Lipids and clays interactions in a productive marine water column (Antofagasta Bay, Chile)

Abstract : Organo-Clay Interactions (OCIs) are a poorly investigated topic despite the potential benefits in both fundamental and industrial studies fields. Indeed, Organic Matter (OM) sorption on clays is a mechanism involved in several research areas: past and actual environmental problematics, hydrocarbons sources rocks or soil sciences considerations. In actual environmental issues, new research axes are emerging on OCIs applications on organic pollutants treatments. In the actual environmental management field, OCIs are investigated in order to characterize the role of clay minerals on landfill barriers strengthening (Sánchez-Jiménez et al., 2011). OCIs investigations in paleoenvironmental field are primordial for the calibration of climatical and paleoecological biomarkers proxies. On the topic of formation and evolution of hydrocarbon source rocks, OCIs play an important role on the early steps of diagenetic processes. Several models of OM preservation paths are already admitted (Boussafir and Lallier-Verges, 1997). However, those schemes do not consider the function of the mineral fraction and particularly the role of the clayey fraction on sedimentation and preservation of organic matter. In order to test the role of clays on early stages of petroleum source rocks genesis, our study aims to investigate the efficiency of three clays on OM sorption in a marine productive water column. This study was carried out off the eastern Pacific upwelling (Antofagasta bay, Chile) which is considered as one of the four most productive up-welling areas of the South American coast. A combined system of upwelling and surface currents (Humboldt current) sustains a proliferating biomass. Natural and synthetic montmorillonites and natural kaolinite samples have been disposed along the oceanic water column at three depths corresponding to different water oxygenic conditions (oxic, transition and anoxic). After different times of exposure, the samples have been removed and characterized via different analytical methods (Py-GC-MS, Dissoveld Organic Carbon Analyser, XRD and infrared spectroscopy). The contributions of this work can be declined in three points: (i) we observed an increase of the dissolved organic carbon (DOC) concentrations in the water of clay traps compared to marine ones. This evidence has been detected in the traps water after 2 days of interactions, demonstrating a good attraction effect of clays on ambient organic material. Drouin (2007) observed similar result in lacustrine area. (ii) The py-GC-MS molecular results show that fatty acids are the preferentially adsorbed molecular lipids family during the interaction process. The anoxic zone of the water column has been defined as the most propitious area for the organo-clay aggregation process. (iii) X-ray analysis have highlighted an extension of montmorillonites (001) plans spacing suggesting the intercalation of some organic compounds. However, previous studies (Drouin et al., 2010) and some of our results suggest that clays specific surfaces play a major role on the organic sorption. A comparison between theoretically absorbable amount of organic material and effectively quantified ones permitted us to conclude that organic molecules occupied a little part of external clay surface area.
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Adoum Mahamat Ahmat, Mohammed Boussafir, Claude Le Milbeau, Régis Guégan, Lydie Le Forestier. Lipids and clays interactions in a productive marine water column (Antofagasta Bay, Chile). 26th International Meeting on Organic Geochemistry, Sep 2013, Tenerife, Spain. ⟨insu-01384979⟩

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