Paleomagnetism

Variations in the Earth's magnetic field, as recorded by magnetic particles in rocks and sediments, may be used as a means of stratigraphie correlation. Major reversals of the Earth's magnetic field are now well known and have been independently dated in many localities throughout the world. Consequently, the record of these reversals in sediments can be used as time markers or chronostratigraphic horizons. In effect, the reversal is used to date the material by correlation with reversals dated independently elsewhere. However, as all episodes of "normal polarity" have the same magnetic signal, and all episodes of "reversed polarity" are similarly indistinguishable from just the polarity signal, it is necessary to know approximately the age of the material under study to avoid miscorrelations.

In addition to aperiodic global-scale geomagnetic reversals, smaller amplitude, quasi-periodic variations of the Earth's magnetic field have also occurred. These secular variations were regional in scale (over distances of 1000-3000 km) and can be used to correlate well-dated "master chronologies" with undated records exhibiting similar paleomagnetic variations.

4.1.1 The Earth's Magnetic Field

The magnetic field of the Earth is generated by electric currents within the Earth's molten core. The exact mechanism of its formation is not agreed upon, but for our purposes it is sufficient to consider the field as if it were produced by a bar magnet at the center of the Earth, inclined at -11° to the axis of rotation (Fig. 4.1a). At the Earth's surface, we are familiar with this global field through magnetic compass variations. If a magnetized needle is allowed to swing freely, it will not only rotate laterally to point towards the magnetic pole, but also become inclined vertically, from the horizontal plane. The angle the needle makes with the horizontal is called the inclination (Fig. 4.1b). The inclination varies greatly, from near 0° at the Equator to 90° at the magnetic poles. If the needle is weighted, to maintain it in a horizontal plane, it will remain pointing towards magnetic north, and the angle it makes with true (geographical) north is called the declination (Fig. 4.1b).

The Earth's magnetic field is considered to be made up of two components — a primary and fairly stable component (the dipole field), which is represented by the bar magnet model, and a much smaller residual or secondary component (the non-dipole field), which is less stable and geographically more variable. Major changes in the Earth's magnetic field are the result of changes in the dipole field, but minor variations may be due to non-dipole factors (see Section 4.1.5).

ROTATION

ROTATION

Paleomagnetism Magnetic Dip

FIGURE 4.1 (a) The Earth's magnetic field.The main part of the Earth's magnetic field (the dipole field) can be thought of hypothetically as a bar magnet centered at the Earth's core.The lines of force represent, at any point, the direction in which a small magnetized needle tries to point.The concentration of these lines is a measure of the magnetic field strength, (b) Declination and inclination. Declination is a measure of the horizontal departure of the field from true north; inclination is a measure of dip from the horizontal.The resultant force is a vector representing declination, inclination, and field strength.

FIGURE 4.1 (a) The Earth's magnetic field.The main part of the Earth's magnetic field (the dipole field) can be thought of hypothetically as a bar magnet centered at the Earth's core.The lines of force represent, at any point, the direction in which a small magnetized needle tries to point.The concentration of these lines is a measure of the magnetic field strength, (b) Declination and inclination. Declination is a measure of the horizontal departure of the field from true north; inclination is a measure of dip from the horizontal.The resultant force is a vector representing declination, inclination, and field strength.

Because of the nature of the (dipole) field and the way in which it is generated, any change in its characteristics will affect all parts of the world. Records of significant magnetic field variations in a stratigraphic column (magnetostratigraphy) can thus be used directly to correlate sedimentary sequences in widely dispersed locations, regardless of whether they have common fossils or even similar facies. The broader significance of the magnetic characteristics of sediments, in a wide range of paleoen-vironmental applications, is well documented by Thompson and Oldfield (1986).

4.1.2 Magnetization of Rocks and Sediments

So far we have referred to paleomagnetic variations and their usefulness without considering how such variations are recorded. It has been known for over 50 years that molten lava will acquire a magnetization parallel to the Earth's magnetic field at the time of its cooling. This is known as thermoremanent magnetization (TRM). The same phenomenon has been observed in baked clays from archeological sites; iron oxides in the clay, when heated above a certain temperature (the Curie point) realign their magnetic fields to those at the time the clay was baked. In this way, archeological sites of different ages have preserved a unique record of geomagnetic field variations over the last several thousand years (Aitken, 1974; Tarling, 1975).

Igneous rocks are not the only recorders of paleomagnetic field information; lake and ocean sediments may also register variations through the acquisition of de-trital or depositional remanent magnetization (DRM). Magnetic particles become aligned in the direction of the ambient magnetic field as they settle through a water column. Providing that the sediment is not disturbed by currents, slumping, or bio-turbation, the magnetic particles will provide a record of the magnetic field of the Earth at the time of deposition. Verosub (1977) considers that the acquisition of magnetization by sediments may occur after deposition due to the mobility of magnetic carriers within fluid-filled voids in the sediment. Once the water content of the sediment drops below a critical level (depending on the sediment characteristics) the magnetic particles can no longer rotate and magnetization becomes "locked in" to the sediment. This post-depositional DRM provides a more accurate record of the ambient magnetic field than simple depositional DRM, but in some circumstances it may lead to distinct regional differences because some sediments became realigned post-depositionally while others, perhaps more densely packed, did not (Coe and Liddicoat, 1994).

Unlike thermoremanent magnetization, detrital remanent magnetization is not an "instantaneous" event. Once the molten lava has cooled, perhaps in a matter of minutes, the ambient field becomes a permanent fixed record. In sediments, the record is subject to disturbance (e.g., by burrowing organisms) that may raise the water content of the sediment enough for magnetic particles to rotate again, changing the magnetization until the sediment is sufficiently dewatered to fix the record once more. Thus, the sedimentary record of the Earth's magnetic field, although continuous, should be considered as a smoothed or average record, unlikely to record short-term variations except in unusual circumstances where sedimentation rates are sufficiently high. Furthermore, it has been demonstrated by Verosub

(1975) that sediment disturbance may result in apparent reversals that would be hard, if not impossible, to detect in cores of non-laminated sediments (Fig. 4.2). This may have contributed to erroneous reports of short-term variations of the Earth's magnetic field (excursions; see Section 4.1.5).

Finally, it is now also recognized that iron minerals in some sediments undergo post-depositional chemical changes that result in a magnetization characteristic of the Earth's magnetic field long after initial deposition. This is known as chemical remanent magnetization (CRM); identification of the minerals typically affected in a sample can provide a warning that errors may be expected.

FIGURE 4.2 Problems of paleomagnetic stratigraphy illustrated by a varved sedimentary record, (a) A folded varved sediment sequence; shaded layers represent winter (clay) sediments; and the unshaded layer the summer (silt) deposits, (b) Paleomagnetic record obtained on a hypothetical core through the fold shown in (a), intersecting points A, C, and D. Because of sediment deformation an apparent paleomagnetic excursion is recorded. In uniform, fine-grained sediments such deformation would probably not be visible, so that the presence of an excursion might be erroneously reported (Verosub, 1975).

4.1.3 The Paleomagnetic Timescale

Most of the early work on establishing a chronology or timescale of paleomagnetic events was carried out on lava flows. It was demonstrated that at times in the past the Earth's magnetic field has been the reverse of today's and that these periods of reversal (chrons) lasted hundreds of thousands of years. Potassium-argon dating methods enabled dates to be assigned to periods of "reversed" and "normal" fields so that eventually a complete chronology spanning several million years was constructed (Cox, 1969). Indeed the development of this chronology went hand-in-hand with the theory of plate tectonics because new lavas, produced at the centers of spreading (e.g., the Mid-Atlantic Ridge) were found to record identical paleomagnetic sequences on either side of the ridge (Opdyke and Channell, 1996). Careful study of lava flows and sea-floor paleomagnetic anomaly patterns has so far enabled a fairly accurate chronology of reversals to be constructed for the Cenozoic (Cande and Kent, 1992, 1995) and less certain chronologies have been constructed for even longer periods of time (Harland et al., 1990). Major periods of normal or reversed polarity are termed polarity chrons or epochs, the most recent of which are named after early workers in the field. Thus we are currently in the Brunhes chron of "normal" polarity, which began -780,000 yr B.P. Prior to that the Earth experienced a period of reversed polarity, the Matuyama chron, which began in late Pliocene times (Fig. 4.3).

In addition to major polarity epochs in which reversals persist for periods of -106 yrs or more, the igneous record has also shown that reversals have occurred more frequently, but less persistently, for periods known as polarity events (or subchrons). These are intervals of a single geomagnetic polarity generally lasting 104-105 yrs within a polarity chron. During the last 2 million yrs, several such events are thought to have occurred, all within the Matuyama reversed polarity chron. Thus the Jaramillo (0.99-1.07 Ma B.P.) and the Olduvai (1.77-1.95 Ma B.P.) subchrons are periods of normal polarity, named after the locality of the lava samples studied. The dating of these relatively brief events is subject to change as new analyses are carried out (particularly by more accurate 40Ar/39Ar dating of samples). Figure 4.3 gives the current status of the polarity timescale.

All of the preceding discussion has referred to studies of polarity changes observed in lavas, but the widest application of paleomagnetism to paleoclimatic studies has been in the identification of reversals in sedimentary deposits, notably in marine sediments and in loess deposits (Hilgen, 1991; Rutter et al., 1990). Studies of detri-tal remanent magnetization in ocean sediments may, in favorable circumstances, give a paleomagnetic record comparable even in detail with the terrestrial volcanic record (Opdyke, 1972). It is common in studies of undated ocean cores to plot the polarity sequence changes with depth and to assign an age of 0.78 Ma to the first major reversal (the Brunhes/Matuyama boundary). Younger ages are then derived by interpolation, assuming a zero age for the uppermost sediments and a constant sedimentation rate. This provides a first-order time-frame within which far more detailed radiometric or biostratigraphic checks and adjustments can be made (see Section 6.3.3). The Quaternary record of 8180 in marine sediments, and its relationship to

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