IMPROVED PREPARATION METHOD AND AMS DATING RESULTS FOR POLLEN FROM MODERN AND FOSSIL SAMPLES FROM CONTRASTING LAKES IN THE SIERRA NEVADA, CALIFORNIA

High-resolution chronologies for lakes and meadows are crucial for terrestrial paleoenvironmental reconstructions. Here we present an improved method for extracting pollen for AMS measurement using low-carbon heavy liquids and bleach in place of the commonly used hydrofluoric acid and acetolysis steps (respectively), to produce the cleanest possible sample for efficient sorting by flow cytometry and radiocarbon dating. We report radiocarbon dates for pollen from Mono Lake and Fallen Leaf Lake (FLL), both located in the northern Sierra Nevada, but characterized by very different sediments. FLL contains homogeneous, organic- and pollen-rich sediment and was selected as an ideal system due to the pollen-rich sediments and the available high-resolution age model based on abundant macrofossils (Noble et al., 2016 Quat. Sci. Rev.). In contrast, Mono Lake presents heterogeneous sediments with tephras and calcareous-rich layers with highly variable pollen concentrations. Due to this heterogeneity, several adjustments to the protocol were required during the pollen extraction and some layers did not produce sufficient pollen grains to be reliably dated. At least 50,000 pollen grains were sorted and dated when possible, but sufficient pollen was not always available in a standard sample of 1-2 cc of sediment; the amount of sediment was increased to 3-4 cc in some cases. The smallest sample dated with a reliable result contained ~16,000 grains and 0.042 mg C; more typical samples with ~50,000 grains yielded ~ 0.270 mg C and had better analytical uncertainties than smaller samples. Modern samples were collected along surface transects from the shore of Mono Lake into the mountains and dated to obtain a representation of the age of the pollen deposited into the lake. Comparison of dates on macrofossils and pollen in the same layers from Fallen Leaf Lake cores shows that pollen concentrates can produce radiocarbon dates of similar accuracy and precision as macrofossils, suggesting that pollen can be used to build a high-resolution chronology when terrestrial macrofossils are rare or completely absent in sediment cores.