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Preliminary report on paleomagnetism of cave deposit.

1981; Society of Geomagnetism and Earth, Planetary and Space Sciences; Volume: 33; Issue: 5 Linguagem: Inglês

10.5636/jgg.33.325

2185-5765

Hiroo Inokuchi, Hayao Morinaga, Katsumi Yaskawa,

Geology and Paleoclimatology Research

Most enterable limestone caves ceases to grow larger because of the lowering of the water table.They are suffering decrease in size from the secondary deposition of calcium carbonate in the form of stalactites on the ceiling, draparies and curtains on the walls and stalagmite on the floor.The stalagmites growing broader, flatter cones on a cave floor are known as flowstone.The growing rate of these cave deposits are variable with the circumstancial condition in a range between 0.1mm/yr and 0.01mm/yr.Recently we found that some of these secondary deposits of lime are carrying weak but considerably stable remanent magnetization of the order of 10 to 10 emu/cc.Since these cave deposits are gradually growing year by year, the past geomagnetic field variation must be recorded continuously in the sequence of thin films of secondary lime from the inner side to the outer.The magnetic minerals have not been analysed yet because their quantities are too little and their size seems too small to separate them from calcite.The purpose of this short note is to show the result of paleomagnetic measurement of a piece of flowstone collected from a cave at Gujo-Hachiman, Gifu, Central Japan.The direction and the intensity of its natural remanent magnetization (NRM) seems to be varied continuously from the bottom to the top of the sample as shown in Fig. 1.The sample used in the paleomagnetic study was quarried out perpendicularly to flowstone layers in the shape of square prism (ca.3x3x18cm) for the purpose of the analysis of carbon isotope by N. Nakai and his colleagues of Nagoya University.Therefore, it was oriented neither azimuthally nor zenithally in situ.Considering the fact that usually the layers in flowstone are not horizontal but rather parallel to its basal surface, it is clear that the declination and inclination of NRM in Fig. 1 do not correspond directly to those of geomagnetic field in the period of the deposition of this sample.The period is determined by 14C method as 27,000 yr B. P, for the upper end and 32,000 yr B. P. for the lower end (Nakai, personal communication, 1979).Thus, both the declination and the inclination of magnetization of this sample should be required certain uniform corrections over the whole sequence of layers of the sample to infer the past geomagnetic field variation.In order to know whether the result actually reflects the past geomagnetic field variation, the following comparison was made.Figure 2 shows the inclination and intensity variation of NRM of sediments cored from the bottom of Lake Biwa (YASKAWA et al., 1973).The age of this sediments was given by 14C for the upper 10 m and for the rest by fission track method using zircons separated from several tuff layers lying in the sediments.The results of the present study are also plotted