Testing Pollen Sorted by Flow Cytometry as the Basis for High-Resolution Lacustrine Chronologies

Abstract

Documenting leads and lags in terrestrial records of past climate change is critical to understanding the behavior of Earth’s natural climate system and making reliable predictions of future climate conditions. However, uncertainties of several hundred years in age models make it difficult to distinguish synchronicity and feedbacks in paleo archives. In lakes this is often due to the lack of terrestrial macrofossils in climate-sensitive locations, such as high alpine or dryland settings. The potential of radiocarbon (14C) dating of pollen has long been recognized, but the difficulty of cleanly separating pollen from other kinds of organic carbon has limited its usefulness. Here we report 14C ages on pollen separated by flow cytometry, from a set of closely spaced samples from Mono Lake, California. The accuracy of the pollen ages is tested using well-dated bracketing tephras, the South Mono and North Mono-Inyo tephras. In spite of the purity of the sorted samples, the pollen dates are older than the bounding tephras by ~400 yr, similar to some other pollen-dating studies. While improvements in sample preparation protocols are planned, understanding the geological processes involved in the production, preservation, and deposition of pollen at each site will be critical to developing robust high-resolution age models.

Erroneously Old Radiocarbon Ages From Terrestrial Pollen Concentrates in Yellowstone Lake, Wyoming, Usa

“…Third, reworked pollen is suggested as a major source of old-carbon contamination in lacustrine settings where extensive old lacustrine deposits occur at the surface (Mensing and Southon 1999;Zimmerman et al 2019) or redeposition occurs from glacial melt (Neulieb et al 2013;Zhang et al 2017). There is no evidence of a Holocene glacial meltwater source in the Yellowstone Lake watershed.…”

“…carbonates, algal remains, humic acids) from the sample. Although some studies suggest dating errors as a result of incomplete separation of non-pollen material (Regnéll 1992;Richardson and Hall 1994) or incorporation of reworked pollen grains (Mensing and Southon 1999;Zimmerman et al 2019), pollenconcentrate ages typically produce more reliable chronologies than bulk-sediment ages where reservoir effects are a concern (e.g. Brown et al 1989;Vandergoes and Prior 2003;Newnham et al 2007;Fletcher et al 2017).…”

Flow cytometric analysis of pollen and spores: An overview of applications and methodology

“…Gates created using biparametric scatterplots of these parameters can therefore be used to circumscribe mature pollen, spores, or microspore events before measurement, counting, and/or sorting [9,23]. Natural autofluorescence of sporopollenin in the pollen wall can also be used for taxonomic identification and sorting of environmental pollen samples (e.g., airborne pollen [24]) and fossil pollen from sediments [23,25]. While such autofluorescence is relatively uniform within species, users should be aware that exceptions have been observed [26].…”

“…Since the pollen cells are long dead, these applications rely mostly on natural autofluorescence of sporopollenin in the pollen wall or whole pollen light scatter properties (SSC and FSC),which may allow their assignment to a particular (group of) plant species[23,32]. Promising applications include using flow sorting to obtain fossil pollen samples in sufficient concentration and purity to allow either radiocarbon dating of layers in a sedimentary sequence[23,25] or extraction and sequencing of ancient DNA[33].Because the objective of flow sorting is to obtain the correct particle type in as pure a form as possible, methods should be validated using samples of known composition. Recovery of some particle types may be more difficult than others; for example, vegetative nuclei may be more prone to disruption than sperm nuclei[22].…”

Breakthrough in purification of fossil pollen for dating of sediments by a new large-particle on-chip sorter

“…The most powerful feature of the presented sorting method is its capability to sort most pollen taxa, i.e., from the smallest types to large types of up to 170 m at high throughput, which allows efficient concentration of fossil pollen from virtually all sedimentary deposits for 14 C dating or other analytical applications. This is a breakthrough achievement compared to conventional particle sorters that can sort particles up to a size of only 100 m (sortable particle volume of 0.52 nl) at a throughput of 10,000 Hz (30) or conventional large-particle sorters, such as those by Union Biometrica, which sort particles in the size range of 20 to 400 m (sortable particle volume of 4.0 pl to 34 nl) at low-throughput rates of maximum 20 particles/s (17,31). Compared to these two kinds of conventional sorters previously used for pollen purification, the sorting performance [defined as "sortable particle volume" × "throughput" (14)] of the new device (sortable particle volume of up to 7.0 nl; throughput of 5000 Hz) is on April 22, 2021 http://advances.sciencemag.org/ Downloaded from about 7.5 and 59 times higher, respectively.…”