Combined effect of carbonate and biotite dissolution in landslides biases silicate weathering proxies
Combined effect of carbonate and biotite dissolution in landslides biases silicate weathering proxies
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DOI：
10.1016/j.gca.2017.07.014
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发表时间：
2017-09
影响因子：
5
通讯作者：
R. Emberson;A. Galy;N. Hovius
中科院分区：
文献类型：
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作者：
R. Emberson;A. Galy;N. Hovius
对硅酸盐岩石溶解作用的长期估计通常源自一系列同位素代理，例如保存在沉积物档案中的放射性锶同位素比 (87Sr/86Sr)。为了使这些系统能够公平地代表硅酸盐风化作用，陆地表面水中同位素比的变化应与整体硅酸盐溶解的变化相对应。这假设作为给定同位素来源的硅酸盐矿物相以与总体硅酸盐风化成比例的速率溶解。基岩滑坡会挖出大量新鲜岩石，以便在暂时储存中风化，而这些沉积物的快速风化主要是通过最具反应性相的溶解来控制的。在这项研究中，我们测试了这样一个假设：在新西兰快速侵蚀的西南阿尔卑斯山（WSA）中，这些不稳定矿物的优先风化可以将锶源的溶解与实际硅酸盐风化速率脱钩。我们发现，滑坡中相对放射性的方解石和黑云母的快速溶解导致锶的局部高通量，其同位素比率在来源之间没有提供明显的区分。这些较高的放射性锶通量与未系统升高的滑坡中的硅酸盐风化速率形成鲜明对比。在山带范围内，放射性锶通量与大型（> 100 km2）流域中近期发生的山体滑坡量无关，但硅酸盐风化通量却与此相关。这种脱钩可能首先是由于碳酸盐矿物中锶含量的广泛变化，其次是由于最近的山体滑坡中黑云母和碳酸盐释放的放射性锶的组合。这项研究支持了之前的工作，表明锶同位素作为研究 WSA 硅酸盐风化的系统的效用有限。然而，至关重要的是，在基岩滑坡是主要侵蚀过程的环境中，如果滑坡溶解所暴露的最具反应性的相对选择的代理有不成比例的贡献，则同位素代理中可能存在随机和系统偏差。这清楚地表明，海洋锶的同位素组成是山脉快速隆升和侵蚀时期的代表，而不是相关的硅酸盐风化增强的代表。
Long-term estimates of the dissolution of silicate rock are generally derived from a range of isotopic proxies, such as the radiogenic strontium isotope ratio (87Sr/86Sr), which are preserved in sediment archives. For these systems to fairly represent silicate weathering, the changes in isotopic ratios in terrestrial surface waters should correspond to changes in the overall silicate dissolution. This assumes that the silicate mineral phases that act as sources of a given isotope dissolve at a rate that is proportional to the overall silicate weathering. Bedrock landsliding exhumes large quantities of fresh rock for weathering in transient storage, and rapid weathering in these deposits is controlled primarily by dissolution of the most reactive phases. In this study, we test the hypothesis that preferential weathering of these labile minerals can decouple the dissolution of strontium sources from the actual silicate weathering rates in the rapidly eroding Western Southern Alps (WSA) of New Zealand. We find that rapid dissolution of relatively radiogenic calcite and biotite in landslides leads to high local fluxes in strontium with isotopic ratios that offer no clear discrimination between sources. These higher fluxes of radiogenic strontium are in contrast to silicate weathering rates in landslides that are not systematically elevated. On a mountain belt scale, radiogenic strontium fluxes are not coupled to volumes of recent landslides in large (>100 km2) catchments, but silicate weathering fluxes are. Such decoupling is likely due first to the broad variability in the strontium content of carbonate minerals, and second to the combination of radiogenic strontium released from both biotite and carbonate in recent landslides. This study supports previous work suggesting the limited utility of strontium isotopes as a system to study silicate weathering in the WSA. Crucially however, in settings where bedrock landsliding is a dominant erosive process there is potential for both random and systematic bias in isotope proxies if the most reactive phases exposed for dissolution by landslides disproportionately contribute to the proxy of choice. This clearly suggests that the isotopic composition of marine Sr is a proxy for periods of rapid mountain uplift and erosion rather than for the associated enhanced silicate weathering.