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| Titel |
Organic matter remains in the Kess Kess mounds of the Hamar Laghadad (Anti Atlas, Morocco): record of microbial biomineralization |
| VerfasserIn |
Fabio Demasi, Roberto Barbieri, Adriano Guido, Adelaide Mastandrea, Barbara Cavalazzi, Franco Russo |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042442
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| Zusammenfassung |
| Carbonate Mud Mounds are well documented in the geological record, and span from
Proterozoic to recent times, in shallow- and deep-water settings. They are a significant
expression of the history of Earth’s microbial life. The origin of carbonate mud-mounds has
long been debated and the discovery of seep- and vent-related ecosystems from
different geotectonic settings, associated to authigenic carbonate mounds, allowed
the re-interpretation of some mounds as the product of chemosynthetic microbial
mediation.
We analyzed the carbonate mounds, informally called “Kess-Kess”, cropping
out in the Hamar Laghdad Ridge, eastern Anti-Atlas, SE Morocco. These mounds
are the most spectacularly exposed carbonate buildups of the world and, due to
differential erosion, they show their original shapes and the relationships with associated
strata.
The origin of these buildups is still under debate and the most consistent hypotheses are
related to submarine hydrothermal vents or hydrocarbon seapage in which bacteria and/or
archaea plaied a prominent role in the carbonate biomineralization.
To investigate the possible remains of prokaryote metabolic activity we studied the
micrite precipitation processes through microfacies and biogeochemical analyses.
The more indicative micrite texture is stromatolitic with very fine wrinkled lamination
organized in antigravitative pattern. High resolution SEM observations suggest the presence
of widespread trace of organic phantoms.
The geochemical characterization of extracted organic matter was performed through the
functional group analyses by FT-IR spectroscopy. The infrared spectra showed bands
between 600 and 3000 cm-1. They contain stretching aliphatic bands (Ï
CHali) at 2950,
2925 and 2850 cm-1, and deformation bands of methyl (δCH3; 1365 cm-1) and
both methyl and methylene [δ(CH2 + CH3); 1458 cm-1] groups. The spectra also
display the band assigned to carbonyl and/or carboxyl groups (νC=O; 1740 cm-1).
The νC-O vibration appears between 1300 and 1100 cm-1. We recorded also the
band νC=C probably related to unsatured compounds (alkene and/or carboxylic
acids).
The organic matter correlated to the fine laminated micrite is characterized by the
presence of stretching C=C vibrations. The lack of bands in the 700-900 cm-1 and
3000-3100 cm-1regions permits to exclude that νC=C band belongs to aromatic compounds.
We attribute this band to alkene and/or unsatured carboxylic acids that could have been
synthesized by bacteria and/or archaea communities, which caused the precipitation of
carbonates through their metabolic activities.
We used the A Factor (2930+2860 cm-1)/(2930+2860+1630 cm-1) and a C Factor (1710
cm-1)/(1710 + 1630 cm-1) in order to quantify changes in abundances of aliphatic and
carbonyl/carboxyl groups. These factors can be used in a similar manner to the traditional
H/C – O/C elemental ratios or to Rock-Eval pyrolysis parameters, as Hydrogen Index (HI) –
Oxygen Index (OI), for the classification of kerogen types and maturation level of organic
compounds. In the analyzed samples, the A factor is ~0.70 while the C factor is ~0.63.
These parameters indicate a marine origin for the organic compounds and a low thermal
evolution. Considering the thermal maturity of the organic compounds, further
analyses in Gas Cromatography-Mass Spectrometry could confirm the presence of
specific bacterial/archaeal biomarkers. These analyses will clarify the microbial
metabolic activities that induced biomineralization processes in the Kess Kess Mound. |
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