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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Comparison of U-Th, paleomagnetism, and cosmogenic burial methods for dating caves: Implications for landscape evolution studies
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Comparison of U-Th, paleomagnetism, and cosmogenic burial methods for dating caves: Implications for landscape evolution studies

机译：U-Th，古磁铁矿和宇宙成因掩埋法对洞穴测年的比较：对景观演化研究的启示

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Caves are useful in landscape evolution studies because they often mark the level of previous water tables and, when dated, yield incision rates. Dating caves is problematic, however, because their ages are only constrained by the oldest deposits contained within, which may be far younger than the cave itself. We dated cave deposits in the Sierra Nevada using U-Th dating of speleothems, paleomagnetic dating of fine sediment, and cosmogenic Al-26/Be-10 burial dating of coarse sediment. The sampled caves formed sequentially as the water table lowered, providing an important stratigraphic test for the dating methods. Large discrepancies between deposit ages from similar cave levels demonstrate that, even when accurately determined, deposit ages can seriously underestimate the timing of cave development. Drip-type speleothems are most prone to this minimum age bias because they can accumulate long after caves form, and because the U-Th method is limited to similar to 400 ka. Paleomagnetic dating requires correlation with the global reversal chronology, and is hindered by a lack of continuous stratigraphy. The fine sediment analyzed for paleomagnetism is also highly susceptible to remobilization and deposition in cave passages well above base level. Cosmogenic Al-26/Be-10 dates bedload material deposited when caves were at or very near river level, and can date material as old as similar to 5 Ma. In the Sierra Nevada, speleothem U-Th ages and sediment burial ages from the same cave levels differ by as much as an order of magnitude. These results suggest speleothem ages alone may significantly underestimate cave ages and thereby overestimate rates of landscape evolution. Cosmogenic burial dating of coarse elastic sediment appears to be the most reliable method for dating cave development in mountainous regions. (c) 2005 Elsevier B.V. All rights reserved.
机译：洞穴在景观演化研究中很有用，因为它们通常会标记以前的地下水位，并且在注明日期时会产生切口率。但是，约会洞穴是有问题的，因为它们的年龄仅受其中最古老的沉积物的约束，这些沉积物可能比洞穴本身年轻得多。我们使用内陆山脉的U-Th年代，细沉积物的古磁性年代和具有宇宙成因的Al-26 / Be-10埋藏的粗沉积物对内华达山脉的洞穴沉积物进行了年代测年。随着地下水位的降低，采样的洞穴依次形成，为测年方法提供了重要的地层测试。与相似的洞穴水平相比，沉积年龄之间的巨大差异表明，即使准确地确定了沉积年龄，也会严重低估了洞穴发育的时间。滴灌型鞘翅目最容易出现这种最小年龄偏差，因为它们可以在洞穴形成后很长一段时间内积累，并且由于U-Th方法限于类似于400 ka的情况。古地磁测年需要与全球逆转年表相关，并且由于缺乏连续地层学而受到阻碍。分析古磁性的细沉积物也很容易在远高于基准面的洞穴通道中迁移和沉积。宇宙成因的Al-26 / Be-10可以确定洞穴处于或接近河流高度时沉积的床荷物质，并且可以确定年代早于5 Ma的物质。在内华达山脉中，相同洞穴高度的蛇形U-Th年龄和沉积物埋葬年龄相差一个数量级。这些结果表明，仅鞘翅目年龄可能大大低估了洞穴年龄，从而高估了景观演化的速度。粗弹性沉积物的宇宙成因埋葬测年似乎是对山区洞穴发育进行测年的最可靠方法。 （c）2005 Elsevier B.V.保留所有权利。

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