by oceanic currents reflect:

1. Shear of wind against the water surface

2. Deflection of water once it starts moving due to the Coriolis effect (to the right in the Northern Hemisphere; to the left in the southern hemisphere).

3. Pressure gradients in water where water will flow from areas where the surface elevation is higher to areas where the surface elevation is lower.

4. Physical barriers to water movement (continental landmasses).

Average flow of surface currents

Although it seems counter-intuitive, the average flow direction of surface currents is nearly perpendicular to the flow of wind.

Ekman transport

→ Water initially starts flowing in the directions of prevailing winds. As soon as water starts to move, the Coriolis effect causes it to deflect to the right (in the Northern Hemisphere).

→The movement of the uppermost layer of water then shears the layer below, which is deflected even farther to the right. Each successive layer deflects father to the right than the layer above. The water surface also exerts a frictional force on the layer below.

→Both friction and the Coriolis effect produce gradual deflection and a reduction in velocity with depth, creating a pattern of flow called Ekman spiral. The net flow direction of the surface current from this spiral perpendicular to the surface wind is called Ekman transport.

Ekman transport and the formation of gyres

The result of Ekman transport alongside pressure gradient forces and physical barriers produces the circular flow of gyres. Gyres are found in all the major ocean basins, clockwise ones in the Northern hemisphere and counterclockwise one in the Southern Hemisphere.

The Great Pacific Garbage Patch

A gyre of marine pollution (plastic, chemical sludge, and other materials) located in the North Pacific gyre. A similar garbage patch is found in the North Atlantic gyre.

Downwelling

Caused by winds that result in water moving towards the coast, an oversupply of water develops along shore that must sink.

Upwelling

Caused by winds that result in water moving away from the coast; a deficit of water develops along shore that is filled by deeper water.

• Important for bringing up nutrients from deep waters that promote planktonic growth that feeds other aquatic organisms; changes in upwelling and downwelling are important to fishing industries.

El Niño-La Niña and southern oscillation

An atmosphere-ocean phenomena that occurs in the equatorial Pacific Ocean between South America and Papa New Guinea. An oscillating change in the trade winds results in warm surface waters shifting to different areas across the Pacific and changing upwelling patterns on the west coast of South America (Peru and Ecuador).

• The oscillation occurs every 4-7 years bracketed by the most extreme conditions called El Niño and La Niña

• El Niño

Name given to the poorer fishing conditions off the coast of Peru and Ecuador occurring in late December some years. Fish feed of plankton that are sustained by nutrients brought from upwelling deep waters. During this time, warm-water currents flow eastward from the central pacific, preventing the upwelling conditions needed to supply nutrients, causing fish to migrate elsewhere.

• La Niña

Associated with a return to good fishing due to upwelling being supported off the coast of South America. Wetter conditions are found over Indonesia/Papa New Guinea/Australia.

Ocean Basins: Continental Margins

Continental margins, interface of continental-oceanic lithosphere, are divided into active (plate boundary at interface) and passive (no plate boundary at interface) margins.

Passive Margin Basin

develop after rifting and separation of continental lithosphere and have a transition from shallower to deeper through several features with variable slopes:

• Continental shelf → flat platform from the shoreline up to 100s of km offshore, usually less than 200 m deep, underlain by continental crust.

• Continental slope → A steeper, typically mud-draped slope marking of the edge of the continental shelf. It descends to depths of 4 km, underlain by continental crust.

• Continental rise → a gently sloping apron of sediment formed by the deposition of sands at the base of the continental slope. May include large submarine fans underlain by serval km offshore sediment.

• Abyssal plain → Beyond the continental rise is a vast, nearly horizontal plain sitting at 4.5-6 km depth. The flattest surface on earth, usually underlain by oceanic crust. Sediments consist mainly of very fine clay, wind blown dust, and shells of microscopic organisms.

Submarine canyons

Relatively narrow and deep valleys that downcut into continental shelves and slopes. Start forming offshore from major rivers (from times of lower sea-level), but are further cut by erosion from turbidity currents (avalanches of sediment + water). When these currents reach the base of the continental slope they deposit graded beds in submarine fans.

Passive Margin basic sediment

Largely derives from sand and mud than wash off the continent, transported by rivers, along with the shells of marine creatures living on and above the seafloor.

Ocean Basins: Active continental margins

Coincide with a plate boundary and host many earthquakes and different architecture.

• Subduction leads to a trench— a deep, elongate trough at the boundary of the subducting plate and accretionary prism (contorted and faulted sediment and basalt that lies along the edge of the coast). The continental slope corresponds to the face of the accretionary prism after the shelf and submarine canyons form and transport sediment into the trench.

Mid-ocean ridges

Sea-level is shallower a spreading centres where a submarine belt forms and is highly fractured due to spreading and generation of transform faults. Sea-level increases in depth away from the spreading centre and transitions to the abyssal plain as the oceanic cutest ages and thickens, the surface sinks, and sediment accumulates.

Ocean sediments: terrigenous

Huge fans of sediment are formed offshore from the major rivers that drain the Himalayan mountains. Patterns like this are seen wherever major rivers discharge their load into the ocean.

Oceanic sediments

Further from land, sedimentation rates decrease and the sediments derive from:

• Minerals precipitated from hydrothermal fluids (e.g. black smokers)

• Local (volcanic) detritus

• Long-transported very fine aeolian dust

• Microscopic organisms with calcareous or siliceous parts (e.g. foraminifera and radiolarians)