25th International Meeting on Organic Geochemistry IMOG 2011

P-238
Rhizoliths at the Nussloch loess profile (SW Germany) –
assessment of source vegetation using stable isotopes and nalkane
molecular proxies
Martina Gocke, Guido Wiesenberg
Department for Agroecosystem Research, University of Bayreuth, Bayreuth, Germany (corresponding
author:martina.gocke@uni-bayreuth.de)
Rhizoliths are a special type of pedogenic carbonates
which formed mainly under arid and semiarid climatic
conditions. They represent former roots that were
encrusted by secondary carbonate, often leading to
selective preservation of the former root tissue. Such
concretions have been described previously in
paleosols and various terrestrial sediments including
loess. In contrast to other types of pedogenic
carbonates, rhizoliths were formed during rather short
periods, i.e. during the life span of the root. Therefore,
they are used as sensitive paleoenvironmental
archives. It has been speculated that rhizoliths are
formed solely around tree roots, or even solely around
roots of coniferous trees. In modern soils, they were
observed to form under various wooden plants like
pine, peach or vine, but sometimes also under
herbaceous plants. However, source plants of
rhizolith formation are commonly unknown, because
the rhizoliths found in terrestrial sediments are not
connected to the aboveground plant parts any more.
Our aim was to assess the origin of rhizoliths, which
occur locally abundant at the late Pleistocene loesspaleosol
sequence at Nussloch, SW Germany. These
rhizoliths are considerably younger than the
surrounding sediment (� ≈ 12-14 ka, [1]). They
appear in a depth between 0.8 m and 8 m below
present soil surface, with diameters of up to ~ 5 cm
and length between some cm and 1.5 m. Within the
depth interval of highest rhizolith abundance (0.8-6.9
m), rhizoliths of different shape and size were
sampled. Small amounts of organic remains
preserved within the rhizoliths allowed stable C
isotopic (δ 13 C) and lipid analyses of organic matter
(OM).
As to be expected under humid temperate climatic
conditions of SW Germany, δ 13 C of rhizolith OM
indicated C3 plants as source vegetation for all
samples, with values between -24 and -25.5‰.
Alkane composition revealed predominance of long
chain (C25+) odd homologues, thus confirmed the
higher plant origin of rhizoliths. The carbon preference
index (CPI) of rhizolith OM was comparable to
modern root biomass, which is lower than
aboveground plant biomass due to the presence of
associated microbial biomass and missing
epicuticular waxes. The most abundant long chain
alkane in most rhizoliths was either C27 or C29,
suggesting tree or shrub source vegetation rather
than grass plants which are usually dominated by C31
long chain alkane [2]. On the other hand, variation of
the alkane composition between rhizoliths of different
depth concerning e.g. CPI and n-C27/n-C31 ratio (Fig.
1) indicate that rhizoliths at Nussloch derive from
various source plants.
However, these results do not exclude the possibility
of rhizolith formation taking place under smaller plants
like herbs. Therefore, further investigations on
rhizoliths, combined with comparison to modern
analogues, are under progress to further elucidate
source vegetation of rhizoliths in terrestrial sediments.
Fig. 1: n-C27/n-C31 ratio of rhizoliths from various
depth intervals.
References
[1] Gocke M., Pustovoytov K., Kühn P., Wiesenberg
G.L.B., Löscher M., Kuzyakov Y. 2011, Chemical
Geology.
[2] Schwark L., Zink K., Lechterbeck J. 2002,
Geology.
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