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

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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.