When mud volcanoes sleep: Insight from seep geochemistry at the Dashgil mud volcano, Azerbaijan

The worlds >1500 mud volcanoes are normally in a dormant stage due to the short duration of eruptions. Their dormant stage activity is often characterized by vigorous seepage of water, gas, and petroleum. However, the source of the fluids and the fluid–rock interactions within the mud volcano conduit remain poorly understood. In order to investigate this type of activity, we have combined satellite images with fieldwork and extensive sampling of water and gas at seeping gryphons, pools and salsa lakes at the Dashgil mud volcano in Azerbaijan. We find that caldera collapse faults and E–W oriented faults determine the location of the seeps. The seeping gas is dominated by methane (94.9–99.6%), with a δ13C (‰ V-PDB) in the −43.9 to −40.4‰ range, consistent throughout the 12 analysed seeps. Ethane and carbon dioxide occur in minor amounts. Seventeen samples of seeping water show a wide range in solute content and isotopic composition. Pools and salsa lakes have the highest salinities (up to 101,043 ppm Cl) and the lowest δ18O (‰ V-SMOW) values (1–4‰). The mud-rich gryphons have low salinities (<18,000 ppm Cl) and are enriched in 18O (δ18O = 4–6‰).