Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean Rise and Fall of Sea Levels 

Tethys would have had a huge mid-ocean ridge in the form of a jagged submarine mountain belt at some place in the equatorial region between Laurasia in the north and Gondwana to the south that would have been along one of the rifts that originally split Pangaea into 2 pieces. The author³ suggests mantle hotspots would have existed, away from the ridge as well as on it and could possibly have produced islands and archipelagos. As mentioned previously not all hot spots are situated beneath mid-ocean ridges. Hot spots beneath Pangaea have been implicated in the rifting and breakup of the supercontinent, though the hot spot that produced the Hawaiian Islands is located far from the mid-Pacific Ridge.

Also starting out as an arm of the Tethys was one of the other rifts that originally formed at the time Pangaea started breaking up, but as it spread it later formed the South Atlantic Ocean. According to the author³ when the drilling ship drilled on DSDP Leg 75 the cores showed that the ocean had been spreading for a bit over 100 My, and later still another phase of volcanic activity occurred along the ancestral Walvis Ridge that was probably related to a hot spot. The evidence in the cores indicated that this had given rise to seamounts and islands that were surrounded by coral. India had rifted from Gondwana further to the east and the beginnings of the ocean that would become the Indian Ocean occurred at this time.

The Late Cretaceous was a time when sea levels were very high around the world. The author³ suggests it was probably at least 200 m higher than at present, though the best estimates suggest it was more likely to be closer to 300 m (1,000 ft) higher. At the time of highest sea levels about half the area of land of the present would have been above water, about 18 % of the land remaining above water, most of which would have been low-lying land of islands and small continents that were green and fertile. The global sea level is closely linked to active seafloor spreading. 

Apart from the diurnal cycles of the tides, the shoreline has rarely remained the same, but has fluctuated widely over time. The distribution of terrestrial fossils, such as dinosaurs and plants, can infer these changes, as in the Cretaceous rocks of a region, for example, versus the fossils of marine reptiles or plankton that occur in rocks from a different region but from the same age, the shoreline being situated at some point between the 2. Coastal sediment can be found that were deposited in deltas, estuaries or beaches, as well as fossil creatures that were inhabitants of such environments, oysters or bird footprints on intertidal muds that are more accurate indicators of the location of ancient shorelines. Seismic profiles produce visual records of the subsurface that are like geological cross-sections displaying succession and arrangement of the layers of sediment. This allows the working out of the way the position of the coastline has changed over time, which indicates the sea level rises and falls. The ocean sediment records are much more complete than those from outcrops on land, but geologists are still disputing the details of these records, though the general pattern is not disputed. During the Permian the sea level dropped to one of the lowest points of the past 500 My of the Phanerozoic. This coincided with both the final completion of Pangaea and the end of Permian mass extinction event. The sea level rose by more than 300 m over the next 170-180 My, reaching a maximum in the Late Cretaceous. This rise occurred in a series of stepped increases and smaller decreases called supercycles. The sea level has dropped by a similar series of supercycles since then until reaching its present level that is relatively low. This one megacycle of change covered the entire existence of the Tethys Ocean.  The supercycles are imposed on shorter cycles and these, in part, on cycles of change that are of even higher order.

The author³ suggests that the part of the explanation of the short-term and long-term cycles of changing sea levels that is known is best explained in a book New Views on an Old Planet, by Tjeerd van Andel. The author³ says there are 3 main causes of changing sea levels that are confidently known.

The first is directly related to seafloor spreading, especially to the amount of spreading and the spreading rate. A larger volume of new oceanic crust that is hot is produced by rapid spreading than by slower spreading. The hotter crust rises, displacing a larger volume of seawater, as the hot rock is less dense. Extending the overall length of oceanic ridges that are present at any one time has the same effect of displacing more seawater. The first half of the megacycle of Tethys rising sea levels is explained by this scenario. Spreading rates where high as new oceans opened up and the mid-ocean ridges increased in overall length. The lowered sea level at the start of the Megacycle contributed to by the crumpling that occurred as continents collided and pushed up mountain ranges, as when Pangaea fused together its constituents continents, resulting in even less continental mass submerged in the oceans.

The second was linked to plate tectonics, especially in regard to transition of ocean to continent. Among the parameters involved were how far the spreading centre was away, whether subduction had begun, if a deep sea trench had been formed, and haw far a high mountain range is from the coast. The author³ suggests that these factors could explain the occurrence of supercycles of varying length, though it would be more complex to work through the overlapping effects of these parameters.

The locking up of large volumes of seawater in glaciers as ice caps is the third factor, though the ice must be on land for this to affect sea-level as floating ice has very little effect on sea levels. Over the last 2 My the climate fluctuated through periods of glacials and interglacials. The geologically recent past has been characterised by high-amplitude, short-term climatic fluctuations. During the time the Tethys existed the seas were more or less free of ice. The author³ suggests it is necessary to return to a plate tectonics explanation for the second half of the Tethyan megacycle, the gradually falling sea level since its high point in the Cretaceous and eventually the closure of the Tethys Ocean.

As well as a fluctuating sea level on a global scale, there were also local changes that may have Affected only a small area of coastline, more generally explained as being related to local earth movements such as tectonic uplift and subsidence, and by the sudden deluge of sediment from the mountain ranges that had recently arisen, to be deposited in large deltas that built outward from the coast. There is also the loading and unloading as ice accumulates, leading to local depression of the continental surface, and when the ice melts to unloading in which the continental rocks rebound, being uplifted once again.

Sources & Further reading

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