The permeability, connectivity, and reactivity of fluid reservoirs in oceanic crust are poorly constrained, yet these reservoirs are pathways for about a quarter of the Earth's heat loss and seawater‐rock exchange within them impact ocean chemical cycles. We present results from the second‐ever cross‐hole tracer experiment within oceanic crust and the first conducted during a single expedition and in slow‐spreading crust west of the Mid‐Atlantic Ridge at North Pond. Here we employed boreholes that were drilled by the Integrated Ocean Drilling Program (IODP Sites U1382 and U1383) that were instrumented and sealed. A cesium‐salt solution and bottom seawater tracer experiment provided a measure of the minimum Darcy fluid velocity (2 to 41 m d‐1) within the upper volcanic crust, constraining the minimum permeability of 10‐11 to 10‐9 m2. We also document chemical heterogeneities in crustal fluid compositions, rebound from drilling disturbances, and nitrification within the basaltic crust, based on systematic differences in borehole fluid compositions over a 5‐year period. These results also show heterogeneous fluid compositions with depth in the borehole, indicating that hydrothermal circulation is not vigorous enough to homogenize the fluid composition in the upper permeable basaltic basement, at least not on the time scale of 5 years. Our work verifies the potential for future manipulative experiments to characterize hydrologic, biogeochemical, and microbial process within the upper basaltic crust.
During expedition MSM37 on the German RV Maria S. Merian, bottom water temperature and sediment temperature profiles were measured in the vicinity of North Pond (western flank of Mid‐Atlantic Ridge) during exploratory dives with Remotely Operated Vehicle Jason II. In addition, push cores were taken at locations with high sediment temperature gradients. We could identify two locations where sediment temperature gradients exceed 1 K/m and bottom water temperatures showed an anomaly of up to 0.04 °C above background. We interpret these observations as clear indication of low‐temperature diffuse venting of fluids that have traveled through the uppermost crust. We can safely assume that the observed phenomena are widespread at ridge flank settings where sediment cover is thin or absent, and hence, we can explain the efficient heat mining on ridge flanks. Due to the difficulties of locating diffuse low‐temperature discharge sites and due to the fact that discharge can occur through thin sediment cover as well as through sediment‐free basement outcrops, it will be very difficult to quantify fluxes of energy and mass from low‐temperature diffuse venting in ridge flank settings; however, thermal anomalies may be used to locate sites of discharge for geochemical, microbial, and hydrologic characterization.
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).