Abstract Radiocarbon dated chronologies are the most extensively applied dating technique when investigating the last ∼45,000 years. In lake and marine sediments, a single macrofossil or several microfossils are the preferred sample material because they have a 14C age that accurately reflects the time of deposition. However, absence of macro- or microfossils are a recurring challenge. Often, it is necessary to use bulk sediment samples for radiocarbon dating, but they frequently yield ages clearly exceeding the depositional timeframe due to occurrence of organic material of older origin. Until now, it has not been possible to assess if the dating result of a bulk sediment sample is reliable, or to adequately explain the mechanisms behind age disagreement between bulk and macro- and microfossils. In this study, we investigate the age offset between paired bulk and macrofossil ages in sediment cores from three lakes in SE and E Greenland. Furthermore, we distinguish three quantifiable main carbon sources in the samples: a) recent terrestrial organic fragments, b) recent aquatic organic matter, and c) reworked land-derived old carbon, through pyrolysis organic geochemistry, organic petrographic microscopy, and isotopic fractionization. Our results show that the offset between bulk (humic fraction) and macrofossil radiocarbon ages in three similar lakes range from 9 14C yr to 20.1 14C kyr. We observe a reduced age offset between bulk- and macrofossil ages, and thereby a higher reliability of bulk (humic fraction) ages, correlated with increased values of TOC, C/N ratio, and high CO2 release in pyrolysis (S3 value). We recommend that future studies presenting bulk sediment chronologies apply pyrolysis organic geochemistry, organic petrographic microscopy, and isotopic fractionization as demonstrated here, to select reliable sediment intervals for bulk dating and retrieve more robust results.

中文翻译：

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提高大块沉积物放射性碳测年的可靠性

摘要 放射性碳测年是研究过去 45,000 年应用最广泛的测年技术。在湖泊和海洋沉积物中，单个大型化石或多个微型化石是首选的样品材料，因为它们具有 14C 的年龄，可以准确反映沉积时间。然而，缺乏宏观或微观化石是一个反复出现的挑战。通常，有必要使用大块沉积物样本进行放射性碳测年，但由于出现较早的有机物质，它们经常产生明显超过沉积时间范围的年龄。到目前为止，还无法评估大块沉积物样本的年代测定结果是否可靠，或者无法充分解释大块与宏观和微观化石之间年龄差异背后的机制。在这项研究中，我们调查了 SE 和 E 格陵兰岛三个湖泊的沉积物核心中成对的体积和大型化石年龄之间的年龄偏移。此外，我们区分了样品中三种可量化的主要碳源：a) 最近的陆地有机碎片，b) 最近的水生有机物质，以及 c) 通过热解有机地球化学、有机岩相显微镜和同位素分馏重新加工的陆源旧碳。我们的结果表明，三个类似湖泊的体积（腐殖质部分）和大型化石放射性碳年龄之间的偏移范围从 9 14C yr 到 20.1 14C kyr。我们观察到大体化石年龄和大化石年龄之间的年龄偏移减少，因此体积（腐殖质部分）年龄的可靠性更高，这与 TOC、C/N 比值的增加和热解过程中高 CO2 释放（S3 值）相关。