Comment on "Possible source of ancient carbon in phytolith concentrates from harvested grasses" by G. M. Santos et al. (2012)

Santos et al. (2012) address the important issue that ¹⁴C dating of the carbon occluded in silica phytoliths (PhytOC) isolated from contemporary plant materials can produce ages that are incompatible, being often several kyr older, with both their known recent origin and the ¹⁴C age of the bulk plant material. In their article, Santos et al. (2012) propose that the anomalously old ¹⁴C carbon dates of PhytOC from harvested plant materials are based on plants taking up "old" dissolved soil carbon to the plant by roots during nutrient uptake. They then propose that this old soil-derived carbon is subsequently partitioned from the general plant biomass into either the silica phytoliths they produce or as recalcitrant organic matter elsewhere in the plant. We suggest that the full data available for PhytOC ¹⁴C dating do not support this hypothesis. Santos et al. (2012) also address the important issue of contamination of PhytOC by general plant biomass material that can occur with procedures that incompletely extract phytoliths. Whilst we agree that such contamination needs to be avoided when examining the nature of PhytOC, we also point out that the converse problem, i.e. removal of PhytOC by over-vigorous extraction procedures, can also have important adverse consequences.