Abstract
Abstract. Mud volcanoes (MVs) are the most prominent indicators of active methane/hydrocarbon venting at the seafloor on both passive and active continental margins. Their occurrence in the western Mediterranean is patent at the West Alboran Basin, where numerous MVs develop overlaying a major sedimentary depocentre containing overpressured shales. Although some of these MVs have been studied, the detailed biogeochemistry of expelled mud so far has not been examined in detail. This work provides the first results on the composition and origin of organic matter, anaerobic oxidation of methane (AOM) processes and general characteristics on MV dynamics using lipid biomarkers as the main tool. Lipid biomarker analysis was performed on MV expelled material (mud breccias) and interbedded hemipelagic sediments from Perejil, Kalinin and Schneider's Heart MVs located in the northwest margin of the Alboran Sea. The n alkane distributions and n alkane-derived indices (CPI and ACL), in combination with the epimerization degree of hopanes (22S/(22S+22R)) indicate that all studied mud breccia have a similar biomarker composition consisting of mainly thermally immature organic matter with an admixture of petroleum-derived compounds. This concordant composition indicates that common source strata must feed all three studied MVs. The past or present AOM activity was established using lipid biomarkers specific for anaerobic methanotrophic archaea (irregular isoprenoids and dialkyl glycerol diethers) and the depleted carbon isotope composition (δ13C) of crocetane/phytane. The presence of these lipid biomarkers, together with the low amounts of detected glycerol dialkyl glycerol tetraethers, is consistent with the dominance of anaerobic methanotrophs of the ANME-2 over ANME-1, at least in mud breccia from Perejil MVs. In contrast, the scarce presence or lack of these AOM-related lipid biomarkers in sediments from Kalinin and Schneider's Heart MVs, suggests that no recent active methane seepage has occurred at these sites. Moreover, the observed methane concentrations support the current activity of Perejil MV, and the very low methane seepage activity in Kalinin and Schneider's Heart MVs.
Highlights
Mud volcanoes (MVs) are mainly formed due to an extensive discharge of hydrocarbon-rich fluids from deeper sedimentary units
The lower interval from 135–209 cm consisted of stiff mud breccia moderately bioturbated in the uppermost part
In agreement with previous geophysical and stratigraphic data, confirm that the source strata of the organic matter in mud breccia belong to overpressured deep units occurring at depth in the West Alboran Basin (Unit VI and Unit V, Early to Middle Miocene in age)
Summary
Mud volcanoes (MVs) are mainly formed due to an extensive discharge of hydrocarbon-rich fluids from deeper sedimentary units. This phenomenon, commonly occurring in petroliferous regions, results from upward transport of deepgenerated water and hydrocarbons to the subsurface The origin of hydrocarbon gases at MVs can be either thermogenic (formed by maturation of buried organic matter in the subsurface as a consequence of increasing temperature and pressure) or biogenic (produced by anaerobic microorganisms from organic matter at low temperatures), or a mixture of both (Milkov et al, 2003; Stadnitskaia et al, 2007, 2008; Mastalerz et al, 2007, 2009; Etiope et al, 2009). López-Rodríguez et al.: Origin of lipid biomarkers in mud volcanoes
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