A library made of dust
The Luochuan Loess Sequence is a thick blanket of wind-blown silt, a geological archive documenting 2.6 million years of climate history. This particular section of the Chinese Loess Plateau, which covers 640,000 square kilometers in total, has a sediment profile reaching depths of 135 to 170 meters. The dust itself originates from deserts in central Asia, carried by wind and deposited here over millennia.
The sequence is visibly striped with alternating bands of yellowish loess and reddish-brown paleosols. The pale, unweathered loess layers correspond to cold, dry glacial periods—ice ages—when strong winter monsoons dominated. The darker, clay-rich paleosol layers are ancient soils that formed during warmer, wetter interglacial periods, when powerful summer monsoons allowed for vegetation and soil development. Together, these layers form a continuous terrestrial record of glacial-interglacial cycles that can be correlated with deep-sea oxygen isotope records. The Luochuan section contains at least 33 distinct paleosol layers, representing 33 major warm climate cycles.
Reading the layers
Scientists analyze the physical and chemical properties of the layers to reconstruct past climates with remarkable detail. One of the important proxies is magnetic susceptibility. The paleosol layers have high magnetic susceptibility because the warm, wet conditions of the summer monsoon favored the formation of strongly magnetic minerals through chemical weathering. In contrast, the dry, windy glacial periods resulted in less soil formation and thus lower magnetic susceptibility in the loess layers.
Grain size is another indicator; coarser grains in a loess layer suggest stronger winds during that particular cold period. The mineralogy of the dust itself also tells a story. The reddish color of the paleosols comes from the mineral hematite, which forms under wet summer monsoon conditions, while the yellowish loess contains more goethite, shows a drier climate. Embedded within these layers are tiny magnetic particles that aligned with Earth's magnetic field as the dust settled. This has allowed scientists to identify and date major geomagnetic reversals, such as the Brunhes-Matuyama reversal around 780,000 years ago, providing firm anchor points for the entire timeline.