25th International Meeting on Organic Geochemistry IMOG 2011

P-406
Biogeochemistry of lake Soppensee (Switzerland) using a
biomarker and a compound-specific isotope approach
Merle Gierga 1 , Rienk Smittenberg 1 , Irka Hajdas 2 , Lukas Wacker 2 , Stefano Bernasconi 1
1 ETH Zürich - Geological Institute, Zürich, Switzerland, 2 ETH Zürich - Laboratory of Ion Beam Physics,
Zürich, Switzerland (corresponding author:merle.gierga@erdw.ethz.ch)
We present the results of a biogeochemical study of
the sedimentary record of the Swiss lake Soppensee
covering the last 14,000 years BP. The small,
eutrophic, hard-water lake is situated on the central
Swiss Plateau (596 m a.s.l.). This is a very well dated
[1] and studied [e.g. 2,3] lake with undisturbed organic
carbon-rich and partially laminated sediments. The
master chronology of the record was established by
the use of a varve chronology and 14 C dating of
terrestrial macrofossils [1].
We show biomarker profiles, in combination with
compound-specific isotope analysis (δ 13 C and radiocarbon,
Δ 14 C), in order to gain insight in a number of
topics: a) The limnological and environmental history
of the lake; b) The potential of using specific
compounds for either radiocarbon dating of the
sediment core, or the extent to which terrestrial lipids
are pre-aged on land. The latter provides insight in
terrestrial carbon build-up through time; c) Early
diagenetic processes taking place in the lake environment.
The main lipid signature derives from higher
terrestrial plants. We have identified a suite of sterols,
long-chain alkyl lipids, and various pentacyclic
triterpenoid alcohols (PTAs). In the early Holocene,
just after deglaciation of the area, we found a range of
long-chain alkenones in some sediment layers that
are typically not found in fresh water environments,
but in cold, brackish systems [4,5].
We found a large number of specific degradation
products of the PTAs, like des-A triterpenoids and �2triterpenes,
together with partially unsaturated
carotenoids from photosynthetic sulfur bacteria, �2
and �3 sterenes, thiosterols and phytadienes. These
are all diagenetic products and provide evidence for
the processes of reductive dehydration, sulfurization
and the two-step process of sulfurization and
subsequent desulfurization described by Hebting
[6,7]. We also report the occurrence of 13�malabarica-14(27),17,21-triene,
which appears to be
a novel biomarker for anoxia, or even photic zone
euxinia.
Individual long-chain n-alkanes and fatty acids were
purified by preparative gas and liquid chromatography
for subsequent radiocarbon analysis. As in other
systems, they show an increasing age difference with
increasing age of deposition. This gives a measure for
residence time of these plant-derived compounds in
soils before entering the lake.
References
[1] Hajdas, I., Michczynski, A., Radiocarbon 52,
(2010), 1027-1040
[2] Lotter, A., J. Paleolimn. 25 (2001), 65-79
[3] Fischer, A., (1993), Diss. ETH Zürich No. 11924
[4] Toney, J. et al., GCA 74 (2010), 1563-1578
[5] Zink, K.G. et al., GCA 65 (2001), 253-265
[6] Hebting, Y. et al., Org. Lett. 5 (2003), 1571-1574
[7] Hebting, Y., et al., Science 312 (2006), 1627-1631
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