25th International Meeting on Organic Geochemistry IMOG 2011

P-172
Microbially mediated carbonate formation from gypsum and oil
Vincent Grossi 1 , Marion Baudrand 1 , Rich Pancost 2 , Christophe Lécuyer 1 , Marie-
Magdeleine Blanc-valleron 3 , Mahmoud Aref 4 , Jean-Marie Rouchy 3 , Giovanni Aloisi 1,5
1 CNRS - Université de Lyon, Villeurbanne, France, 2 University of Bristol, Bristol, United Kingdom, 3 CNRS -
Muséum National d'Histoire Naturelle, Paris, France, 4 Cairo University, Giza, Egypt, 5 CNRS - Université
Pierre et Marie Curie, Paris, France (corresponding author:vincent.grossi@univ-lyon1.fr)
Large amounts of evaporitic gypsum are present
in sedimentary basins worldwide, which may come
into contact with organic matter(OM)-rich sediments.
If the temperature is below 80°C, microbial
communities which include sulfate-reducing bacteria
(SRB) may develop, dissolving gypsum and
promoting carbonate and sulfur precipitation:
OM + CaSO4, 2H2O � S0 + CaCO3 + H2O
In order to study this biogeochemical process in
hydrocarbon-rich environments, we investigated
sulfur-bearing carbonate samples from Egypt which
are in close stratigraphical contact with Miocene
evaporitic gypsum deposits and with petroleum
seepage.
Mineralogical and petrographical observations
indicate the presence of carbonate pseudomorphs
after gypsum crystals which are often associated with
euhedral elementary sulfur. The studied carbonates
are composed of calcite and/or dolomite. A new semiautomatic
analytical method was developed to
differentiate between carbon and oxygen stable
isotopic compositions of each carbonate pool in these
natural mixtures of calcite and dolomite [1]. All
carbonates are depleted in 13 C (-24.5‰ < δ 13 C (vs
PDB) < -2.8‰) but only calcite bears a clear OM
isotopic signature (-15‰ < δ 13 C < -25‰) similar to
that of oil associated to it. This suggets that: 1) calcite
was precipitated from a 13 C-depleted fluid produced
by a bacterial activity and, 2) (part of) the carbon
incorporated in calcite comes from petroleum
hydrocarbons.
GC-MS analysis of the lipids present in the
carbonates show the presence of more or less
biodegraded petroleum hydrocarbons. The most
degraded sample also contains specific bacterial (e.g.
macrocyclic non-isoprenoid dialkyl glycerol diethers,
DGDs) and archaeal (e.g. archaeol) lipids indicating
the presence of a mixed microbial community
characteristic of this specific ecosystem [2; Fig. 1].
The compound specific stable carbon isotopic
composition (δ 13 C) of petroleum hydrocarbons ranges
from -24 to -28‰. The isotopic compositions of
bacterial DGDs are slightly depleted (-28 to -30‰)
compared to hydrocarbons whereas the isotopic
signature of archaeol is slightly enriched (~ -23‰).
Fig. 1. Chromatograms of biodegraded oil and polar
microbial lipids present in diagenetic carbonates from Egypt.
The data allow proposing a model for the formation
of diagenetic carbonates by SRB using petroleum
hydrocarbons as carbon source and gypsum sulfate as
terminal electron acceptor. The heavier carbon isotopic
composition of carbonates compared to bacterial lipids
suggests however that an additional, isotopically
heavier, carbon source has been incorporated into
carbonates.
References
[1] Baudrand, M., Aloisi, G., Lécuyer, C., Martineau, F., et al.
(2011) Appl. Geochem., in press.
[2] Baudrand, M., Grossi, V., Pancost R., Aloisi, G. (2010)
Org. Geochem. 41, 1341-1344.
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