Mixed sources contribute to the molecular isotopic signature of methane-rich mud breccia sediments of Kazan mud volcano (eastern Mediterranean)

Abstract

We have investigated the molecular carbon isotopic signature of mud breccia sediments from a methane-seep environment on Kazan mud volcano in the eastern Mediterranean. Many different classes of lipids have been identified and attributed to methane-oxidizing archaea, sulfate-reducing bacteria, methane-oxidizing bacteria, and sulfide-oxidizing bacteria, as well as older organic matter associated with the ascending mud diapirs in the region. Of particular interest is the record of glycerol dialkyl glycerol tetraethers (GDGTs) derived from various types of archaea. A geochemical depth profile of the upper 30 cm of sediment allows the assessment of vertical variability in the microbial community, which proves to be diverse based on molecular isotopic analyses, and the importance of present-day microbes relative to paleo-organic matter. In this environment, it seems that anaerobic oxidation of methane (AOM) has progressed at relatively low rates or for a shorter time compared with other seep sites, based on the high relative abundance of organic matter associated with the ascending mud matrix rather than with AOM, and the carbon isotopic composition of GDGT-derived biphytanes. The presence of many different biomarkers of AOM-related microbes with varying depth trends in both concentration and carbon isotope composition suggests substantial variability in the microbial community on a small vertical scale (∼30 cm).