Australia: The Land Where Time Began

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Possible Global Ice Volume Changes and Geomagnetic Excursions and Earth Orbital Eccentricity

A study of climate and geomagnetic records for the past 400,000 years has been carried out to investigate a possible relationship between major global ice volume, geomagnetic variations and short-term climate cooling. It is suggested by calculations that rotational instabilities leading to magnetic excursions, that lead in turn to rapid cooling by several proposed mechanisms, can be caused by redistribution of the water mass of the Earth. Such times of double coincidences of magnetic excursions and the advance of glaciers at times of major changes of ice volume have occurred at about 13,500, 30,000, 110,000 and 180,000 BP. The last 4, and possibly 5, times when major eccentricity of the orbit of the Earth occurred were followed closely by magnetic excursions, and several of these excursions were accompanied by catastrophic cooling and rapid ice buildup. Therefore glaciation may be a result of insolation changes and the effects of geomagnetism on climate, which may result from Milankovitch cycle parameters.

 

It was considered (Verosub & Banerjee, 1977) that the 2 events with the best supporting evidence during the past few hundred thousand years are the Lake Mungo Excursion at about 30,000 BP and the Blake Event at about 110,000 BP. The Blake Event was first detected in deep-sea cores and has also been reported in a long sediment core from lake Riwa, Japan.

 

The Blake Event apparently occurred at the same time as a climate cooling with a build-up of ice that followed immediately, and terminated, the Ermian, the last interglacial stage. It has been found by studies of oxygen isotope ratios in deep-sea cores (Shackleton, 1976, 1977; Johnson, 1978) and sea level changes recorded in tropical reef terraces (Mathews, 1972) suggesting that at about 115,000-110,000 BP the sea level dropped 60-70 m in less than 10,000 years (Andrews & Mahaffy, 1976). The ice sheet growth rate is believed to have been enough to have produced a volume of 28 x 106 km3, equivalent to the combined volume of the Antarctic and Greenland ice sheets of the present in less than 10,000 years.

 

Author's1 discussion

In this paper the author cautions that the relationships proposed here must be considered to be highly speculative until proven or disproved. This is because coincidences of events, such as global ice volume changes, magnetic excursions, and rapid glaciations that occur on short time scales and climate cooling, do not prove the cause-and-effect relationships. According to the author1 there have been other magnetic excursions reported that don't appear to have occurred at the same time as ice volume changes and rapid cooling events that were recorded in oxygen isotope curves in deep -sea cores (Verosub & Banerjee, 1977). There are many excursions that have been reported that were poorly dated, and there is also the problem of several reported magnetic events that have been questioned. Many deep-sea cores may preserve records that have poor resolution of events that are shorter than a few thousand years as deposition took place slowly, the sediment was disturbed by bioturbation, and or dissolution of calcium carbonate. The result is that most deep-sea cores are not suitable for study of magnetic excursions that occurred on very short time scales and climatic events. The author suggests future work on sediments that had a high deposition rate in lakes and/or anaerobic basins have promise of resolving these problems.

Eccentricity, climate and the magnetic field

The author1 suggests it is notable that magnetic excursions closely followed the last 4, and possibly 5, periods of maximum eccentricity of the orbit of the Earth. It appears that brief periods (104 years) of minimum ice cover, the interglacial periods, that followed times when there was rapid melting of the ice in the Northern Hemisphere, appear to coincide with eccentricity maxima (Hays et al., 1976). Magnetic excursions may result from such changes in the distribution of water mass (Doake, 1977).

It has been argued (Wollin et al., 1977, 1978) that the eccentricity of the Earth's orbit directly modulates the geomagnetic field by the resulting variations in precession. A relationship between geomagnetic field strength and a 41,000 yr cycle of the tilt of the Earth has been reported (Kent & Opdyke, 1977), and debate is continuing regarding the entire question of modulation of the geomagnetic field by orbital parameters. The author1 suggests the excursions may be the result of orbital variations directly affecting the core-mantle dynamics and /or global ice mass and volume distribution could result that possibly affect the rotation of the Earth which thereby set up instabilities in the core. Some excursions, e.g., the Lake Mungo Excursion, were not associated with an orbital eccentricity maximum, though they do appear to have followed periods when rapid melting of ice occurred.

If the excursion occurs at an unfavorable time in the Milankovitch insolation cycle, as occurred with the 13,500 BP Gothenburg Excursion, then the glaciers advanced only briefly, though a full glacial stage may be initiated if the geomagnetic trigger occurs at a "sensitive" phase in the Milankovitch cycle.

If these factors are taken into consideration, the catastrophic cooling episodes that may have ushered in the last 3 or 4 major glacial stages could have been triggered by magnetic excursions that resulted directly or indirectly from astronomical forcings. According to the author1 the Milankovitch insolation  hypothesis for glacial-interglacial cycles may be only part of the solution to the problem of the initiation of glaciation, if these relationships are correct. The atmospheric-oceanic circulation changes and climatic cooling that have been proposed to accompany magnetic excursions, might reinforce the insolation effects, and the related feedback mechanisms, such as changes in atmospheric and oceanic circulation and global albedo.

Sources & Further reading

  1. Rampino, Michael R. "Possible Relationships between Changes in Global Ice Volume, Geomagnetic Excursions, and the Eccentricity of the Earth's Orbit." Geology 7, no. 12 (December 1, 1979 1979): 584-87.
Author: M. H. Monroe
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Last updated 22/09/2013 
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading