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AMS radiocarbon dating of pollen concentrates in a karstic lake system

机译：AMS Radiocarbon在喀斯特湖系统中的花粉浓缩物

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In lake sediments where terrestrial macrofossils are rare or absent, AMS radiocarbon dating of pollen concentrates may represent an important alternative solution for developing a robust and high resolution chronology suitable for Bayesian modelling of age-depth relationships. Here we report an application of the heavy liquid density separation approach (Vandergoes and Prior, Radiocarbon 45:479-492, 2003) to Holocene lake sediments from karstic Lake Sidi Ali, Morocco. In common with many karstic lakes, a significant lake C-14 reservoir effect of 450-900 yr is apparent, evidenced by paired dates on terrestrial macrofossils and either aquatic (ostracod) or bulk sediment samples. AMS dating of 23 pollen concentrates alongside laboratory standards (bituminous coal, anthracite, IAEA C5 wood) was undertaken. Concentrates were prepared using a series of sodium polytungstate (SPT) solutions of progressively decreasing density (1.9-1.15 g/cm(3)) accompanied by microscopic analysis of the resulting residues to allow quantification of the terrestrial pollen content. The best fractions (typically precipitating at 1.4 -1.2 g/cm(3)) yielded dateable samples of 0.5-5 mg (from sediment samples of similar to 15 g), with C content typically similar to 50% by weight. Terrestrial pollen purity ranges from 29% to 88(mu = 67%), reflecting the challenge of isolating pollen grains from common aquatic algae, e.g. Pediastrum and Botryococcus. A Poisson-process Bayesian depositional model incorporating radiocarbon (pollen and macrofossil) and Pb-210/Cs-137 data is employed. As all pollen samples incorporate some non-terrestrial organic matter, we assume an exponential outlier distribution treating each pollen concentrate datum as an old outlier and terminus post quem. This approach yields strong data-model agreement, and differences between the prior and posterior age distributions are furthermore consistent with theoretical offsets anticipated for the known reservoir ages and sample-specific terrestrial content. This application of the pollen concentrate dating approach reinforces the importance of microscopic inspection of the residues during the separation and sieving stages. Sample specific differences mean that the pollen concentrate preparation cannot be reduced to a simplistic "black box" protocol, and dating and subsequent age-model development must be supported by detailed analysis of the microfossil content of the sediments. (C) 2017 Elsevier B.V. All rights reserved.
机译：在苏术大甲状腺炎是稀有的或不存在的湖泊沉积物中，花粉浓缩物的AMS RadioCarbon约会可以代表一种用于开发适用于年龄深度关系的贝叶斯建模的强大和高分辨率年表的重要替代解决方案。在这里，我们报告了重型液体密度分离方法（Vandergoes和Priain，RadioCarbon 45：479-492,2003）到摩洛哥喀斯特湖锡施湖沉积物的应用。与许多岩溶湖泊共同，明显的C-14水库效应为450-900岁，可通过成对的陆地宏甲基汞和水生（Ostracod）或散装沉积物样品来证明。 AMS约会23种花粉浓缩物，并沿着实验室标准（烟煤，无烟煤，IAEA C5木材）进行。使用一系列逐渐降低密度（1.9-1.1.15g / cm（3））的一系列磷酸钠（SPT）溶液制备浓缩物（1.9-1.1.15g / cm（3）），伴随着所得残基的显微分析，以允许定量陆地花粉含量。最佳级分（通常在1.4-1.2g / cm（3）中沉淀）产生0.5-5mg（从类似于15g的沉积物样品）的可调性样品，C含量通常类似于50重量％。陆地花粉纯度范围从29％到88（mu = 67％），反映了孤立普通水生藻类的花粉谷物的挑战，例如， Pediastrum和Botryococcus。采用泊松过程贝叶斯沉积模型，其包含radiocarbon（花粉和致癌物质）和PB-210 / CS-137数据。随着所有花粉样品都包含一些非陆地有机物，我们假设指数异常分布将每个花粉浓缩物数据视为旧的异常值和终端职位。该方法产生强大的数据模型协议，并且之前和后期分布之间的差异还与预期的已知储层年龄和特异性种群含量的理论抵消一致。花粉浓缩剂的这种应用增强了分离和筛分阶段在分离和筛分阶段的微观检查的重要性。样品特定差异是指花粉浓缩制剂不能降低到简单的“黑盒子”方案，并且必须通过对沉积物的微基含量的详细分析来支持约会和随后的年龄模型开发。 （c）2017 Elsevier B.v.保留所有权利。

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