When the carbon being dated is not what you think it is: Insights from phytolith carbon research

• Post by: Doris Barboni
• September 14, 2018
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During phytolith formation, a small amount of carbon (C) can become occluded in the siliceous structure (<1% of the phytolith dry weight). Since the major process by which plants fix carbon is photosynthesis, researchers assumed that the carbon occluded in phytoliths (phytC) must reflect the atmosphere’s CO₂ value at the time when the plant was growing. This assumption underlies radiocarbon dating of phytC for archaeological reconstructions as well as the idea that phytolith sequester a significant amount of carbon from the atmosphere. Nevertheless, systematic investigation of literature data led us to question the assumption that phytC comes exclusively from photosynthetic pathway.

We launched a comprehensive study on phytC and its molecular makeup. We also evaluated phytolith structure, cavities and porosity levels among other features. Through below- and above-ground C manipulation experiments, we showed that a proportion of phytC is from soil-C acquired by the plant trough their roots during nitrogen (N) acquisition. Since soil-C is part of a carbon continuum with multiple organic compounds, it makes phytC a heterogeneous pool of carbon with a large array of ¹⁴C ages of distinct oxidation-resistance. This fact precludes its use as a dating tool in archaeology and diminishes its atmospheric sequestration impact.

In order to give a broader view of these issues, and to address speculative explanations for the skewed phytC ¹⁴C data in the literature, we summed our findings in a review and link those to information on N uptake by plants, C-N transport in plant cells, and Si-cell molecular formation.

Three-dimension reconstruction of a grass short cell phytolith structure using micro-scale X-ray imaging and the software XMReconstructor. It shows internal cavities connected to the outside.