Geomagnetic excursion

A geomagnetic excursion, like a geomagnetic reversal, is a significant change in the Earth's magnetic field. Unlike reversals, however, an excursion does not permanently change the large-scale orientation of the field, but rather represents a dramatic, typically short-lived change in field intensity, with a variation in pole orientation of up to 45° from the previous position. These events, which typically last a few thousand to a few tens of thousands of years, often involve declines in field strength to between 0 and 20% of normal. Excursions, unlike reversals, are generally not recorded around the entire globe. This is partially due to them not being recorded well within the sedimentary record, but also because they likely do not extend through the entire geomagnetic field. One of the first excursions to be studied was the Laschamp event, dated at around 40000 years ago. This event was a complete reversal of polarity, however, as it later turned out, though with the reversed field 5% of the normal strength.[1] Since this event has also been seen in sites around the globe, it is suggested as one of the few examples of a truly global excursion.[2]

Causes

Scientific opinion is divided on what causes geomagnetic excursions. The dominant theory is that they are an inherent aspect of the dynamo processes that maintain the Earth's magnetic field. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. In such cases, this spontaneous disorganization can cause decreases in the magnetic field as perceived at the Earth's surface. In truth, under this scenario, the Earth's magnetic field intensity does not significantly change in the core itself, but rather energy is transferred from a dipole configuration to higher order multipole moments which decay more rapidly with the distance from the Earth's core, so that the expression of such a magnetic field at the surface of the Earth would be considerably less, even without significant changes in the strength of the deep field. This scenario is supported by observed tangling and spontaneous disorganizations in the solar magnetic field. However, this process in the sun invariably leads to a reversal of the solar magnetic field (see: solar cycle), and has never been observed such that the field would recover without large scale changes in field orientation.

The work of David Gubbins suggests that excursions occur when the magnetic field is reversed only within the liquid outer core; reversals occur when the inner core is also affected.[3] This fits well with observations of events within the current chron of reversals taking 3–7000 years to complete, while excursions typically last 500–3000 years. However, this timescale does not hold true for all events, and the need for separate generation of fields has been contested, since the changes can be spontaneously generated in mathematical models.

A minority opinion, held by such figures as Richard A. Muller, is that geomagnetic excursions are not spontaneous processes but rather triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core–mantle boundary, and possibly mantle-core shear forces and displacements resulting from very large impact events. Supporters of this theory hold that any of these events lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field for a period of time necessary for it to recover.

Richard A. Muller and Donald E. Morris suggests geomagnetic reversal due to very large impact event and following rapid climate change. The impact triggered a little ice age and change of water redistribution more to poles alters the rotation rate of crust and mantle. If the sea-level change is sufficiently large (>10 meters) and rapid (in a few hundred years), then the velocity shear in the liquid core disrupts the convective cells that drive the Earth's dynamo.[4]

Except for recent periods of the geologic past, it is not well known how frequently geomagnetic excursions occur. Unlike geomagnetic reversals, which are easily detected by the change in field direction, the relatively short-lived excursions can be easily overlooked in long duration, coarsely resolved, records of past geomagnetic field intensity. Present knowledge suggests that they are around ten times more abundant than reversals, with up to 12 excursions documented within the current reversal period Brunhes–Matuyama reversal.

Effects

Due to the weakening of the magnetic field, particularly during the transition period, greater amounts of radiation would be able to reach the Earth, increasing production of beryllium 10 and levels of carbon 14.[5] However, it is likely that nothing serious would occur, as the human species has certainly lived through at least one such event; Homo erectus and possibly Homo heidelbergensis lived through the Brunhes–Matuyama reversal with no known ill effect, and excursions are shorter-lived and do not result in permanent changes to the magnetic field. The major hazard to modern society is likely to be similar to those associated with geomagnetic storms, where satellites and power supplies may be damaged, although compass navigation would also be affected. Some forms of life that are thought to navigate based on magnetic fields may be disrupted, but again it is suggested that these species have survived excursions in the past. Since excursion periods are not always global, any effect might well only be experienced in certain places, with others relatively unaffected. The time period involved could be as little as a century, or as much as 10000 years.

Possible relationship to climate

There is evidence that geomagnetic excursions may be associated with episodes of rapid short-term climatic cooling during periods of continental glaciation (ice ages).[6]

Recent analysis of the geomagnetic reversal frequency, oxygen isotope record, and tectonic plate subduction rate, which are indicators of the changes in the heat flux at the core mantle boundary, climate and plate tectonic activity, shows that all these changes indicate similar rhythms on million years’ timescale in the Cenozoic Era occurring with the common fundamental periodicity of ∼13 Myr during most of the time.[7]

Occurrence

Geomagnetic excursions for the Brunhes geomagnetic chron are relatively well described.[8]

Geomagnetic excursions in the Matuyama, Gauss and Gilbert chrons are also reported and new possible excursions are suggested for these chrons based on analysis of the deep drilling cores from Lake Baikal and their comparison with the oceanic core (ODP) and Chinese loess records.[9]

See also

Notes and references
  1. "Ice age polarity reversal was global event: Extremely brief reversal of geomagnetic field, climate variability, and super volcano". Sciencedaily.com. Science Daily. 2012-10-16. Retrieved 2013-07-28.
  2. Roperch, P.; Bonhommet, N.; Levi, S. (1988). "Paleointensity of the earth's magnetic field during the Laschamp excursion and its geomagnetic implications". Earth and Planetary Science Letters. 88 (1–2): 209–219. Bibcode:1988E&PSL..88..209R. doi:10.1016/0012-821X(88)90058-1.
  3. Gubbins, David (1999). "The distinction between geomagnetic excursions and reversals" (PDF). Geophysical Journal International. 137 (1): F1–F4. Bibcode:1999GeoJI.137....1C. doi:10.1046/j.1365-246X.1999.00810.x. Archived from the original (PDF) on 3 March 2012. Retrieved 19 April 2012.
  4. Muller, Richard A.; Morris, Donald E. (1986-11-01). "Geomagnetic reversals from impacts on the Earth". doi:10.1029/gl013i011p01177. Cite journal requires |journal= (help)
  5. Helmholtz Association of German Research Centres (16 October 2012). "An extremely brief reversal of the geomagnetic field, climate variability and a super volcano". Retrieved 2 November 2014.
  6. Rampino, Michael R. (1979). "Possible relationships between changes in global ice volume, geomagnetic excursions, and the eccentricity of the Earth's orbit". Geology. 7 (12): 584–587. Bibcode:1979Geo.....7..584R. doi:10.1130/0091-7613(1979)7<584:PRBCIG>2.0.CO;2.
  7. Chen, J.; Kravchinsky, V.A.; Liu, X. (2015). "The 13 million year Cenozoic pulse of the Earth". Earth and Planetary Science Letters. 431: 256–263. Bibcode:2015E&PSL.431..256C. doi:10.1016/j.epsl.2015.09.033.
  8. Roberts, A.P. (2008). "Geomagnetic excursions: Knowns and unknowns". Geophysical Research Letters. 35 (17). doi:10.1029/2008GL034719.
  9. Kravchinsky, V.A. (2017). "Magnetostratigraphy of the Lake Baikal sediments: A unique record of 8.4 Ma of continuous sedimentation in the continental environment". Global and Planetary Change. 152: 209–226. doi:10.1016/j.gloplacha.2017.04.002.
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