Can radiocarbon dating chironomid head capsules increase chronological accuracy in lake sediments?

从湖泊沉积物中解释过去的环境变化对于理解过去、现在和未来的全球变化变得越来越重要。为了了解环境和气候变化的速度，并比较来自不同沉积源的发现，这些序列的准确定年是至关重要的。目前，大多数湖泊沉积物序列都是通过放射性碳测定年代的。这些日期可能会有相对较大的误差，而且测定不同材料的年代可能会得出不同的年代。因此，这些不确定性被带入到环境重建的解释中。我们的目标是减少这些不确定性。放射性碳测年大多数湖泊沉积物是有问题的，因为旧的碳从周围的土壤进入湖泊。因此，我们需要找到一种方法，使这种情况最小化或完全消除。解决这一问题的一个潜在方法是放射性碳测定摇蚊幼虫头囊的年代。摇尾虫幼虫通常是淡水生态系统中占优势的海底无脊椎动物（仅次于寡毛纲）。头部胶囊由一种叫做几丁质的坚固聚合物制成，保存在湖泊沉积物中。之前的两项研究表明，通过放射性碳测年，并将其与有机湖泥测年进行比较，摇壶鱼产生了更年轻的年代。这表明有机泥浆中含有古老的碳，因此手摇体年代更准确。然而，到目前为止还没有对这一假设进行检验。我们的目标是从一系列已知年龄的冰岛湖泊沉积物中提取的chironomid头部胶囊样本，包括头部胶囊的壳聚糖提取物（最近开发的）和整个头部胶囊（散装甲壳素）本身的放射性碳定年。然后，我们将评估样品的放射性碳浓度，并试图将它们与最近的已知大气放射性碳浓度相匹配，这一时期由于炸弹试验而出现了非常明显的放射性碳“峰值”。这将使我们能够测试使用手摇仪作为测年工具的准确性，即它们的放射性碳浓度（年龄）与它们死亡并融入湖泊沉积物的时间相匹配的程度。如果这种准确性能够得到证明，那么就意味着，通过测定摇壶器的年代，我们可以准确地说出湖泊沉积物中其他物质所显示的变化发生的时间。这些分析的结果将使我们能够检验(1)壳聚糖提取物或散装壳质样品与已知年龄相比是否提供准确的年龄估计；(2)壳聚糖萃取物是否总是比散装萃取物更年轻，突出了（尚不清楚的）由于依赖散装甲壳素样品而产生的年代误差；(3)手摇测年相对于其他形式的湖泊沉积物放射性碳测年的可能价值。据推测，使用这些新协议的仔细定年将提供一种重要的创新方法，从广泛的沉积档案中得出年表，其中许多目前难以确定日期。

Interpreting past environmental change from lake sediments is becoming increasingly important for understanding past, present and future global change. In order to understand rates of environmental and climatic change, and compare findings from different sedimentary sources accurate dating of these sequences is fundametal. Currently most lake sediment sequences are dated by radiocarbon. These dates can be subject to relatively large errors and dating different material can give different ages. Thus these uncertainties are carried forward into interpretation of the environmental reconstructions. We aim to reduce these uncertainties. Radiocarbon dating most lake sediments is problematic due to old carbon entering the lake from surrounding soils. We therefore need to find a method whereby this is minimised or eliminated entirely. One potential method which gets round this problem is to radiocarbon date chironomid (non-biting midge) larval head capsules. Chironomid larvae are often the dominant bottom living invertebrates (after oligochaetes) present in freshwater ecosystems. The head capsules are made from a robust polymer called chitin, and are preserved in lake sediments. Two previous studies have shown that by radiocarbon dating chironomid head capsules, and comparing this to dating the organic lake mud, younger dates are produced by the chironomids. This suggests that the organic mud contains old carbon, and therefore that the chironomid dates are more accurate. However, as yet no tests have been undertaken on this hypothesis. We aim to radiocarbon date samples of chironomid head capsules from a range of recent Icelandic lake sediments of known age including both a chitosan extract (recently developed) from head capsules and the whole head capsules (bulk chitin) themselves. We will then assess the radiocarbon concentrations of the samples, and attempt to match them to known atmospheric radiocarbon concentrations from the recent past, a period with a very clear radiocarbon 'spike' due to bomb testing. This will allow us to test the accuracy of using chironomids as a dating tool, i.e. how well their radiocarbon concentration (age) matches when they died and became incorporated into the lake sediment. If this accuracy can be demonstrated then it means by dating the chironomids we can tell excactly when changes indicated by other material in the lake sediments were taking place. The results from these analyses will allow us to examine (1) Whether either the chitosan extract or bulk chitin samples provide accurate age estimates when compared to their known age; (2) Whether the chitosan extracts are always younger than the bulk extracts, highlighting (as yet unknown) dating errors arising from reliance on bulk chitin samples; (3) The likely value of chironomid dating against other forms of radiocarbon dating for lake sediments. It is hypothesised that careful dating using these new protocols will provide an important innovative approach to deriving chronologies from a wide range of sedimentary archives, many of which are at present difficult to date.