Abstract
Since 1991, ODP and IODP have instrumented >30 subseafloor boreholes in sedimented young ocean crust and subduction settings with long-term sealed-hole Circulation Obviation Retrofit Kit (CORK) observatories. We review CORK results from young oceanic crust on the flanks of the Juan de Fuca Ridge (JFR), Costa Rica Rift, and Mid-Atlantic Ridge (MAR). DSDP experience showed that holes drilled through marine sediments into underlying oceanic basement often allowed open exchange between formation and ocean, perturbing if not totally disturbing the in-situ hydrogeological state. This motivated the CORK approach to seal select holes, with instrumentation to record the recovery to in-situ formation state and to monitor natural hydrologic, tidal, geodynamic, and oceanographic signals. The original design had a single seal at seafloor; later designs allowed for separately monitoring multiple zones sealed by packers in a single hole. The sensor strings have always included pressure and temperature monitoring, and many have included osmotically driven OsmoSamplers for geochemical and microbiological objectives. Important results from paired CORK sites in all three ridge flanks include very small lateral pressure and temperature differentials in basement despite significant differences in basement topography. This requires vigorous off-axis hydrothermal circulation in highly permeable young oceanic basement, and the formation pressure responses to seafloor tidal loading also imply high upper basement permeabilities. These results have provided key ground-truth for numerical simulations of warm and cool off-axis hydrothermal circulation. At the JFR CORKs, a long-term tracer injection experiment documented a component of flow within off-axis basement that is normal to the spreading direction, consistent with the ridge-parallel strike of basement tectonic structures, and showed that most of the tracer transport occurred within a small fraction of the upper crust. At the North Pond MAR CORKs, seafloor temperature records document episodic renewal of oxygenated bottom water at times scales of 3-18 months in the isolated sediment pond. Without JOIDES Resolution past 2024, future crustal observatory objectives can still be supported by ROV deployment of CORK-Lites or new models into existing reentry holes.