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

Alexandre, Anne

Corbineau, Rémi

Reyerson, Paul,

Santos, Guaciara,

English

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Is 14C-AMS analysis of phytoliths a suitable tool ?
Rémi Corbineau, Guaciara Santos, Anne Alexandre, Paul Reyerson
To cite this version:
Rémi Corbineau, Guaciara Santos, Anne Alexandre, Paul Reyerson. Is 14C-AMS analysis of phytoliths
a suitable tool ?. American Geophysical Union Fall Meeting 2011, Dec 2011, San Francisco (California),
United States. ￿halshs-00803013￿
Is 14C-AMS analysis of phytoliths a suitable dating tool ? 
Rémi Corbineau2-1, Guaciara M. Santos3, Anne Alexandre1, Paul E. Reyerson4 
1CEREGE, CNRS, Aix-Marseille Université, France, 2LAMM, CNRS, Aix-Marseille Université, France,
3Earth System Science, KCCAMS/UCI, USA, 4Depart. of Geography, University of Wisconsin-Madison, USA.
B31A-0315
contacts: corbineau@cerege.fr/gdossant@uci.edu/alexandre@cerege.fr/reyerson@wisc.edu
Introduction
Phytoliths are micrometric amorphous silica 
particles that form in living plant cells.
Fossil phytolith assemblages are used as 
vegetation, climate, and archaeobotanical 
indicators.
Quantification of phytoliths helps to assess the 
global biogeochemical cylcle of silicon.
Carbon (phytC) can be trapped during phytolith 
formation and represents less than 2% by dry 
weight.
Radiocarbon dating of phytC is immediatly attractive because phytC is fully protected 
from oxidizing environments (Wilding 1967). Therefore, it would contribute to obtaining 
new absolute chronologies and would bring new insights on the global silicon cycle.
I 30mm
Typical phytolith structures
 found in grass leaf tissues. 
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scale bar = 10mm
II State of the art: phytC investigations
Chemical composition
d13C signature
Attempts were made to use phytC d13C 
signatures as C3/C4 plant proxies using 
IR-MS (Kelly et al. 1991, Mclaran and Umlauf 2000, Smith 
and White 2004, Webb and Longstaffe 2010).
Direct 14C-AMS dating
- aliphatic compounds
- graphite or coal (if heated at 400°C)
- glycoproteins but no DNA
- sugars
- uronic acid
- lignin
- proteins
- lipids
- aromatic hydrocarbons (if heated at 400°C)
F
ig
u
re
 fro
m
 S
m
ith
 a
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d
 W
h
ite
 2
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4
.
Phytolith d13C values are more negative 
than grass cellulose d13C values.
Inexplicably old values/Inverted chronologies.
2. Measurements of PhytC from modern 
plants (Santos et al. 2010).
For the first time :
- Backrground assessments (BLANK)
- Accuracy tests (<1%)
- Replication tests
Bulk plant tissues are modern.
 
PhytC always yields ages of several 
thousand years (from 2,280 BP to 8,040 BP).  
Were the phytolith extraction 
protocols efficient to remove all 
exterior carbon from phytoliths ?
1. Attempts to obtain absolute 
chronologies (e.g. comparaison 14C-AMS phytC/TL 
on tephra) (Kelly et al. 1991, Mulholland and Prior 1993, 
Mclaran and Umlauf 2000, Piperno and Stothert 2003, Prior 
et al. 2005, Rieser et al. 2007).
Grass from rural area in Madison, USA
Bulk plant
 
Atmospheric CO
2 
phytC
Modern (n=2)
 
Modern (n=1)
 
8,040 +- 560 (n=1) yrs BP
GC-MS
Perry et al. 1987, Pironon et al. 2001, 
Smith and Anderson 2001.
Raman spectroscopy
Pironon et al. 2001.
Protein stains + PCR
Elbaum et al. 2009.
III Material and Methods
Dry ashing at 500°C (a)
Microwave digestion (b)
Parr et al. 2001
Wet oxidation using H
2
SO
4
+H
2
O
2
Kelly et al. 1999
Optical microscopic images from Parr et al. 2001.
OM 
30µm 30µm
OM 
a b
Organic matter (OM) still remains.
Application to FACE grass samples
Phytolith samples were obtained from harvested Sorghum bicolor (a C4 silicon accumulator) 
grown within a FACE (Free-Air CO
2
 Enrichment) experiment at Maricopa, Arizona, during 
1998. In these experiments, controlled plots are exposed to enriched CO
2
 concentrations with 
known 14C and d13C values. 
Sorghum was exposed to fossil CO
2 
(dead or 14C-free)
 
during a full growing season in one hand 
(7E FACE), and to modern CO
2 
(modern-air) in the other hand (8E FACE). Soils are the same.
Evaluation of phytolith extraction protocols commonly used
Set up of new phytolith extraction protocols
1. Combination of published extraction protocols.
(Geiss 1978, Kelly et al. 1999, Parr et al. 2001, Prior 2005, Piperno 2006) 
2. Checking for potential sources of old C contaminant:
 
- Phytoliths samples extracted in two different laboratories.
(UW-Madison and CEREGE). 
- 14C-AMS analysis of a sample of 14C free SiO
2
 powder subjected to the phytolith 
extraction procedure as a BLANK.
- 14C-AMS analysis of laboratories ambient air.
 
 3. Checking for samples purity: SEM-EDS.
14C-AMS target preparation and measurement
50-300mg of phytoliths and SiO
2
 were:
- baked at 160°C before evacuation,
- combusted at 900°C in quartz tube with CuO and silver wire.
  (Santos et al. 2010)
 
Graphitization: Hydrogen reduction with Fe powder as catalyst (Santos et al. 2004).
Targets were measured in the AMS (NEC 0.5MV 1.5SDH-2) compact system (Southon et al. 2004).
a b
SEM-EDS analysis of (a) phytoliths (high peaks of Si) 
and support, and (b) organic particle (high peak of C). 
Si Si + C
 
Bulk plant tissue from 8E FACE:
Modern
PhytC concentrates from 7E FACE and 8E FACE:
1,175+-280 yrs BP
Bulk plant tissue from 7E FACE:
3,580+-10 yrs BP
14C-AMS measurements of 7E FACE and 8E FACE 
(bulk plant tissue and phytC for each sample).
1. Photosynthesized bulk plant tissues from 7E FACE sample shows distinct 
14C value due to fossil CO
2
 "tag" relative to 8E FACE control plot 
(subjected to modern-air CO
2
).
2. PhytC from both samples shows similar 14C signatures, different from 
those  of atmospheric and tagged CO
2
.
d13C measurements: d13C values obtained for bulk plant tissue FACE 7E and 8E are 
respectively -9.7 ‰ (n=2) and -11.2 ‰ (n=2) while d13C value of phytC from FACE 8E is 
-21.6‰ (n=2).
IV Results
Protocol 1 Protocol 2
SEM images and EDS analysis of phytoliths (high peak of Si).
Acid digestion (H
2
SO
4
, H
2
O
2
, HNO
3
, KClO
3
, KOH).
 
(Combined from Kelly 1999, Piperno 2006, Prior 2005).
1. All particles analysed by EDS are siliceous.
2. No Ca was analyzed.
SEM images and EDS analysis of phytoliths (high peak of Si).
40µm 40µm 40µm
5µm
O
C Al
Au
Pd
O
C
Si
Au
Pd
O
C
Al
Au
Pd
C
Al Au
Pd
Si
Al
Si O Si
Dry ashing (500°C) + acid digestion (HNO
3
, HClO
4
, H
2
O
2
).
(Combined from Kelly 1999, Parr et al. 2001, Piperno 2006).
Phytoliths are partly dissolved.
1µm
20µm 10µm
O
C
Al
Si
Au
Pd
Two phytolith extraction protocols leading to purity 100% 
14C values obtained for the FACE samples V Discussion - Conclusions - Perspectives
While phytC is commonly assumed to originate from plant tissues and/or photosynthesis, these 
preliminary results suggest that phytC from the FACE grasses may come from a similar 
soil source.
These results will soon be confirmed or infirmed by :
14C analysis of FACE samples extracted using the same extraction protocols in a 
different lab (lab 2: UW). 
 
14C analysis of phytC from archaeological samples of known ages.
  
14C analysis of phytC from grass planters fed by nutrients with differents d13C and 
14C values.
The unexpected radiocarbon values of phytC highlight that 14C-AMS of phytoliths may not 
be a suitable dating tool but brings new thrilling questions on plants nutrition and the related 
soil carbon cycle.
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Is 14C-AMS analysis of phytoliths a suitable tool ?

Santos, Guaciara, M.

Reyerson, Paul, E.

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

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