BUILDING A CHRONOLOGY BASED ON POLLEN CONCENTRATES: AMS MEASUREMENTS AND FLOW CYTOMETRY SORTING

When terrestrial macrofossils are rare or completely absent in sediment cores, radiocarbon dating of pollen grains may represent the most valuable resource to obtain a chronology. Previous studies have demonstrated that the two most challenging aspects of this application are the isolation of the pollen grains from other organic material, to eliminate possible C-bearing materials of different ages, and obtaining enough carbon to produce a useful age. We present an improved method for extracting and purifying pollen concentrates through the flow cytometry sorting process and AMS measurements. The application of flow cytometry to the concentrated pollen allows the separation of individual pollen grains from other particles such as microcharcoal and other microfossils and provides the purest pollen concentrate. We present the results of AMS measurements of fossil samples sorted by flow cytometry from cores from Mono and Fallen Leaf Lakes, both located on the eastern side of the Sierra Nevada. These two lakes present very different sediments: Fallen Leaf Lake contains homogeneous, organic- and pollen-rich sediment while Mono Lake presents very heterogeneous sediments with volcanic ashes and calcareous-rich layers with various pollen concentrations. Also, modern samples were collected along surface transects from the shore of Mono Lake towards the Sierra to test the extent to which pollen could be aged on the land surface. Samples were chemically treated to separate the pollen from other organic and inorganic material before being sorted; the different sediment types required adjustments during the chemical treatment, and showed different reactions and final pollen concentrations. When possible, at least 50,000 pollen grains were sorted by flow cytometry and dated, but this amount of pollen was not always available in a standard sample of 1-2 cc of sediment. The smallest sample dated with reliable results contained ~16,000 grains and 0.042 mg C. The correlation between the number of pollen grains and the carbon content is not consistently linear as expected, maybe relating to the preservation and conservation process. Comparing two different types of sediments and examining modern pollen are steps toward understanding if and where pollen can be used to build a reliable high-resolution chronology.