A long-term increasing aridification and cooling trend at the Chinese Loess Plateau during the Pliocene

Abstract

The development of Asian aridification and cooling is a research hotspot nowadays. Loess-Red Clay sequence in the Chinese Loss Plateau provides an excellent terrestrial archive of such paleoclimatic evolution. This paper applied the magnetic “unmixing” technique to isolate the magnetic components of the Red Clay sequence for the first time. The Jiaxian section in the north edge of Chinese Loess Plateau is sensitive to paleoclimate changes because it is in the transitional zone of the monsoon and non-monsoon area. Systematic magnetic measurements and XRD analysis consistently indicate that the magnetic components of Red Clay are predominantly magnetite, maghemite and hematite. The strongly developed paleosols exhibit high content of soft magnetic minerals, higher Ms and Mrs, and lower Bc and Bcr. While the weakly developed paleosols and loess samples display the opposite features. With increasing alternating field, the contributions of soft components to SIRMx mT are cleaned gradually (SIRM is the saturation isothermal remanent magnetization; SIRMx mT represents the residual SIRM after alternating field demagnetization at x mT). The significant contributor to SIRM100 mT is hematite, an indication of the intensity of chemical weathering. Variations in the ratios of SIRM100 mT/SIRM, SIRM100 mT/SIRM60 mT, and SIRM100 mT/SIRM30 mT document a long-term up-section decreasing trend during the last 5–2.6 Ma, with similar trends to the long-term up-section increase in δ18O values of marine records. Decreased hematite content indicates weakened chemical weathering, which further implies an increasing aridification and cooling and a gradually weakened summer paleomonsoon mainly in the depositional area (Chinese Loess Plateau). In some loess layers, where hematite is mainly of aeolian origin, the mineral magnetic signals may indicate aridification and cooling in the Asian interior (source region). This long-term aridification and cooling trend is probably attributed to the comprehensive activity of ongoing global cooling, progressive uplift of the Tibetan Plateau and changes in land–sea distribution.