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.

中文翻译：

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来自冲积层洞穴系统的山谷切口年表


这篇综述探讨了从充满冲积层的洞穴系统中建立山谷切口年代学的潜力，并涵盖了自 1997 年以来的总共 30 个案例研究。当基岩溶解的条件最佳时，石灰岩中的洞穴发展非常快（∼104 年），并且许多洞穴包含同种异体下沉溪流沉积的冲积物，此后将沉积物保存了数百万年。洞穴网络显示垂直连续的次水平通道，这些通道表示地下水位的过去位置，溪流入口和出口洞穴表示相邻谷底的先前高程。随着山谷切口增加局部地势（下降洞穴形成），预计废弃洞穴的水位将成倍增加，但沉积物堆积或冰川积冰也可能提高当地的基底水平并淹没旧洞穴或产生新的洞穴（上升洞穴成因）。确定洞穴所承载的冲积沉积物的年龄，依赖于使用两种陆地宇宙成因核素 （TCN） （通常为 26Al 和 10Be）对富含石英的碎屑进行埋藏测年，并在同一样品中测量。对现有文献中年龄-海拔数据模式的系统检查揭示了从直观一致的山谷切口历史到违反直觉的年龄倒置和其他异常情况。在这里，对这些异常情况进行分析和分类，以确定相应的不一致是可以避免的程度，从而提供可预见的困难和陷阱的系统目录。强调了三个不确定性领域。第一个与喀斯特过程有关：洞穴网络几何形状、洞穴通道对包气和溶发过程的反应，以及洞穴年龄与沉积物之间的历时联系。 综述了洞穴形成的多种途径。他们强调了“洞穴水平”概念背后的歧义，作为基底古海拔的代表，它可能不如河流填充或层状阶地提供的海底信息精确。不确定性的第二个来源在于冲积层提供的时间信息。地下喀斯特地貌中的沉积物动力学产生了复杂的地层配置，为沉积后沉积物在洞穴内或洞穴之间进行返工提供了机会。此外，TCN 埋藏年龄对石英颗粒群有效，但不一定对包含它们的整个地层序列有效，也不一定对包含石英颗粒的洞穴有效。不确定性的第三个来源在于埋葬测年方法本身，因为 26Al 和 10Be 核素清单无法明确记录在洞穴最终埋葬之前是否可能发生了更古老的埋葬事件。该审查建议：（i） 采样策略应取决于对 speleogens 和 speleothems 的诊断，以及对冲积填充物的详细沉积学和地层学分析;（ii） 测年应侧重于单个床载碎屑而不是沙子，因为这有助于区分卵石种群并检测沉积物混合;（iii） 应测量现代河道冲积层和储存在集水区的旧矿床中的 26Al/10Be 比率，以检测沉积物级联特有的某些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，讨论了四种情况，其中应保留最年长或最年轻的年龄。