Magnetostratigraphy

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The Magnetostratigraphy , and magnetic stratigraphy is in the Earth's history , a branch of stratigraphy that deals with permanently magnetized rock units involved and their sequence. It is based on the changes in the polarity of the earth's magnetic field (colloquially known as pole shift ), which have occurred very frequently in the history of the earth. However, this method is only useful in combination with other methods of stratigraphy (e.g. biostratigraphy , lithostratigraphy , chronostratigraphy , or theRadiocarbon method ), however, can then achieve an even finer resolution than, for example, biostratigraphy alone. The result is a polarity-time scale that shows the polarity changes in the earth's magnetic field in a time sequence.

history

The proof of the reversal of the earth's magnetic field from paleomagnetic measurements was first achieved by Bernard Brunhes from the Puy de Dôme observatory in 1905.

In the 1950s, Keith Runcorn , Edward A. Irving , PMS Blackett, and others found evidence of continental drift from the paleomagnetic reconstruction of polar migration. During the 1960s, the Earth's oceans were first studied intensively. The measurement of the magnetic field in the predominantly basaltic rocks of the ocean floor resulted in a pattern of strips of different widths that ran parallel to the mid-ocean ridges . The stripes alternately showed a different polarity and thus provided evidence of a multiple reversal of the earth's magnetic field during the last 150 million years of earth's history (see also plate tectonics ). These changes in polarity were later found in sediments using more refined methods .

Basics of the methodology

Essentially, four paleomagnetic phenomena can be recorded by the method: the polarity of the earth's magnetic field at that time, the position of the two poles of the magnetic dipole (which provides information on the apparent pole shifts), the non-dipole component of the magnetic field (secular variation) and the Magnetic field intensity . For magnetostratigraphy and thus the relative age determination of a rock or the correlation of different rock sequences, only the polarity is relevant.

The corresponding information comes from the conservation of the paleomagnetic field as a result of a natural remanent magnetization of the rock, which is particularly pronounced with a high proportion of ferrimagnetic minerals (mostly magnetite ). The magnetization takes place in different ways, u. a. by thermoremanence , chemoremanence or sedimentation remanence .

The reconstruction of the paleo-geomagnetic field from the information handed down in rocks is made more difficult by the fact that the original (primary) magnetization can change or be overprinted in the course of the geological history of a rock. For example, if a rock is heated above the Curie temperature of a certain ferrimagnetic mineral it contains, with subsequent cooling, the magnetization caused by this mineral is thermally reset (if the temperature exceeds the Curie point of all ferromagnetic minerals in the rock, complete remagnetization occurs). Furthermore, for example, magnetite or hematite can be newly formed in sedimentary rock that has already been magnetized by sedimentation during the diagenesis , whereby in addition to the paleomagnetic field at the time of deposition, the paleomagnetic field at the time of new mineral formation is also preserved. Therefore, several paleo-geomagnetic fields are very often handed down in a rock, the magnetic information of which is superimposed.

Using special methods with the help of which information on individual paleomagnetic fields can be removed from the total magnetic information of a rock (e.g. thermal demagnetization), this can be measured indirectly and thus isolated or determined.

units

In magnetostratigraphy, the prefix 'magneto-' is used to describe all aspects of remanent magnetization (e.g., 'magnetointensity', 'magnetopolarity' etc.). In magnetostratigraphy, only the frequent changes in polarity of the magnetic field are currently used for stratigraphy and thus for relative age dating. The current orientation of the earth's magnetic field is referred to as normal, the opposite orientation as reverse. The chronological sequence of the measurable magnetic field reversals can provide indications of the relative age with complete documentation in the sediment.

Each unit of magnetopolarity is a rock body that is distinguished from another rock body with different polarity by a certain remanent polarity. A stratotype must be determined for each unit ; how long the interval lasts need not be included in the definition. For the correlation of the units with other stratigraphic time scales, however, biostratigraphic or geochronological data are necessary. The upper and lower limits of a unit are marked by a change in magnetopolarity in the rock. These changes can be caused by an actual change in the polarity of the earth's magnetic field documented in the sediment, or by deposit gaps during which one or more reversal events took place.

The basic unit of magnetostratigraphy is the 'polarity zone'. When confusion with other uses of polarity is possible, it is recommended that the term 'magnetopolarity zone' be used. Should a further formal subdivision be possible in further more detailed investigations, this can be referred to as a 'polarity subzone'. Several polarity zones can be grouped in 'polarity superzones'. The rank of a polarity zone can also be changed should this prove necessary. The name for a formally defined magnetopolarity zone should be composed of a geographical name and the addition 'polarity zone'.

The magnetostratigraphic timescale

Geomagnetic polarity in the Upper Cenozoic
  • normal polarity (black)
  • reverse polarity (white)

    • The Global Magnetic Polarity Time Scale ( GMPTS for short ) goes back to the Jurassic . The polarity zones (anomalies) are counted separately in the Cenozoic including the Upper Cretaceous and then again from the Lower Cretaceous and provided with letters (see below). The counting direction is backwards, so the youngest zone has the number 1.

    The polarity zones of the Cenozoic era are provided with the letter 'C' (from English Cenozoic ). They begin with the C1 anomaly of the present time and end with the C34 anomaly in the chalk, and always consist of a younger part with predominantly normal polarity and an older part with predominantly reverse polarity. The two parts can be of different lengths. The most recent four polarity zones were given proper names: Brunhes (named after Bernard Brunhes , mostly normal), Matsuyama (named after Motonori Matsuyama , mostly revers), Gauss or Gauss (named after Carl Friedrich Gauß , mostly normal) and Gilbert (named after William Gilbert , predominantly reverse). The Brunhes reversal (from reverse to normal) occurred 780,000 years ago. However, since this reversal there have been a number of other brief polarity reversals from normal to reverse, which are also referred to by names.

    The “M anomalies” begin with the M0 anomaly in the Lower Aptian , where the “M” stands for Mesozoic Era. The M anomalies are counted back in the earth's history to M 41; the latter anomaly is dated to the Bathonium . The C34 and M0 anomalies are special. The C34 anomaly is also known as the "Cretaceous Magnetic Quiet Zone". This is an approximately 41 million year period (from about 83.5 to 124.5 Ma ) of mostly normal polarity. Strictly speaking, the associated part with reverse polarity is the M0 anomaly. In the meantime, however, three very brief periods of time with reverse polarity were also found in the C34 anomaly, two events in the Albium and one event in the middle section of the Aptium. The M anomalies are still relatively clear up to M25 ( Kimmeridgian ), whereas the anomalies M26 to M41 are characterized by very rapid changes in polarity. Their normal parts contain many short-term reversals to reverse polarity, and the reverse parts contain many short-term normal polarities.

    Intensive work is currently underway on extending the global magnetopolarity time scale back to the Paleozoic .

    Geochronological phases

    In order to be able to refer to specific places in the long magnetostratigraphic pattern, it is hierarchically divided into named sections. A chron (early epoch ) typically lasts 1,000,000 years, has a dominant polarity throughout and is delimited by phases of dominating reverse polarity. A chron may be interrupted by subchrons (earlier events ) up to a duration of about 100,000 years. For example, the Gauss-Chron lasted almost 1 Ma and contains Kaena and Mammoth as subchrons with a duration of 70,000 and 110,000 years respectively. There are also classifications that distinguish megachrones , hyperchrone and superchrone , which largely correspond to the geological ages.

    The above classifications have no physical basis - the time between polarity reversals shows a broad, unstructured spectrum, which indicates a chaotic process as the cause. Towards the short end, the spectrum is limited by the duration of the polarity reversal itself, which, including the periods of weakness before and after the actual polarity reversal, is several thousand years. Processes in this time range that do not lead to a sustained field reversal are called geomagnetic excursions . Examples are the Laschamp event and the Mono Lake excursion .

    Other uses

    In addition to the reversals of the previous magnetic field (paleomagnetic field), the direction of the paleomagnetic field can also be measured, e.g. B. to create a polar wandering path that shows the drift of the continental plates in the context of plate tectonics. With increasing data density, u. U. the polar wandering curve can then also be used to correlate Precambrian rocks.

    literature

    Individual evidence

    1. ↑ Ogg, Gabi (2012): Chapter 8. Magnetostratigraphic polarity units . In: Homepage of the Geologic Time Scale Foundation . (accessed March 10, 2013)
    2. ↑ Galbrun, Bruno (1997): Did the European dinosaurs disappear before the KT event? Magnetostratigraphic evidence (PDF; 988 kB). In: Earth and Planetary Science Letters 148 (1997), pp. 569-579. (English)
    3. ↑ Lerbekmo, JF & KC Coulter (1985): Magnetostratigraphic and Lithostratigraphic Correlation of Coal Seams and Contiguous Strata, Upper Horseshoe Canyon and Scollard Formations (Maastrichtian to Paleocene), Red Deer Valley, Alberta. In: Bulletin of Canadian Petroleum Geology, Vol. 33 (1985), No. 3. (September) , pp. 295-305. (English)
    4. ↑ Ogg, Gabi (2012): Chapter 10. Relation between different kinds of stratigraphic units . In: Homepage of the Geologic Time Scale Foundation . (accessed March 10, 2013)
    5. ^ Ojha, Tank Prasad (2009): Magnetostratigraphy, Topography and Geology of the Nepal Himalaya: A GIS and Paleomagnetic Approach. Dissertation at the Department of Earth Sciences at the University of Arizona. ProQuest . UMI No. 3352636.
    6. ↑ James G. Ogg: Magnetic Polarity Time Scale of the Phanerozoic . In: Thomas J. Ahrens (Ed.): Global earth physics a handbook of physical constants. AGU reference shelf series . tape 1 . American Geophysical Union, Washington, DC 1995, ISBN 0-87590-851-9 , pp. 240 .
    7. ↑ Molostovskii, EA, DM Pechersky and I. Yu Frolov (2007): Magnetostratigraphic Time Scale of the Phanerozoic and Its Description Using a cumulative distribution function (PDF; 188 KB). In: Physics of the Solid Earth , 2007, Vol. 43, No. 10, pp. 811-818. ISSN  1069-3513 . doi : 10.1134 / S1069351307100035