Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean Stirring

According to the author³ the currents of the ocean are more than 2000 times more powerful than any river on land. The currents of the ocean never stop, continuously moving vast volumes of water around the globe. The currents of the world's oceans are powered by the winds, that are in turn powered by the Sun, so the energy supply is effectively limitless, the constant movement of air being transmitted to the ocean over which it blows. The distribution of the landmasses on the surface of the Earth affects the flow of currents around the world, so the currents have changed a number of times as the continents have been shunted around. The Gulf Stream in the western North Atlantic is said transport more than 55 million m³ of water per second, about 1000 times the total discharge from the top 20 rivers combined. The Canary Current is a broader return flow in a southerly direction that travels slower along the eastern margin of the basin. Resulting from the north-south orientation of continents and of the oceans between them, in each ocean, to the north and south of the equator, there are similar gyres.

The Southern Ocean, that encircles Antarctica, is the only ocean in which there currents encounter no impediments to their flow. The flow of the currents in the Southern Ocean very effectively insulates the polar region from the equatorial waters.

Abyssal currents that flow along the floor of the oceans, that are linked to the surface currents, are powerful and slow-moving. These deep currents are powered by density differences caused by variations in water temperature and salinity. As sea water freezes at the poles it excludes 70 % of the salt as it becomes ice, forming salt-enriched water that is denser so sinks to the sea floor to become part of the global thermohaline circulation system as it flows towards the equator. The surface and bottom currents collectively form vast oceanic network of currents that circulates and transfers heat energy, as well as other nutrients, and sediment around the world. Wally Broecker called it the 'global conveyor belt'. The high temperature differential between the poles and the equator is the main driver of this ocean conveyor current, coupled with seasonal sea ice development. It has been estimated that any particular water molecule would take 1000-1500 years to travel 1 complete cycle.

It has been found that the oceans are a crucial factor in the workings of the climate systems of the Earth, storing heat, and obviously moisture, in immense quantities, serving to moderate change and then prolonging the new conditions once changes have occurred. As the oceans hold more the 50 times as much CO₂ as the atmosphere, the cold high-latitude waters hold more dissolved oxygen than the warmer waters of the lower latitudes. The ocean surface acts like a 2-way control valve for gas exchange between the oceans and the atmosphere that opens in response to the 2 main factors of gas concentration and the stirring of the oceans.

Given the ocean circulation resulting from the present distribution of continents, the author³ suggests the present is a temporary warm spell in an icehouse world, in which the average surface temperature of the sea surface ranges from 27^o C at the equator to -1.5^o C near Antarctica. The water deeper than 2000 m are uniformly cold at 1-4^o C. Landmasses in the polar region are covered by ice that is thick and permanent, and the extensive sea-ice forms annually. Trends towards global warming are beginning to temper these extremes, though the global temperature are not yet near the greenhouse world of the Cretaceous.

Another important aspect of ocean circulation is the upwelling that occurs in the oceans and the stirring of nutrients essential for primary production. Carbon, nitrogen, phosphorus and silicon, as well as trace elements such as iron that is used for chlorophyll and other pigments, that are brought abundantly to the oceans from rivers that carry billions of tons of chemical and organic detritus to the oceans. The same elements are present in the cells of animals, being released back to the oceans by the decaying of dead animals as bacteria recycle dead and decaying organisms, as well as their faeces. There is a tendency for these organic materials to rapidly fall to the seafloor taking the vital nutrients with them. In upwelling regions they are brought to the surface most effectively, where cooler waters from a depth of several hundred metres are brought to the surface, as well as by thorough mixing of the upper ocean layer during major storms.

Where different masses of water meet in the oceans and deeper water is allowed to reach the surface by circulation upwelling occurs. This occurs in 1 prominent belt in the equatorial region and 2 high latitude belts. In some coastal areas where the prevailing surface currents flow away from the land dragging the deeper  waters to the surface it is even more pronounced. This type of upwelling occurs off the west coast of West Africa, Chile-Peru and California.

Sources & Further reading

Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.