Early and deep diagenetic imprint in surficial pore fluids from the Olimpi mud volcano field (Eastern Mediterranean)

This study provides high-resolution pore-fluid profiles of surficial sediments (down to 40 cm) from four submarine mud volcanoes (MVs) of the Olimpi Mud Volcano Field (OMVF) in the Eastern Mediterranean, including the Gelendzhik, Heraklion, Moscow and Milano MVs. Here, we present major ions (Na + , K + , Mg² + , SO 4 ² - , Cl - ), sulfide, methane, dissolved inorganic carbon (DIC), δ¹³C DIC , ammonium, phosphate and silicate concentrations. These results were evaluated in relation to both early and deep diagenetic processes shaping pore-fluid chemistry. The four MVs can be classified into two geochemical groups: Gelendzhik and Heraklion, dominated by deep-sourced hypersaline fluids from Messinian salt dissolution and Moscow and Milano MVs, characterized by pore fluids largely reflecting seawater-derived compositions. In the hypersaline group, signatures of deep processes persist such as smectite–illite conversion at Heraklion and ammonium and methane upward migration, demonstrating that near-surface pore fluids retain the imprint of deep diagenesis. Organic matter oxidation via sulfate reduction (OSR) and anaerobic oxidation of methane coupled to sulfate reduction (AOM-SR) were also active, even within the hypersaline environments of the Gelendzhik and Heraklion MVs, as evidenced from stoichiometric ratios of ΔDIC and ΔSO 4 2- and δ 13 C DIC isotopic data. In the hypersaline Gelendzhik MV, the diagenetically added DIC, representing the isotopic signature of mineralized organic matter, is estimated at −53.1‰, further indicating active AOM-SR under extreme salinity. Overall, our findings demonstrate that deep-sourced fluids shape near-surface pore-fluid chemistry, generating pronounced heterogeneity among MVs and provide rare geochemical evidence of microbial resilience in hypersaline submarine environments.