MAGNETIC STRATIGRAPHY OF CHINESE LOESS AS A RECORD OF NATURAL FIRES

Magnetic susceptibility records of paleosols and loess show high correlation with oxygen-isotope stratigraphy of ocean sediments [Kukla, 1987], providing a global paleoclimatic record. Different models have been put forth to explain the nature and cause of susceptibility variations, but consensus has not yet been achieved. Our low-temperature studies reveal a secondary magnetite component in paleosols that is characterized by a higher Verwey transition(115K) than that for the magnetite (100 K) in unaltered loess. The same shift in the Verwey transition can be achieved by heating and cooling loess samples. This is consistent with a new hypothesis that the magnetic signal from paleosols may be produced by natural fires in the past. Natural fire intensity is sensitive to the amount of annual precipitation, so that increased fire-induced susceptibilities should reflect an increase in the humidity of regional climate.