Late Quaternary variations in paleoerosion rates in the northern Qilian Shan revealed by 10Be in fluvial terraces

Understanding the pace of erosion through space and time may provide insight into how tectonic and climatic processes conspire to drive landscape evolution. However, determining whether erosion rates in a given locality have varied with past climate change remains challenging, and the mechanisms invoked to explain variations in past erosion rates are not well understood. In this study, we evaluate past rates of erosion during the late Quaternary along the northern Qilian Shan, the range bounding the northeastern margin of the Tibetan Plateau. We measured the concentrations of cosmogenic 10Be from fluvial deposits atop 5 terraces along the Hongshuiba River and average 10Be concentrations at the time of terrace formation. We also determined terrace ages by combining 5 10Be depth profiles and 15 OSL samples of overlying loess. Our results confirm previous dating of this terrace sequence using cosmogenic exposure ages of boulders and depth profiles in terrace gravels and indicate that terrace treads span glacial-interglacial cycles over the past ~200 ka. We exploit this exceptionally dense sampling of 10Be to evaluate differences in cosmogenic 10Be concentrations at the time of terrace formation along the Hongshuiba River. Analysis of 48 individual determinations of “inherited” 10Be concentrations reveal that fluvial sediment in terraces abandoned during the last glacial and penultimate glacial periods are 3–4 times greater than concentrations in sediment atop terraces formed during interglacial times. Paleoerosion rates within the.Hongshuiba watershed appear to have been ~200 m/Myr during glacial episodes but increased to ~500–700 m/Myr during interglacial times, similar to recent erosion rates inferred from 10Be in modern sediment. These results point to likely changes in the mean residence time of sediment with the watershed that fluctuate in association with 100-ka glacial-interglacial cycles. We suggest that reductions in erosion rates associated with glacial conditions may have been driven by changes in catchment hydrology during cold and dry periods.

了解侵蚀在空间和时间上的速率可能有助于深入了解构造和气候过程是如何共同推动地貌演化的。然而，确定某一地区的侵蚀速率是否随过去的气候变化而变化仍然具有挑战性，而且用于解释过去侵蚀速率变化的机制也尚未得到很好的理解。在这项研究中，我们评估了祁连山北部（青藏高原东北边缘的山脉）晚第四纪期间的过去侵蚀速率。我们测量了红水坝河沿岸5级阶地上河流沉积物中宇宙成因核素¹⁰Be的浓度以及阶地形成时的平均¹⁰Be浓度。我们还通过结合5个¹⁰Be深度剖面和15个上覆黄土的光释光（OSL）样本确定了阶地的年龄。我们的研究结果证实了先前使用阶地砾石中巨石的宇宙暴露年龄和深度剖面对该阶地序列进行的测年，并表明阶地台面跨越了过去约20万年来的冰期 - 间冰期旋回。我们利用这种对¹⁰Be异常密集的采样来评估红水坝河沿岸阶地形成时宇宙成因¹⁰Be浓度的差异。对48个“继承性”¹⁰Be浓度的单独测定分析表明，末次冰期和倒数第二次冰期期间废弃的阶地中的河流沉积物的¹⁰Be浓度比间冰期形成的阶地顶部沉积物中的浓度高3 - 4倍。红水坝流域内的古侵蚀速率在冰期似乎约为200米/百万年，但在间冰期增加到约500 - 700米/百万年，这与从现代沉积物中的¹⁰Be推断出的近期侵蚀速率相似。这些结果表明，流域内沉积物的平均停留时间可能会随着10万年的冰期 - 间冰期旋回而发生变化。我们认为，与冰川条件相关的侵蚀速率降低可能是由寒冷干燥时期流域水文学的变化所驱动的。