Freeze-dried is as good as frozen: Evaluation of differential preservation of pollen grains in stored lake sediments

Abstract

In Quaternary studies, sediment storage methods (e.g., freezing, freeze-drying, vacuum-drying) vary across paleoecological disciplines. Typically, palynologists store sediment cores in a freezer or refrigerator; however, sediments already collected and stored (freeze-dried) for paleolimnological studies are available and potentially useful for pollen analysis. Here, we investigate the effect of freeze-drying on pollen grain structure compared to pollen recovered from frozen sediments. Pollen was recorded for eight deterioration categories for paired samples of freeze-dried and frozen sediment obtained from a small alpine lake (Sentinel Lake) in Banff National Park, Canada. No statistically significant differences in pollen deterioration were detected between storage methods for identified pollen taxa and groups (Pinus, Picea, Abies, Alnus, non-arboreal, arboreal). There were also no statistically significant differences between freeze-dried and frozen samples and their mean pollen sum, number of indeterminable pollen grains and number of taxa detected per sample. Instead, deterioration of pollen for both storage methods increased with the depth of sample taken from the core. These results demonstrate freeze-drying as a useful method for storing sediments intended for palynological research. Our findings highlight how limnologists and palynologists can collaboratively share freeze-dried archival sediment samples to reduce field costs and strengthen interpretation of data through analysis of multiple proxies.