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

放射性碳测年摇蚊头胶囊能否提高湖泊沉积物的年代准确性？

• 批准号: NE/D007828/1
• 负责人: Peter Langdon
• 金额: $ 4.39万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FD007828%2F1
• 关键词: radiocarbon; dating; chironomid; head; capsules

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.

通过湖泊沉积物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率，并比较不同沉积源的发现，对这些序列进行准确的测年是基础。目前，大多数湖泊沉积物序列均通过放射性碳测年。这些日期可能会出现相对较大的误差，并且不同材料的年代测定可能会给出不同的年龄。因此，这些不确定性被带入环境重建的解释中。我们的目标是减少这些不确定性。由于旧碳从周围土壤进入湖泊，对大多数湖泊沉积物进行放射性碳测年是有问题的。因此，我们需要找到一种方法，最大限度地减少或完全消除这种情况。解决这个问题的一种潜在方法是对摇蚊（非咬性蠓）幼虫头部胶囊进行放射性碳测年。摇蚊幼虫通常是淡水生态系统中存在的主要底层无脊椎动物（仅次于寡毛类）。头部胶囊由一种称为甲壳素的坚固聚合物制成，保存在湖泊沉积物中。之前的两项研究表明，通过放射性碳测年摇蚊头部胶囊，并将其与有机湖泥测年进行比较，摇蚊产生了更年轻的日期。这表明有机泥含有古老的碳，因此摇蚊年代更准确。然而，迄今为止尚未对这一假设进行测试。我们的目标是对来自一系列已知年龄的近期冰岛湖泊沉积物中的摇蚊头部胶囊样本进行放射性碳测年，其中包括从头部胶囊中提取的壳聚糖提取物（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已知的大气放射性碳浓度相匹配，这一时期由于炸弹测试而出现非常明显的放射性碳“峰值”。这将使我们能够测试使用摇蚊作为年代测定工具的准确性，即它们死亡并融入湖泊沉积物时的放射性碳浓度（年龄）匹配程度如何。如果这种准确性能够得到证明，那么这意味着通过对摇蚊进行年代测定，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何时发生。这些分析的结果将使我们能够检查（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，突出显示由于依赖散装甲壳素样品而产生的（目前未知的）年代测定错误； (3) 摇蚊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，使用这些新协议进行仔细的年代测定将为从广泛的沉积档案中推导出年表提供一种重要的创新方法，其中许多沉积档案目前难以确定年代。