Application of lipid biomarkers to detect sources of organic matter in mud volcano deposits and post-eruptional methanotrophic processes in the Gulf of Cadiz, NE Atlantic

Abstract

The Gulf of Cadiz is known as an active mud volcano (MV) and fluid venting region discovered during the Training Through Research (TTR) expeditions in 1999 (TTR-9). Here we report a study of lipid biomarkers in the mud breccia matrix from three MVs in this region. The lipid biomarker composition revealed strong compositional resemblance as well as similar thermal maturity properties for the studied MVs. This indicates that the primary source of the erupted material for these MVs is located in similar litho-stratigraphic units. In both areas, upward migrated fluid went through the sedimentary series of the allochthonous Olistrostrome and through the Upper Cretaceous horizons. The relatively immature characteristics of organic matter from both mud breccia rock clasts and matrix together with the more mature properties of hydrocarbon gases from the same sampling locations indicate that source-strata for the initial fluid are located deeper in the subsurface than the source-strata for the erupted sedimentary material. The presence of microbial lipid biomarkers derived from archaea and sulfate reducing bacteria in the mud breccias revealed anaerobic oxidation of methane (AOM) as a dominant microbial process in these habitats. Our data suggest that the AOM-derived lipids are, at least in part, fossil in origin (i.e. they do not reflect active AOM communities) and thus reflect AOM activity in these MVs integrated over time. The low concentrations of AOM-related biomarkers in these MVs suggest relatively low intensities of AOM in the studied MV deposits. The distribution of biomarkers suggests an abrupt and brief cold seep activity in the past and the absence of a continuous methane influx in the studied locations.