An integrated study of the grain-size-dependent magnetic mineralogy of the Chinese loess/paleosol and its environmental significance

[1] To investigate the grain- size- dependent properties of the magnetic minerals in Chinese loess/ paleosol samples ( Touxiangdao, Xining, Qinghai province, China), magnetic extracts were divided into two size fractions by gravitational settling. On the basis of hysteresis measurements, thermal demagnetization of low- temperature saturation isothermal remanent magnetization, and nonmagnetic studies ( SEM and XRD) we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples. The results show that the oxidation degree ( nonstoichiometry) of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered. In ascending order of the oxidation degree of our samples, we find ( 1) loess- coarse particles ( LC) are multidomain ( MD) magnetite with slight oxidation, ( 2) paleosol- coarse ( PC) particles are also MD magnetite but with a higher oxidation degree compared to LC, ( 3) loess- fine ( LF) particles are pseudo- single domain ( PSD) magnetite with a high oxidation degree, and ( 4) paleosol- fine ( PF) particles are PSD maghemite. Single domain ( SD) and superparamagnetic ( SP) maghemite mainly stay in the residues. Further thermomagnetic analysis of PF ( PSD maghemite) revealed that this natural maghemite has a Curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50% after a 700 degrees C heating/ cooling cycle. These new results identify the sources of multicomponent NRM in Chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components.