The next chapter of direct phytolith 14C dating: debunking the myth of occluded photosynthetic carbon exclusivity

Abstract:

Radiocarbon dating of carbon (C) encapsulated in phytoliths (phytC) is currently used in many Earth Science disciplines for absolute chronologies and paleoclimatic reconstructions; however, the usefulness of phytC has been hampered by inadequate extraction methods[1] and uncertainties regarding its origin as purely photosynthetic [2,3,4]. An early investigation measuring isotopes from Gramineae spp. grown in free-air C enrichment experiments (FACE), showed that part of of its phytC is from a non-photosynthetic source, thus indicating a dual origin[5]. To demonstrate that non-photosynthetic sources within phytC could be from soil C stocks, we measured ¹⁴C-AMS phytC extracted from a set of Sorghum bicolor growing on known ¹⁴C and d¹³C bulk substrates and hydroponic solutions. The phytolith concentrates and a silica blank were extracted at UCI, CEREGE and Wisconsin using an improved protocol [1,2]. We also measured CO₂ fluxes and isotopic signatures of microbial respiration, percentage of biomass and phytolith extracts produced, and isotopic signatures of the local air and bulk-plant during the growing season of 2012. This allowed comparison of the belowground substrate and nutrient C contributions to phytC ¹⁴C results. Meanwhile, NanoSIMS analyses of phytolith polished sections was used to locate phytC in the phytolith siliceous structure [6]. These results will be shown and discussed.

[1] Corbineau et al. 2013 R. Paleobot. Palyn. 197: 179

[2] Santos et al. 2010 T. Radiocarbon 52:113

[3] Santos et al. 2012a Biogeosci. 9:1873

[4] Santos et al. 2012b Biogeosci. Discussion 9:C6114

[5] Reyerson et al. 2013 AGU Fall meeting 2013 (Abstract ID: 1803125).

[6] Alexandre, et al., submitted.