Australia: The Land Where Time Began

A biography of the Australian continent 

Tethys Ocean Explanation of Low Oxygen levels 

During the Cretaceous, that is now known to have been a greenhouse world, the Tethys had opened as a broad, low-latitude ocean that allowed the equatorial currents to flow unhindered and increase circulation. The currents passed through Tethys and continued around the world without deflecting from the tropics and continued to gain heat. Warm water was peeled off as gyres along the way, taking the warm water towards the poles. Based on the reconstructions of plate tectonics from that time the author3 suggests there would have been no landmasses at either pole, and there is no evidence of floating ice from the high latitude sediments from that time. In rocks of the Antarctic and Canada there are fossil forests that would have been at 85o latitude in the Cretaceous, equivalent to the Central Arctic Ocean of the present. There are also coals, coals that formed equivalent to north of Oslo, Norway of the present. There were also coral reefs that formed far from the subtropical distribution of the present.

Oxygen isotope studies of the shells of planktonic organisms have shown that during the Mid-Cretaceous the surface temperatures of the ocean surface waters in the equatorial region were 25-30o C and decreased to 10-15o C at the poles. Fossil shells of animals that lived at a depth of 2000 m below the ocean surface have given an isotope treading indicating that at that depth the temperature was 15o C.

In the Cretaceous sea water would have contained less oxygen because it holds less the higher the temperature rises. Added to this was another problem, without the high temperature differential between the poles and the equator and no seasonal sea-ice to drive it, ocean circulation would have been sluggish at best. At that time the sea levels around the world were much higher than at the present and continued to rise, resulting in wide areas of the coastal land of the continents around the Tethys being inundated by the sea, and marginal seas and large lagoons formed, in all of which circulation had the potential to have restricted circulation leading to stagnation. The author3 suggests that the same would have also applied to the fingers of the Tethys that continued into the South Atlantic that was still narrow at that time, the area where he began his study of the black shales. In the partially restricted shelf areas and marginal seas that were partially enclosed would have contributed to a lowering of the dissolved oxygen in these areas, and the oxygen levels would have also been declining as the organic material was recycled by the microorganisms, as they use oxygen in the process. This made it inevitable that there would be areas where there would have been oxygen starvation or even stagnation, all this made worse by the warm temperatures of the water, as it could hold less oxygen.

According to the author3 it was very likely the levels of productivity in these warm surface waters would have been high in the Tethys. He suggests that especially in the wide areas of the shelves the levels of productivity would have been high, some of which would have been carried to the deep ocean as ocean currents flowing the continental slopes. He says that this was found to be the situation when the ship he was on was drilling in the South Atlantic, organic debris from the Walvis Ridge being carried by currents into the deep Angola Basin, along with fine clays and silts. As organic material that was buried rapidly cannot easily be reached by scavengers and recycling bacteria they can't decompose it. It is well preserved if it is buried deep enough in fine clay-rich sediment to prevent percolating sea water bringing oxygen to it.

The author3 suggests that widespread, prolonged episodes of black shale accumulation between 125-85 Ma in the Tethys resulted from a number of factors including low oxygen levels and high productivity of organic matter. Such a result was called an 'oceanic anoxic event' by Prof High Jenkyns, and the 'Black Death' by the author3.

Sources & Further reading

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



Author: M. H. Monroe
Last Updated 10/04/2012



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