Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate
Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate
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DOI：
10.1016/j.gca.2019.09.044
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发表时间：
2020-01-15
影响因子：
5
通讯作者：
Brantley, Susan L.
中科院分区：
文献类型：
--
作者：
Gu, Xin;Rempe, Daniella M.;Brantley, Susan L.
在已发表的对许多集水区的比较中，已经观察到化学风化的速率随着物理侵蚀的速率而增加，但是将这些过程结合起来的机制还没有得到很好的理解。我们通过研究美国宾夕法尼亚州海相页岩集水区的化学风化和孔隙度剖面来研究这个问题（Susquehanna shale Hills Critical Zone Observatory， SSHCZO）；美国加州（鳗河临界带天文台，ERCZO）；台湾（福山实验林）。这些地点的原岩组成、原岩孔隙度和风化层深度大致相似，但流域的侵蚀速率差异较大(福山1 ~ 3 mm yr(-1), > 0 ~ 0.2 mm yr(-1), ERCZO 0 ~ 0.4 mm yr(-1), > 0 ~ 0.4 mm yr（-1), SSHCZO 0 ~ 0.01 mm yr(-1)）。自然实验并没有完全孤立侵蚀作为一个变量：年平均降水量沿侵蚀梯度变化(福山bbb为4.2 m yr(-1)， SSHCZO为1.9 m yr(-1), czo >为1.1 m yr(-1))，因此侵蚀最快的地点经历的年平均温度几乎是其他两个地点的两倍。尽管侵蚀速率相差约100倍，但黄铁矿和碳酸盐枯竭的深度（此处定义为风化层厚度）大致相同，这与这些矿物的化学风化作用在三个地点与侵蚀保持一致。这些矿物一直被观察到是反应最深的，它们一直反应到100%耗尽。在三个集水区中的两个，这些矿物在狭窄的反应锋上风化。另一方面，对于造岩粘土矿物绿泥石，随着侵蚀/降水的增加，地表风化深度间隔较宽，损耗程度逐渐减小。因此，粘土的化学风化作用跟不上侵蚀速率。但最大的区别可能在于，在快速侵蚀部位，微裂缝占总孔隙度的30-60%，而在缓慢侵蚀部位，溶蚀可能与次生孔隙直接相关。我们认为，微裂缝增加了深度氧的流入，减小了基体内部扩散受限区域的尺寸，加速了黄铁矿和碳酸盐在高侵蚀速率条件下的风化作用。因此，微压裂是页岩物理侵蚀和化学风化耦合的过程。(C) 2019 Elsevier Ltd.版权所有。
The rate of chemical weathering has been observed to increase with the rate of physical erosion in published comparisons of many catchments, but themechanisms that couple these processes are not well understood. We investigated this question by examining the chemical weathering and porosity profiles from catchments developed on marine shale located in Pennsylvania, USA (Susquehanna Shale Hills Critical Zone Observatory, SSHCZO); California, USA (Eel River Critical Zone Observatory, ERCZO); and Taiwan (Fushan Experimental Forest). The protolith compositions, protolith porosities, and the depths of regolith at these sites are roughly similar while the catchments are characterized by large differences in erosion rate (1-3 mm yr(-1) in Fushan >> 0.2-0.4 mm yr(-1) in ERCZO >> 0.01-0.025 mm yr(-1) in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr(-1) in Fushan > 1.9 m yr(-1) inERCZO > 1.1 m yr(-1) in SSHCZO), so the fastest eroding site experiences nearly twice the mean annual temperature of the other two.Even though erosion rates varied by about 100x, the depth of pyrite and carbonate depletion (defined here as regolith thickness) is roughly the same, consistent with chemical weathering of those minerals keeping up with erosion at the three sites. These minerals were always observed to be the deepest to react, and they reacted until 100% depletion. In two of three of the catchments where borehole observations were available for ridges, these minerals weathered across narrow reaction fronts. On the other hand, for the rock-forming clay mineral chlorite, the depth interval of weathering was wide and the extent of depletion observed at the land surface decreased with increasing erosion/precipitation. Thus, chemical weathering of the clay did not keep pace with erosion rate. But perhaps the biggest difference among the shales is that in the fast-eroding sites, microfractures account for 30-60% of the total porosity while in the slow-eroding shale, dissolution could be directly related to secondary porosity. We argue that the microfractures increase the influx of oxygen at depth and decrease the size of diffusion-limited internal domains of matrix, accelerating weathering of pyrite and carbonate under high erosion-rate conditions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales. (C) 2019 Elsevier Ltd. All rights reserved.