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Geomagnetic secular variation recorded in the stable magnetic remanence of the young sediments collected from Osaka Bay.

1980; Society of Geomagnetism and Earth, Planetary and Space Sciences; Volume: 32; Issue: 9 Linguagem: Inglês

10.5636/jgg.32.581

2185-5765

Masayuki Hyodo, Katsumi Yaskawa,

Magnetic and Electromagnetic Effects

Recently we took a core of sediments from Osaka Bay, the diameter and the length of which are 20 cm and 700 cm, respectively.The diameter of 20cm is enough to make several specimens at any given depth level.We took out 4 to 5 specimens from every 3 cm depth level, so as to average out the error due to sampling etc.The remanent magnetization of argillaceous sediments is known to be weak but very stable and to have kept the exact orientation of the ambient magnetic field at the time of deposition.Their magnitude of intensity of remanent magnetization is generally of the order of 10-6 emu/cc for the argillaceous sediments in the inlandsea and lake bottom, where the sedimentation rate is considerably high, and 10-4 emu/cc for those in the ocean bottom.The sedimentation rate of the latter is almost less than a few centimeters in a thousand years and it means that we can hardly obtain any information concerning the geomagnetic secular variation of the period of several thousand years out of the deep ocean-bottom sediments.The young argillaceous sediments in the lakes or in the inland-sea have retained such informations and are most suitable for the investigation of geomagnetic secular variation on account of their high sedimentation rate.It is necessary, however, to make considerably big specimens to carry out measurements exact enough to deduce the past geomagnetic field, say at least 10cc, because the sensitivity of a usual magnetometer is equal or less than 10-6 emu and the intensity of magnetization of those sediments is almost always less than 10-6emu/cc.On the other hand, the diameter of core sampler is mostly between 5 and 8cm and, because of the difficulty of coring operation under the same condition, we are compelled to carry out the analyses in various fields with an extremely restricted amount of sample at each depth level of a core.Therefore, it is almost impossible to get several specimens for magnetic measurement from every depth level from such a thin core sample.To determine the orientation and the intensity of the past geomagnetic field through the palaeomagnetic investigation of a certain volcanic rock, we usually prepare several differently oriented specimens, more than 4, to measure their