Origin of lipid biomarkers in mud volcanoes from the Alboran Sea, western Mediterranean

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.

Novel lipid biomarkers for detecting microbial oxidation of methane in the environment

“…Overall, the coincidence of 17β,21β(H) and 17α,21β(H) configurations next to thermally mature 22S/(22S+22R) homohopane values and immature signals of the Ts/(Ts/Tm) index clearly suggest the presence of more than one source of organic matter in the mud breccias as observed in other studies about MVs e.g. in the Mediterranean or Black Sea (López-Rodríguez et al, 2014;Stadnitskaia et al, 2007Stadnitskaia et al, , 2008. The aforementioned n-alkanes, pristane and phytane distributions as well as the CPI and OEP indices also confirm this hypothesis, having both immature organic matter and moderately mature or petroleum derived compounds in the investigated mud breccias.…”

“…The high abundance of CH 4 in tandem with the probably marine derived organic matter in the mud breccias suggest that the Sicilian MVs could also host sulfate dependent AOM as found previously in other terrestrial and marine MVs via molecular and lipid biomarker based techniques (e.g. Stadnitskaia et al, 2005;Alain et al, 2006;López-Rodríguez et al, 2014;. The relatively high abundant 10-methyl C 16 FA in the Maccalube di Aragona and Comitini MVs (Fig.…”

“…Indeed, previous studies mainly about marine MVs also suggested that the lipid biomarkers (e.g. n-alkanes and hopanes) in the mud breccias originate from different mature geological sections in the subsurface (Stadnitskaia et al, 2007;López-Rodríguez et al, 2014).…”

“…Materials expelled from MVs are a mixture of mud breccias, waters and gas which may not be all co-genetic in their source strata (Stadnitskaia et al, 2007(Stadnitskaia et al, , 2008López-Rodríguez et al, 2014). Therefore, lipid biomarker distributions and indices can help to characterize the organic geochemical signatures of potential source strata (Peters et al, 2005).…”

“…nalkanes, acyclic isoprenoids or hopanes) can give insight into the source organic matter and maturity degree (Seifert and Moldowan, 1978;Peters et al, 2005) of the erupted material and provide key information about the geological history of the area. However, to the best of our knowledge only a few studies on marine MVs (Stadnitskaia et al, 2007;López-Rodríguez et al, 2014) and none on terrestrial MVs explored the source organic matter and its thermal maturation history as they mostly focus on the gas composition or the microbial lipid biomarker inventory and microbial communities.…”