The Earth's history consists of a continuing series of events which have occurred in the Earth and it's immediately surrounding space. Most of the evidence for these events has been lost to us, but traces of some of these events have been recorded in the Earth's crust. The most conspicuous of these includes traces of the past gravity field within which all phenomena of the Earth occur. Based on the principle of uniformitarianism, we can interpret layered rocks as “fossils” of past gravity fields. Another phenomenon recorded is the effects of the Earth's geomagnetic field. Geological evidence clearly indicates that the main magnetic field of the Earth has been that of a geocentric dipole, and, unlike the gravity field, it has changed in both direction and intensity frequently. These variations in the magnetic field are undoubtedly global in extent, and all past geologic processes have operated within the geomagnetic field. Geomagnetism cannot compare with the effects of gravity as a dominant process, but it is effective in protecting the Earth's surface from the bombardment of charged particles projected from the Sun and from outer space, and which have been considered to affect the atmosphere and hydrosphere, along with the organisms within them. The frequent changes in direction and intensity of the geomagnetic field thus provide many isochronous horizons throughout natural and artificial materials, and can also be considered an important cause for past climatic and biologic events. Paleomagnetism, then, is an important element in recording Quaternary history and is therefore of great importance in Quaternary research. The symposium, “Paleomagnetism and Quaternary Research, ” was held on the 6th of February, 1977, on the occasion of the annual meeting of the Japan Association for Quaternary Research at Satio Ho-on Kai Museum of Natural History, Sendai. This special issue of THE QUATERNARY RESEARCH contains the papers presented and discussed there. Taking into account the close connection between paleomagnetism and Quaternary research, as indicated by the compatible juxtaposition of the two terms in the title of the symposium, no particular order of discourse was followed in collecting the papers. However, a brief note on some aspects of paleomagnetism is included in the introduction for the sake of general understanding, considering that this special issue of the journal is one of interdisciplinary association.
Many geomagnetic polarity variations have been revealed by recent measurements of lacustrine and marine sediments which accumulated at high sedimentation rates during the Brunhes Normal Polarity Epoch. An attempt has been made to clarify the characteristics of individual geomagnetic reversal events occurring in the Quaternary by comparison of the virtual geomagnetic pole (VGP) path with those of Neogene geomagnetic transitions. The VGP paths during the excursions and reversal transitions show particular patterns according to geologic age, as follows: 1. VGPs of intermediate direction of magnetization were mostly located in the central Pacific region during the Early Miocene. 2. From the Middle Miocene to the Pliocene, VGPs migrated across the eastern Atlantic and African regions. 3. In the Early Pleistocene, VGP paths took their way across the Indian Ocean and the south Pacific. 4. At the Matuyama-Brunhes polarity transition, the poles were located in the western Pacific. 5. Although we do not yet have sufficient pole path data for the Blake and older excursions in the Brunhes Normal Polarity Epoch, available data indicate a younger oscillating variation pattern, and the pole paths are located in the equatorial region of the central Pacific as well as in the eastern Atlantic. Another conspicuous characteristic of the VGP paths recognized in Japan is that none have been located in the central and western Atlantic region throughout the Late Cenozoic.
This article outlines the present status of micropaleontology of Quaternary deep-sea sediments of the Pacific in relation to magnetostratigraphy. The occurrance of five groups of planktonic microfossils-foraminifera, calcareous nannofossils, radiolaria, silicoflagellates, and diatoms-has been compared with the geomagnetic polarity record of North Pacific deep-sea sediment cores. The relationship between the first and last appearances of characteristic fossils as well as changes in coiling direction of some planktonic foraminifera and the paleomagnetic reversals has been especially examined. Within the geographic limitations for extending the isochronous surfaces represented by the biologic events, regional sequences have been established. Magnetostratigraphy has allowed the correlation of the land-based Oga and Choshi sections with the oceanic sections. The composite scheme, which has 27 micropaleontologic events during the last 1.8m.y. in the North Pacific, will be useful in the interpretation of local magnetostratigraphic and biostratigraphie intervals. Fluctuation of surface temperatures due to Pleistocene glaciation and interglaciation may be a global phenomenon. Attempts to reconstruct paleoclimates from Pleistocene deep-sea sediments of the Pacific have been based upon various data sources-percent carbonate, diatom flora, foraminiferal fauna, oxygene isotopic ratios. In the Northern Hemisphere the paleontologic evidence suggests cooling commenced near the Jaramillo Event and that the Brunhes Normal Polarity Epoch is characterized by longer cold intervals than in the preceding epochs. Marked interglacial events occurred at about 500, 000 and 120, 000 years ago. The subarctic boundary of the Northern Pacific does not appear to have moved more than 6 degrees in latitude during the Quaternary.
This paper briefly reviews the history and present status of magnetostratigraphy, and re-examines the usefulness of magnetostratigraphy in absolute chronology. At present, two different kinds of time scale are used as standards for magnetostratigraphic age determination. The first is that time scale developed from a combination of paleomagnetic polarity data and radiometric age dates derived from volcanic rock sequences on land. The second is based on the interrelationships between the sequence of geomagnetic reversals and marine planktonic microfossils. Usually, these time scales are assumed to be equivalent. Based on the collection of radiometric dates made on magneto- and biostratigraphically-studied sedimentary sections, the relationships between the geomagnetic-radiometric and geomagnetic-microbiostratigraphic time scales were examined. A distinct discrepancy was noticed between the radiometric age and the estimated absolute age from the magnetic-microbiostratigraphic scale, as indicated in Fig. 5. This discrepancy is worthy of serious consideration by uses of the geomagnetic time scale in absolute chronology. Further examination of various methods of measurement is needed to increase the reliability of magnetostratigraphy, microbiostratigraphy and radiometric chronology. In Japanese marine sedimentary sections, there exist many pyroclastic intercalations that may provide suitable materials for such examinations.
Since 1951, many studies have been published on the magnetostratigraphy of the Plio-Pleistocene strata of the Kinki district. In the 1970's, especially, various sections of the Osaka and Kobiwako Groups, including bottom sediments of Lake Biwa, as well as their correlatives in the Tokai district, have been studied repeatedly. The sediments are mostly lacustrine and fluvial, and are frequently intercalated with volcanic ash. The Osaka Group also contains interbedded marine clay layers. The sediments, therefore, are suitable for establishing a combined litho-, tephro-, magneto- and biostratigraphy and radiometric chronology. Such a chronology can be used for testing the reliability of primary remanent magnetization of the sediments by the measurements of multiple samples from various localities and correlated by means of volcanic ash intercalations. Based on the latest studies, including unpublished results of studies by the present writer, the Plio-Pleistocene magnetostratigraphy of the Kinki district has been compiled (Fig. 2). Important points include: 1. The Gauss/Matuyama boundary is drawn between the Kosaji and Hazama Volcanic Ash in the Kobiwako Group, and between the T2 and Shimakumayama Volcanic Ash (2.3-2.4m.y.), in the Osaka Group. 2. The Matuyama/Brunhes boundary is about 10m above the Biotite Volcanic Ash (0.7m.y.) in the Kobiwako Group, and under the Ma4 clay layer in the Osaka Group. 3. Five normal polarity events have been recognized in the Matuyama Reversed Epoch. They are, in ascending order, as follows: a-. Shiba event (about 2.3m.y.) b-. Sushirodani event (about 2.2m.y.) c-. Naka-Nango event (1.7-1.8m.y.) d-. Pink-Komyoike event (1.1-1.2m.y.) e-. Kamikatsura event Some of the five events remain uncertain as to correlation with those reported previously in the Matuyama Epoch. 4. Five reversed polarity events or excursions were detected in the Brunhes Normal Epoch. The older three were named Biwa I, II and III events, and the younger two may be correlatives with the Lashamp and Blake events. The age of the Biwa III Event has been estimated to be about 0.33m.y..
Paleomagnetic study and potassium-argon dating of igneous rocks have led to an establishment of a geomagnetic polarity time scale for the past 4.5m.y., Late Cenozoic polarity changes have been recognized throughout the world in deep-sea sediments and outcropping sedimentary sequences. However it has been suggested that the establishment of reliable magnetostratigraphy is difficult in some cases because of post-depositional remagnetization resulting from chemical precipitation and the acquisition of VRM. Sometimes experimental techniques have not allowed distinction between the original geomagnetic record from the secondary magnetization. It is thought therefore, that the most convincing way to demonstrate the reliability of magnetostratigraphy is to reproduce the results in separate sections showing different lithology and sedimentation rates. In accordance with this concept, a field stability test of remanent magnetism of the pyroclastic sediments deposited in and around the Aizu Basin, Fukushima Prefecture, was conducted. The following conclusions can be drawn concerning the reliability of magnetostratigraphy: (1) The DRM of pyroclastic materials in the Nanaorizaka Formation is as stable as the TRM of welded tuffs considered to be their correlatives in the Seaburiyama and Shirakawa Formations. (2) Stability of the DRM is also supported by the folding test made on the key tuffaceous bed within the Nanaorizaka Formation. (3) The reliability of the magnetostratigraphy of the Fujitoge Formation was demonstrated by reconfirmation of the same reversed polarity magnetozone in four separate sections with differing sedimentation rates and lithologies.
Since the correlation between variations of the Earth's magnetic field and climatic changes first became an attractive subject for scientific study, much attention has been given to the investigation of deep-sea sediment cores. Recently, the trend has been toward investigating increasingly minute subdivisions within the cores studied. Viewed from a micropaleontological standpoint, however, the size of the unit subdivision selected for study within the sediment core is of gravest import. In order to grasp the time significance and paleoecological implications of such subdivisions, some problems related to biostratigraphy and deep-sea sedimentation especially with respect to sedimentary disconformities, rates of sedimentation, the mixing of sediments by artificial as well as natural causes, and dissolution are reviewed.
(1) Magnetization acquired after deposition is called “post-depositional detrital remanent magnetization” (PD-DRM). Natural remanent magnetization (NRM) of very fine-grained deposits is regarded as a kind of PD-DRM, since the loose unconsolidated upper part of fine-grained sediments-say several tens of centimeters-does not have the stable NRM, the direction of which usually does not change with depth. The thickness of this upper part generally coincides with the depth at which magnetic particles are fixed by friction due to compaction, and it should be noted, especially in case of stratigraphic correlation, that there is a time lag in the magnetic record in a sediment. This time lag can be obtained dividing thickness by depositional rate. Since the depth at which magnetic particles are fixed is a function of the internal friction due to compaction as well as the applied magnetic field, it is possible to use this depth as a convenient parameter to indicate the degree of compaction. (2) There are two contrary speculations concerning the correlation between climatic changes and geomagnetic intensity-variations. One maintains that an increase of the magneic moment corresponds to the surface coldness of the Earth. The other view insists that a decrease of the geomagnetic field is a cause of an ice-age. Both vieupointo are derived from paleomagnetic studies of fine-grained sediments; the former from investigations of deep-sea cores, and the latter mainly from study of a 200m-long core taken from Lake Biwa. Since the rate of sedimentation is usually a few hundred to thousands of times greater in lakes than in the ocean, there must be some difference in time scale in these presumptive arguments. Moreover, there is another important difference in the nature of their arguments. The former concept regards the intensity of magnetization of sediments as a linear function of that of the past geomagnetic field, but the latter does not. To normalized the intensity variation of magnetization due to the difference in the quantity of the magnetic substances in the sediments, the latter concept requires as an indicator of the intensity of the past geomagnetic field, that the value be obtained through dividing the intensity of magnetization by that of saturation remanent magnetization. It is pointed out that these differences in the two viewpoints lead to contrary conclusions.
The encounter between the solar wind and the Earth's paleomagnetic field formed the paleomagnetosphere around the Earth. From a paleomagnetospherological viewpoint, the entire Quaternary Period consists of stable stages (intervals when the geomagnetic field polarity was either normal or reversed) and transition stages (transition intervals between the stable stages). Observation of the virtual geomagnetic pole during equatorial crossings in all four recent transition stages suggests that the Earth was in a perpendicular condition for about 2, 000 years during each transitional stages, i.e. the virtual dipole axis was perpendicular to the Earth's rotational axis. This observational inference is verified by a theoretical analysis of three models related to the mechanism of geomagnetic polarity reversal. The magnetosphere in the perpendicular condition is generally called the Type 3 Magnetosphere, in contrast to the Type 1 Magnetosphere in a parallel condition. It can be mathematically demonstrated that both the size and the shape of the paleomagnetosphere show drastic changes only when it changes from Type 1 to Type 3 during transitions. Several possible relationships between drastic changes of the paleomagnetosphere and observed drastic changes in paleoclimateand the fossil record during the transitional stages in the Quaternary were examined. During the transition stages, auroral particles, solar cosmic rays, and galactic cosmic rays are found to precipitate within the equatorial polar caps whose centers are situated on the geographic equator. Cro-Magnon man, in the Ice Age, left scratched drawings resembling auroral patterns on the inner walls of many Lascaux caves. An investigation is being made to determine if such drawings are related to brilliant auroral curtains that could surely have been observed by them in one of the equatorial polar caps during the geomagnetic transitional stage of the Laschamp Event. Finally, it is proposed that missing sunspot phenomena, similar to those that actually occurred in the Seventeenth to Eighteenth centuries, could possibly have also occurred during the Quaternary, giving rise to changes in paleoclimate and the paleontological and archaeological records.
Some of the possible mechanisms for changes in the earth's surface temperature due to a depression of the geomagnetic field are described and commented upon. For example, as a result of the field depression, charged particles, such as galactic and solar cosmic rays, are expected to impinge upon air molecules in the stratosphere, even at lower latitudes. This may lead to an increase in NO concentration, and a subsequent decrease in ozone content in the atmosphere. Depletion of ozone would cause climatic changes to some extent and/or some biological effects. A brief discussion is also given of some effects of magnetospheric deformation.
A numerical computation of the secular variation of the production rate of atmospheric radiocarbon was carried using the secular variation of atmospheric radiocarbon concentration. In order to show the relation between atmospheric radiocarbon concentration and geomagnetism, the computed secular variation in radiocarbon production rate was converted to that of the geomagnetic moment, assuming a simple dependence of the production rate on the geomagnetic moment. The results of these computations of secular variation of the geomagnetic moment showed an essential discrepancy from the observed values in the period of de Vries's peak. This discrepancy shows that the simple dependence of the radiocarbon production rate on the geomagnetic moment does not hold in the computation of the secular variation of paleogeomagnetic moment from radiocarbon production rates. The results of this study indicate that the variation of the radiocarbon production rate depends not only on the variations in paleogeomagnetisms, but also upon the modulation of the magnetosphere by solar activities.