Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean

At this time the Tethys Ocean straddled the equator, and according to the author³ there were surface gyres that spun clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. This pattern was the result of the Earth's rotation about its axis, the Coriolis force. At the equator the northern and southern arms of the supergyres combined at the equator to flow as a powerful current to the west, pulling the water at the eastern archipelago end of the Tethys Ocean which would have created a broad region of upwelling. The upwelling water would have been cooler and rich in nutrients derived from the sea floor, as well as recycling those present in the water column above the seafloor, as occurs off the coast of Namibia and Peru. The model of ocean circulation was produced by physical oceanographers who believe the equatorial upwelling in Tethys would have extended about 2/3 of the way across Tethys Ocean. There would also have been 2 other regions of upwelling in the extreme north and the extreme south of the ocean.

Life proliferated especially in the upwelling water, and as there would have been plenty of sunlight for the photosynthesising organisms, leading to a rich plankton bloom that forms the basis of the food webs in any ocean, allowing life in the ocean to proliferate greatly in the Tethys of the Late Permian. Among the tiny consumers that fed on the even tinier phytoplankton, or scavenging the constant rain of organic detritus that reached the ocean floor, were seed-shrimp ostracods. There would also have been trilobites, a subphylum of arthropods, on the sea floor, though they were approaching the end of their presence in the life of the oceans, that lasted for about 750 My. There were also conodonts, minnow-sized eel-like fish, that were very common, Most of the species present were not like any that are known at present, all being from the Palaeozoic Era, and the unique reefs that looked different from those of the present, that were common along the margins of volcanic islands, that harboured a rich variety of life that included such animals as giant lamp shells and crinoids, and there would have been many schools of fish, though the author³  describes them as 'strangely different from those of the present'. The top marine predators then, as now, were the sharks.

Not long after the Tethys Ocean had been separated from the Panthalassa Ocean a catastrophe occurred that almost wiped life from the face of the earth at the close of the Permian, the End Permian Mass Extinction Event.

According to the author³ by the end of the Permian the entire supercontinent of Pangaea was under tensional stress, and quite suddenly deep fractures were appearing all over Pangaea. He suggests this may have been connected to the massive weight and extent of the giant supercontinent causing heating of the mantle beneath it. Parts of Pangaea that had drifted over existing hotspots would have been especially true, though he suggests there is also the possibility that the presence of the supercontinent caused the development of hotspots or superplumes. He also suggests that further research is required before the situation in Pangaea and the Earth in general, at this time.

During the Triassic the fractures developed further until they extended through the continental crust and became rift valleys. As these valleys widened large amounts of sediment were deposited in them, becoming the sites of red beds and evaporites, and as the water from the Tethys Ocean eventually flooded them the sediment deposited was of marine origin. They would also have been the sites where lava erupted as the valleys widened and the future seabed on either side of the fractures extended. They were also the sites where opening of the Tethys Ocean began, and later, the beginnings of the North Atlantic Ocean and the South Atlantic Oceans opened.

Where the Hudson River passes through New Jersey the banks are lined by the cliffs of the Palisades National Park, that are almost vertical and are composed of dark grey basaltic rocks, that in places have become rusty brown, that are the solidified lava that was intruded into the country rock of these rifts formed in the Triassic. This 'sill' (type of intrusion) is typically the feeding vent for nearby lava flows. The author³ says these rocks were the remains of one of the first attempts at the breakup of Pangaea. Extending for more than 1000 km from the Carolinas to Nova Scotia, the Newark Rift System is the formation that the Palisades Sill is a part of. It was one of many rift systems that didn't complete their development to the point where they separated a block of continental rock enough for it to break away from the main landmass of Pangaea.

If the attempted rift had been successful the author³ suggests Manhattan Island and New York may have ended up next to Casablanca in Morocco. In Morocco, the Ziz Valley is a similar rift that failed to develop. It has since been exhumed and the High Atlas Mountains have been uplifted, raising it to its present elevated position, exposing sediments deposited on its floor. The features uncovered are salt deposits and sun-baked mud flats, both indicating that a cut-off arm of the Tethys Ocean had been subjected to evaporation. This was followed by deposits of fully marine limestone containing gigantic ammonites, sponge mounds and reefs of algae. This was evidence that the sea had flooded the area allowing the deposition of turbidite sediments that are characteristic of deep water environments. Even so, the Ziz Valley failed to complete the breakaway.

The rift that was successful in forming an ocean was the one that passed through the Straits of Gibraltar, cutting southwest along the axis of the Central Atlantic Ocean of the present. When the Tethys Ocean flooded in it came from the east, past Morocco and Nova Scotia, extending as far south as Florida and the Bahamas. Laurasia then began slowly drifting away from Gondwana.

Around this time the region from Yucatan along the Panamanian Isthmus was being cut by the Panthalassa Ocean that flooded in from the west. A proto-Gulf of Mexico had begun to form, but spreading stopped before completion. The salt deposits characteristic of many early rifts was formed as these early seas evaporated. The salt deposits in the Gulf of Mexico are very thick, leading the author³ to suggest that there were probably many cycles of flooding and evaporation. He says that long after these evaporites had been buried beneath hundreds or even thousands of metres of sediment from the adjacent land, the great weight of the sediment caused them to deform. As the salt is of lower density than the overlying sediment they are buoyant, finger-like protrusions of salt, salt domes, forced their way up through the sediment. There are places where the salt has forced it way to the surface through several kilometres of sediment to flow out onto the sea floor. This bizarre behaviour of salt under pressure has led directly to the seafloor of the northern part of the Gulf of Mexico being pockmarked with a maze of domes and hollows. This feature is common in buried evaporites around the world, being easily visible on seismic profiles.

After the spreading ceased in the Gulf of Mexico it began to the south of Yucatan Peninsula leading to the formation of a proto-Caribbean. Spreading eventually opened a passage around Florida and Cuba that allowed the waters of the Tethys and Panthalassa to mix forming a single massive ocean. At first the gap between the 2 supercontinents that resulted from the breakup of the former single, much larger, supercontinent was the Tethys Seaway, but the gap continued to expand.

The author³ has suggested that the joining of all the landmasses to form Pangaea was one of the main causal factors Triassic as life recovered from its near extinction, one of the main episodes of evolutionary radiation.

The author³ has listed a number of effects of the breakup of Pangaea:

1. The breaking up of Pangaea and the addition of new areas of seafloor spreading, where the ocean bed rose up at the spreading ridges, the sea level would have risen and spread over much of the continental areas.
2. The climate of the globe would have been changed by the formation of the narrow seaway that eventually grew wider until it was an ocean, as it crossed the interior of the most severely arid continental interior known in Earth history. There would have been climatic amelioration around the world and in Pangaea the addition of moisture to the coastal areas, as well as the new coastal areas on either side of the seaway.
3. The circulation pattern of the ocean was changed by this connection between Tethys and Panthalassa. With no barrier to flow from the east to the west around the equator for the first time in about 150 My, and as the ocean deepened, currents in the depths as well as the surface could pass unhindered around the world. The blocked access of Tethyan waters to the Panthalassa Ocean to the west meant that the circulation in the Tethys formed 2 great gyres that moved warm water to the higher latitudes and brought cooled water from the high latitudes to the equatorial regions. This pattern ameliorated the climatic extremes resulting from latitude difference. The climate of the higher latitudes cooled once the barrier of Pangaea was breached and the warm waters of the equator flowed continuously around the globe, no longer moving warm water to the poles and cool water from the poles.
4. The length of coastline increased with the number of continents that separated from Pangaea, and later from Laurasia and Gondwana, increased rainfall as more land became close to coasts, so could be reached by coastal rain, increasing erosion rates contributed more sediment, and hence minerals, to the oceans. The availability and variability of ecospace increased with the longer coastlines, shelf areas that were flooded by rising sea levels. Put simply there were more areas of different ambient conditions that were available for life to exploit. The number and distribution of regions of upwelling of nutrient-rich water, that together with the increased addition of mineral nutrients, as a result of increased rates of erosion, especially iron, an essential ingredient for high primary productivity in the oceans.
5. As the landmasses moved away from each other the species carried on the blocks became increasingly divergent from other members of the original species on Pangaea they evolved along divergent lines as a result of geographical isolation. A feature of this divergence was that as they were on a separate landmass they were less affected by other competitors as they evolved along particular lines. The same situation exists for shallow-water animals that are separated by stretches of deep water that is wide enough to prevent migration. The northern and southern margins of Tethys began to develop faunal assemblages that were distinctly different from this time.

A greatly increased space was made available by the collective effect of these factors for the adaptive radiation to take place on a grand scale as many new ecological niches became available for the experimentation and diversity that developed. The base of the food chain in the oceans and on the land had improved supply of food as levels of primary production increased. The seas in the Jurassic were extremely fecund, the author³ stating that 'there was a remarkable growth and vibrancy of the ocean world while dinosaurs ruled the land'.

Portrait of the Tethys Sea

At the opening of the Cainozoic India was approaching eastern Asia, the Middle East was approaching Russia and Africa was colliding with Europe. The Tethys Ocean was to become a narrow seaway between continents in the Era.

Following the KT extinction event the remaining taxa didn’t recover immediately following the great environmental changes that had taken place at the end of the Cretaceous. The author³ suggests it probably took about 1-5 My for life to recover from this near-death experience, though the nature and duration of the recovery varied for different groups. There were great losses among the oceanic plankton, many fewer species being present after the KT boundary than before, the species numbers of coccoliths being greatly reduced, and among the foraminifera only the smaller species survived the mass extinction event.  The dinoflagellates and diatoms had been less affected, diversifying and recolonising the oceans so that the basic food stock of the oceans was replenished. Some animals, such as bivalves, sea snails, crabs, sea urchins, bryozoans and teleosts that had survived the extinction event began the next era in prominent positions in the new ecology. Shortly after new forms evolved to occupy particular niches, such as sand dollars, a flattened sea urchin the size of a biscuit that are the only sea urchins able to burrow into the sand of beaches. And corals displaced the rudists, though the author³ suggests their diversification appears to have taken longer.

Desert dollars

The pyramids of Giza are made of blocks of the nummulitic limestone of the Giza Plateau, a rock type that had been deposited in the Tethys Ocean 40-50 Ma. Nummulites are small fossils that have a disc shape, that are found in various sizes from a few millimetres to a few centimetres across, with an ornamentation of a spiral pattern on their outer shell, though many have been smoothed by exposure to sand and wind to such an extent that this pattern is no longer apparent on the shells. They are single-celled benthic foraminifers, among the largest known, that used calcium carbonate to construct their disc-like, multichambered shells. They were completely different from the smaller related forms that lived in the plankton and ate coccoliths. The nummulitic limestone was formed of lithified thick shell banks and dunes that were in turn formed by the sweeping into these banks and dunes of the animals that appear to have carpeted the seafloor in shallow, warm water.

The same type of limestone is also present in North Africa and the Middle East. The northern shoals of Tethys have been traced through southern Europe and central Asia by their distinctive signatures, occasionally in the form of white cliffs more than 20 m in height.

On the Black Sea coast, a bit inland, there is a double line of nummulitic crags that sparkle in bright sunlight. Light reflecting from tiny calcite spar crystals forming the strong cement holding the individual animals together is the source of the sparkles. It was these same crags that were the ‘jaws of the Valley of Death’ that had doomed the ‘Charge of the Light Brigade’ in the Battle of Balaclava in 1854.