Australia: The Land Where Time Began

A biography of the Australian continent 

As on land the topography of the seafloor is varied and just as dramatic as that on land. The large oceans are divided into basins, their floors often being at quite different depths, and the basins are separated by submarine mountain ranges. At high latitudes cold-kitchen area bottom waters pile up behind barriers such as the mountain ranges until they reach the spill point. This usually occurs at narrow passes that cut across the barrier as occurs on land where mountain passes cut through ranges. The huge mass of dense water being funneled through a narrow gateway has its width greatly restricted causing it to accelerate. As the bottom currents pass through such deep ocean passageways they can be very erosive scouring loose sediment and cutting into bare rock.

The dense water of a narrow bottom current flowing at high velocity cascades down slopes and spreads out below when it enters an adjoining basin, referred to as submarine waterfalls as they fall from immense heights with a tremendous amount of power, though as the slopes are gentle Stow¹ compares them to cataracts or rapids in rivers flowing across land. He describes their scale as awesome, whatever the slope. Beneath the Denmark Strait is the most impressive submarine waterfall known, falling over the Greenland-Iceland Ridge, the water flowing down-slope into the North Atlantic Basin at the rate of 5,000,000 m³/second and dropping 3.5 km, forming giant eddies and turbulent whirlpools in the deep ocean, being neither heard nor seen from the surface. It dwarfs the Angel Falls, Venezuela, the tallest waterfall in the world on land that drops 1 km. The Cuaira Falls on the border of Paraguay and Brazil at13,000 m³/second has the largest average flow rate on land is more than that of Niagara Falls that is 400 times less than its submarine counterpart. It is only in recent years that the tremendous power of these bottom currents has been discovered.   The flows of these currents are constrained against the western coasts of the continents by the rotation of the Earth and the Coriolis force, where they become narrow bottom currents with high energy after they enter the ocean basins, then being known as western boundary undercurrents, currents that comprise most of the deep water transport of water, chemicals, heat and nutrients through the global conveyor belt. Huge amounts of dissolved chemicals as well as the finest of the sediments are carried for thousands of miles in these strong currents along the beds of the oceans. When this fine sediment finally settles out of the currents it forms contourite drifts, giant mounds that have an elongate shape, sometimes with symmetrical waves of sediment formed on their surface. It has been found that these contourites sometimes grow in the same place for as much as 20 My, reaching hundreds of metres thick and covering an area of seafloor the size of Cuba.

It has been found that complex mechanisms cause the velocity of these currents to respond to small climatic changes as when under intermediate climatic conditions as at the present when slightly more cold water is generated at the high northern latitudes. As so much water was locked up in glaciers during the last glacial maximum when the climate was extremely cold, less cold water was produced at high latitudes. Stow¹ suggests there is likely to be a progressive shut-down of deep water circulation at the much warmer temperatures that are approaching due to climate change (global warming). The sediments being deposited are influenced by these changes, with slightly less of the coarser size at high velocities and slightly finer at the slower velocities being deposited. The author¹ reports a large research program that involved drilling through the Eirik Drift off the southern tip of Greenland, the preliminary results indicate that the velocity of the bottom current tracked the major climatic changes over the last 20,000 years, tough on a shorter timescale there was a considerable amount of variation, but these had still to be decoded at the time the book was written.

When considering the Tethys Ocean, there was an equatorial circulation pattern, in contrast to the interpolar deep-water circulation pattern of the present that is dominated by cold, dense water from high latitudes. The equatorial circulation pattern of the Tethys Ocean contributed to a global climate that was more equable, with warm water surrounding the poles and palm trees growing along the northern shores of Siberia. At that time it was warm-water currents that formed along the ocean margins that dominated the deep water circulation. Dense waters along barren shorelines of the continents were formed by the intense evaporation that made the water more saline, therefore denser, as only pure water is removed by evaporation. As this dense, saline water sank it formed the warm deep-water currents of the basins. These warm waters spread across the global ocean floor. The author¹ suggests there are 2 regions at the present where this phenomenon is occurring, the Mediterranean Sea and the Red Sea, both semi-enclosed basins.

It has been suggested by ocean modelers that the bottom currents would have tracked the surface currents flowing to the west. The Tethys Ocean became increasingly compartmentalised during the Cainozoic into at least 3 sectors, the western central and the eastern sectors as the ocean narrowed. Between these separate basins there were topographic barriers deep beneath the surface over which the powerful bottom currents found the lowest path to flow over waterfalls to the seafloor. The Stow¹ suggests, though he admits it is as yet unproven, that a similar gateway-waterfall couplet lies beneath the seafloor of the present near the Straits of Gibraltar separating the central and western Tethys Basin through a large part of the Cainozoic. In this region there is sediment that forms a broad cone-shaped swathe of sediment below the present seafloor of this region that slopes upwards towards Gibraltar … but it requires further research. The ancient equivalents of the marine contourites that were drilled and cored through at sea have been difficult to find in the rock record on land. Part of the problem is the controversial state of the appearance of contourites, and also because they are piles of mud that are rather indistinctive. The author¹ claims that as he has observed many examples of contourites on the floor of the deep sea he is confident he can recognise their subtle, distinctive features in rocks on southern Cyprus that were once part of the floor of the Tethys Ocean that have been dated to 30-40 Ma. He suggests they are the first well-documented examples of contourites that have been described. They were discovered by the Stow¹ and Dr Gisela Kahler, and Dr. Costas Xenophonotos of the Cyprus Geological Survey. They indicate a period of intensified bottom currents that was possibly the last gasp of the equatorial circulation prior to its replacement by the interpolar circulation that has dominated ever since the Tethys Closed.

According to the authors² deep ocean currents and bottom water can flow from one ocean basin to another by flowing through the narrow gaps of fracture zones, lateral jogs in the spreading centre. Of the many fracture zones on the mid-ocean ridges the Romanche Fracture Zone is one of the most important, being a pathway through the MAR near the equator for the Atlantic abyssal circulation. The Eltanin Fracture Zone and the Udintsev Fracture Zone are examples in the Pacific Ocean that steer the Antarctic Circumpolar Current (ACC)².

Sills, straits and passages

The separate ocean regions are connected by features such as sills, straits and passages.. A ridge above the average level of the floor of an ocean region separating 2 ocean basins, or a landward basin from an ocean basin, as in the case of a fjord, is called a sill. The depth of a ridge is defined as the depth from the surface to the deepest part of the ridge - the maximum depth at which water can flow directly across the ridge. The authors² compare an oceanic sill to a topographic saddle, the sill depth being equivalent to the saddlepoint. Sills in the deep ocean connect deep basins, the density of the water flowing over the ridge.

All 3 constrictions, sills, straits and passages are horizontal structures. Strait is usually used in connection with landforms such as the Strait of Gibraltar connecting the Mediterranean Sea to the Atlantic Ocean, and there is also a sill at the connection at the outlet of the Mediterranean. Surface structures as well as submarine topography can be referred to as passages and channels, as when it refers to fracture zones connecting 2 basins. The constricted flow through a strait or over a sill can be hydraulically controlled by the minimum witth of a strait and the maximum depth of a sill.

Oceanography