Diagenetic formation of gypsum and dolomite in a cold-water coral mound in the Porcupine Seabight, off Ireland

In the Porcupine Seabight, located south-west of Ireland, cold-water corals such as Lophelia and Madrepora have built mounds up to 200 m high (De Mol, 2002; Foubert & Henriet, 2009; Henriet et al., 1998; Huvenne et al., 2002). Until now, little attention has been paid to the diagenetic processes that occur within these cold-water coral mounds (Foubert & Henriet, 2009; Noé et al., 2006). Evidence is presented that changing sedimentary conditions have a significant effect on the early diagenesis of the sediment and may lead to the formation of secondary minerals. This study focuses on the discovery of authigenic gypsum in a gravity core, retrieved from the top of Mound Perseverance, a cold-water coral mound in the Porcupine Basin. The occurrence of gypsum in such an environment is intriguing, since this classic evaporitic mineral, is normally undersaturated with respect to sea water. Sedimentological, petrographic and isotopic evidence point to diagenetic formation of the gypsum, tied to oxidation of sedimentary sulphide minerals (i.e. pyrite). This oxidation is attributed to a phase of increased bottom currents which caused reduced sedimentation or erosion and enhanced inflow of oxidizing fluids into the mound sediments. The oxidation of pyrite produced acidity, causing carbonate dissolution and subsequently leading to pore-water oversaturation with respect to gypsum and dolomite. Calculations based on the isotopic compositions of gypsum and pyrite reveal that between 21.6 % and 28.6 % of the sulphate incorporated into the gypsum derived from pyrite oxidation. The dissolution of carbonate increased the porosity in the affected sediment layer but promoted lithification of the sediments at the sediment-water interface. Thus, authigenic gypsum can serve as a signature for diagenetic oxidation events in carbonate-rich sediments. These observations demonstrate that the sedimentary and fluid flow regime, steered by environmental factors, have an important effect on the diagenesis of coral mounds.