Biogeochemical processes in sediments associated to cold-water coral ecosystems: From living reefs to ancient mounds

Syn- to post-depositional microbially-mediated diagenetic processes in cold-water coral reef associated sediments are addressed here to assess the coupling between sulfur and iron cycles, and carbonate formation and dissolution. Organic carbon mineralization at cold-water coral reefs on the Norwegian margin occurred in: a) the reef surface framework consisting of living and dead coral thickets and b) the underlying sediments. Several cold-water coral mounds in the Gulf of Cadiz were influenced by ascending hydrocarbon-rich fluids and the diagenetic imprint of a sulfate-methane transition zone (SMTZ) was evidenced. Carbonate preservation in the cold-water coral reef associated sediments was controlled by the availability of reactive iron. The interplay of low rates of sulfate reduction and the input of terrigenous Fe-rich material led to a buffering of the pore-water carbonate system in the Norwegian reef sediments. Conversely, processes that led to a scavenging of Fe-oxides such as the production of hydrogen sulfide facilitated carbonate dissolution. This was manifested in distinct layers of dissolved corals in Gulf of Cadiz mound sediments.