Valley incision chronologies from alluvium-filled cave systems

IF 10.8 1区 地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY Earth-Science Reviews Pub Date : 2024-11-01 DOI:10.1016/j.earscirev.2024.104963
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Abstract

This review explores the potential for establishing valley incision chronologies from alluvium-filled cave systems, and covers a total of 30 case studies since 1997. Caves in limestone develop very fast (∼104 years) when conditions for bedrock solution are optimal, and many contain alluvium deposited by allogenic sinking streams, preserving the sediment thereafter for millions of years. Cave networks display a vertical succession of sub-horizontal passages which indicate past positions of the water table, with the stream in- and outlet caves indicating the former elevation of the adjacent valley floor. Abandoned cave levels are expected to multiply as valley incision increases local relief (descending speleogenesis), but sediment aggradation or glacier ice accumulation may also raise the local base level and flood older caves or generate new ones (ascending speleogenesis). Establishing the age of alluvial sediment hosted by caves relies on burial dating of quartz-rich clasts using two terrestrial cosmogenic nuclides (TCNs) – commonly 26Al and 10Be – measured in the same sample. Systematic examination of age–elevation data patterns in the existing literature reveals situations ranging from intuitively consistent valley incision histories to counter-intuitive age inversions and other anomalies. Here those anomalies are analyzed and classified in order to establish the extent to which the corresponding inconsistencies are avoidable, thereby providing a methodical catalogue of foreseeable difficulties and pitfalls. Three domains of uncertainty are emphasized. The first relates to karst processes: cave network geometry, cave passage response to vadose and phreatic processes, and diachronous links between cavity age and sediment. The multiple pathways of speleogenesis are reviewed. They highlight ambiguities behind the concept of ‘cave level’, which, as a proxy for base-level paleoelevations, may be less precise than subaerial information provided by fluvial fill or strath terraces. The second source of uncertainty lies in the chronological information provided by the alluvium. Sediment dynamics in subterranean karst generate complicated stratigraphic configurations, with opportunities for postdepositional sediment reworking within or between cave levels. Furthermore, a TCN burial age is valid for a population of quartz grains but not necessarily for the entire stratigraphic sequence containing them nor for the cave that contains it. The third source of uncertainty lies in the burial dating method itself, because 26Al and 10Be nuclide inventories cannot unequivocally document whether older burial events might have occurred prior to final burial in the cave. The review recommends that (i) sampling strategies should be contingent on a diagnosis of speleogens and speleothems, and on a detailed sedimentological and stratigraphic analysis of the alluvial fills; (ii) dating should focus on individual bedload clasts rather than on sand because this helps to discriminate between pebble populations and to detect sediment mixing; (iii) 26Al/10Be ratios in modern channel alluvium and in older deposits stored in the catchment should be measured for the purpose of detecting whether certain features endemic to the sediment cascade could explain apparent burial age anomalies in the cave sediments. In situations where 26Al/10Be determinations generate wide age dispersion, four scenarios are discussed in which either the oldest or the youngest age should be retained.
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冲积层填充洞穴系统的河谷侵蚀年代学
这篇综述探讨了从充满冲积物的洞穴系统中建立山谷侵蚀年代学的可能性,涵盖了自 1997 年以来的 30 个案例研究。当基岩溶蚀条件最佳时,石灰岩溶洞的发育速度非常快(104 年~104 年),而且许多溶洞都含有冲积物,这些冲积物是由同源下沉溪流沉积而成的,其后的沉积物保存了数百万年。洞穴网络呈现出亚水平通道的垂直演替,这表明了地下水位过去的位置,溪流出入洞穴表明了邻近谷底过去的海拔高度。随着河谷内切使当地地势升高(降水成洞),废弃洞穴的水位预计会成倍增加,但沉积物侵蚀或冰川冰积也可能使当地基底水位升高,淹没老洞穴或产生新洞穴(升水成洞)。要确定洞穴所在冲积沉积物的年龄,需要利用两种陆地宇宙成因核素(TCNs)--通常是 26Al 和 10Be--在同一样本中测量富含石英的碎屑的埋藏年代。对现有文献中的年龄-海拔数据模式进行系统研究后发现,既有直观一致的山谷侵蚀历史,也有违背直觉的年龄倒置和其他异常情况。本文对这些反常现象进行了分析和分类,以确定相应的不一致性在多大程度上是可以避免的,从而有条不紊地列出可预见的困难和陷阱。本文强调了三个不确定性领域。第一个领域与岩溶过程有关:洞穴网络的几何形状、洞穴通道对岩浆和岩相过程的响应,以及洞穴年龄与沉积物之间的非同步联系。对岩浆形成的多种途径进行了回顾。它们强调了 "洞穴水平 "概念背后的不确定性,作为基底古海拔的替代物,"洞穴水平 "可能不如河道填土或地层阶地提供的次生信息精确。不确定性的第二个来源是冲积层提供的年代信息。地下岩溶中的沉积物动力学产生了复杂的地层结构,在洞穴内部或洞穴层位之间存在沉积物沉积后再加工的机会。此外,TCN 埋藏年龄对石英颗粒群有效,但不一定对包含这些石英颗粒的整个地层序列有效,也不一定对包含这些石英颗粒的洞穴有效。不确定性的第三个来源在于埋藏年代测定方法本身,因为 26Al 和 10Be 核素清单无法明确记录在洞穴最终埋藏之前是否发生过更早的埋藏事件。审查报告建议:(i) 取样策略应取决于对岩浆和岩浆物的诊断,以及对冲积层填充物的详细沉积学和地层学分析;(ii) 测定年代应侧重于单个岩床碎屑,而不是砂粒,因为这有助于区分卵石群和检测沉积物混合情况;(iii) 应测量现代河道冲积物和集水区较古老沉积物中的 26Al/10Be 比值,以探测沉积物级联的某些特有特征是否可以解释洞穴沉积物中明显的埋藏年龄异常。在 26Al/10Be 测定产生较大年龄差异的情况下,讨论了应保留最古老或最年轻年龄的四种情况。
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来源期刊
Earth-Science Reviews
Earth-Science Reviews 地学-地球科学综合
CiteScore
21.70
自引率
5.80%
发文量
294
审稿时长
15.1 weeks
期刊介绍: Covering a much wider field than the usual specialist journals, Earth Science Reviews publishes review articles dealing with all aspects of Earth Sciences, and is an important vehicle for allowing readers to see their particular interest related to the Earth Sciences as a whole.
期刊最新文献
Mapping the evolution of marine carbon during the last deglaciation: δ13C perspectives on the deglacial ocean carbon cycle Numerical modelling of coupled climate, tectonics, and surface processes on the eastern Himalayan syntaxis Valley incision chronologies from alluvium-filled cave systems Floodplain lakes: Linking hydrology to ecology and conservation A review of machine learning applications to geophysical logging inversion of unconventional gas reservoir parameters
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