Climate preconditions the Critical Zone: Elucidating the role of subsurface fractures in the evolution of asymmetric topography
Climate preconditions the Critical Zone: Elucidating the role of subsurface fractures in the evolution of asymmetric topography
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DOI：
10.1016/j.epsl.2019.01.039
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发表时间：
2019-05
影响因子：
5.3
通讯作者：
N. West;E. Kirby;A. Nyblade;S. Brantley
中科院分区：
文献类型：
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作者：
N. West;E. Kirby;A. Nyblade;S. Brantley
虽然人们很早就知道岩石性质强烈影响岩石向表层的化学和物理转变，但最近的研究强调了浅层地下机械压裂的中心作用。关于裂缝如何与地表过程共同演化的相互竞争的假说表明，地形应力要么增强或抑制裂缝扩展的潜力，要么表明裂缝的生长取决于地下水文和风化反应之间的耦合。这两个端元模型都产生了可预测的和系统的地下裂缝分布的空间模式。为了阐明导致不对称山坡地形和临界区结构发展的过程，我们结合了对钻孔中地下裂缝分布的直接观测、从地震折射调查推断的地下岩性的间接测量，以及宾夕法尼亚州中部一个小型页岩下垫面集水区地下样品的地球化学变化。结果表明，地震纵波速度剖面与井中观察到的地下裂缝密度和化学亏损带是一致的，但这些分布随山坡倾角而变化。简单的霜裂模型表明，虽然现代气候不利于地下分离冰的生长，但不对称的裂缝分布可能是由于过去冰缘气候中与方面相关的推动冻融的小气候的细微差异造成的。裂隙密度、坡度、风化层深度和风化层运移效率之间的空间协方差与控制该流域长期结构的小气候条件所造成的化学机械破坏是一致的。因此，气候驱动的破坏可以解释今天观察到的不对称地形。
Although it has been long understood that rock properties strongly modulate the chemical and physical transformation of rock to regolith, recent studies highlight the central role of mechanical fracturing in the shallow subsurface. Competing hypotheses for how fractures co-evolve with surface processes suggest either that topographic stresses enhance or dampen the potential for fracture propagation, or alternatively suggest that fracture growth depends on the coupling between subsurface hydrology and weathering reactions. Both end-member models produce predictable and systematic spatial patterns of subsurface fracture distributions.To elucidate the processes responsible for the development of asymmetric hillslope topography and critical zone structure, we combine direct observations of subsurface fracture distributions in boreholes, indirect measures of subsurface rock properties inferred from seismic refraction surveys, and geochemical variations from subsurface samples at a small, shale-underlain catchment in central Pennsylvania. Our results reveal that seismic P-wave velocity profiles are consistent with subsurface fracture densities and zones of chemical depletion observed in boreholes, but that these distributions co-vary with hillslope aspect. Simple models of frost cracking show that, while modern climates are not conducive to subsurface segregation ice growth, asymmetric fracture distributions could have been achieved by subtle differences in aspect-related microclimates driving freeze-thaw during past periglacial climates. The spatial co-variance among fracture density, hillslope gradient, regolith depth, and regolith transport efficiency is consistent with chemo-mechanical damage arising from microclimatic conditions governing the long-term architecture of this watershed. Thus, damage driven by climate could explain the asymmetric topography observed today.