RADIOCARBON DATING POLLEN GRAINS SORTED BY FLOW CYTOMETRY FROM LAKE SEDIMENTS OF MONO LAKE (CALIFORNIA)

Building a reliable chronology is a crucial step in most studies looking at environmental changes through time (e.g., ecological, climatic, volcanic, geomagnetic), but macrofossils that can be radiocarbon dated are not always present in the lake sediments used in many of these studies. In cases when terrestrial macrofossils are rare or absent, pollen can represent a valuable option in building high-resolution radiocarbon-based chronologies. Nevertheless, separating and concentrating pollen samples for AMS measurement present some difficulties related to the types of sediments and to the preservation and, in particular, the concentration of pollen. In addition, pollen grains are subjected to various processes before and after deposition into the sediments (e.g., degradation and bioturbation) that contribute to altering their concentration and distribution. Different chemical and physical approaches have been used to extract and concentrate pollen from sediments. Recently, we have worked to develop methodologies that allow flow cytometric sorting of pollen from such pollen-concentrate samples.

Here we present preliminary results of AMS measurements on modern and fossil samples from the Mono Lake basin, in the eastern Sierra Nevada of California. The modern samples are stream sediments, mineral soils and moss cushions, collected along surface transects from the shore of the lake towards the hills, to obtain a set of samples representative of the age of the pollen subjected to erosion and subsequent deposition in the lake. The fossil pollen samples are concentrated from lake sediment core subsamples. Pollen from both modern and fossil specimens has been extracted following a modified version of a protocol normally used for pollen analysis. The samples were treated with HCl, KOH, sodium polytungstate (Polygee), and NaClO (sodium hypochlorite) in order to obtain an “as clean as possible” pollen concentrate. The pollen concentrates were then sorted by flow cytometry to separate the pollen grains from other organic particles that remained and are potential sources of carbon contamination.

Our results should provide some indication of the potential for significant contribution of pollen that has aged on the landscape into the lake sediments in Mono Lake basin.