Paleomagnetic chronology is a method to determine the stratigraphic age by using the earth’s magnetism.his method does not directly measure the age of rocks, but indirectly obtains the age of rocks by measuring the positive and negative polarity changes of natural residual magnetism of rocks and comparing with the standard paleomagnetic polarity chronology.Therefore, it can not be called paleomagnetic dating, and the age obtained can only be called contrast age.
The paleomagnetic polarity chronology is based on the measurement of a series of paleomagnetic polarity events in stratigraphic sections, and assisted by a large number of K-Ar dating data, to compile the polarity change events of different time scales into the earth’s polarity schedule.The paleomagnetic polarity chronology is global.It is the standard of paleomagnetic polarity correlation in the world.At present, the polarity chronology used for Quaternary research is a quaternary paleomagnetic polarity chronology, which is based on the geomagnetic polarity schedule since 5mab. P. which was drawn up by A. Cox and others in 1969 according to more than 140 data of land and ocean, and supplemented and revised by many researchers.
The earth is a uniform magnetized sphere, and its magnetic field is equivalent to that of a magnetic dipole placed in the center of the earth.The angle between the magnetic axis of the magnetic dipole and the earth’s axis is 11.5°. The extension line of the magnetic axis intersects the ground at two points, which are called the geomagnetic north pole (N pole, positive pole) and the geomagnetic south pole (S pole, negative pole). The rocks (magnetic minerals) in the earth’s crust are all magnetized by this geomagnetic field, and the direction of magnetization is consistent with that of the geomagnetic field at that time, so the characteristics of the geomagnetic field at that time are recorded. Nowadays, as long as we use rocks to measure the geomagnetic elements at that time, we can restore the geomagnetic characteristics at that time. Magnetic minerals in magmatic rocks, sedimentary rocks and metamorphic rocks can be magnetized, but all magnetic minerals show magnetism only when they are below the Curie point (generally 500-650 ℃), while they lose magnetism when they are above the Curie point. In the process of magma condensation and crystallization, when the temperature of magma decreases to Curie point, some magnetic minerals in the magma get magnetism and are magnetized due to the influence of the geomagnetic field at that time. When the magma completely condenses and crystallizes into magmatic rock, the arrangement direction of magnetized minerals is fixed, recording the characteristics of the geomagnetic field at that time. The sedimentary rocks record the geomagnetic field in the process of deposition. Some fine magnetic minerals can rotate freely in the water medium. When they are deposited at the bottom of the water medium, they are directionally arranged (magnetized) under the influence of the geomagnetic field at that time, and then they are fixed by the overlying pressure of the sediments, which records the characteristics of the geomagnetic field at that time.
The total magnetic field intensity (T) at any point on the earth is a vector, which can be divided into seven variables: magnetic declination (D), magnetic inclination (I), horizontal magnetic field intensity (H), East horizontal magnetic field intensity (Y), North horizontal magnetic field intensity (X) and vertical magnetic field intensity (Z). The other three variables can be obtained by knowing the three vectors of X, Y, Z or H, D, Y. By measuring the elements of natural residual magnetic field from the samples, the basic data of paleomagnetism are obtained.Geomagnetic elements (magnetic dip angle, magnetic declination angle) and magnetic pole position change with time. The change time of magnetic pole position is long but not significant. For example, in the past 20 million years (since Miocene), the magnetic pole position of volcanic remanence has always changed around the geographical pole. Since Quaternary, the magnetic pole position of China has been concentrated in the range of 80 ° to 90 ° north latitude and moved around the earth’s axis. The change period of geomagnetic polarity direction is 0.01 ~ 1MA, so the polarity change is more suitable for Quaternary sediment age measurement. It is a basic feature of paleomagnetic history that paleomagnetic polarity alternates in positive and negative directions. Positive polarity (positive magnetization) means that the polarity direction of rock remanence is consistent with that of modern earth, its magnetic dip angle is positive (northern hemisphere), and its magnetic declination is close to zero. Reverse polarity (or reversal of magnetic polarity) means that the polarity direction of rock remanence is opposite to that of the modern earth, its magnetic dip angle is negative, and its magnetic declination angle is close to 180 degrees. In the history of positive and negative changes of geomagnetic polarity of the earth, the time unit with a dominant polarity and a longer duration is generally about 1 MA, which is called polar time (epoch, period). The short period (10000 to 100000 years) of polarity reversal in polarity time is called polarity sub time (event). During the polarity time, there are some short-term events of polarity direction change, which reflects the relationship between the general trend of polarity change and the small change.
Since the age of the studied geological body is obtained by comparing the paleomagnetic method with the standard paleomagnetic polarity chronology, and the whole history of the earth has magnetic field, this method is not limited by time and can be used for the age study of the whole Quaternary period. Although the method is not limited by time, it is also restricted by the following conditions in practical application: ① The Quaternary strata studied should be sedimentary continuous without sedimentary discontinuity. If there is sedimentary discontinuity, the polarity event may be lost and the correlation result may be wrong;② The stratigraphic section should be relatively thick, and the section too thin is not suitable for paleomagnetic chronology; ③ The grain size of the sediment in the profile should be relatively fine, and clay, silty clay and clayey silt are the best. Gravel and sand are not conducive to the use of this method; ④ The Quaternary sedimentary strata are not affected by the later dike intrusion; ⑤ Paleomagnetic chronology usually needs the assistance of other dating methods.
Paleomagnetic sampling requirements: First, samples were taken from fresh stratigraphic sections. Sampling tools should not be magnetic, iron tools should not be used, usually copper tools or plastic products. Secondly, orientational samples must be taken. The occurrence of strata and the upward and northward directions must be marked on the sample box. If sampling is carried out in the core of the borehole, the upward direction shall be indicated and shall not be reversed. Third, take two samples from the same height in each sampling layer for testing and standby. Fourth, usually use 2cm × 2cm × 2cm plastic box sampling, also use cylindrical sampling box, depending on the test instrument. Fifthly, in the Quaternary loose layer sampling, first clear out a table, draw the north and East directions on the table, and then buckle the sample box on the layer (the straight line on the box is aligned with the north, and the small round hole is placed on the East Side) gently press to take out the sample. Sixth, the vertical spacing of sampling layers should not be more than 1m (or be relaxed as appropriate), and the samples should be continuously sampled at equal intervals in the vertical direction.
The magnetic inclination (I) and magnetic declination (d) were measured by magnetometer or superconducting magnetometer. Based on the first two measurements, especially the polarity column made from the magnetic dip angle, and then compared with the standard polarity chronology, the age of sediments can be inferred indirectly. If a few mammalian fossils or other chronological data are found on the section, the effect will be better. Paleomagnetic methods have been widely used in core research of loess, lacustrine sediments, continental shelf and plain boreholes.
In the application of paleomagnetic methods, it is difficult to compare the measured paleomagnetic polarity column with the standard paleomagnetic polarity chronology. Different scholars often have different results on the same paleomagnetic polarity column. Therefore, in the process of paleomagnetic pole correlation, the determination of the correlation point is the key. In the study of Quaternary paleomagnetism, there are two kinds of paleomagnetic poles: The first is that the end time of the paleomagnetic polarity column is the present, such as the sedimentary section in the ocean and the present lake. It is relatively simple to compare this kind of paleomagnetic pole. If the sediments of the tested section are continuous, the density of the samples is relatively high, and there is no loss of polarity events, then the present (the top of the polarity column) can be determined as the contrast point, which can be compared with the polarity events of the standard paleomagnetic polarity chronology in reverse time. The other situation is more complicated. The obtained paleomagnetic polarity column is incomplete. Only the paleomagnetic polarity column of a certain period does not continue to the present day, and the upper paleomagnetic record is missing, such as the paleomagnetic polarity column of early Pleistocene and Middle Pleistocene. The correlation of this kind of paleomagnetic pole is very complex. The key problem is to determine the correlation point according to fossils, dating data and the general characteristics of paleomagnetic pole. Then, it can be compared with the standard paleomagnetic polarity chronology upward (in the direction of new age) or downward (in the direction of old age).