黄土高原北部丘陵沟壑区近160年土壤侵蚀量演变及其对ENSO事件的响应

对形成于1851~1861年的靖边（JB）聚湫内22.75 m沉积序列进行完整钻探,并基于XRF Core Scanning的元素分布划分旋回和年际冻融层,在高精度定年的基础上计算旋回及年际产沙量和产沙模数。结果表明：JB聚湫沉积序列由126个旋回叠加而成,且形成于1855~2014年的78 个年份中。JB聚湫内旋回产沙量和产沙模数变化范围分别为0.27×10 ⁴~22.44×10 ⁴t和0.09×10 ⁴~7.82×10 ⁴t/km ²;年际产沙量和产沙模数范围分别为0.27×10 ⁴~90.73×10 ⁴t/a和0.09×10 ⁴~23.40×10 ⁴t/(km ²·a)。 ENSO事件显著影响JB流域强降水作用下的侵蚀产沙,且在El Ni?o次年和La Ni?a年份发生侵蚀产沙的频率较高,可能是这些年份中东亚季风增强后带来更多强降水事件的结果。研究结果有效延长了黄土高原北部丘陵沟壑区小流域土壤侵蚀演变历史,充分理解ENSO事件驱动下该地区侵蚀产沙的响应,为黄土高原北部坝库建设、水土流失治理和防洪减灾等提供理论依据。

Erosion Flux Change and Its Response to ENSO Events During the Past 160 Years Within the Hill-gully Area at the Northern Loess Plateau

As one of the most severe soil erosion areas and typical fragile eco-environment regions, the soil erosion processes on the Chinese Loess Plateau (CLP) include surface soil erosion and weathering, hydrological transport and biological activities in the catchment, all of which belong to the processes of Earth’s Critical Zone. Nevertheless, the understanding of historical soil erosion flux and its driving mechanism is limited, which significantly restricts the reasonable evaluation of erosion control effect on the CLP since 1950s. The sediment sequences in the check dams and landslide-dammed reservoirs on the CLP, which are composed of different numbers of deposition couplets and annual freeze-thaw layers, are one of the best geological carriers for high-resolution reconstructing historical erosion flux, storm intensity and frequency, eco-environment change and land use. Located at the hilly-gully area of the northern Loess Plateau, Jingbian (JB) landslide-dammed reservoir was the result of natural collapse of loess slope at the left of the gully during the period of 1851-1861, which had conserved erosion sediments and documented natural and human information in the catchment during the past 160 years. Based on a drilled 22.75 m JB sediment sequence, Rb/Zr ratios by XRF core scanning and correlation between Ca/Zr and Fe/Zr were used to plot deposition couplets and identify annual freeze-thaw layers. Meanwhile, couplets and annual (specific) sediment yields were calculated when the chronology of the JB sequence was built by the combination of ¹³⁷Cs contents, annual freeze-thaw layers, modern and history rainfall records. The results showed that the JB sediment sequence consisted of total 126 couplets with 78 annual freeze-thaw layers, which were formed between 1855 and 2014. The range of couplet sediment yields and specific sediment yields in the JB catchment were 0.27×10 ⁴-22.44×10 ⁴t and 0.09×10 ⁴-7.82×10 ⁴t/km ², respectively. And the range of annual sediment yields and specific sediment yields were 0.27×10 ⁴-90.73×10 ⁴t/a and 0.09×10 ⁴-23.40×10 ⁴t/(km ²·a), respectively. ENSO events significantly affected soil erosion and sediment yield under heavy rains in the JB catchment. Further, compared with El Ni?o years, the sediments in the JB catchment was more prone to be eroded and deposited in the La Ni?a years and next years of El Ni?o, which may be mainly contributed on the heightened East Asian monsoon along with more precipitations within the JB catchment in these years. These results are helpful for extending the history of soil erosion flux in the small catchments and for understanding the response of soil erosion for ENSO events at the hilly-gully area, which will provide the theory basis for dam construction, soil and water loss management, flood control on the northern CLP.