25th International Meeting on Organic Geochemistry IMOG 2011

P-055
Organic matter preservation in Cariaco Basin, a pyrolytic study
Melesio Quijada 1 , Armelle Riboulleau 1 , Pierre Faure 2 , Olivia Bertrand 2
1 FRE 3298 CNRS Géosystèmes, UFR des Sciences de la Terre, Université de Lille 1, Villeneuve d'Ascq,
France, 2 UMR 7566 G2R, Université Henri Poincaré, Vandeuvre-les-Nancy, France (corresponding
author:ma.quijada-hermoso@gmail.com)
The Cariaco Basin is an east-west trending pull-apart
basin (Schubert 1982), located on the continental
shelf of the northern coast of Venezuela. The limited
exchange with open ocean and the high oxygen
demand from organic matter (OM) decomposition,
makes that its water column is anoxic below
approximately 275 m. (Thunell et al., 2000). Cariaco
Basin is the second largest euxinic basin on Earth
and has been widely used for studying
biogeochemical processes.
OM preservation pathway in the sediments of the
Cariaco Basin has previously been studied with a
6000 years focus (Aycard et al. 2003). This study
revealed the presence of melanoidin moieties in
kerogens, pointing to an important role of the
degradation-recondensation pathway. Sulfurization
appeared as a minor process, though from its high
content of inorganic sulfur in the water column and
sediment, Cariaco Basin presents favorable
conditions for sulfurization of OM.
Classical treatment for kerogen isolation may induce
artifactual formation of melanoidin-like moieties
leading to inaccurate interpretation of preservation
processes (Allard et al. 1997, Quijada et al. 2009).
To overcome this problem, we analyzed by flash
pyrolysis 13 kerogen samples isolated using a
sequential hydrolysis method avoiding artifacts
formation (Riboulleau et al. 2005; Quijada et al.
2009). The studied kerogens ranged in age from 0.4
to 135 ky, allowing to survey relatively long-term
diagenetic evolutions.
The total ion current (TIC) of flash pyrolysates
showed a complex mixture dominated by n-alkanes,
n-alkenes, alkylbenzenes and alkylthiophenes.
Alkylpyrroles and branched alkanes are also present
in relatively high abundance. Phenols, furans,
alkylindanes and alkylbenzothiophenes are minor
constituents of these pyrolysates.
A large number of C0- to C4-alkylbenzenes was
identified. Their abundance increases with depth
suggesting several contributors (algae and
cyanobacteria lipids, bacterial isoprenoids) and
progressive condensation of the organic matrix with
time. Alkylthiophenes are the second (in abundance)
most important compounds found in the TIC trace of
all samples. Two different groups of alkylthiophenes
were identified: (i) C1- to C5- alkylthiophenes,
suggesting sugars sulfurization (Sinninghe Damsté et
al. 1998; Kok et al. 2000; van Dongen et al. 2003); (ii)
isoprenoid thiophenes dominated by 3-methyl-2-
(3,7,11-trimethyldodecyl)-thiophene, described as an
early stage diagenetic product coming from the
incorporation of sulfur into chlorophyll-derived phytol
or archaebacterial phytenes and their diagenetic
products (Brassell et al. 1986; Sinninghe Damsté et
al. 1998). These two groups of thiophenes present a
global increase with depth, suggesting progressive
incorporation of their precursors to the
macromolecular organic matrix through sulfurization.
The relative abundance of pyrolitic yields
(alkylbenzenes, alkylthiophenes, alkylpyrrols) of
kerogens isolated by sequential hydrolyses are
significantly different to those of kerogens isolated by
the classical treatment, and characterized by a higher
alkylthiophene abundance. This observation confirms
that artifactual formation of melanoidin during kerogen
isolation during the previous study lead to wrong
interpretation of OM preservation process in the
Cariaco Basin.
In addition, while Aycard et al. (2003) showed a
substantial increase of sulfur incorporation with time
(6 ky), we here evidence that incorporation of
inorganic sulfur into organic matter continues with
depth, suggesting that natural sulfurization is not
restricted to early diagenesis but also operates on a
longer period of time (130 ky).
References:
- Aycard et al. (2003). Org. Geochem. 34, pp. 701-
718
- Brassell et al. (1986). Nature. 320, pp.160-162.
- Eglinton et al. (1994). Org. Geochem. 22, pp. 781-
799.
- Kok et al. (2000) Geochim. Cosmochim. Acta. 64,
pp. 2689-2699.
- Schubert (1982). Mar. Geol. 47, pp. 345–360.
- Sinninghe Damsté et al. (1998) Earth Planet. Sci.
Lett.164, pp. 7-13.
- Thunell et al. (2000). Limnol. Oceanogr. 45, pp. 300-
308
- van Dongen et al. (2003). Org. Geochem. 34, pp.
1129-1144.
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