Australia: The Land Where Time Began
Sea Level Change and Climate Change
Prior to the Cambrian not a lot is known about the water levels of the Earth’s oceans but from the Cambrian, about 542 Ma the situation becomes clearer. Some broad trends can be discerned bound up with climate change, but also with the distribution of the tectonic plates as they moved across the face of the Earth. The sea levels of the present are lower than they have been for most of the last 500,000 years.
Eustatic refers to changes in global sea level, differentiating them from local or regional variations reflecting the rises and falls of adjacent landmasses as they respond to tectonic or isostatic forces. The amount of water locked up in ice sheets and glaciers and the depth of the ocean basins determine the global sea level that typically changes by more than 100 m as the climate moves between glacial and interglacial conditions. The ocean basins can hold a lot more water when they are wide and deep than when they are of more limited in size and shallower for various reasons. At such times the oceans spread across vast tracts of low-lying land around the margins of the basins. During the Cretaceous about 80 Ma was the last time this happened when warm shallow seas covered much of Europe, Africa and North America.
According to the author1 the amount of water that can be held in the ocean basins is closely associated with the way in which the continents are spread around the surface of the Earth. The sea level has tended to be low at times when the continents have been clustered together into supercontinents such as Rodinia and Pangaea, and when the continents are spread out, great distances between them, the sea level is high. This change of sea level as the continents cluster and spread out has been happening throughout the history of the Earth. About 300 Ma, coincident with the formation of the supercontinent Pangaea, the sea level was low. It was also low 600 Ma when the continents clustered together in the supercontinent of Pannotia. When the continents were widely dispersed during the Ordovician and Cretaceous periods the sea level was high.
The explanation given by the author1 is that following the breakup of a supercontinent the individual fragments are separated by young oceans between the continents which have spreading ridges that are producing oceanic plates that are slowly moving the continents further apart by produce large quantities of hot lava to form new sea floor that takes some time to cool and become more brittle. Before it cools the lithosphere of the ocean floor it less dense than older, cooler lithosphere so it floats higher, with the result that the oceans were shallower until the oceanic plate cooled and sank lower, increasing the depths of the oceans. By the time the continents were once again gathered together in a supercontinent the floors of the oceans were cool so floated lower allowing the oceans to hold large quantities of water, and this resulted in lower global sea levels.
At times when the continents cluster at the poles, as they are at the present, and have been throughout the Quaternary, they form a substrate for the accumulation of ice sheets which extracts very large quantities of water from the oceans thereby reducing the sea levels even further than they would otherwise be.
The history of changing sea levels has a distinctive M shape with 2 peaks and 3 troughs. After the breakup of Pannotia in the Late Precambrian sea levels reached a peak about 450 Ma in the Ordovician at which point they were 200 m higher than present levels. When Pangaea formed they fell dramatically, reaching a low point in the Permian about 250 Ma. At this point they were at similar levels to those of the present. They rose to about 170 m higher than the present levels, and peaked in the Late Cretaceous about 80 Ma. Since that time they have been gradually falling, with the exception of the times in glacials and interglacials when it dropped and rose again.
7,500 years ago a similar flood has been suggested to have filled the Black Sea by water from the Mediterranean. William Ryan and Walter Pitman first suggested this in their book Noah’s Flood in 1997, the authors suggesting it may have been the origin of the Great Flood that is mentioned in a number of ancient texts such as the Bible and the Epic of Gilgamesh, an Akkadian story. According to the author1 the filling of the Black Sea may have been much less spectacular than the flooding of the Mediterranean Basin according to more recent research.
At the end of the last glaciation rapidly rising sea levels eventually inundated most, if not all, of the land that had been exposed by lowered sea level during the glaciation. Beringia, the 1500 km wide land bridge which connected Alaska to Siberia, allowing many large mammals to move between the 2 regions, and 12,000 years ago it allowed humans to reach North America. Other land bridges that went under water as the sea levels rose were the English Channel between the UK and France and in Australia, connected Tasmania to the mainland in the south and New Guinea to mainland Australia in the north. In Asia the islands of java, Sumatra and Borneo were all connected to mainland Asia before the sea level rose. There are no similar ‘land bridges’ that are in immediate danger of being flooded by the rising sea levels, though the author1 suggests that if nothing is done to curb the rising atmospheric carbon dioxide, eventually thermal expansion of the oceans, as well as meltwater being increasingly added from polar ice sheets and glaciers. In the longer term, over the coming centuries there may be big ocean rises as a result of the loss of the Antarctic ice sheets and the Greenland ice sheet. If this happens there could be problems for such low-lying countries as the Netherlands.
At times when the climate is in transition from glacial to interglacial and back again, according to the author1 vast amounts of mass was shifted around the Earth over period of 10,000 years or less, a very short time in terms of the geological timescale. There are 45 times the volume of water in the oceans of the present that in the polar ice sheets and other smaller areas of land ice. Enormous volumes of water were moved from the oceans to build up the continental ice sheets and the smaller ice caps on mountains and glaciers at the height of the glacial phases. There were about 52 million km3 of ice, about 4 % of the total volume of the ocean, at the Last Glacial Maximum. Though a small fraction, this is a huge weight to be moved around the planet over a short time, the rearrangement of the Earth’s mass on this scale being enough to cause adjustments in the spin of the Earth. In 2011 the magnitude 9 earthquake in Japan moved the Pacific Plate 30-40 m to the west, which was enough to cause the Earth to spin a bit faster, shortening the day by a bit less than 2 microseconds. Following the earthquakes in Sumatra in 2004 and Chile in 2010 caused spin adjustments that were a response to mass movements associated with the quakes. The position of the Earth’s figure axis, the mass around which the mass of the Earth is balanced, separated from its N-S axis by about 10 m, can be altered by major earthquakes. According to the author1 this appears to have been displaced by about 17 cm by the Japan event, which contributed to a slight change in the wobble of the Earth as it moves along its orbit around the Sun.
Major geological events can change the spin of the Earth, its day length and the way in which it wobbles in space. It can be seen by geodetic measurements monitoring with extreme precision the shape of the Earth that its shape changes with the seasons as mass is continuously being redistributed by the oceans and the atmosphere, which is reflected in day length changes of 1 millisecond, this effect being 500 times greater than that caused by the mass changes associated with the Japanese earthquake. The author1 has described how over a year reallocations of the mass of the planet resulting from the annual movement of surface water has been found to be associated with a seasonal response of the volcanoes around the world to the extent that their eruptions are ‘forced’ by tiny changes in stress and strain within or beneath them. As the active volcanoes of the present appear to be responding to deformation of the Earth that are on a broad scale associated with the movement of a mass of water that is 5,000 times less than the mass that was swapped back and forth from the oceans to the ice sheets during the major transitions of the climate the author1 suggests it would not be un reasonable to expect the response from the solid Earth at such times that was similar, or even possibly greater.
Much slower changes in the Earth’s shape and spin characteristics are involved in the expansion and contraction of ice sheets. More mass is concentrated closer to the spin axis as high latitude ice sheets expand, progressively increasing the spin of the Earth, in the same manner as an ice skater spins faster when she brings her arms closer to her body. When the climate swings to an interglacial the mass of the ice is moved to the oceans from the spin axis reducing the spin of the Earth, as occurs when the figure skater extend her arms.
All fluid systems of the Earth – atmosphere, hydrosphere and asthenosphere – the latter being where magma is generated, have their behaviour affected by the accelerations and decelerations of the spin of the Earth. Such changes in the spin rate have been found to influence deformation of the crust, as has been seen in China. Tiny variation in day length resulting from fluctuations of the spin rate over decades have been linked to changes of stress in the crust that lead to movements of several millimetres in faults that were being studied for other reasons. According to the author1 the shortening by 4 seconds of the day resulting from the formation of the ice sheets in the glacial period, and the day length increasing by 4 seconds as the ice sheets melted, returning their water mass to the ocean basins, though occurring over a much longer time period, appears likely to have had some effect on the stress and strain in the crust. These stress changes associated with the variation of day length that was much greater would have had some influence on the geological systems such as volcanoes that were still active, earthquake faults and possibly the movement of large rock volumes that were unstable.
According to the author1 there would have also have been modification of the shape of the Geoid, as well as affecting the spin rate, and therefore the length of the day, as repeated mass transfer occurred from the ice sheets to the oceans and back again. The author1 suggests this can be thought of as the form of the Earth that is defined by gravity rather than the topography of the Earth. The Geoid is effectively coincident with the mean sea level and can be visualised as the shape of the Earth if it was entirely covered by water, though it is not a perfect sphere, taking the form of an oblate spheroid, flattened at the top and the bottom and bulging in the middle, as a result of centrifugal force linked to the spin of the Earth. The diameter of the Earth is a bit less than 40 km greater at the equator than when measured from pole to pole. The topography of the Geoid arises from differences in the strength of gravity at various points on the surface of the Earth, that reflect the mass distribution within the Earth and geophysical processes that operate in the Earth’s deep interior.
The Geoid is much more subdued than that described by high mountains and deep ocean trenches, which define a maximum height difference of almost 20 km. There is less than 200 metres variation between the peaks and the troughs over the entire Earth. The shape of the Geoid was significantly impacted by huge weight of the continental ice sheets that accumulated during the last glacial period that resulted in it being depressed beneath Europe and North America. In the north-eastern part of North America the negative anomaly is still present though it is being gradually being reduced as the lithosphere rebounds. The weight of the ice sheets had displaced the asthenosphere, the plastic part of the mantle, to the south by depressing the lithosphere during the last glaciation and it is still flowing back to higher northern latitudes. Associated with the post-glacial rebound the migration of mass, in its own right, is estimated to be still affecting the rotation rate of the Earth, with a tendency to slow it by a bit over 0.5 milliseconds per century. The variations of stress and strain associated with distortion of the form of the Earth, taken together with excursions of the asthenosphere, as well as related adjustments of the spin rate of the Earth and day length can, according to the author1, reasonably expected to have had an influence on geological systems such as volcanoes that are active and earthquake faults at times of major climate changes. The 52 million km3 of water that is cyclically redistributed around the Earth is the driving potential for geological phenomena of changes of stresses and strains within the Earth that are latent hazards, and quantification of this process is difficult. According to the author1 it is probably best to surmise that a dynamic situation seems to be conducive to a response that is more lively from geological systems that are primed and poised.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|