Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean Global Change and Ocean Circulation

The author3 suggests that physical changes in the natural world are responsible for both evolutionary radiation and extinctions; often there is a combination of drivers or cause and effect. He points out that catastrophist theories have never been proposed as a mechanism for kick-starting some of the dramatic evolutionary periods that have occurred on Earth, only for the ending of periods.

When the continents were beginning to approach the groupings of the present, and the size and shape of the oceans were changing. As the Indian subcontinent was getting close to central and eastern Asia the crust of the Tethys Ocean was being subducted along the northern margin of Tethys into a line of deep submarine trenches, narrowing the Tethys to a broad seaway between the continents, some extensions passing around India and into Europe. At its deepest point the Tethys Ocean was still oceanic at this time. Tethys expanded to flood larger areas of the continents when sea levels were high, as occurred in the mid-Palaeogene, but the flooding didn’t last long. As Northwest Africa drifted much closer to Spain only a narrow gap linked the central and western parts of Tethys, and the gateway of Gibraltar came into existence.

According to the author3 most tectonic reconstructions of this period name this western Tethys Ocean the South Atlantic Ocean. He says the South Atlantic had widened into an ocean in its own right by this time, and in the Northern Hemisphere the incipient North Atlantic Ocean had begun to open. The Labrador Sea was formed by an arm of the mid-ocean ridge running between Canada and Greenland that began spreading. Another branch ran in a north-westerly direction between Scotland-Norway and Greenland. The North Atlantic of the present began to open as it was the ridge separating Scotland-Norway and Greenland that predominated. This ridge cut through at its northernmost extremity to link to the Arctic Ocean.

According to the author3 a few million years after the superplume and volcanic activity that formed the Deccan Traps at the KT boundary a major plume and hotspot developed associated with rifting between Greenland and Europe. A very large amount of lava from this hotspot that erupted at this time is present in Greenland and across a submarine ridge through Iceland to the Faeroe Islands and Scotland. This prolonged episode that is now centred on Iceland formed the basalt steps of the Giant’s Causeway in Northern Island and those at Fingal’s Cave on the Isle of Staffa.

The last movement of continents began when Australia finally separated from Antarctica about 36 Ma near the end of the Eocene, and began drifting north leaving Antarctica centred on the South Pole.

It is difficult to be certain that another defining point in Earth history has been reached because the movement of the plates are so slow, an arrangement of plates with their continents and oceans positioned correctly for what was to happen next. At about 36 Ma the global circulation of the oceans changed from equatorial to interpolar. Prior to the change water had flowed unimpeded around the broad equatorial Tethys, that is associated with the ‘Black Death’ of the author and the blooming of the plankton in the chalk seas, no longer being the dominant pattern of circulation. Cold water built up around Antarctica that was now isolated, and ice began forming for the first time both on land and on the sea during the winter months with no sunlight. The Antarctic waters were very cold and as they became denser began falling to the floor of the ocean and slowly spread north through the basins of the other oceans. It is at this point that the temperature of the bottom water of the oceans dropped by almost 5o C, as indicated by oxygen isotope ratios that took place over a period of 100,000 years. According to the author3Now that really is impressively short as a geological timescale and also a very severe drop’. He suggests that a tipping point had been reached in the global ocean instigating a ‘psychrosphere’ of cold bottom-water circulation. The global climate began to destabilise at this time and trend towards the ice age.

In the Northern Hemisphere, though the Arctic Ocean surface water was already connected to the North Atlantic by this time an impediment to the exchange of cold waters remained. The main obstruction was the Greenland-Iceland-Scotland ridge that had been formed by the activity of a hotspot and volcanic activity that produce large quantities of basalt larva. Though it was completely submarine, Iceland not breaking the surface until about 16 Ma, it was high enough to prevent bottom water from escaping. It has been suggested that about 30 Ma this ridge was breached allowing the high latitudes cold waters to add to the psychrosphere. At this point global temperatures dropped further.

These cold waters have been invoked as a contributory factor in the burst of evolutionary radiation of the cetaceans. There are areas of upwelling water, especially along the western coasts of continents that are associated with the flow of deep water at the present. These upwellings bring nutrient elements to the surface where they allow the plankton to bloom. These high levels of productivity among the planktonic organisms attract fish as well as animals high up the food chain. At present this occurs off Peru and Namibia. The author3 suggests that in the latest Eocene it was just such blooms that led to the evolution of new baleen whale species.

 

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 10/04/2012

Oceanography
 

 
 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading