The carbon isotopic composition of occluded carbon in phytoliths: A comparative study of phytolith extraction methods

Abstract

Phytoliths are microscopic siliceous particles that precipitate within epidermal cells of plant leaves. Phytolith morphology and its carbon isotopic composition (δ13C) are often used to understand the past vegetation composition. To extract phytoliths from modern plants, two different wet-oxidation, dry ashing and microwave digestion techniques have been used. However, the impact of these techniques on the δ13C value of phytoliths of different plant species (trees, shrubs and grasses) is not well understood. The present study compares the recovered materials (RM) from these four extraction methods by scanning electron microscope (SEM) and carbon isotopic composition (δ13CRM). We find that the recovery yield is high in one of the wet-oxidation methods, showing significant organic residues (tissues) as evident from the SEM images. In other three methods, the SEM images show near-complete removal of bulk plant residues. However, the wide variation (up to 14‰) seen in the δ13CRM values (even within a species) suggests a difference in response of bulk tissues and phytoliths towards the extraction methods. The difference in organic constituents within the cell wall and lumen phytoliths also possibly contribute to the variation in δ13CRM values. The characteristic difference observed in 13C/12C ratio between the RM and bulk tissue (Δ13CRM-Bulk) suggests the predominance of lumen phytoliths in grasses and cell wall phytoliths in trees/shrubs species. Since the same extraction methods are also used to remove the organic residues adhering to the phytoliths in soils/paleosols, careful consideration is required before employing phytolith δ13C value for vegetation reconstruction in paleoecological and archeological studies.