Pore water signatures and gas hydrates occurrence in and around the Olimpi mud volcano field, south of Crete

Abstract

Mud volcanoes in the Mediterranean Sea have been subject of scientific research since the 1970s and contributed to the understanding of these seafloor features and their role in the subduction budgets worldwide. Recently, the need to better characterize the connection of these mud volcanoes with the deeper lithologies of the accretionary prism and its implications led to expedition POS410 (2011) and SO278 (2020). During these expeditions, mud volcanoes which had been investigated in ODP Leg 160 (1996) were resampled and complemented with new sites along the Mediterranean Ridge accretionary complex. We present pore water data from six mud volcanoes and two brine pools in and around the Olimpi Mud Volcano Field (OMVF). The data highlight strong depletion in Cl (<200 mM), Na (∼200 mM), Mg and K (with concentrations ∼0 mM) and striking water isotope ratios (δ18O of +9.79‰ V-SMOW and δD of −26.33‰ V-SMOW), indicating dehydration of clay minerals at depth. At five locations, the interstitial waters are characterized by extremely high salinities (Cl > 5000 mM and Na >6500 mM), therefore drastically expanding the previous reports of highly saline pore waters in the OMVF. The measurements further reveal that Gelendzhik mud volcano, located in the western part of the OMVF along a major strike-slip fault, shows an unusual pore water downcore profile and its water isotopes signature differ strongly from the other structures (δ18O of +13.63‰ and δD of +1.83‰). Upon further investigation through XRF scans and hydrates stability calculations we tied the anomalous low salinity values of the Gelendzhik MV pore waters results from the presence of gas hydrates in the sediments. This is the first, indirect evidence of gas hydrates in the OMVF, several years after this hypothesis was formulated by De Lange and Brumsack (1998) and then quickly discarded.