Complex Fluid Pathways and the Role of Shallow Evaporites in Mud Volcano Systems in the Gulf of Cadiz and Mediterranean Ridge

The plate boundary between Africa and Eurasia, ranging from the transform-dominated Gulf of Cadiz in the west to the reverse-faulting Hellenic subduction zone in the east, has been studied to identify mud volcanoes (MVs) of various origins. Different salinity patterns, either freshening or salinization signals, have been identified in different MV fluids through various research expeditions. During the R/V Meteor cruise M149, contrasting fluid signatures were detected at the summit (Cl depletion) and moat/rim (Cl enrichment) of the Ginsburg and Yuma MVs in the Gulf of Cadiz, whereas previous ODP Leg 160 reported similar spatial distributions of Cl concentrations at the Milano MV in the Olimpi MV field on the Mediterranean Ridge. Focusing on three MVs with contrasting fluid signatures, we suggest complex fluid pathways, rarely acknowledged in previous studies, are responsible for fluid expulsion.

By utilizing pore water geochemistry, advection-diffusion modeling, and high-resolution seismic profiles, we trace fluid origins, quantify fluxes, and constrain migration pathways. The Cl-depleted summit fluids originate from clay dehydration and are channeled by central conduits, reaching high advection velocities (up to 50 cm/yr). The Cl-enriched moat fluids exhibit slower advection velocities (0.3-1.5 cm/yr) and show additional evaporite effects. At the Ginsburg MV, one MeBo core of up to 40 m length collected at the moat and high-resolution seismic profiles across the whole MV allow to constrain moat fluid sources and further explain the structural implications behind this spatial variation in chemical and fluid fluxes. Fluid formation temperatures have been calculated using water isotopes and Mg-Li geothermometer. The resulting low temperatures suggest source depths atop the Allochthonous Unit of the Gulf of Cadiz (AUGC) (~0.8 kmbsf), consistent with seismic data across the Ginsburg MV (showing high-amplitude reflections at the same depths) but differently than the summit sites, where the source is deeper within the AUGC (~2.2 kmbsf). We relate moat seepage occurrence to fractures formed due to edifice subsidence, marked by stacked enhanced reflectors. Upon comparing the three MVs with one younger and one inactive MV, we suggest that peripheral seepage of MV edifices is a widespread process that appears at a specific evolutionary stage, during which it represents an important component of the fluid budget.

The findings from this study not only offer insights into the complex mechanisms of fluid circulation within MV structures but also provide a new approach to investigating the role of shallow evaporites in MV fluid dynamics, particularly in regions like the Mediterranean Ridge where evaporites are extensively present. By focusing on rim sites around MVs, the influence of evaporites can be more effectively identified, as peripheral fluids are more susceptible to being overprinted by salinization signals compared to the typically freshening summit fluids.