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Basement formation pressures and temperatures were recorded from 1997 to 2017 in four sealed‐hole observatories in North Pond, an isolated ∼8 × 15 km sediment pond surrounded by thinly sedimented basement highs in 7–8 Ma crust west of the Mid‐Atlantic Ridge at ∼23°N. Two observatories are located ∼1 km from the southeastern edge of North Pond where...
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... Thus, they have yielded very little temperature data. However, the CORK seals should have remained intact, so the pressure logging systems in all three holes should still be recording data at 20 min sampling intervals (Becker et al. 2022). ...
... 2) Utilizing in situ pressure values along with pre-sealing temperature profiles to estimate permeabilities of the Negative anomalies in the SP log were interpreted as potential permeable zones of radial flow of ocean bottom water from the hole into basement . Also shown are temperatures from the CORK thermistor cable (green circles) 10 years after the hole was sealed and an interpreted natural temperature profile (dashed line) that is also depressed in uppermost basement becaue of vigorous cool hydrothermal circulation in basement beneath the North Pond sediment cover (Becker et al. 2022). The right side shows eruptive volcanic cycles (Roman numerals I-X) interpreted from geophysical logs and apparent correspondence to the sequence of aphyric (A#) and phyric (P#) basaltic units described in the recovered core (Shipboard Scientific Party 1978). ...
... The right side shows eruptive volcanic cycles (Roman numerals I-X) interpreted from geophysical logs and apparent correspondence to the sequence of aphyric (A#) and phyric (P#) basaltic units described in the recovered core (Shipboard Scientific Party 1978). Figures modified from Becker et al. (2022). ...
We review the contributions to our understanding of the hydrogeology of the oceanic crust as gained by monitoring subseafloor temperatures and pressures in CORK (“Circulation Obviation Retrofit Kit”) sealed hole observatories that penetrate through marine sediments into igneous basement. CORKs were installed during 1991–2012 in 17 holes drilled by the Ocean Drilling Program and Integrated Ocean Drilling Programs in igneous oceanic crust (aged 0.4–24 Ma) in the thickly sedimented eastern Pacific Ocean and in the thinly sedimented north Atlantic Ocean. Results indicate that the igneous crust in all these settings is highly transmissive and supports vigorous lateral fluid flow and (or) convection beneath the sediment cover driven by very small horizontal pressure gradients. Inter-CORK experiments suggest that the horizontal permeability of the young oceanic basement at these sites may be anisotropic, with higher values in the direction parallel to the axis of spreading and tectonic fabric and lower values in a ridge-normal cross-strike direction. Tracer experiments applied to ridge-parallel flow suggest that only a small fraction of the crust is well connected. CORKs also record the response of subseafloor formation fluid pressures to seafloor tidal loading, and, once the tidal responses are filtered out, response to tectonic strain events. Interpretations based on these responses and original two-dimensional modelling of ridge-normal fluid circulation in the crust suggest a scale dependence of permeability of igneous oceanic crust (higher values at larger scales up to tens of km), although recent three-dimensional models suggest a more limited scale effect. CORKs have also provided important geochemical and microbiological results from the igneous oceanic crust, although they are not reviewed in this paper.
... Recent numerical studies of coupled heat transfer and fluid flow require high crustal permeabilities of 10 10 to 10 9 m 2 (Price et al., 2022(Price et al., , 2023, similar to the aforementioned studies on the Cocos Plate. These predictions by numerical studies are consistent with CORK observations (Becker et al., 2022;Wheat et al., 2020). However, flow dynamics below North Pond appear to be more complex than outcrop-to-outcrop circulation for "discharge-dominated" systems in the two examples for intermediate-and fast-spreading crust given above (Becker et al., 2022;Price et al., 2022). ...
... These predictions by numerical studies are consistent with CORK observations (Becker et al., 2022;Wheat et al., 2020). However, flow dynamics below North Pond appear to be more complex than outcrop-to-outcrop circulation for "discharge-dominated" systems in the two examples for intermediate-and fast-spreading crust given above (Becker et al., 2022;Price et al., 2022). ...
Cold and diffuse hydrothermal circulation on mid‐ocean ridge flanks impacts heat and fluid fluxes between the seafloor and the ocean. One mode of this circulation is given by outcrop‐to‐outcrop flow, where seawater circulates through a crustal aquifer that connects two or more recharging and discharging seamounts or basement highs that outcrop through the less permeable sediment cover. The physical mechanism driving this flow is a lateral pressure gradient that is sustained by contrasting the hydrological properties of the recharging and discharging outcrops. To investigate the physical controls of this pressure gradient, we performed two‐dimensional numerical simulations of coupled heat transfer and fluid flow. We have modified aquifer permeability, outcrop permeability and width, outcrop distance, and sediment thickness to assess their mutual effects on the lateral pressure differences. We have also investigated how different flow patterns, resulting from changes in these parameters, manifest themselves in seafloor observables such as flow rates, aquifer temperatures, and heat flow. Our models show that outcrop‐to‐outcrop flow generally occurs for aquifer permeabilities ≥10⁻¹⁴ m², depending on the basal heat input. High aquifer permeabilities correspond to fast flow rates and low fluid temperatures, whereas the maximum lateral pressure differences arise for lower permeabilities. The permeability and the geometric shape of the outcrops determine the flow direction, while the aquifer temperature is also affected by the distance between the outcrops. Thicker sediments increase the lateral pressure difference and the flow rate. Our models thus provide constraints for predicting subseafloor hydrothermal ridge flank flow behavior from regional field data.
