Magnetostratigraphy

The magnetostratigraphy, and magnetic stratigraphy, is in the earth's history a branch of stratigraphy that deals with permanently magnetized rock units and their temporal sequence. It is based on the change of the polarity of the geomagnetic field ( colloquially pole shift ), which have very often occurred in the Earth's history. The method is, however, only in combination with other methods of stratigraphy useful (eg biostratigraphy, lithostratigraphy, chronostratigraphy, or the radiocarbon method), but an even finer resolution can be considered, for example, reach the biostratigraphy alone. The result is a polarity time scale that represents the polarity change in the earth's magnetic field in a time sequence.

History

The detection of the reversal of the geomagnetic field from paleomagnetic measurements succeeded for the first time Bernard Brunhes from the observatory of the Puy de Dome in 1905.

In the 1950s Keith Runcorn, Edward A. Irving, PMS Blackett and others from the paleomagnetic reconstruction of polar wandering found evidence of continental drift. During the sixties of the 20th century, the Earth's oceans were first studied extensively. The measurement of the magnetic field in the predominantly basaltic rocks of the ocean floor showed a pattern of stripes of different widths, which ran parallel to the mid-ocean ridges. The strips shown alternately set a different polarity, and thus provided evidence of a multiple reversal of the geomagnetic field during the last 150 million years of Earth's history (see also Plate Tectonics). This change in polarity with refined methods were later found in sediments.

Basic methodology

Essentially four paleomagnetic phenomena can be detected by the method: the polarity of the then earth's magnetic field, the position of the two poles of the magnetic dipole ( the instructions are on the apparent polar wander ), the non -dipole component of the magnetic field ( secular variation ) and the intensity of the magnetic field. For the magnetostratigraphy and thus the relative age determination of a rock or the correlation of different rock sequences only the polarity is relevant.

The relevant information was derived from the conservation of the Paläomagnetfeldes due to a natural remanent magnetization of the rock, the (mostly magnetite) is in a high proportion of ferrimagnetic minerals particularly pronounced. The magnetization is carried out in various ways, including through Thermoremanenz, Chemoremanenz or Sedimentationsremanenz.

The reconstruction of the paleo- geomagnetic field from the traditional in rocks information is made ​​more difficult because the original (primary) magnetization during the geological history of a rock change and can be overprinted. For example, a heating of the rock above the Curie temperature of a certain contained ferrimagnetic mineral followed by cooling at a thermal reset caused by this mineral magnetization ( exceeds the temperature is the Curie point of all ferromagnetic stomach tables minerals of the rock, a complete re-magnetization ). Furthermore, for example, magnetite or hematite in an already magnetized sedimentary sedimentary rocks during diagenesis are newly formed, whereby in addition to Paläomagnetfeld the time of deposition and the Paläomagnetfeld preserved at the time of mineral formation. Very often, therefore, several paleo- Erdmagnetfelder have been handed down in a rock whose magnetic information overlap.

Be removed by special methods, by means of which information on individual Paläomagnetfeldern from the total magnetic information of a rock ( eg thermal demagnetization ), they can be indirectly measured and thus isolated and determined.

Units

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

Each unit of the magneto polarity is a body of rock, which is distinguished by a certain residual polarity of another rock body with different polarity. For each unit, a stratotype must be determined; how long the interval need not be included in the definition. However, biostratigraphic or geochronological data are necessary for the correlation of stratigraphic units with different time scales. The upper and lower boundaries of a unit are marked by changing the polarity of Magneto in the rock. These changes may be due to or by deposition gaps by an actual documented in the sediment change in the polarity of the geomagnetic field, during which took place one or more reverse events.

The basic unit of magnetostratigraphy is the polarity zone '. If confusion with other uses of the polarity are possible, it is recommended to use the term, magnetic polarity zone '. Should any further more detailed investigations, a more formal subdivision be possible, this can be referred to as polarity subzone '. Several polarity zones can be grouped into, Polarity super zones '. The rank of a polarity zone can be changed, should it prove necessary. The formal name for a defined magnetometer polarity zone should be composed of a geographical name and the suffix, polarity zone '.

The magnetostratigraphic time scale

• Normal polarity ( black)
• Reverse polarity ( white)

The Global Magneto- polarity time scale (Global Magnetic Polarity Time Scale, abbreviated GMPTS ) now extends back to the Jura. The polarity zones (anomalies ) are in the Cenozoic, including the Upper Cretaceous and then counted separately from the Lower Cretaceous indicated with letters (see below). The counting direction is backwards in time, the youngest zone so has the number 1

The polarity zones of the Cenozoic are with the letter 'C ', respectively (of English. Cenozoic ). You start with the C1 anomaly of the present time and end with the C34 anomaly in the Cretaceous, and always consist of a younger fraction predominantly normal polarity and an older share with predominantly reverse polarity. The two components 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, predominantly reverse), Gaussian or Gauss ( after Carl Friedrich Gauss named, mostly normal) and Gilbert (named after William Gilbert, predominantly reverse). The Brunhes reversal ( from reverse to normal) occurred 780,000 years ago. However, there were a number of other since this reversal brief polarity reversals from normal to reverse, which are also referred to by name.

The "M- anomalies " begin with the M0 anomaly in the Lower Aptian, the 'M' stands for Mesozoic. The M- anomalies are counted back into the Earth to M 41; the latter anomaly is dated in the Bathonian. The C34 and M0 anomalies are a special represents the C34 anomaly is also called " Cretaceous Magnetic Quiet Zone" ( Cretaceous magnetic quiet zone). This is an approximately 41 million years lasting period (from about 83.5 to 124.5 Ma) of predominantly normal polarity. The strictly speaking belongs to share with reverse polarity is the M0 anomaly. In the meantime, three very brief periods of time were found with reverse polarity in the C34 anomaly, two events in the Albian and an event in the middle portion of the Aptiums. The M- anomalies are formed relatively clear up to M25 ( Kimmeridgian ), the anomalies M26 to M41, however, are characterized by very rapid change in polarity. Your ordinary shares contain many short-term reversals to reverse polarity and the reverse shares many short-term normal polarities.

On the extension of the Global Magneto - polarity time scale back to the Paleozoic Era is currently working intensively.

Geochronological phases

Depending on the strength and duration of the magnetostratigraphic time scale can be divided into different phases. This is achieved by so-called Chron (formerly epochs) made ​​which have a duration of 100,000 to 1,000,000 years. For example, the Gauss Chron took nearly 1 Ma, some of the Subchrons contained therein ( earlier events), Kaena and Mammoth, but only about 100,000 per year. The Laschamp event as well as the polarity excursions, lasted less than 1,000 years. There are also organizations that Megachrone, Hyperchrone and Superchrone distinguish which coincide largely with the geological eras.

Other Applications

In addition to the inversions of the previous magnetic field ( Paläomagnetfeld ) and the direction of the Paläomagnetfeldes can be measured, for example, to create a Polwanderpfad that represents the drift of the continental plates in the framework of plate tectonics. With increasing data density can u.U. the Polwanderkurve are then also used for correlation of Precambrian rocks.