Is 14C-AMS analysis of phytoliths a suitable tool ?

Résumé 0

14C AMS analysis of occluded carbon in phytoliths (phytC) is a promising dating tool for palaeoenvironmental and archaeological studies. In order to assess the accuracy of this method, different tests were recently carried out on large phytolith concentrates of phytC samples extracted from soils and harvested plants, in association with blank samples of SiO2 powder to check the absence of carbon contamination during the treatments. Despite this precaution, 14C values from recent harvested plants were inexplicably old (2 - 8 ka years BP). Nevertheless, we noticed that many chemical extraction protocols that were used did not lead to samples totally free of organic matter. In order to tackle this problem, and as a first step, the efficiency of common extraction protocols from the literature were tested. Samples were analyzed by SEM/EDX in order to assess the purity of the siliceous material following extraction. As a result of these tests, a new extraction protocol combining acid digestion, oxidation and dry ashing to acquire pure samples of phytoliths from harvested plants is proposed. In a second step, modern and well dated archaeological materials (harvested plants grown within a FACE experiment and plant residues from a 17th century French mummy) were analyzed in 14C-AMS. Results should allow either to demonstrate the reliability of 14C-AMS analysis of phytolith occluded carbon as a dating tool or trigger further investigations of possible sources of old occluded carbon in phytoliths if the 14C ages are still older than expected.