Flow cytometry method for purification of fossil pollen from Lake Van varved sediments for AMS radiocarbon dating – a new approach

为重建气候和环境变化而制作精确和可靠的地质年代学框架是一项挑战。在这方面，在2010年ICDP运动期间钻探的土耳其凡湖记录具有非同寻常的潜力，这是因为保存了每年一次的层状沉积物，可用于结合放射性碳测年的火山年代学。花粉颗粒可用于AMS放射性碳测量，特别是在湖泊沉积物中，那里很少或根本没有陆地植物大型化石(如在凡湖)。因此，当获得的材料具有足够的纯度和浓度时，这是必不可少的。目前提取化石花粉的方法都有很大的缺点，即在工作量和丰度之间的权衡，以及在准备进行放射性碳测年的样品中化石花粉的纯度。在这里提出的研究项目中，我们将应用一种新颖和创新的方法，能够快速纯化化石花粉，用于基于Tennant等人的基于流式细胞术的AMS放射性碳测年。(2013年)。到目前为止，这种方法既没有用于建立沉积记录的系统时间框架，也没有用于改进连续的变化年代学。我们对该方法进行了测试，作为初步工作，基于面包湖样品。AMS技术的最新发展，即减少14C分析所需的样本量和用流式细胞仪纯化花粉，成功地结合在一起，使测年结果在基于VVE计数的预期范围内。基于VOLVE计数和14C测量之间的交叉核对，我们将首次为凡湖记录制作一个涵盖过去约14,000年的强大日历年时间尺度。这是进行高分辨率花粉分析以确定这段时间段内气候突然和极端变化的显著时期的先决条件。

Producing precise and robust geochronological frameworks for reconstructing climatic and environmental changes is a challenge. In this respect, the Lake Van record from Turkey drilled during an ICDP campaign in 2010 has an extraordinary potential caused by the preservation of annually laminated sediments which can be used for a varve chronology in combination with radiocarbon dating. Pollen grains can be used for AMS radiocarbon measurements especially in lake sediments, where little or no terrestrial plant macrofossils are present (such as in Lake Van). Therefore, it is essential when the obtained material is of sufficient purity and concentration. Current methods of extracting fossil pollen all have significant drawbacks regarding the trade-off between effort and abundance as well as purity of fossil pollen in the samples to be prepared for radiocarbon dating. In the proposed research project presented here, we will apply a novel and innovative method enabling rapid purification of fossil pollen for AMS radiocarbon dating based on flow cytometry after Tennant et al. (2013). This method has neither been used so far for establishing a systematic time frame of a sedimentary record nor for improving a continuous varve chronology. We have tested the method as preliminary work based on Lake Van samples. The successful combination of the recent development in AMS technique, i.e., downscaling of sample amount needed for 14C analysis and purification pollen by flow cytometry, led to a dating result within the range of expectation based on varve counting. Based on a cross check between varve counting and 14C measurements, we will produce for the first time a robust calendar-year time scale for the Lake Van record encompassing the last ca 14,000 years. This is a precondition for high-resolution pollen analyses to identify remarkable periods of abrupt and extreme climate change during this time interval.