[1] Areas of the seafloor at mid-ocean ridges where hydrothermal vents discharge are easily recognized by the dramatic biological, physical, and chemical processes that characterize such sites. Locations where seawater flows into the seafloor to recharge hydrothermal cells within the crustal reservoir are by contrast almost invisible but can be indirectly identified by a systematic grid of conductive heat flow measurements. An array of conductive heat flow stations in the Endeavour axial valley of the Juan de Fuca Ridge has identified recharge zones that appear to represent a nested system of fluid circulation paths. At the scale of an axial rift valley, conductive heat flow data indicate a general cross-valley fluid flow, where seawater enters the shallow subsurface crustal reservoir at the eastern wall of the Endeavour axial valley and undergoes a kilometer of horizontal transit beneath the valley floor, finally exiting as warm hydrothermal fluid discharge on the western valley bounding wall. Recharge zones also have been identified as located within an annular ring of very cold seafloor around the large Main Endeavour Hydrothermal Field, with seawater inflow occurring within faults that surround the fluid discharge sites. These conductive heat flow data are consistent with previous models where high-temperature fluid circulation cells beneath large hydrothermal vent fields may be composed of narrow vertical cylinders. Subsurface fluid circulation on the Endeavour Segment occurs at various crustal depths in three distinct modes: (1) general east to west flow across the entire valley floor, (2) in narrow cylinders that penetrate deeply to high-temperature heat sources, and (3) supplying low-temperature diffuse vents where seawater is entrained into the shallow uppermost crust by the adjacent high-temperature cylindrical systems. The systematic array of conductive heat flow measurements over the axial valley floor averaged ∼150 mW/m 2 , suggesting that only about 3% of the total energy flux of ocean crustal formation is removed by conductive heat transfer, with the remainder being dissipated to overlying seawater by fluid advection.
New hydrostations plus a comprehensive compilation of existing data have allowed us to characterize the dissolved silica plume located at midwater depths in the North Pacific. The North Pacific silica plume is a global‐scale anomaly, extending from the North American continental margin in the east to beyond the Hawaii‐Emperor seamount chain in the west. Inventory of the plume between 2000 and 3000 m depth indicates that it contains 164 Tmols (164 × 1012 mols) of anomalous dissolved silica and is maintained by a horizontal flux of approximately 1.5 Tmols/yr from the east. The source region of this plume has been previously suggested to be Cascadia Basin in the NE Pacific. Biochemical and geothermal processes within this small region can produce approximately one third of the required flux, but the majority of silica contained within the North Pacific plume may originate in crustal fluid venting from the warm upper basement aquifer that underlies the easternmost Pacific plate.
The circulation of hydrothermal fluid within upper oceanic crust constrains the global composition of seawater and is also responsible for many of the dynamic chemical and biological processes that alter the underlying volcanic rocks that form the sea floor. The heat of crustal formation drives this fluid circulation, and the impact on the overlying ocean is most easily observed at mid‐ocean ridge spreading centers. Previous efforts to quantify the heat associated with crustal formation have lacked information regarding the partitioning of thermal energy between discrete, high‐temperature vent fields, ubiquitous low‐temperature diffuse venting, and the pervasive conductive heat flux through the volcanic rocks.
[1] A new compilation of CTD data sheds light on the circulation and properties of bottom water within Cascadia Basin. A relatively high salinity water mass is revealed in a region of likely bottom water inflow east of the Blanco Fracture Zone. Water mass distributions and the deep geostrophic shear field are consistent with a general cyclonic circulation within the basin and suggest a northward-flowing boundary current next to the deep continental slope. Shifts in temperature-salinity relationships are consistent with a vertical transition from waters of northern origin to waters of southern origin approaching the seafloor. This vertical water-mass transition complicates the interpretation of temperature anomalies. We redefine the temperature anomaly due to geothermal activity and the thickness of the layer influenced by seafloor heating. Simple heat budget calculations lead to a residence time of about one year for the bottom water, shorter than suggested by previous studies. Citation: Hautala, S. L., H. P. Johnson, and T. Bjorklund (2005), Geothermal heating and the properties of bottom water in Cascadia Basin, Geophys. Res. Lett., 32, L06608,
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