Deep-Circulating Groundwater From the Tibetan Plateau Constructed the Loess Plateau

China's Loess Plateau (LP) is a quintessential example of a desert transitional environment characterized by a distinctive soil profile comprising of a red clay layer overlain by loess. The source of the sedimentary materials that formed the plateau, as well as the mechanism for the red clay/loess transition, are yet unexplained. This study attributes this transition to changes in groundwater temperature. Isotopic analyses, including delta O-18-delta H-2, H-3 in water bodies, and Pb isotopes in soil, reveal that the LP is predominantly recharged by deep-circulating groundwater from the Tibetan Plateau. During soil formation, groundwater traversing the lithosphere reached supercritical temperatures, extracting and transporting mineral elements that precipitated to form features characteristic of the red clay layer, such as dolomite and iron-manganese deposits. This process persisted until approximately 2.6 Ma, when groundwater temperatures dropped below supercritical levels, initiating the accumulation of loess.