25th International Meeting on Organic Geochemistry IMOG 2011

P-402
New insights into soil organic carbon build-up from compoundspecific
radiocarbon analysis
Axel Birkholz 1 , Rienk Smittenberg 1 , Irka Hajdas 2 , Lukas Wacker 2 , Jostein Bakke 3,4 ,
Stefano M. Bernasconi 1
1 ETH Zürich, Geological Institute, Zürich, Switzerland, 2 ETH Zürich, Laboratory of Ion Beam Physics, Zürich,
Switzerland, 3 Bjerknes Centre for Climate Research, Bergen, Norway, 4 University of Bergen, Department of
Geography, Bergen, Norway (corresponding author:axel.birkholz@erdw.ethz.ch)
With ~1500 Gt, Soil Organic Carbon (SOC) is the
second largest active carbon pool in the world, and
hence plays an important role in the global carbon
cycle. However, there are still many uncertainties
about the reactivity of the SOC in response to climatic
and environmental changes. In this respect,
especially the role of refractory SOC is important, as
this pool is ultimately responsible for long-term carbon
storage. It is, for instance, still uncertain how fast the
large amount of terrigenous carbon at higher latitudes
was accumulated after deglaciation, and if this buildup
is still ongoing or not [1]. To understand the longerterm
dynamics of this large carbon pool, one needs to
resort to sedimentary records, as experiments are not
possible at the time scales involved.
In order to gain more insight into SOC dynamics,
we expanded on the successful approach of
Smittenberg et al. [1] by analyzing the radiocarbon
content of soil-derived molecular compounds
preserved in a well-dated sedimentary record. When
compared to the depositional age of the sediment, the
age of the terrestrial lipids and organic carbon
fractions gives an average residence time of these
lipids and fractions in the SOC pool. Changes in the
age-differential between sediment and soil-derived
organic compounds over time give an estimate of
SOC build-up and evolution.
Since a large portion of terrigenous carbon is stored
at higher latitudes, we investigated the area of Lake
Lusvatnet, located on Andoya, Lofoten, Norway
(69°N), which was deglaciated shortly after the last
glacial maximum, unique for Scandinavia. In contrast
to the generally peat-dominated landscape the
catchment area of lake Lusvatnet is characterized by
relatively steep slopes and a relatively large amount
of mineral soil. In 2007 a ca. 5.5 m long lake sediment
core was taken. A preliminary age model suggests
that the core dates back to ca. 15 ka cal. BP. We
analyzed samples from twelve depth-horizons, each 4
cm thick, collected every 50 cm starting from the top
of the core, together with a higher resolution record
from the bottom of the core. Due to sediment
characteristic this part is thought to show the time of
deglaciation (14.6 to 11.6 ka cal. BP). In addition, a
short core was retrieved from the same location in
2008.
We present selected biomarker profiles especially
from the sediment deposited during the time of
deglaciation. In addition we present TOC contents,
� 13 C and � 15 N data for bulk samples. The data are
compared with other sedimentary, paleoenvironmental
and –climatic information available for
the region.
For radiocarbon analysis, compounds with an
unequivocal terrigenous source were isolated from
the total lipid extract, in order to separate them from
the aquatic – derived compounds. The isolated
compounds are branched glycerol dialkyl glycerol
tetraethers (brGDGTs), long chain n-alkanes and long
chain fatty acids (LCFAs). For the isolation and
purification of these compounds semi-preparative
High-Performance Liquid Chromatography - Mass
Spectrometry and preparative capillary Gas
Chromatography were used. Radiocarbon dating was
performed using an Accelerator Mass Spectrometer
interfaced with a gas source, allowing the analysis of
very small samples [2]. As expected, the isolated
compounds show a significant age difference to their
depositional age. We discuss the results in the
framework of SOC build-up and evolution.
References
[1] Smittenberg, R.H., Eglinton, T.I., Schouten, S., Damste,
J.S.S. (2006) Science 314, 1283-1286.
[2] M. Ruff, et al., Radiocarbon 49 (2007) 307.
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