Does decreasing paraglacial sediment supply slow knickpoint retreat?
Does decreasing paraglacial sediment supply slow knickpoint retreat?
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DOI：
10.1130/g32018.1
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发表时间：
2011-06-01
期刊：
影响因子：
5.8
通讯作者：
Codilean, Alexandru T.
中科院分区：
文献类型：
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作者：
Jansen, John D.;Fabel, Derek;Codilean, Alexandru T.
在苏格兰西部的四条河流中，有一个相对基准面下降的严格记录，冰川后基岩侵蚀的速率是通过测量在向后退的裂缝点下游的河床阶地上的原位宇宙成因Be-10的浓度来量化的。Be-10暴露年龄沿河道梯度表现出上游年轻化和恒定年龄两种明显的趋势，分别可作为断裂点后退和扩散输运受限切口的诊断。研究表明，基岩河道切割和区域阶地的形成始于消冰期后不久（约11.5 ka），裂口退缩率在全新世早期至中期达到峰值。此后，侵蚀速率下降了两个数量级，在全新世晚期收敛到与单位河道面积水流功率无关的低速率。我们推断，冰期后裂缝点退缩的这种区域性减缓是全新世期间副冰川沉积物供应枯竭的结果，导致基岩侵蚀的“工具”缺乏。我们的研究结果表明，主要河流侵蚀的事件可能与冰川旋回一致，并且冰川-间冰期过渡后的沉积物消耗可能是导致冰川景观中河流基岩侵蚀速率变化的重要原因。
In four rivers in western Scotland for which there is a well-constrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantified by measuring the concentration of in situ cosmogenic Be-10 on strath terraces downstream of headward-retreating knickpoints. Along-channel gradients in Be-10 exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a deficiency in "tools" for bedrock erosion. Our results imply that episodes of major fluvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.