Deep Cold-Water Dolomitization of Neogene Sediments, Belgica Carbonate Mound Province, Porcupine Basin, North Atlantic Ocean

Gregg, Jay M.¹, Tracy D. Frank² (1) Oklahoma State University, Stillwater, OK (2) University of Nebraska - Lincoln, Lincoln

Dolomitization in the deep cold-water environment of the Belgica Carbonate Mound Province likely involves complex pore water chemistry and microbial activity. The Mound Province is located approximately 100 km off the southwest coast of Ireland in water depths of between 550 and 1025 m. Largely unlithified Miocene sediments in the region are comprised of mixed siliciclastics (silts and clays) and carbonates (nannoplankton and skeletal debris). These are overlain by unlithified Plio-Pleistocene carbonate mounds composed of nannoplankton and varying amounts of clay to silt sized siliciclastics, stabilized by cold water corals. Cores were collected from 11 holes drilled at 3 sites within and adjacent to the Challenger Mound during IODP Expedition 307. Minor amounts of dolomite were observed within the mound, whereas significant (>30%) dolomite was observed in several meter scale lithified intervals beneath and adjacent to the mound. Dolomite is characterized by micron to decimicron sized limpid crystals. Saturation profiles for aragonite, low-Mg calcite, and dolomite derived from pore water show that dolomite is oversaturated and calcite and aragonite undersaturated within the upper 50 m of Challenger Mound, approximately 10 m above the base of Challenger Mound, and throughout the underlying Miocene section. Sulfate, methane, and alkalinity profiles suggest a link between dolomite precipitation and microbial sulfate reduction. The distribution of dolomite as predicted by the pore water data has been confirmed by X-ray diffraction and petrographic analysis, which also indicates that dolomite is associated with lithified intervals in which nannoplanktonic carbonate has undergone significant diagenetic alteration.