Intensity of the geomagnetic field in Japan for the past 10,000 years

The ancient geomagnetic field intensity was obtained by comparing the magnitude of natural remanent magnetization with that of thermoremanent magnetization induced in a known field. Throughout the study the step-wise heating method was used; measurements of both partial thermoremanent magnetization and the change in natural remanent magnetization were made for each temperature interval. The material studied was pottery from eastern Japan covering the period from 9300 to 2500 years ago. It was concluded that the geomagnetic field at 2000 B.P. was about 1.6 times as large as that at present. The field intensity showed the minimum of about half the present value at nearly 5500 B.P. Assuming a simple periodic change of the field intensity for the convenience of further calculations, the secular variation gives a sinusoidal curve as a first approximation. The secular variation of atmospheric C14 concentration calculated for the observed variation in field intensity is in good agreement with values obtained from tree-ring samples. TMs supports the suggestion that the variation of the geomagnetic field intensity plays an important role in the control of the concentration of C14 in the atmosphere.