METHODOLOGICAL ADVANCES IN RADIOCARBON DATING OF POLLEN

There are two primary methodological challenges in radiocarbon dating of pollen: (1) acquiring enough pollen for robust AMS measurements and (2) quantitatively removing unwanted organic and inorganic materials. Previous attempts to date pollen have utilized heavy liquids, sieving, and mouth pipetting but the success and implementation of these approaches were limited because of incomplete isolation of pollen and the substantial amount of time required to process the samples. Recently, flow cytometry has been used in conjunction with chemical pretreatment techniques to successfully isolate pollen from lake sediments from Lake Suigetsu (Japan) and Mono Lake, California. Flow cytometry detects light-scatter and fluorescence characteristics of particles using laser excitation in a fluidics-based system and has resulted in exceptionally pure pollen extracts. We applied similar chemical pretreatment techniques and flow cytometry to date pollen recovered from alluvial sediments at White Sands National Park that contain ancient human footprints and megafaunal tracks. Each sample consisted of ~75,000 grains of conifer (mostly pine) pollen and yielded ages that were statistically indistinguishable from ages obtained previously from the same stratigraphic levels. However, background (or blank) levels in that study (equivalent to <30,000 ¹⁴C years) were much higher than standard blank values (equivalent to >40,000 ¹⁴C years), which places an artificial constraint on the practical limit of dating terrestrial pollen. Here we describe a new source of radiocarbon-free pollen from the Ziegler Reservoir fossil site near Snowmass Village, Colorado from sediments dating to ~100,000 years before present that we are using to establish new blank levels. We also describe new pretreatment protocols that have dramatically reduced the time required to process the samples. When fully realized, these advances will provide new opportunities for determining the age of sediments that do not contain contemporaneous macrofossil remains in a wide variety of geologic settings and hydrologic environments worldwide.