Laschamp event

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The Laschamp event (also the Laschamp excursion ) was a brief reversal of the earth's magnetic field , which took place about 41,000 (± 2000) years ago and lasted for about 440 years. The phases of reversal and the return to the current polarity each lasted about 250 years and the reversed field only reached 25 percent of the strength of the normal field. During this time, more cosmic radiation reached the earth, which would otherwise be more strongly deflected by the stronger earth's magnetic field. The geomagnetic excursion discovered in 1967 is the first to be found in Brunhes-Chron , which has been going on for 780,000 years . It took place during the Vistula High Glacial and can be traced worldwide. It therefore represents an important temporal fixed point (marker) for research into the climatic history of the Young Pleistocene .

Type locality and designation

View from the Puy de Côme to the Puy de Louchardière

The Laschamp event was named after its type locality , the mugearitic lava flow emanating from the Puy de Laschamp . This volcanic cone is located southwest of Clermont-Ferrand near the village of Laschamps ( Saint-Genès-Champanelle ) in the French Massif Central and belongs to the Chaîne des Puys . The neighboring lava flows from Olby ( Hawaiit ) and from Puy de Louchardière also registered the Laschamp event.

The Laschamp event is identical to the Skjong event found in Norwegian cave sediments .

Geographical distribution

Effluent rock that formed during the Laschamp event has also been found on Iceland and in the Auckland Volcanic Field . The signal is also found in ice cores (on Greenland and Antarctica) and in deep-sea cores ( Ocean Drilling Program and other projects), for example in the Black Sea , the North Atlantic , the Greenland Basin, the Gulf of Mexico , the South Atlantic , and the Indian Ocean Ocean and in the Arctic Ocean northeast of Svalbard . Furthermore, it can be measured in caves stalagmites and in lake sediments .

stratigraphy

The Laschamp event falls in the Huneborg Stadium (Huneborg I, a significant cold relapse) and correlates with the Dansgaard-Oeschger event DO10 (considerable climatic fluctuation). It occurred shortly before the Heinrich event H4 (accelerated ice advance) and only relatively shortly before the Campanian ignimbrite was deposited (the eruption of the Phlegraean Fields about 39,400 years ago). The next field excursion was the Mono Lake excursion around 35,000 years BP.

Dating

The dating of geomagnetic events is indirect: a clue for the event must be found in a stratigraphy, and the stratigraphy must show other, datable features, as close as possible.

In effluent rock, the strength and direction of the field strength at the time of cooling below the Curie point is recorded as magnetization. For quantitative measurements the hysteresis curve of the sample must be determined. The almost identical crystallization age is dated using the argon-argon and potassium-argon methods , which, however, have to be calibrated because of the long half-life. Several lava flows were examined for an acceptable temporal coverage. One result of these investigations is (41,300 ± 300) years (before 2000, b2k) as the central point in time of the event.

The weakening of the magnetic field increases the rate of production of radionuclides in the stratosphere . 14 C has a complex and slow dynamics of distribution between atmosphere, biosphere and ocean. 10 Be - bound to dust - is quickly washed out and is therefore a good marker for the event in ocean sediments and ice sheets. Annual layers can be counted in Greenland ice cores. The 10 Be signal in it is evaluated both as an absolute amount per unit area and year and as a concentration and extrapolated to the global production rate. Dating by counting gives (41,250 ± 1630) years b2k. Most of the uncertainty arises in the deep layers, which are many times thinner due to flowing in the width. In the publications of the Greenland Ice Core Project , the sum of the counting uncertainties at all uncertain points is specified as the 95 percent error interval, which is probably too conservative for modern methods (counting using multiple signals).

characterization

In 2012, the investigation of drill cores from the sediment of the Black Sea showed a phase with reversed field of 440 years and 250 years for polarity reversals. In 2004, Laj and others assumed it would take around 1000 years to reverse the polarity of the field. During the polarity reversal, the field strength in the Black Sea fell to less than 10 percent and reached 25 percent in the opposite direction.

The magnetic inclination angle had changed from + 30 ° (i.e. 30 ° N) to -60 ° (or 60 ° S) in the course of the Laschamp event, and then returned to + 60 °. The declination turned from northwest over north to south and then returned to north again.

The virtual geomagnetic pole (English Virtual Geomagnetic Pole or VGP for short ) ran clockwise through a large loop that was centered at 150 ° east longitude north of New Guinea. Starting from North Greenland and after completing two switchbacks through North America, he steered through the Pacific towards Antarctica (polarity reversal). The way back ran through the Indian Ocean, India, Tibet and finally ended in Northeast Siberia.

parameter

During the Laschamp event, the generation rate for 10 Be reached a peak value of 0.85 atoms / cm 2 year, which has not been reached in the last 60,000 years (in comparison: the normal values are generally around 0.4 atoms / cm 2 year). The paleointensities of the magnetic field measured in the Laschamp lava flow fluctuated between 1 and 2.8 VADM (English Virtual Axial Dipole Moments - moments of the virtual axial dipole, measured in 10 22 Am 2 ). The peak value of the normal field reached around 47,000 years BP, however, was 13 VADM (normal values ​​are around 6 to 8 VADM).

Web links

Individual evidence

  1. ↑ a b Carlo Laj et al .: Dynamics of the earth magnetic field in the 10-75 kyr period Comprising the Laschamp and Mono Lake excursions: New results from the French Chaîne des Puys in a global perspective . Earth and Planetary Science Letters 387, 2014, pp. 184-197, doi : 10.1016 / j.epsl.2013.11.031 ( online ).
  2. ↑ Norbert Bonhommet and Jean Babkine: Sur la présence d'aimantation inversée dans la Chaîne des Puys. Comptes rendus hebdomadaires des séances de l'Académie des sciences ( weeklies on the meetings of the Academy of Sciences ) Series B 264, Paris 1976, pp. 92-94.
  3. ↑ a b c d e Norbert R. Nowaczyk et al .: Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments . In: Earth and Planetary Science Letters . tape 351–352 , 2012, pp. 54–69 , doi : 10.1016 / j.epsl.2012.06.050 ( online [PDF]).
  4. ↑ a b Hervé Guillou et al .: On the age of the Laschamp geomagnetic excursion . Earth planet. Sci. Lett. 227, 2004, pp. 331-343, doi : 10.1016 / j.epsl.2004.09.018 ( online ).
  5. ↑ Achim Brauer et al .: The importance of independent chronology in integrating records of past climate change for the 60e8 ka INTIMATE time interval . Quaternary Science Reviews 106, 2014, pp. 47-66, doi : 10.1016 / j.quascirev.2014.07.006 (full text).
  6. ↑ January Mangerud et al .: Paleomagnetic correlations between scandinavian ice-sheet fluctuations and greenland Dansgaard- oeschger events, from 45.000 to 25.000 yr BP . Quaternary Research 59, 2003, pp. 213-222, doi : 10.1016 / S0033-5894 (03) 00010-3 ( online ).
  7. ↑ a b Muscheler et al .: Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10 Be and 14 C records . Earth and Planetary Science Letters 219, 2004, pp. 325-340, doi : 10.1016 / S0012-821X (03) 00722-2 .
  8. ↑ Katrine K. Andersen et al .: The Greenland Ice Core Chronology 2005, 15–42 ka. Part 1: constructing the time scale . Quat. Sci. Rev. 25, 2006, pp. 3246-3257, doi : 10.1016 / j.quascirev.2006.08.002 ( online ).
  9. ↑ a b Carlo Laj et al .: High resolution global paleointensity stack since 75 kyr (GLOPIS-75) calibrated to absolute values . In: JET Channell u. a. (Ed.): Timescales of the Paleomagnetic Field (=  Geophysical Monograph ). tape 145 . American Geophysical Union, 2004, p. 255-265 .