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.
Two expeditions to Dorado Outcrop on the eastern flank of the East Pacific Rise and west of the Middle America Trench collected images, video, rocks and sediment samples and measured temperature and fluid discharge rates to document the physical and biogeochemical characteristics of a regional, low‐temperature (~15°C) hydrothermal system. Analysis of video and images identified lava morphologies: pillow, lobate, and sheet flows. Glasses from collected lavas were consistent with an off‐axis formation. Hydrothermal discharge generally occurs through pillow lavas, but is patchy, sporadic, and sometimes ceases at particular sites of discharge. Year‐long temperature measurements at five of these discharge sites show daily ranges that oscillate with tidal frequencies by 6°C or more. Instantaneous fluid discharge rates (0.16 to 0.19 L s‐1) were determined resulting in a calculated discharge of ~200 L s‐1 when integrated over the area defined by the most robust fluid discharge. Such discharge has a power output of 10‐12 MW. Hydrothermal seepage through thin sediment adjacent to the outcrop accounts for <3% of this discharge, but seepage may support an oxic sediment column. High extractable Mn concentrations and depleted δ13C in the low but variable organic solid phase suggest hydrothermal fluids provide a source for manganese accumulation and likely enhance the oxidation of organic carbon. Comparisons of the physical and geochemical characteristics at Dorado and Baby Bare Outcrops, the latter being the only other site of ridge‐flank hydrothermal discharge that has been sampled directly, suggest commonalities and differences that have implications for future discoveries.
Benthic octopods cluster on bare rock on Dorado Outcrop, a ~3000 m deep basalt exposure. The outcrop hosts intermittent discharge of relatively cool (up to 12.3 °C) hydrothermal fluid that carries about half as much oxygen as bottom seawater (~54 μM vs. 108 μM). We analyzed 231 hours of video footage and still images taken by sub-sea vehicles in 2013 and 2014 that documented the clustered octopods, members of the poorly-known genus Muusoctopus. The largest cluster (102 octopods) occurred in a 19 m2 area of fluid discharge, where the basalt was sediment-free; individual octopods were also seen across the outcrop. The clustered octopods appeared to be brooding eggs and a total of 11 egg clutches were confirmed. None of the 186 eggs closely examined showed embryonic development. The intermittent fluid discharge may clear the basalt of sediment and attract gravid octopods which then spawn. However, the increased temperature and limited oxygen of the discharging fluids may threaten the octopods’ survival. Octopods in/near areas of discharging fluid had significantly higher estimated respiration rates (3.1–9.8 contractions/minute) than did octopods away from discharging fluid (0.8–6.0 contractions/minute). Warm fluids likely increase the octopods’ metabolic rate and thus their oxygen demand but provide only limited oxygen. The resultant physiological stress is hypothesized to eventually kill eggs near fluid discharge. We hypothesize, because these eggs do not survive, the population is sustained by a larger pool of undetectable females that brood their eggs inside cool conduits of this and perhaps other, unstudied basalt outcrops.
