Hydrothermal circulation within the Endeavour Segment, Juan de Fuca Ridge

Abstract

[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.

Surficial permeability of the axial valley seafloor: Endeavour Segment, Juan de Fuca Ridge

“…There are currently three generalized models proposed to describe the circulation pathways of fluid in hydrothermal systems at medium‐to‐fast‐spreading mid‐ocean ridges: (1) seawater enters the subsurface along deep, normal faults at one or both edges of the axial valley and then flows across‐axis to emerge as high temperature outflow in the center of the valley or on the side opposite the recharge zone [ Williams et al ., ; Johnson et al ., ], (2) fluid circulation takes place primarily within individual faults that are oriented along‐strike and parallel to the ridge axis in distinct slot convection cells [ Rabinowicz et al ., ; Wilcock , ; Tolstoy et al ., ], and (3) fluid convects in a basement layer with uniformly isotropic permeability as annular rings with broad circular recharge areas surrounding narrow up‐flow regions [ Coumou et al ., ; Tivey and Johnson , : Johnson et al ., ]. Each of these models appears to be applicable for specific hydrothermal vent systems located on spreading centers within a different geological environment.…”

“…The extent of the 2011 ROV image survey is shown in purple. Bathymetric data were obtained via SM2000 multibeam sonar [ Tivey and Johnson , ; Johnson et al ., ] with contour lines shown every 10 m.…”

Contrasting styles of deep-marine pyroclastic eruptions revealed from Axial Seamount push core records

“…The former is supported by the common association of active hydrothermal venting on the MOR and crustal discontinuities (i.e. faults; Fornari and Embley, 1995;Johnson et al, 2010), fluid flow models of submarine caldera complexes (Gruen et al, 2012), and the presence of sulfide deposits along caldera ring-faults of deeply eroded subaqueous caldera complexes (Stix et al, 2003;Mueller et al, 2009). Caldera ring faults are not only sites of active hydrothermal venting, but also serve as important magma conduits (Browning and Gudmundsson, 2015).…”

Activity and abundance of denitrifying bacteria in the subsurface biosphere of diffuse hydrothermal vents of the Juan de Fuca Ridge

“…We based our range of values for residence times on data from Johnson et al (2010) and Foustoukos et al (2009). Johnson et al (2010) calculated a crustal residence time of 2.1 yr (assuming 10 % porosity) for the across-axis flow in the Endeavour Segment axial valley, Juan de Fuca Ridge, using conductive heat flow measurements.…”

“…We based our range of values for residence times on data from Johnson et al (2010) and Foustoukos et al (2009). Johnson et al (2010) calculated a crustal residence time of 2.1 yr (assuming 10 % porosity) for the across-axis flow in the Endeavour Segment axial valley, Juan de Fuca Ridge, using conductive heat flow measurements. Foustoukos et al (2009) estimated a crustal residence time of only a few tens of hours for the very shallow circulation cells feeding low-T diffuse vents adjacent to the Endeavour high-temperature fields, using CO 2 / CO equilibrium data.…”