MORE DATES, BETTER DATES: PROGRESS IN CHEMICAL CONCENTRATION, FLOW-CYTOMETRIC PURIFICATION AND RADIOCARBON DATING OF POLLEN FROM TERRESTRIAL SEDIMENTARY ARCHIVES

The drive for higher-resolution and more-precise chronologies in recent years is especially intense in the radiocarbon-datable part of Earth's timescale, the last ~50,000 yr, because of the immediate relevance of Holocene paleoenvironmental records and efforts to reconstruct the history of human habitation and associated land-use practices. The lack of sufficient terrestrial macrofossils in lake, meadow, alluvial fan, and other sedimentary archives is a challenge that many studies have attempted to address with ¹⁴C dating of pollen, due to its atmospheric carbon source and near-ubiquity in terrestrial environments. Significant strides have been made in recent years using flow cytometry for achieving nearly pure pollen samples, but several tasks remain, including routine sample flow from chemistry lab to flow cytometry facility to accelerator mass spectrometry (AMS) facility; robust dating of old and small samples; establishment of reliable ¹⁴C-dead background materials; investigation of flow cytometry for eliminating specific types of unwanted pollen; and widespread application of the technique to identify settings where the technique is useful.

Here we report initial results from a partnership to develop and apply pollen ¹⁴C dating to sediments from western U.S. lakes across the spectrum of sample size and age. While pollen dates from Holocene sediments in lakes with past lake-level highstands, such as Mono Lake (eastern Sierra Nevada), may be affected by old pollen eroded from exposed sediments, in sediments from highstands such contamination may be limited. In some cases, even an additional uncertainty of several hundred years may be acceptable in a pollen date, if a large gap with no other datable material exists. Very old sediments (>40,000 yr) at Clear Lake in California's Coast Ranges have been successfully dated with pollen ¹⁴C dates. Very small samples (<100 micrograms C) have also been successfully dated; however, with old and small samples, analytical uncertainties grow significantly. Two main goals of our current work are identification of key controls in the chemical concentration of pollen samples (e.g., very fresh base and bleach solutions are critical) and identifying and characterizing an abundant source of ¹⁴C-dead material that can be prepared alongside unknown samples, for example from the >65,000 yr old Wilson Creek sediments at Mono Lake.