MAGNETIC-SUSCEPTIBILITY OF CHINESE LOESS AND ITS BEARING ON PALEOCLIMATE

Magnetic susceptibility measurements are reported from 468 samples spanning a 160 m-thick section of loess near the city of Baoji, Shaanxi Province. An independent absolute chronology is provided by geomagnetic polarity reversals, which indicate that the sediments represent the geological interval 0 to 2.5 million years ago. Fourier analysis of the susceptibility time series reveals strong spectral peaks at quasi-periods of 100,000 and 40,000 years in agreement with the astronomical theory of climatic forcing; the longer period is due to the eccentricity of the Earth's orbit, the shorter to the obliquity of the ecliptic. The expected peak near 20,000 years (due to precession) appears only weakly. However, we do find strong evidence for a 400,000 year quasi-periodicity, particularly prior to 1.32 million years ago; this is firmly predicted by astronomical calculations, but has rarely been recognized in geological data.