A centennial-resolution terrestrial climatostratigraphy and Matuyama–Brunhes transition record from a loess sequence in China
Masayuki HyodoKenta BanjoTianshui YangShigehiro KatohMeinan ShiYuki YasudaJunichi FukudaMasako MikiBalázs Bradák
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Abstract Terrestrial records of the last geomagnetic reversal often have few age constraints. Chronostratigraphy using suborbital-scale paleoceanic events during marine isotope stage 19 may contribute to solving this problem. We applied the method to an 8 m long, high-resolution paleomagnetic record from a loess sequence in China and revealed millennial-to-sub-centennial scale features of the Matuyama–Brunhes (MB) transition. All samples were subjected to progressive thermal demagnetization with 14–15 steps up to 650–680 °C. As a result, 96% of the samples yielded a high-quality remanent magnetization. The MB transition terminated with a 75 cm thick zone with nine polarity flips. The polarity flip zone, dated at about 779–777 ka, began between the warm events “I” and “J” and terminated at the end of the cooling event coincident with the lowest axial-dipole strength interval. Most polarity flips occurred within 70 years. The virtual geomagnetic poles (VGPs) in the upper polarity flip zone clustered in the SW Pacific region, where the MB transitional VGPs from lavas of the Hawaiian and Canary Islands and lacustrine deposits of Java also clustered. These sites were probably dominated by dipolar fields. The absence of transitional fields across polarity flips implies a short time span for averaging fields due to a thin loess-magnetization lock-in zone. The reverse-to-normal polarity reversal dated at about 778 ka in Lingtai occurred at the end of the SW Pacific VGP zone, an important key bed for MB transition stratigraphy. The reversal is a good candidate for the main MB boundary. We found an excursion at about 766 ka spanning about 1 ka.Keywords:
Magnetostratigraphy
Geomagnetic reversal
Polarity (international relations)
Polarity reversal
Natural remanent magnetization
Magnetostratigraphy
Polarity (international relations)
Natural remanent magnetization
Geomagnetic reversal
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A paleomagnetic investigation was carried out to analyze magnetostratigraphic information and to evaluate the relationship between paleoenvironment and magnetic properties in sedimentary sequences of piston cores recovered from the abyssal basin of the southwestern Pacific. Pateomagnetic results revealed that the sediments had a stable remanent magnetization and recorded both normal and reversal polarities. The age of sediments was from late Pliocene and Pleistocene determined by matching the polarities with the geomagnetic time scale. The sedimentation rates were in the range of 0.63-1.85 mm/ year which were extremely low rates. The results of the paleomagnetic analyses indicated that intervals of the magnetically stable layers as well as high value of susceptibility were significantly affected by the input changes which resulted input of large-quantity materials of relatively stable magnetic carriers.
Magnetostratigraphy
Abyssal zone
Geomagnetic reversal
Natural remanent magnetization
Sedimentation
Abyssal plain
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This study is an attempt to isolate and identify the primary magnetization in red cherts. Red chert samples from a middle Triassic sequence in the Inuyama area (35.4°N, 137.0°E), central Japan have a multicomponent magnetization, which is delineated by thermal demagnetization. Two of the components have high coercivity and high blocking temperatures; one is carried by hematite and the other by magnetite. The former was concluded to be the primary magnetization for two reasons: (1) it predates intraformational folding, and (2) it records geomagnetic reversals. The other component, which accounts for the dominant part of natural remanent magnetization, was probably acquired at a time long after the deposition. Two paleomagnetic results were drawn from the Middle Triassic red chert sequence: (1) The mean inclination of the samples in the sequence was 1.4° ± 6.8°; therefore the paleolatitude amounts to 0.7° ± 3.4°, in contrast to the paleolatitude expected from the paleopole of Eurasian continent, which is about 70°, and (2) the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the Middle Triassic are considerably longer (more than several million years) than the average of those in the Cenozoic.
Magnetostratigraphy
Natural remanent magnetization
Geomagnetic reversal
Sequence (biology)
Red beds
Polar wander
Geomagnetic pole
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ABSTRACT The history of geomagnetic polarity reversals in the Cenozoic and Late Mesozoic is well known since the Late Jurassic (Oxfordian). A continuous record of polarity has been derived for this time interval from the interpretation of oceanic magnetic anomalies. Most of the polarity chrons in this oceanic record have been verified and dated in coordinated magnetostratigraphic and biostratigraphic studies. This has led to the generation of progressively refined and improved geomagnetic reversal time‐scales that provide a framework for absolute dating of palaeontological zonations. By serving as a basis for statistical analysis of reversal frequency they provide information relevant to processes in the Earth's core. The rate of reversals since the Late Cretaceous shows a steady increase on which a cyclical variation appears to be superposed. A stochastic model for reversals predicts a Poisson distribution of polarity interval lengths. The polarity time scales contain many fewer short (± 50 kyr) polarity chrons than a Poisson distribution, and it has been suggested that a gamma renewal process with index greater than unity is a more appropriate statistical model. The statistical arguments give no convincing reason for abandoning the model and other, physical reasons must be sought to explain the incompleteness of the reversal record. The discovery and verification of short chrons in the oceanic record may best be investigated by deep‐tow magnetometer surveys. The reversal history before the Late Jurassic is not well known. Magnetostratigraphy in coeval Early Jurassic sections has not given correlatable records and it has not been possible to compile a definitive polarity sequence. Evaluation of geomagnetic polarity history for the Early Mesozoic and the Palaeozoic will require unambiguous magnetostratigraphy in well‐dated sections where verification of the polarity pattern is possible at the fossil zone or stage level.
Geomagnetic reversal
Magnetostratigraphy
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Polarity reversal
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Magnetostratigraphy is the element of stratigraphy that deals with the magnetic characteristics of rock units, obtained by measuring the direction and intensity of magnetism in rocks of different ages. The magnetic characteristic most often used, however, is the polarity of magnetic remanence. The polarity is said to be normal (north-seeking magnetization gives a northern hemisphere pole, as today) or reverse (north-seeking magnetization gives a southern hemisphere pole). The polarity is established from isolating of the primary remanent magnetism of the sample, using paleomagnetic investigation. Magnetostratigraphy defines the sequence of geomagnetic polarity reversals recorded during deposition of a geological formation. This approach can be a very valuable technique for the subsurface correlation of marine and continental sequences and is the only correlation technique which is independent of any facies control. It provides an important complement to biostratigraphy for core correlating and dating. In general the rock magnetic stratigraphy involves the utilization of magnetic properties of sediments and sedimentary rocks as a means of (1) stratigraphic correlation, (2) identification of sediment sources and transport mechanism, (3) characterization/detection of paleoenvironmental change.
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Polarity (international relations)
Natural remanent magnetization
Rock magnetism
Sequence Stratigraphy
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Paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal, as one would expect for an axial geocentric dipole model. Specifically, in magnetostratigraphic studies of Tertiary sediments, which include large data sets of paleomagnetic directions of both polarities, it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity. Such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions. We investigate the conventional explanation for such asymmetry, which involves the presence of a persistent, partially unremoved present day field magnetization in the studied rocks. Both plate motion history and inclination shallowing, which we evaluate in detail, can play an important role in the paleomagnetic record. Our analysis shows that alternatively, it is plausible that the inclination anomaly is due to a contribution of an axial octupole, in accordance with observations of the recent (0–5 Ma) geomagnetic field recorded in lava flows.
Magnetostratigraphy
Polarity (international relations)
Geomagnetic pole
Antipodal point
Geomagnetic reversal
Polar wander
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Magnetostratigraphy
Sequence (biology)
Geomagnetic reversal
Longitude
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We present a paleomagnetic record of the lower Cochiti polarity transition and new results from the lower and upper Nunivak transitions obtained from the Lower Pliocene Suva Marl ( Fiji, (18°S, 178°E)). This formation has yielded a well‐defined magnetostratigraphy that, combined with 40 Ar/ 39 Ar ages from interbedded tuff beds, provides excellent chronostratigraphic control [ Clement et al, 1997]. We previously presented the upper Cochiti and the upper and lower Nunivak polarity transition records from this unit [ Clement et al., 1995, 1996]. New results were obtained from oriented block samples spanning these reversals. Each of these reversals was sampled at duplicate sections separated by 4 to 8 km. The agreement between the records obtained from different sections suggests that the paleomagnetic records are not artifacts of local sedimentary disturbances or random degrees of incompletely removed overprints. Instead the similarities in these and other records of these same reversals from different types of paleomagnetic recorders suggest that they provide information about the behavior of the geomagnetic field as it reverses.
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Marl
Polarity (international relations)
Geomagnetic reversal
Geomagnetic pole
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Magnetostratigraphy
Rock magnetism
Natural remanent magnetization
Geomagnetic reversal
Environmental Magnetism
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Paleomagnetic measurements were performed on whole cores and discrete samples from approximately 4 km of sediment recovered during Leg 145 of the Ocean Drilling Program.The obtained magnetic polarity sequences enabled the establishment of a chronostratigraphic framework from the middle Miocene (about 18.3 Ma) to the present.The quality of the magnetic record is related directly to the sedimentology: clay-rich sediments deposited since 2.6 Ma and before 5.4 Ma preserve a relatively strong and stable magnetic signal, whereas diatom-rich sediments deposited between 5.4 and 2.6 Ma generally contain a much weaker and less stable paleomagnetic signal.Despite these problems, Sites 884 and 887 have provided some of the longest continuous polarity sequences ever obtained in the history of ocean drilling, with continuous records obtained back to 12.6 Ma and 18.3 Ma, respectively.Sediments at Site 881 are stably magnetized through the upper part of the thick diatom-rich interval and a continuous magnetic polarity record was obtained to the 4.03 Ma (C3n.In; Cochiti Subchron).However, extended core barrel drilling was employed below the Cochiti Subchron, at which point the paleomagnetic data substantially diminished in quality.Rotary drilling with the extended core barrel, when employed in Leg 145, seriously compromised the fidelity of the paleomagnetic record.Sites 882 and 883 provided continuous records to just below 2.6 Ma (i.e., the Gauss/Matuyama boundary).Low remanence intensities and unstable paleomagnetic behavior prevented retrieval of a reliable magnetostratigraphy in the diatomrich interval below 2.6 Ma at these two sites.The magnetostratigraphy from the more stably magnetized sediments of Site 884 can be transferred to the diatom-rich intervals of Sites 882 and 883, however, by correlation of susceptibility records.This permits identification of the Gilbert/Gauss boundary and the normal subchrons within the Gilbert Chron, thus extending the records from Sites 882 and 883 to beyond 5 Ma.The chronological constraints provided by the magnetostratigraphy are important for studying the nature of several important events that are observed throughout the North Pacific Ocean.The diatom ooze that begins near 5.4 Ma and extends to about 2.6 Ma defines a time of extreme silica deposition in the North Pacific Ocean.This episode was observed at all Leg 145 sites and represents a short period of extreme productivity at high latitudes that is associated with an early Pliocene period of climatic amelioration.The interval beginning near the Gauss/Matuyama boundary at 2.6 Ma is marked by high fluxes of diatoms, clay, volcanic ash, ice-rafted detritus, and quartz.The consistent appearance of abundant dropstones and other terrigenous material at the Leg 145 sites near 2.6 Ma heralds the onset of major glaciation in the Northern Hemisphere.This suggests that the onset of glaciation in the source regions for this lithogenic material, Siberia and Kamchatka, was synchronous with the onset of Laurentide and Fennoscandian glaciation.Increased fluxes of terrigenous clays near 2.6 Ma are also recorded at all of the Leg 145 sites and may be associated with the resurgence of bottom-current activity.An increase, by more than an order of magnitude, of volcanogenic material near 2.6 Ma in the North Pacific Ocean has also been known for many years, however, the Leg 145 sites provide much better-dated sequences for determining the timing of the onset of the northwest Pacific portion of this oceanwide volcanic episode.The long, continuous records obtained at Sites 884 and 887 permit the first direct correlation of the North Pacific Ocean diatom and other microfossil zonations to the magnetic reversal time scale in sediments older than latest Miocene.
Magnetostratigraphy
Geomagnetic reversal
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