Assessment of the controls on (234U/238U) activity ratios recorded in detrital lacustrine sediments
Assessment of the controls on (234U/238U) activity ratios recorded in detrital lacustrine sediments
复制标题
DOI：
10.1016/j.chemgeo.2020.119698
复制
发表时间：
2020-06
期刊：
影响因子：
3.9
通讯作者：
A. Francke;A. Dosseto;J. Just;B. Wagner;B. Jones
中科院分区：
文献类型：
--
作者：
A. Francke;A. Dosseto;J. Just;B. Wagner;B. Jones
在最近气候变暖和土地利用加剧的背景下，了解地质时间尺度上的景观演化变得越来越重要。铀同位素（234 U/238 U）活度比可用于重建细粒碎屑物质的全流域侵蚀，因为较轻的核素234 U在风化剖面、运输过程中和最终沉积后从小于63 μm的颗粒中损失到周围的孔隙空间中。因此，（234 U/238 U）放射性比是基岩粉碎成碎屑沉积物后所经过时间的量度。当应用于沉积物中的碎屑颗粒时，粉碎年龄可以推导出沉积物停留时间的过去变化，即沉积物在山坡上储存和搬运的时间。然而，234 U的活化也受到234 U从碎屑颗粒中优先氧化和浸出的控制。关于沉积物矿物学和铀结合自生或内生matter.Here，我们扩展了以前发表的（234 U/238 U）的活动比和铀浓度的碎屑物质沿着一个5.43米长的，晚冰期全新世沉积记录从奥赫里德湖（北马其顿，阿尔巴尼亚）的控制讨论。奥赫里德湖被选择来研究侵蚀如何响应人类活动的干扰和气候变化，这是常见的通过晚更新世和全新世在地中海地区。我们比较了铀浓度和同位素数据的氧化还原敏感，矿物学和地球化学代理。矿物学和地球化学数据表明，铀浓度和（234 U/238 U）活度比不受控制。岩石磁性氧化还原代用指标表明铀从碎屑物质在更氧化环境中的活动。（234 U/238 U）活度比值与岩石磁性数据之间没有相关性，表明氧化还原环境对同位素信号没有控制作用。Monte Carlo模拟表明，沉积后优先动员的234 U只有微不足道的影响calculatedsediment停留时间。我们的模型表明，沉积前的淋溶可以导致较短的沉积物停留时间，但低（234 U/238 U）的活动比在寒冷和干燥的间隔意味着反冲可能是主要的过程，解释损失的234 U从碎屑颗粒。这可以通过本文研究的时间尺度（> 10，000年）来解释，在此期间，优先沥滤并不明显。
Understanding landscape evolution on geological time scales has become increasingly important in the light of recent climate warming and intensified land use. Uranium isotopes ((234U/238U) activity ratios) can be used to reconstruct catchment-wide erosion from fine-grained detrital matter, as the lighter nuclide234U is lost from grains <63 μm into surrounding pore space in a weathering profile, during transportation, and after final deposition. Thus, (234U/238U) activity ratios are a measure for the time elapsed since comminution of bedrock into detrital sediment. When applied to detrital grains in sedimentary deposits, thecomminution ageallows derivation of past variations insediment residence time,i.e. the amount of time for which sediments are stored on hillslopes and transported.Loss of234U is mainly attributed to recoil of234Th during α-decay of238U. However, mobilisation of234U is also controlled by preferential oxidation and leaching of234U from detrital grains. Further considerations are required concerning the sediment mineralogy and uranium bound to authigenic or endogenic matter.Here, we extend the discussion about the controls on previously published (234U/238U) activity ratios and uranium concentrations of detrital matter along a 5.43 m-long, Late Glacial to Holocene sedimentary record from Lake Ohrid (North Macedonia, Albania). Lake Ohrid is chosen to study how erosion responds to anthropogenic disturbances and climate variability, which are common through the Late Pleistocene and Holocene in the Mediterranean Region. We compared uranium concentration and isotope data to redox-sensitive, mineralogical and biogeochemical proxies. Mineralogical and biogeochemical data show no control on uranium concentration and (234U/238U) activity ratios. Rock magnetic redox proxies indicate uranium mobilisation from detrital matter in more oxic environments. No correlation between (234U/238U) activity ratios and rock magnetic data is observed, which implies that the redox environment has no control on the isotope signal. Monte Carlo simulations reveal that post-depositional preferential mobilisation of234U has only negligible impact on the calculatedsediment residence times. Our model implies that pre-depositional leaching can results in shortersediment residence timesbut low (234U/238U) activity ratios during cold and dry intervals imply that recoil is likely the main processes explaining loss of234U from the detrital grain. This is explained by the time scales studied herein in (>10,000 years), during which preferential leaching is seen to be not significant.