[1] Hydrothermal systems at mid-ocean spreading centers play a fundamental role in Earth's geothermal budget. One underexamined facet of marine hydrothermal systems is the role that permeability of the uppermost seafloor veneer plays in the distribution of hydrothermal fluid. As both the initial and final vertical gateway for subsurface fluid circulation, uppermost seafloor permeability may influence the local spatial distribution of hydrothermal flow. A method of deriving a photomosaic from seafloor video was developed and utilized to estimate relative surface permeability in an active hydrothermal area on the Endeavour Segment of the Juan de Fuca Ridge. The mosaic resolves seafloor geology of the axial valley seafloor at submeter resolution over an area greater than 1 km 2 . Results indicate that the valley walls and basal talus slope are topographically rugged and unsedimented, providing minimal resistance to fluid transmission. Elsewhere, the axial valley floor is capped by an unbroken blanket of low-permeability sediment, resisting fluid exchange with the subsurface reservoir. Active fluid emission sites were restricted to the high-permeability zone at the base of the western wall. A series of inactive fossil hydrothermal structures form a linear trend along the western bounding wall, oriented orthogonal to the spreading axis. High-temperature vent locations appear to have migrated over 100 m along-ridge-strike over the decade between surveys. While initially an expression of subsurface faulting, this spatial pattern suggests that increases in seafloor permeability from sedimentation may be at least a secondary contributing factor in regulating fluid flow across the seafloor interface.

Thermal diffusivity (TD) is a measure of the temperature response of a material to external thermal forcing. In this study, TD values for marine sediments were determined in situ at two locations on the Cascadia Margin using an instrumented sediment probe deployed by a remotely operated vehicle. TD measurements in this area of the NE Pacific Ocean are important for characterizing the upslope edge of the methane hydrate stability zone, which is the climate-sensitive boundary of a global-scale carbon reservoir. The probe was deployed on the Cascadia Margin at water depths of 552 and 1049 m for a total of 6 days at each site. The instrumented probe consisted of four thermistors aligned vertically, one sensor exposed to the bottom water and one each at 5, 10, and 15 cm within the sediment. Results from each deployment were analyzed using a thermal conduction model applying a range of TD values to obtain the best fit with the experimental data. TD values corresponding to the lowest standard deviations from the numerical model runs were selected as the best approximations. Overall TDs of Cascadia Margin sediments of 4.33 and 1.15 × 10-7 m 2 s-1 were calculated for the two deployments. These values, the first of their kind to be determined from in situ measurements on a methane hydrate-rich continental margin, are expected to be useful in the development of models of bottom-water temperature increases and their implications on a global scale.

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