Magnetic properties of the Tertiary red clay from Gansu Province, China and its paleoclimatic significance

The study on magnetic properties of the red clay indicates that the red cla y and loess-paleosol sequence have a common magnetic mineralogy, with magne tite, maghemite, hematite (and possibly goethite) contributing to the magne tic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests th at, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic compone nt produced during pedogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. Th is is supported by the correlation between Rb/Sr ratio and magnetic suscept ibility. This signifies that, like the above loess-paleosol sequence, the m agnetic susceptibility of the red clay can be used as a general proxy paleo climatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Mag netic susceptibility variation in the red clay thus also provides an eolian /pedogenic record of paleoclimatic evolution. Study of the background susce ptibility indicates that, on average, the absolute scale of the paleoclimat ic shift from red clay development to Quaternary loess deposition is simila r to the climatic shift from stage 5 (S1) to stage 2-4 (L1). This may sugge st that during the Quaternary there is an evident strengthening of the abso lute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loe ss Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important environmental chan ge from the underlying Cretaceous red sandstone. The red clay development w as closely related to global drying and climate cooling since the Cretaceou s and closely associated with the abrupt uplift of the Qinghai-Xizang Plate au at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition.