25th International Meeting on Organic Geochemistry IMOG 2011

P-501
Land use and climatic effects on hydrogen isotope
compositions of long-chain n-alkanes and distribution of
tetraether lipids in soils
Janet Rethemeyer 1 , Michael Lappé 1 , Stefan Schouten 2 , Pascal Boeckx 3 , Enno Schefuß 4
1 Institute of Geology and Mineralogy, University of Cologne, Cologne, Germany, 2 NIOZ Royal Netherlands
Institute for Sea Research, Department of Marine Organic Biogeochemistry, Texel, Netherlands, 3 Faculty of
Bioscience Engineering, Ghent University, Ghent, Belgium, 4 Department of Geosciences, University of
Bremen, Bremen, Germany (corresponding author:janet.rethemeyer@uni-koeln.de)
Distributional and isotopic analyses of terrestrial lipid
biomarkers are powerful tools used for paleoenvironmental
reconstructions. These analyses are
commonly performed on marine or lacustrine
sediment records. The abundance as well as the
isotopic composition of the organic compounds from
terrestrial source organisms, however, may be
significantly altered until organic matter burial in
sediments. Intermediate storage in soil as well as
differences in the recorded signals between
ecosystems may be considerable factors affecting the
reliability of the lipid based proxies, which have not
yet been studied in any detail.
We investigated two terrestrial lipid based proxies in
modern soils under different land use and climatic
conditions: (a) the distribution of branched glycerol
dialkyl glycerol tetraethers (GDGTs) used as
indicators for past changes in continental air
temperature and soil pH [1], and (b) the hydrogen
isotopic composition of long-chain n-alkanes as an
indicator for hydrological changes. The study area is
the south-western Ethiopian highland where soil
samples have been taken along a topo-sequence
from 1770 to 2650 m altitude. Mean annual air
temperature and precipitation range from about 15 to
21 °C and 1500 to 2200 mm, respectively. At six
altitudes agricultural, forest and pasture soils (Nitisols)
were analysed.
Bulk soil properties including total organic C, total N
and pH indicate a more intense cultivation and more
pronounced soil degradation, respectively, of the sites
at lower altitude. Soil pH values of all cultivation
variants showed a decrease with altitude from about
pH 5.8 to 5.4, which was less pronounced in the
arable soils. We used the cyclisation ratio of branched
tetraethers (CBT) to estimate soil pH [1]. The results
however, differed strongly from the measured soil pH
values with no consistent trend. This is caused by the
relatively large uncertainty of the CBT index as well
as the low abundance of cyclopentane moeities of
branched GDGTs at low soil pH. We also tested the
methylation index of branched tetraethers (MBT),
used to reconstruct mean annual air temperatures.
The GDGT based data showed a linear increase with
altitude, which is most pronounce for the undisturbed
forest soils. The calculated mean annual air
temperatures however, overestimate the measured
temperature by about 3 to 4.5°C, which may reflect
that main GDGT production by soil bacteria occurs at
warmer periods, suggesting the need for a local
calibration.
The hydrogen isotope composition of long chain nalkanes
(n-C25 - n-C33) derived from higher plants leaf
waxes showed a strong scatter and no clear trend
with altitude. We thus examined the � 13 C composition
of the n-alkanes and found higher values for n-C31
and n-C33 reflecting a higher contribution of C-4 plant
material. C-4 plant have a higher water use efficiency
than C-3 plants and thus differ from C-3 plants in their
apparent hydrogen isotopic fractionation between
precipitation and lipids. After excluding these two
compounds, weighted average δD values for n-C27
and n-C29 alkanes showed a significant correlation
with altitude. The most prominent trend was observed
for the forest sites (R 2 = 0.87) most probably due to
the long term growth of C-3 vegetation while the
isotopic signal in the two other soils is biased by
cultivation. This altitude trend line can be translated
into a δD lapse rate (n-C27 and n-C29) of 17.9 ‰ per
1000 meters which agrees well with global
observations of altitudinal isotopic changes in rainfall.
These results indicate that land use can considerably
bias terrestrial lipid based proxies. The results
emphasize the need for source-area specific
calibrations of proxy parameters.
References
[1] Weijers, J.W.H. et al., 2007. Geochimica et
Cosmochimica Acta 71, 703–713.
622