Redai dili (Jun 2023)
Xiashu Loess Deposits in the Lower Reaches of the Yangtze River and Their Paleoclimatic Significance
Abstract
Quaternary Xiashu loess deposits are widely distributed in the middle and lower reaches of the Yangtze River. The Xiashu loess deposits provide an important archive for studying the area's paleoclimate. We analyzed the magnetic susceptibility of 239 bulk samples from two newly excavated Xiashu loess profiles: the Puwulu and Pancheng. Based on a comparison with previously studied profiles in this region, our new record shows a good correlation in the stratigraphy, with three well-developed paleosols (layers S1, S2, and S3) as the marked layers. This correlation was observed in the Taishanxincun, Laohushan, and Dagang profiles. Despite the good spatial correlation in stratigraphy, the well-developed paleosol layers were thicker than the underlying loess layers. The average sediment accumulation rates in the paleosol layers (6.2 cm/ka, n=19) were higher than those in the loess layers (5.1 cm/ka, n=17). This stratigraphic feature of the Xiashu loess deposits differs from that of the Chinese loess plateau in North China. Combined with previous studies on the origin of the Xiashu loess deposits, the new records suggest a proximal origin; however, the dominant contribution of sediment materials from the floodplains at the foot of Laoshan Mountain. Increased material flux through the primary stream runoff on the floodplain resulted in higher sediment accumulation rates in the paleosols. Based on sedimentary model analysis, the paleosol and loess layers in the Xiashu deposits are continuous sedimentary records on glacial-interglacial scales. After adjusting for fluctuating sediment deposition rates, the revised magnetic susceptibility displayed a clear cyclic variation pattern, which was interpreted as the intensity of pedogenesis. Power spectral analysis suggests that the cyclic pattern of variations shows a strong frequency in the 100 kyr band for both the Pancheng and Puwulu profiles. This may explain the paleoclimatic dynamics of the paleosols that formed. The most developed paleosol layers (S1, S2, S3, S4, and S5) correlated well with the orbital parameter eccentricity with clear 100-kyr ice cycles. Ice age terminations suggest sea-level changes and a far-reaching influence on precipitation in the middle and lower reaches of the Yangtze River. These observations indicate the relationship between the glacial cycle and soil development is closer than previously believed. The coincidence with eccentricity may largely support the idea that changes in global ice volume have promoted the formation of strongly developed paleosols along the incised valleys of the Yangtze River.
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