No abstract

Quaternary records of climate change from terrestrial sources, such as lake sediments and aeolian sediments, in general agree well with marine records. But continuous records that cover more than the past one million years were essentially unavailable until recently, when the high-sedimentation-rate site of Lake Baikal was exploited. Because of its location in the middle latitudes, Lake Baikal is highly sensitive to insolation changes and the entire lake remained uncovered by ice sheets throughout the Pleistocene epoch, making it a valuable archive for past climate. Here we examine long sediment cores from Lake Baikal that cover the past 12 million years. Our record reveals a gradual cooling of the Asian continental interior, with some fluctuations. Spectral analyses reveal periods of about 400 kyr, 600 kyr and 1,000 kyr, which may correspond to Milankovitch periods (reflecting orbital cycles). Our results indicate that changes in insolation were closely related to long-term environmental variations in the deep continental interior, over the past 12 million years.

Theoretical investigations on models of the developmental processes of erosional features show that the time variations in erosional force have more sensitive influence on the time fluctuation of eroded material, such as sediments in lakes, than that of the quantity representing erosional features, such as volume of mountains. Time sequences of grain-size distributions from the Pleistocene sediments in Lake Biwa, central Japan indicate dominant periodicities of about 40,000 and 20,000 yr, very close to those predicted from the Milankovitch theory. The time variations of the grain-size parameters which may be indicators of erosional forces are very similar to that of the caloric summer insolation in the Northern Hemisphere.

The relationship between magnetization and principal stress under a triaxial stress field is deduced theoretically from an equilibrium condition between magnetoelastic and magnetization energies calculated on polycrystalline iron. The relationship is expressed by a formula in which the intensity of magnetization parallel to the principal axis of the stress tensor decreases in proportion to the difference between the largest principal stress and the magnetizing-directional principal stress. A biaxial stress is developed in a cruciform testpiece made of mild steel, and then magnetizations in the direction of principal stress parallel and normal to the surface of the testpiece are measured using U- and C-shaped magnetic sensors, respectively. The measured results are revised considering the spread of magnetic flux. The experimental results fundamentally agree with theoretical predictions. The quantitative discrepancies are discussed.

A secular variation record of the geomagnetic field direction for the last 6.5 kyr has been obtained from the magnetization of sediment cores from Erhai Lake, southwest China. In order to make a comparison with this record, secular variation in eastcentral China was investigated by combining available magnetic field data from historical records and archaeomagnetic measurements since about 350 . The secular variation in Erhai Lake shows features consistent with the combined record, except for the oldest three observed declination swings in Sian from 720 to 900 . Many features of declination and inclination in China also occur in Japan. From 500 to 1000 , declination was westerly ranging from about −20°to −5°in Erhai Lake, east-central China, and Japan.