Palaeomagnetism and diagenesis in sediments

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The cause o f such differences m a y reflect some additional distortion of the p a l a e o m a g n e t i c record in continental sediments, possibly relating to the irregularities in the post-depositional remagnetization of continental sediments because of the p e n e t r a t i o n o f atmospheric and g r o u n d water, whereas m o s t m a r i n e sediments have been deposited in m o r e consistent s u b a q u e o u s conditions.

Studies of post-depositional remanent magnetization and their relevance to the palaeomagnetic record. His experiments with artificial sediments showed that strong changes in the intensity and direction of remanence occurred during both wetting and drying processes. These were not attributable to magnetic viscosity, but could be explained by surface-tension forces changing the orientation of interstitial grains.

Such a process is likely to operate, not only under natural changes in the level of the water table, but also in samples extracted from sediments if their interstitial fluid or gases change. Although NoEl demonstrated that some features in studies of Scandinavian Quaternary varved sediments could be explained by such processes, it is very difficult to simulate natural compositions and processes in the laboratory correctly, and one of the main aims of this study is to examine the post-depositional remagnetization in a range of continental sediments drift deposits, varved clays and lake sediments during their watering and drying, and to estimate the magnitude of such changes.

The main questions were: 1 Does wetting and drying in the presence of magnetic field of different direction lead to similar changes of the NRM direction in natural continental sediments, as was shown for the artificial sediment by NoEl ? An example of thermal demagnetization of the PDRM. Samples The following sediments have been studied: 1 Upper Quaternary varved clays from the St Petersburg area, Russia; 2 Upper Quaternary drift deposits and lake sediments from the Voronezh area, Russia; 3 Middle Quaternary loess and drift deposits sediments from the 23 Taman peninsula, Black Sea, Russia; 4 Upper Quaternary loess from the Tashkent area, Uzbekistan.

Thermomagnetic analyses had shown they contained a variety of magnetic minerals from iron hydroxides to hematite. Four cubic 2. One of these was used to determine the maximum critical value of water that could be added without destroying it.


After that the N 1. Symbols as for Fig. Future work will also measure the permeability. Procedure and results Three specimens from 16 samples were placed within a steady field of the order of the local geomagnetic field about 0. Immediately after placing them in the coils, one of specimen from each sample was watered with the critical amount of water. Another was watered with one-half of this value, and the third specimen was not watered at all. These were then left for few weeks for drying out before remeasuring their N R M and susceptibility.

All specimens showed an ability to acquire a strong remanent magnetization as a result of secondary watering, but its value varied strongly for different sediments Table 1 , being smallest in the varved clays and some loess from the Tashkent area. Only in two specimens were the viscous components large Table 1 but even in these specimens, the P D R M was clearly recognizable. Consequently, the observed rotations in the remanence vector because of a mechanical rotation of magnetic particles in pores seem to be the explanation f or this remagnetization.

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As continental sediments are subjected to natural changes in moisture content on repeated, irregular time scales, repeating wetting and drying was attempted using the drift deposits from the Voronezh area. These showed Fig.

The next experiment simulated a major change in the ambient field, such as could occur during a geomagnetic reversal or event. In some cases, the record of 'excursion' was been erased completely Fig. An additional experiment was undertaken using 19 specimens from outcrops in one part of an Upper Quaternary section Kostyonki 14 in the Voronezh area. N Fig. Examples of repeated wetting of continental sediment samples. Filled and open symbols represent downward and upward directions, respectively.


Larger open circle shows the direction of the local field during experiment the spatial position of the stereogram is arbitrary. Examples of the emulation of the excursion of geomagnetic field. Stereoplots of remanence directions of drift sediments from the Voronezh area a initial; b after watering and drying in the presence of a reversed magnetic field.

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These samples were then watered, dried and remeasured Fig. The behaviour of the remanence varied strongly. In some, it did not change at all, whereas in others if had been reversed completely, with other samples showing 'intermediate' behaviour.

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The viscous component in these specimens did not exceed a few per cent of N R M , and no correlation was found between the post-depositional changes of remanence and porosity. Discussion and conclusions These experiments, using different types of continental sediments, confirm the experimental evidence of strong directional changes during wetting and drying obtained by NoEl It is therefore clear that such post-depositional processes can be a fundamental influence on the observed properties of many types of continental sediments, and that these are not readily 26 S.

The experiments were n o t designed to examine the m e c h a n i s m o f this type o f P D R M , but d e m o n s t r a t e d that it was of a mechanical r a t h e r t h a n chemical origin, and there are no reasons for not accepted the surface-tension effects p r o p o s e d by Nofil The oldest sediments studied here have an age of a b o u t ka, and these were still able to be remagnetized by changes in the fluid and gaseous e n v i r o n m e n t within their pore spaces.

This also means that similar such effects m a y occur in all sediments that are allowed to dry out after collection and before m e a s u r e m e n t. A n obvious r e q u i r e m e n t for watering to have any effect on the direction o f r e m a n e n c e is that the direction of the a m b i e n t field must have changed. If the directional change in the field is small, then the r e m a g n e t i z a t i o n effect is also small. Consequently, such changes are particularly significant if wetting takes place after there has been a m a j o r c h a n g e in the orientation of the geomagnetic field, such as during a largescale geomagnetic event, excursion or polarity transition.

It is possible that its record of such behaviour will be 'smeared' over time intervals o f a few h u n d r e d years if there are changes in the water table or i n a d e q u a t e care is taken to avoid drying out before m e a s u r e m e n t. The author is grateful to A. Khramov and V. Kochegura for detailed discussions and advice, and to M. Kovalevsky for measurements of porosity.

Models of inclination shallowing during sediment compaction. Journal of Geophysical Research, 95, COL, R. Overprinting of natural magnetic remanence in lake sediments by a subsequent high-intensity field. Post-depositional detrital remanent magnetization in a synthetic sediment.

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Sedimentology, 3, Field-dependent postdepositional grain realignment in the PDRM process: experimental evidence and implications. Physics of the Earth and Planetary Interiors, 85, Post-depositional realignment of magnetic grains and asymmetrical saw-tooth patterns of magnetization intensity. Earth and Planetary Science Letters, , The influence of freezing processes on the magnetization of deposits.

Geophysical Journal of the Royal Astronomical Society, 62, Postdepositional remanent magnetization of the continental deposits. Fizika Zemli, N12, in Russian. The confirmation of reality of Gothenburg excursion by results of research of varved clays of Leningrad area. About an excursion of geomagnetic field found during study of Holocene deposits in Pre-Tashkent area.

Kostyonki 21 Gmelinskaya stoyanka. Naukova Dumka, Kiev in Russian. Detail stratification, stratigraphic thickness and the age of the Karangat horizon in Black Sea area. However, viscous remanent magnetization VRM of anomalously high thermal stability is an alternative possibility. Normally, such demagnetization temperatures are required to remove only very ancient geological remanences.

The classical 'conglomerate test' of palaeomagnetism fails to discriminate between a VRM and a primary remanence.

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Any lithology sensitive to VRM will yield a negative result, suggesting that the remanence is secondary. However, tests of physical phenomena that caused multiple reorientations of clasts, at rotation rates comparable with the viscous remagnetization rate, provide a more suitable field test for VRM.

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The high thermal stability VRM of the bedrock must have been acquired in the last 0. Two new field tests detected the viscous nature of the high stability remanence. The first was a 'debris flow test', in which limestone blocks spin slowly with respect to the geomagnetic field. The blocks record variably tilted or helicoidal, multicomponent vectors. The second was a 'masonry test', in which samples of limestone from buildings of known age were exposed to the geomagnetic field for known periods, up to years in this study.

This shows that unusually stable VRM may be common in carbonates. Finally, the masonry test permits an empirical estimate of viscous remagnetization rates over hundreds of years. This established chronometric curves that date ground disturbances of geotechnical or geomorphological interest, and archaeological features of unknown age.

Theory of viscous remanent magnetization Viscous r e m a n e n t magnetization V R M is a time-dependent magnetization with acquisition Sa and decay coefficients Sa that predict, from short-term laboratory experiments, a log time dependence of remanence intensity. F o r magnetite, the coefficients have their largest values for grain sizes near 3 0 - 4 0 n m. As the acquisition and decay coefficients are not constant contrary to the assumption of Heller and M a r k e r t the acquisition rate or remagnetization rate must be calibrated using another dating m e t h o d , e.

Empirically determined acquisition rates reveal a power-law relationship for V R M acquisition over the last years in some English limestones bearing pseudosingle-domain PSD magnetite and will be discussed later. Viscous remanent magnetization of high thermal stability in limestone. Absolute temperature T is the most significant variable.

One also requires Boltzmann's constant k and a frequency factor C that may vary from s -1 Butler to s -1 Pullaiah et al. The choice of these parameters can affect the shape of the nomograms significantly, and the validity of published curves to one's own study should be justified before use. N6el's equation shows the relationship of VRM and thermal remanent magnetization TRM as end-members of a continuous spectrum of behaviour.

This principle led Pullaiah et al. The principle uses theoretically derived temperature-time nomograms that relate the long relaxation time of a VRM at surface temperatures e. Nomograms relating the demagnetization or unblocking temperature TuB necessary for a laboratory test. Laboratory test times, typically lasting less than l h, are those that will remove geological remanence acquired at a lower temperature over geological time. In some cases a much higher laboratory demagnetization temperature TLAB is required, and examples from the literature are illustrated.

Such samples are considered to have anomalously high thermal stability, the subject of this paper. Strictly speaking, the graphs are valid only for SD, non-interacting grains at low fields. Of course, my studies involved archaeological time scales and PSD magnetite.

Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments Palaeomagnetism and diagenesis in sediments
Palaeomagnetism and diagenesis in sediments

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