5th Swiss Geoscience Meeting, Geneva 2007
Radiocarbon dating of old stable soil organic matter
and charcoal fragments found in Alpine soils
Favilli, Filippo1*, Egli, Markus1, Cherubini, Paolo2, Sartori, Giacomo3, Delbos
Evelyne4
1
Department of Geography, University of Zurich, CH-8057 Zurich, Switzerland;
(favilli@geo.uzh.ch)
2
Swiss Federal Research Institute WSL, CH-8903 Birmensdorf, Switzerland;
3
Museo Tridentino di Scienze Naturali, Via Calepina 14, 38100, Trento, Italy;
4
Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
Soil organic carbon is known to contain a stable fraction with an old radiocarbon
age, even if the size and the stabilisation processes leading to the formation of
this old soil carbon pool are still unclear (Eusterhues et al., 2003). We tested
several methods to isolate the oldest possible stable organic matter (SOM) of
two soil profiles developed on a morainic substratum under a Larix decidua Mill.
forest of an Alpine environment in northern Italy. The parent material is
paragneiss and the soils can be classified as Cambic Podzol. Our aim was to
understand the mechanisms governing the long-term residence time of organic
matter (OM) and to develop a quantitative dating method based on OM. The
conceptual approach is based on the findings that partial oxidative degradation
of OM leaves behind intrinsically and chemically resistant as well as mineralprotected organic material (Mikutta et al, 2006). We isolated and dated the most
resistant OM after chemical treatments and compared the obtained radiocarbon
ages with the age of charcoal fragments found in the studied soils. Charcoal
fragments naturally buried in soils are considered biologically inert and
physically stable in relation to isotopic changes in the environment. The age of
charcoal fragments varied between 3300 y calBP in the upper horizon to 10500
y calBP in the lower one. Three different extraction methods for SOM were
tested for the same soil material. These methods were based on the oxidation
of “fresh” OM and, depending on the method, on the dissolution of the minerals.
The first method included an oxidation of OM by 10% NaOCl, followed by
dissolution of minerals with 10% HF or 1M Aqua Regia. The second method
was similar to the first one but with a changed order of the treatments (chemical
oxidation as the last step). The third included only a treatment with 10% H2O2
for 7 days. Results showed that the treatment with 10% NaOCl followed by 10%
HF gave rise to the highest loss in weight in the samples (56%) due to the
dissolution of silicate minerals such as kaolinite (checked with FT-IR) (Fig.1) but
produced some artefacts (cryolite Na3AlF6) that probably led to a coprecipitation of young OM. The treatment with HF 10% produced a loss in total
organic C (TOC) in the range of 50% to 78%. The treatment with NaOCl 10%
followed by Aqua Regia produced a lower loss in weight but almost the same
loss in TOC. The second method (HF or Aqua Regia followed by NaOCl)
showed that the Aqua Regia procedure reduced more organic C (OC) and N
compared to HF. HF dissolved also N-rich compounds which are strongly
associated with minerals. The H2O2 treatment ( 3rd method) was very effective
in oxidising OC (around 90% of the initial OC). The remaining organic matter
was strongly enriched in nitrogen. Before and after the individual treatments, the
organic fraction was dated with 14C (AMS), and the functional groups were
5th Swiss Geoscience Meeting, Geneva 2007
analysed with FT-IR. Additional information about mineral-SOM characteristics
was obtained with SEM-EDS measurements. The individual dissolution and
oxidation mechanisms of the applied methods left behind a different organic
fraction with a consequently different age. The methods including HF (or Aqua
Regia) and NaOCl gave rise to ages decreasing with soil depth which agrees
well with results from others (i.e. Mikutta et al., 2006). A good correlation was
found between the clay content of the sample and the corresponding resistant
OC after the first and the second method. Compared to the other methods, the
H2O2 treatment left behind the organic fraction with the highest ages (up to
17000 y calBP). In contrast to the other methods, the highest 14C age was
measured in the topsoil. The H2O2-resistant OC fraction was enriched in
aliphatic compounds (Fig. 2). In this case, the resistant OC showed no
correlations with clays or Fe-Al mineral phases. Accordingly, the suitability of
each method is discussed. It seems that the H2O2 procedure leaves behind the
oldest OM fractions in a soils and consequently could be used for dating of
Holocene-aged surfaces. The presented results are part of a study focusing at
dating selected Alpine sites of distinct landform surfaces using numeric and
relative dating techniques with the aim to establish an absolute chronology of
surfaces and to correlate and improve several dating techniques.
Keywords: Stable organic matter, Radiocarbon dating, charcoal, soil formation
Rabbi 38 AE Not treated
LAX 1 OE not treated
Rabbi 38 AE 10% HF
Kaolinite
1450
LAX 1 OE Not treated
1190-1127
Aliphatic compounds
LAX 1 OE 10% HF
LAX 1 OE after H2O2
4000
3800
3600
3400
3200
-1
cm
Fig. 1 Effect of HF on silicate
minerals
2000
1800
1600
1400
1200
1000
-1
cm
Fig. 2 Enrichment in aliphatic
compounds after treatment with
H2O2
REFERENCES
Eusterhues, K., Rumpel, C., Kleber, M., Kögel-Knabner, I., 2003. Stabilization of
soil organic matter by interactions with minerals as revealed by mineral
dissolution and oxidative degradation. Organic Geochemistry 34, 1591-1600.
Mikutta, R., Kleber, M., Torn, M.S., Jahn R., 2006. Stabilization of organic
matter: association with minerals or chemical recalcitrance? Biogeochemistry
77, 25-56