Nature2014-06-26 6:25 PM

河流下切——事实与迷信 A signature of transience in bedrock river incision rates over timescales of 104–107 years


河流下切速率是景观中与气候相关的侵蚀变化程度的重要指标,而且所测出的河床下切速率也被认为会约束活跃构造过程。然而,所有这一切都假设河流下切速率能记录关于外部作用力因素的有意义的信息。这项研究分析了来自世界范围内14个地点的155个河流下切测量结果,发现下切速率对测量间隔的依赖性使得人们难以从河流下切速率随时间的变化来推断气候作用力或构造作用力。在为本文配发的一篇News & Views文章中,Roman DiBiase结合从净侵蚀性景观来推断景观演变速度的变化方面的工作讨论了这项研究的实际意义。


Measured rates of river incision into bedrock are commonly interpreted as proxies for rates of rock uplift (see refs 1 and 2, for example) and indices of the strength of climatic forcing of erosion over time (see refs 3 and 4, for example). This approach implicitly assumes that river incision rates are in equilibrium with external forcings over a wide range of timescales. Here we directly test this assumption by examining the temporal scaling of bedrock river incision from 155 independent measurements of river incision compiled from 14 sites. Of these sites, 11 exhibit a negative power-law dependence of bedrock river incision rate on measurement interval, a relationship that is apparent over timescales of 104–107 years and is independent of tectonic and geomorphic setting. Thus, like rates of sediment accumulation, rates of river incision into bedrock exhibit non-steady-state behaviour even over very long measurement intervals. Non-steady-state behaviour can be explained by episodic hiatuses in river incision triggered by alluvial deposition, if such hiatuses have a heavy-tailed length distribution. Regardless of its cause, the dependence of incision rate on measurement interval complicates efforts to infer tectonic or climatic forcing from changes in rates of river incision over time or from comparison of rates computed over different timescales.






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