The western flank of the Mid-Atlantic Ridge is a region underlying the oligotrophic waters of the central Atlantic. The seafloor along portions of this ridge is characterized by sediment-filled depressions, which are surrounded by steep basaltic outcrops. We present pore fluid and sediment solid-phase chemical data from fourteen gravity cores from “North Pond”, a sediment pond where previous drilling work indicated directed flow of seawater within the basement. Sediment lithology is broadly characterized as a nannofossil pelagic sediment containing varying amounts of clay, foraminifers, and Mn-micronodules and typically contains less than 0.3% organic carbon and ~ 70% calcium carbonate. Consistent with its location within an oligotrophic ocean gyre, oxygen and nitrate penetrated deeply into the sediment package. However there is significant spatial variability in the pore fluid nitrate and oxygen profiles, with oxygen generally lower and nitrate higher toward the center of the basin as compared to the edges. In addition, oxygen increased with sediment depth at a number of sites toward the edges of the pond, where sediment cover was thinnest. We interpret these oxygen distributions to indicate that there is upward diffusion of dissolved oxygen from the underlying basaltic basement fluid and the sediment package, and this process appears to be regionally pervasive. Pore fluid molybdenum generally decreases with depth and exhibits spatial variability similar to dissolved oxygen and nitrate. Molybdenum is likely being taken up at depth via adsorption onto manganese oxides, as these sediments are rich in manganese (~ 300–3000 ppm Mn) and molybdenum (~ 2–14 ppm Mo). The strong geographical variations in pore fluid chemistry coupled with the co-variation between molybdenum and oxygen, two species that we would not necessarily expect to be coupled, suggest that diffusion of dissolved constituents into the sediment package from below plays an important role in determining the chemistry of the overlying sediment.
An essential aspect of the forty years of deep-sea scientific drilling has been to maximize the scientific return during each expedition while preserving samples for future investigations. This philosophy also extends to borehole design, providing the community with tens of cased legacy boreholes that penetrate into the basaltic crust, each ripe for future investigations of crustal properties and experiments to determine crustal processes (Edwards et al., 2012a). During Integrated Ocean Drilling Program (IODP) Expedition 336 to North Pond on the western flank of the Mid-Atlantic Ridge at 22N, Hole U1383B (Fig. 1) was planned to be a deep hole, but was abandoned when a 14.75-inch tri-cone bit catastrophically failed at 89.9 meters below the seafloor (mbsf) (Expedition 336 Scientists, 2012). Thisresulted in about 36 meters of open hole below casing, similar to conditions within tens of legacy boreholes. Because the overall experiment required a return to the “natural” hydrologic state in basaltic basement, it was critical to seal the hole to prevent a hydrologic “short circuit”. Thus, a plan emerged at sea to seal Hole U1383B with a simplified Circulation Obviation Retrofit Kit (CORK) termed “CORK-Lite” that could be deployed by a remotely operated vehicle (ROV) on a planned dive series five months later. To prepare for this deployment, a standard ROV platform that is used with CORKs was modified to be self-guiding in the re-entry cone and deployed. The next step was to design a CORK system that could seal the borehole, yet be physically manageable with an ROV, and be ready for shipping and deployment within three months. Several key functional aspects dictated the design of the new CORK-Lite (Table 1).