Possible source of ancient carbon in phytolith concentrates from harvested grasses

[摘要] Plants absorb and transport silicon (Si) from soil, and precipitation of Siwithin the living plants results in micrometric amorphous biosilicaparticles known as phytoliths. During phytolith formation, a small amount ofcarbon (<2%) can become occluded in the silica structure (phytC) andtherefore protected from degradation by the environment after plant tissuedecomposition. Since the major C source within plants is from atmosphericcarbon dioxide (CO₂) via photosynthesis, the current understanding isthat the radiocarbon (¹⁴C) content of phytC should reflect the ¹⁴Ccontent of atmospheric CO₂ at the time the plant is growing. Thisassumption was recently challenged by ¹⁴C data from phytolithsextracted from living grasses that yielded ages of several thousand years (2–8 kyr BP;in radiocarbon years "Before Present" (BP), "Present" beingdefined as 1950). Because plants can take up small amounts of C of varyingages from soils (e.g., during nutrient acquisition), we hypothesized thatthis transported C within the plant tissue could be attached to or evenembedded in phytoliths. In this work, we explore this hypothesis byreviewing previously published data on biosilica mineralization and plantnutrient acquisition as well as by evaluating the efficiency of phytolithextraction protocols from scanning electron microscope (SEM) images andenergy dispersive spectrometer (EDS) analyses from harvested grassesphytolith concentrates. We show that current extraction protocols areinefficient since they do not entirely remove recalcitrant forms of C fromplant tissue. Consequently, material previously measured as "phytC" maycontain at least some fraction of soil-derived C (likely radiocarbon-old)taken up by roots. We also suggest a novel interpretation for at least someof the phytC – which enters via the root pathway during nutrient acquisition –that may help to explain the old ages previously obtained from phytolithconcentrates.

[学科分类] 地球化学与岩石