Geomagnetic Reversal and Change in Foraminiferal Fauna and Sedimentary Environments

Abstract

The purpose of the present study is to find the relationship of a geomagnetic field reversal to the changes in a foraminiferal fauna and the sedimentary environments. In the first part of this paper, quantitative estimations were made of the possible effects of direct and/or indirect radiations on the earth's surface caused by cosmic rays, solar flares and solar wind during the geomagnetic field reversal. The result of estimation suggested that the increased radiation during the geomagnetic field reversal could not be a factor significant to cause the appearance or extinction of marine species.A fossiliferous marine sedimentary section of the middle part of the Kokumoto Formation continuously exposed at Kamiyanagawa in the Boso Peninsula was chosen for the investigation of the behavior of the geomagnetic field during its reversal between the Matuyama and Brunhes Polarity Epochs which took place around 0.69 m.y. ago. The behavior of the geomagnetic field was established in detail through the measurement of the inclination, declination and intensity of the DRM and IsRM of the sedimentary rocks in the middle part of the Kokumoto Formation. Along the same section, analyses were made on the grain-size of the sediments, oxygen isotope ratio in the planktonic and benthonic foraminiferal tests, and the faunal assemblages of the planktonic and benthonic foraminifers.The intensity of the magnetic field fluctuated with a period of approximately 7000 years and did not disappear during the reversal. The migration of the virtual magnetic north pole occurred at the magnetic field reversal between the Matuyama and Brunhes Polarity Epochs roughly along the meridian of 120°E Long. in 4700 years. In the same duration, the depth of the water, under which the sediments were accummulated, fluctuated within the range of 200-300m with a period of approximately 7000 years. No particular relation between the magnetic field reversal and fluctuation of the water depth was recognized. The result of paleotemperature and faunal analyses of the foraminifers showed that the magnetic field reversal at the Matuyama-Brunhes Polarity Epoch boundary may have been related to the changes in the circulation pattern of the upper water-masses which caused more influx of cold water into this area in the northwestern Pacific as compared with the preceding period.