Australia: The Land Where Time Began

A biography of the Australian continent 

Newell's Hypothesis

Norman Newell was the first to suggest there was an explicit connection between mass extinction events and major lowering of global sea levels that resulted in regression of epicontinental seas. According to the author1 the continental shelves of the present, a time of relatively low sea level in relation to the period covered by the entire Phanerozoic, could be seen as a pericontinental sea. A large proportion of the total area of the continents was covered by shallow seas of less than 200 m depth in the geological past, usually known as epicontinental seas. Regression of an epicontinental sea, unlike the oceans, can result in substantial reduction of the area of habitat for benthos, as well as to a lesser extent for the nekton, organisms that inhabit in the water column. The great bulk of biomass (organisms), occur in the shallow pericontinental seas above the continental shelf that surrounds the continents, and on the upper portion of the continental slope, though there are also representatives of all the major phyla, sometimes in an unexpectedly high levels of diversity, in the deep ocean. The author1 suggests that shallow epicontinental seas, usually with low tidal ranges, would have been more vulnerable to environmental changes of factors such as temperature and salinity than the open oceans that would have been more buffered, and therefore more stable, and a significantly reduced area of shallow water habitat at times of marine regression. The changed conditions in the epicontinental seas resulting from marine regressions would be expected to have had a deleterious effect on organisms that were adapted to these shallow water conditions. An example is seen at the present in the destruction of large areas of tropical rainforests, loss of habitat  is believed to be the main reason for greatly increased extinction rates that are occurring among a wide range of organisms.

The author1 suggests Newell's Hypothesis appears plausible for such reasons. Newell suggested there were 6 marine mass extinctions caused by habitat area reduction that resulted from marine regression of the epicontinental seas. Hallam1 suggests that the hypothesis must be tested, and it needs to be determined if extinction events correlate with regressions that result from falling sea levels, as can be deduced from the stratigraphic record. Hallam1 suggests 3 sources must provide the relevant information: the best possible correlations as determined by worldwide biostratigraphic analysis; by determining the shallowing-up and deepening-up successions from the thorough analyses of facies; and the plotting of land and sea distributions over time as determined by palaeogeographic analyses.

In Fig. 5.3 of his book Hallam1 displays his tentative sea level curve for the whole of the Phanerozoic, indicating the 6 mass extinction events of Newell by asterisks. Hallam1 says it is usual to display sea level curves in this form, as 'squiggly' lines, on a wide variety of scales. He suggests self-similarity probably applies, as the degree of 'squiggliness' remains constant with scale, as is true of coastlines, the system obeying fractal laws. It needs to be remembered that many of the sea level changes on a finer-scale, that are of short duration and lesser magnitude, remain controversial, as they are no more than tentative and, as usual, qualitative models of sea level change rates and amounts, the limits they reach depending on the refinement of biostratigraphic control. Fig. 5.1 of the author1 is also best treated as a broad approximation, Hallam suggesting it could be challenged in detail as it is based on a review of existing stratigraphic literature.

At first sight Hallam's Fig. 5.3 appears to strongly confirm Newell's hypothesis, but he suggests it is necessary to study more closely and critically the stratigraphic data associated with the extinction events, and more recent research needs to be incorporated into any such study. Hallam suggests that before carrying out such a study the strongest criticism of the regression hypothesis, at least for the 5 extinction events that were recognised by Raup and Sepkoski, must be considered. The marine regressions of the Quaternary that were dramatically rapid are the best-known, and it is known how sea levels dropped during glacial periods as water was locked up in terrestrial glaciers. There are no known extinction events of marine benthos associated with such marine regressions that exposed vast areas of continental shelf, which clearly contradicts Newell's hypothesis. Hallam suggests there are other possible explanations for this apparent contradiction, one of which is that the sea level falls of the Quaternary were very rapid on a geological timescale, and it can be argued that there may have been limited refugia where the organisms could expand from once the sea level rose again as the glaciers melted, as organisms must be kept in a stressed state for a minimum period of time before they become extinct, and the time span of the glacial phase may have been too short to wipe out the organisms in the refugia. According to Hallam, Jablonski has proposed that, based on the evidence from possible island refugia in the oceans, only limited extinctions would have been caused by regressions in the Quaternary.

When the entire history of the Phanerozoic is considered, it can be argued that the Quaternary was a very unusual time, suggesting that any organisms that survived to the Quaternary probably evolved a wide tolerance of environmental fluctuations. The organisms that produced the vast bulk of the marine fossil record lived in older epicontinental seas that may have been environmentally stable over long periods, such that a deleterious effect could result from even a small environmental change. Hallam cites Ager's dictum about the stratigraphic record, that originally applied to the life of a soldier, 'Long periods of boredom interrupted by moments of terror'.

Sources & Further reading

  1. Hallaml, Tony, 2004, Catastrophes and Lesser Calamities: The causes of mass extinctions, Oxford University Press

 

Catastrophes and Lesser Calamities

The causes of mass extinctions

Author: M. H. Monroe
Email: admin@austhrutime.com
Last updated: 25/03/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading