25th International Meeting on Organic Geochemistry IMOG 2011

P-135
Composition of aliphatic hydrocarbons in prehistoric rice paddy
soils in China
Cornelia Mueller-Niggemann 1 , Jin Zhang 2 , Zhi-Hong Cao 3 , Lorenz Schwark 1
1 Institute of Geosciences, Christian-Albrechts-University of Kiel, 24118 Kiel, Germany, 2 School of Environ.
Science and Technology, Zhejiang A & F University, Lin`an 311300, China, 3 Institute of Soil Science,
Chinese Academy of Sciences, Nanjing 210008, China (corresponding author:cmn@gpi.uni-kiel.de)
Cultivation of rice in China dates back as far as 8000
years as documented by archaeological harboring
rice field management tools and carbonized grains of
cultivated rice species. At the Chuodun site north of
Hangzhou Bay major relics of the Neolithic Majiabang
culture, including ancient paddy rice fields have been
excavated. At this site two prehistoric rice paddy
horizons are overlain by a recent rice paddy soil. The
base of the older prehistoric paddy was dated to
6280a b.p., that of the younger to 3300a b.p. [1].
From here, recent and ancient paddy soils, rice plants
and in-field rice combustion residues have been
analyzed for their lipid content. The results show, that
recent and ancient paddy soils retrieve substantial
input of n-alkyl lipids not only from pristine plant
waxes but in significant amounts from combustion
residues.
Combustion in heaps of wet rice straw in the field
leads to commonly observed PAH but also to a suite
of n-alkanes and n-alkenes. The extract of rice straw
ash containing residual charred plant fragments is
shown in Figure 1. Three suites of acyclic aliphatic
hydrocarbons can be differentiated. Firstly, a series of
n-alkenes ranging from nC15 to nC28 and maximizing
at nC22 reveals a CPI value of 1.0. Secondly, an
envelope of n-alkanes from nC15 to nC35 and
maximizing at nC26 exhibits a CPI value around 1.0.
Only a few primordial wax alkanes with a chain length
of 29, 31 or 33 extend from this envelope.
The even numbered n-alkanes contributing to this
envelope are assumed to derive from direct
dehydration and hydrogenation of even numbered nalcohol
precursors. Formation of n-alkane distribution
with CPI values around 1 has been previously
described [2] for artificial combustion of maize and rye
straw at temperatures between 300 to 500°C. At
temperatures of 300°C sterenes, formed via sterol
dehydration, dominated and n-alkenes with even
carbon number predominance were present in minor
concentrations. At 400°C the aliphatics consisted
almost exclusively of n-alkanes with a CPI of 1.0. By
comparison we infer an average temperature ranging
between 300 and 400°C for the in-field combustion of
rice straw. The n-alkyl lipid signature determined from
the analysis of rice ash is thus taken as an input
indicator for rice cultivation into recent and fossil soils.
The recent paddy soil analyzed from the Chuodun site
revealed an n-alkane distribution similar to that of the
rice straw ash. In soils of the Chuodun archaeological
site and in a variety of rice paddy soils from China,
Java and Sumatera, n-alkenes have not been
detected, arguing for a rapid conversion into their
corresponding saturated analogues.
Fig.1. Distribution and source allocation of
alkanes/alkenes in rice field combustion residues.
Contamination of paddy soils with fossil hydrocarbons
has been observed in other paddy sites but was
accompanied by traces of steranes and hopanes. As
no such triterpane contaminants were detected in the
recent and ancient soils from the Chuodun site, the nalkanes
with even carbon distribution are indicative of
rice ash input and thus of rice cultivation. The 3300
year old paddy contained minor amounts of ashderived
alkanes, preferentially at the base. In the
6620 year old paddy horizon (80 cm thick) all samples
show a prominent ash pattern. Soil lipid analysis
identified rice paddy cultivation and fire management
practices for fertilization in the Neolithic Majiabang
cultural sites in East China.
[1] Cao et al. (2006) Naturwissenschaften 93,232-236
[2] Wiesenberg et al. (2009) Organic Geochemistry
40, 167-174.
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