Rapid enumeration of rare microscopic particles in sedimentary archives using imaging flow cytometry

The accurate enumeration of microscopic particles is essential for deriving a wide range of climate proxies from sedimentary archives. Traditionally, particle counts require visual examination by microscopy, a time-consuming process that often analyses a small aliquot of the total sample material. As a result, rare particle populations and fine-scale variability are often poorly constrained, as a fast method to count an entire sample remains elusive. Here, we present a flow cytometric approach that enables the near-complete quantification of rare particle populations using high-throughput, multispectral imaging. We demonstrate this method using fossil pollen obtained from 0.5 cm³ peat core samples collected from the Falkland Islands. Analysis takes less than 2 hours per sample and images nearly all the particulate matter. These improvements in speed and precision facilitate the detection of smaller-scale fluctuations as well as the robust quantification of rare particle types. Using an isopycnic approach, we further show that this method can accurately track non-native, wind-blown pollen species that constitute <1% of the assemblage. Imaging flow cytometry can thus reveal changes that are impractical to find manually, expanding the range and resolution of climate proxies obtainable from sedimentary records.