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

[摘要] Mud volcanoes (MVs) are the most prominent indicators of activemethane/hydrocarbon venting at the seafloor on both passive and activecontinental margins. Their occurrence in the western Mediterranean is patentat the West Alboran Basin, where numerous MVs develop overlaying a majorsedimentary depocentre containing overpressured shales. Although some ofthese MVs have been studied, the detailed biogeochemistry of expelled mud sofar has not been examined in detail. This work provides the first results onthe composition and origin of organic matter, anaerobic oxidation of methane(AOM) processes and general characteristics on MV dynamics using lipidbiomarkers as the main tool. Lipid biomarker analysis was performed on MVexpelled material (mud breccias) and interbedded hemipelagic sediments fromPerejil, Kalinin and Schneider's Heart MVs located in the northwest margin ofthe Alboran Sea. The n alkanedistributions and n alkane-derivedindices (CPI and ACL), in combination with the epimerization degree ofhopanes (22S/(22S+22R)) indicate that all studied mud breccia have asimilar biomarker composition consisting of mainly thermally immature organicmatter with an admixture of petroleum-derived compounds. This concordantcomposition indicates that common source strata must feed all three studiedMVs.

The past or present AOM activity was established using lipid biomarkersspecific for anaerobic methanotrophic archaea (irregular isoprenoids anddialkyl glycerol diethers) and the depleted carbon isotope composition(δ¹³C) of crocetane/phytane. The presence of these lipidbiomarkers, together with the low amounts of detected glycerol dialkylglycerol tetraethers, is consistent with the dominance of anaerobicmethanotrophs of the ANME-2 over ANME-1, at least in mud breccia from PerejilMVs. In contrast, the scarce presence or lack of these AOM-related lipidbiomarkers in sediments from Kalinin and Schneider's Heart MVs, suggests thatno recent active methane seepage has occurred at these sites. Moreover, theobserved methane concentrations support the current activity of Perejil MV,and the very low methane seepage activity in Kalinin and Schneider's HeartMVs.

[学科分类] 地球化学与岩石