A rapid method for measuring ancient coccolith size

Coccolithophores are single-celled phytoplankton that produce calcite plates called coccoliths. Coccolithophores play a key role in long-term ocean carbon storage through the sedimentation and burial of their calcite (calcium carbonate) exoskeletons. Coccolith size is important in determining how much carbonate is ultimately buried. We present a novel method for measuring ancient coccolith size using imaging flow cytometry combined with cross-polarised light (ISX +PL ). This technique provides a rapid and high throughput alternative to manual and existing automated techniques by enabling the analysis of ≈1000 objects per second without time-consuming slide preparation. We have applied ISX +PL to the fossil record for the first time, and as a proof of method, analysed samples from Goban Spur (DSDP Site 548) and Ceara Rise (ODP Site 925/9) to reconstruct 66 million years of coccolith size. We show that data obtained from ISX +PL is consistent with published coccolith size trends, including the reported decrease in size toward the modern. Changes in coccolith size below 3 μm should be treated with caution because this exceeds the accuracy of the method. The inexpensive, rapid, and less labour intensive approach increases the speed of analysis compared to existing methods. The ISX +PL method enables users to analyse more samples, increase the robustness of their dataset due to a high specimen count (≈10,000 per sample), and explore a wealth of morphological features recorded in the digital images of coccoliths. The method offers flexibility, enabling users to adapt it to their needs—for instance, by targeting specific morphologies—thereby expanding its applicability. • Coccolith size is important in determining how much carbon is stored in the ocean. • We present a rapid, high-throughput method for measuring fossil coccolith size. • We reproduce trends in published size data, giving confidence in the method. • The technique's simplicity, speed and throughput enables analysis of more samples and specimens.