Sedimentary response to glacial‐interglacial cycles in an alluvial fan at the marginal East Asian monsoon zone, northern China

ABSTRACT The rapidly accumulating alluvial fans are important archives of tectonic and climatic processes. However, deciphering how alluvial fans respond to orbital‐scale climate fluctuations remains unclear. While some alluvial fans exhibit a distinct response to glacial‐interglacial cycles, others do not. This detailed outcrop study of the late Quaternary Bantanzi alluvial fan in the Daihai Lake Basin, northern China, reveals two stratigraphic intervals with distinct facies, where a lower stratigraphic succession is dominated by debris flow deposits and palaeosols and a higher stratigraphic unit by flash‐flood deposits and loess. Physical correlations to well‐dated successions on the Chinese Loess Plateau provide a chronological framework. The palaeosols and the elevated values in magnetic susceptibility of the debris flow‐dominated interval, dated 88.1 to 73.4 ka (the last interglacial period MIS5, 130 to 70 ka), indicate a climate with intense seasonal monsoon rainfall, which facilitated chemical weathering and saturation of clay‐rich sediments on hillslopes triggering debris flows. The wind‐blown loess deposits together with regional data in the flash flood‐dominated interval, dated 58.5 to 22.1 ka (the last glacial period MIS 2 to 4, 70 to 14 ka), indicate an arid climate with highly intermittent and short‐duration rainfall. Such rainfall conditions tend to promote rapid run‐off and generate flash floods, as well as hinder hillslope saturation and chemical weathering resulting in a regolith with higher permeability and erodibility, hindering high pore pressures from building up and promoting erosion by run‐off. These distinct climatic conditions between interglacial and glacial periods were controlled by the latitudinal shift of the margin of the East Asian Monsoon. The Bantanzi fan's position at this climatic boundary, coupled with its small size, made it highly sensitive to glacial‐interglacial cycles. Comparison with other fan systems suggests that the position of alluvial fans at distinct climate boundaries, rather than within the climate zones, is an important control on their sensitivity to climate changes and their potential as climate‐change archives.