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，并比较不同沉积来源的发现，对这些序列进行准确的年代测定是至关重要的。目前，大多数湖泊沉积层序都是用放射性碳测年的。这些日期可能会有相对较大的误差，测年不同的材料可能会得出不同的年龄。因此，这些不确定性被带到了对环境重建的解释中。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，因为旧碳是从周围的土壤进入湖泊的。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。解决这个问题的一个可能的方法是放射性碳枣摇蚊(不咬人的蚊子)幼虫头部胶囊。摇蚊幼虫通常是淡水生态系统中主要的底层无脊椎动物(仅次于寡毛类)。头部胶囊是由一种名为甲壳素的坚固聚合物制成的，保存在湖泊沉积物中。之前的两项研究表明，通过放射性碳测年摇蚊头部胶囊，并将其与有机湖泥测年进行比较，更年轻的枣是由摇蚊产生的。这表明有机泥浆中含有旧碳，因此摇摆学测年更准确。然而，到目前为止，还没有对这一假设进行测试。我们的目标是从一系列已知年龄的最近冰岛湖泊沉积物中提取摇蚊头部胶囊的放射性碳年代样本，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。然后，我们将评估样品的放射性碳浓度，并尝试将它们与最近过去的已知大气放射性碳浓度进行匹配，这段时间由于炸弹试验，放射性碳浓度出现了非常明显的“峰值”。这将使我们能够测试使用摇蚊作为测年工具的准确性，即它们死亡时的放射性碳浓度(年龄)与湖泊沉积物中的匹配程度如何。如果可以证明这种准确性，那么这意味着通过测定摇蚊的年龄，我们可以准确地判断出湖泊沉积物中其他物质所指示的变化正在发生的时间。这些分析的结果将使我们能够检验(1)与已知年龄相比，壳聚糖提取物或散装甲壳素样品是否提供了准确的年龄估计；(2)壳聚糖提取物是否总是比散装提取物年轻，突出(尚不清楚)因依赖散装甲壳素样品而产生的测年误差；(3)摇摆体测年与其他形式的放射性碳测年在湖泊沉积物中的可能价值。据推测，使用这些新协议的仔细测年将为从广泛的沉积档案中得出年表提供一种重要的创新方法，其中许多档案目前很难确定年代。

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.