Award: OCE-1536601

This project was one of three collaborative NSF research grants with the overall scientific objective of elucidating hydrogeological processes in young oceanic crust under an isolated 10x15 km sedimented depression called "North Pond" west of the mid-Atlantic Ridge. The main field approach was to utilize the remotely-operated vehicle (ROV) Jason from research vessel Atlantis in October of 2017, to recover geophysical, geochemical, and microbiological data and sensors from subseafloor sealed-hole hydrogeological observatories called ?CORKs? that had been installed by the Integrated Ocean Drilling Program in 2011, thus bringing those 6-year-long experiments to interim conclusion. Since the late 1970?s, North Pond has been recognized as an important type setting for low-temperature, off-axis circulation of formation fluids, with important implications for the thermal and chemical evolution of oceanic crust and the oceans, and, as recognized in the last ~2 decades, the potential to harbor a significant subseafloor microbial ecosystem. Scientific ocean drilling has been a key tool to study these phenomena beneath the seafloor, especially since the development around 1990 of the sealed-hole CORK long-term observatories. The earliest CORKs incorporated temperature and pressure sensors in the sealed holes, with data loggers at the wellhead accessible by manned or unmanned submersibles. Since then, geochemical and microbiological sampling capabilities have routinely been added to CORK instrumentation. This was especially the case for the North Pond CORKs (see attached figure), which were installed in 2011 and 2012 with a mix of NSF and Moore Foundation support, and were particularly focused on microbiological objectives but also included basic long-term temperature and pressure instrumentation financially supported by NSF. This specific project covered the pressure and temperature aspects related to the North Pond CORKs, including recovery and interpretation of seafloor and subseafloor temperature and pressure data from long term instruments that had been recording at two-minute intervals since original installation. The subseafloor pressure and temperature data were expected to be important in constraining directions of in-situ formation fluid flow in the permeable basement beneath the less permeable North Pond sediment cover, and in estimating related hydrological properties such as permeability of the basement formations. All three of the North Pond CORKs provided excellent long-term pressure data that revealed subtle pressure gradients in the North Pond basement formations associated with lateral flow of formation fluids. Formation pressures in the two deep CORKs are slightly above hydrostatic (up to about 1 psi), whereas pressures in the shallowest CORK were closer to hydrostatic. This is probably due to the thermally insulating effect of the relatively impermeable sediment layer in the setting. Despite the fact that the lateral pressure differences are small, they are associated with huge lateral fluxes of the basement formation fluids beneath the sediment cover, indicating that the upper sections of basement are extremely permeable relative to the overlying sediments. Because the sediment pond is of limited lateral extent and surrounded by large exposures of the basement formations, it is inferred that the huge fluxes beneath North Pond are made possible by essentially unlimited access to ocean water via the surrounding basement exposures. The CORK pressures are consistent with other evidence that suggests a dominant flow direction of ocean bottom water and formation fluids from south to north underneath North Pond. This should bring plentiful oxygenated seawater into the basement formations, with important implications for the microbiological ecosystems that were investigated in allied projects. The inferred flow direction also consistent with the main sense of faulting and fracturing to be expected in crust formed at the mid-Atlantic Ridge, which is oriented in a nearly north-south direction. The North Pond CORK pressure data are currently being utilized by graduate students at the University of Miami and University of California at Santa Cruz to provide ground-truth constraints on their computer models of the subseafloor flow systems. The six years of North Pond CORK pressure data are publicly available at the Marine Geoscience Data System/IEDA archive, and can be accessed at http://www.marine-geo.org/tools/search/entry.php?id=AT39-01 Last Modified: 10/04/2019 Submitted by: Keir Becker