The imprint of methane seepage on the geochemical record and early diagenetic processes in cold-water coral mounds on Pen Duick Escarpment, Gulf of Cadiz

The diagenetic history and biogeochemical processes in three cold-water coral mounds located in close proximity to each other on Pen Duick Escarpment in the Gulf of Cadiz were examined. The influence of ascending methane-rich fluids from underlying sediment strata delineated two mound groups: Alpha and Beta Mound showed evidence for the presence of a sulfate-methane transition zone (SMTZ) at shallow depth, whereas Gamma Mound appeared to lack a shallow SMTZ. In the methane influenced Alpha and Beta Mound, upward diffusion of hydrogen sulfide from the shallow SMTZ caused extensive pyritization of reactive iron phases as indicated by values for the degree-of-pyritization >0.7. This secondary pyritization overprinted the sulfur isotope composition of sulfides formed during organoclastic sulfate reduction. The almost complete consumption of reactive iron phases by upward diffusing sulfide limited dissimilatory iron reduction to the top layer in these mounds while organic matter in the pyritized zones below was primarily degraded by organoclastic sulfate reduction. Hydrogen sulfide produced during sulfate reduction coupled to the anaerobic oxidation of methane (AOM) diffused upward and induced aragonite dissolution as evidenced in strongly corroded corals in Alpha Mound. This mound has been affected by strong fluctuations in the depth of the SMTZ, as observed by distinct layers with abundant diagenetic high-Mg calcite with a ¹³C-depleted carbon isotope composition. In the non-methane influenced Gamma Mound low sulfate reduction rates, elevated concentrations of dissolved iron, and solid-phase iron speciation indicated that organic matter mineralization was driven by dissimilatory iron reduction and organoclastic sulfate reduction coupled to oxidative sulfur cycling. The latter process led to ³⁴S-depletion in pyrite of up to 70‰ relative to pore-water sulfate.