Search citation statements
Paper Sections
Citation Types
Year Published
Publication Types
Relationship
Authors
Journals
We have estimated the heat flux from black smoker vents on the Juan de Fuca Ridge to evaluate their importance for heat transfer from young oceanic crust. The velocity and temperature of smoker effluent were measured from the manned submersible Alvin within a few centimeters of vent orifices, using a turbine flowmeter with an attached temperature probe. Exit velocity was calculated from a simple plume model, and vent orifices were measured in photographs and video records. The estimated power output from smokers alone is 49±13 MW for the Plume site, Vent 1 and Vent 3 on the southern Cleft segment near 45°N; 364±73 MW for the main vent field on the Endeavour Segment near 48°N; and 122±61 MW for the Tubeworm field 2 km north. The estimates for the Cleft and Tubeworm fields could be too low because of undiscovered vents. These values constitute only 4% to 14% of the total advective heat flux estimated for these vent fields from measurements in the nonbuoyant plume and of diffuse flow at the seafloor, indicating that most of the heat advected at these hydrothermal vent sites is carried by diffuse rather than focused flow. Values for individual smokers vary from 0.1 to 94 MW, with an average of 6.2 MW at the Endeavour field and 3.1 MW at the Cleft field. Our estimates agree well at all scales with those of Bemis et al. [1993] based on measurements made during the same dives, in some cases simultaneously, up to 50 m high in the buoyant plume. The good agreement between the two techniques implies that little diffuse flow at either high or low temperature is incorporated into the buoyant plumes generated by smokers at these sites. Velocity‐temperature measurements at vents excavated by Alvin could not be modeled successfully, suggesting that vent structures may grow in equilibrium with the force of the exiting water such that orifice size is determined by volume flux. At the Endeavour field the heat flux is focused by faults.
We have estimated the heat flux from black smoker vents on the Juan de Fuca Ridge to evaluate their importance for heat transfer from young oceanic crust. The velocity and temperature of smoker effluent were measured from the manned submersible Alvin within a few centimeters of vent orifices, using a turbine flowmeter with an attached temperature probe. Exit velocity was calculated from a simple plume model, and vent orifices were measured in photographs and video records. The estimated power output from smokers alone is 49±13 MW for the Plume site, Vent 1 and Vent 3 on the southern Cleft segment near 45°N; 364±73 MW for the main vent field on the Endeavour Segment near 48°N; and 122±61 MW for the Tubeworm field 2 km north. The estimates for the Cleft and Tubeworm fields could be too low because of undiscovered vents. These values constitute only 4% to 14% of the total advective heat flux estimated for these vent fields from measurements in the nonbuoyant plume and of diffuse flow at the seafloor, indicating that most of the heat advected at these hydrothermal vent sites is carried by diffuse rather than focused flow. Values for individual smokers vary from 0.1 to 94 MW, with an average of 6.2 MW at the Endeavour field and 3.1 MW at the Cleft field. Our estimates agree well at all scales with those of Bemis et al. [1993] based on measurements made during the same dives, in some cases simultaneously, up to 50 m high in the buoyant plume. The good agreement between the two techniques implies that little diffuse flow at either high or low temperature is incorporated into the buoyant plumes generated by smokers at these sites. Velocity‐temperature measurements at vents excavated by Alvin could not be modeled successfully, suggesting that vent structures may grow in equilibrium with the force of the exiting water such that orifice size is determined by volume flux. At the Endeavour field the heat flux is focused by faults.
Multichannel seismic reflection data acquired between 8°50′ and 9°50′N and between 12°30′ and 13°30′N along the East Pacific Rise provide a three‐dimensional view of the young oceanic crust. Seafloor‐to‐Moho reflection travel times vary by up to 0.9 s within our study areas; the total range of crustal travel times in the 9°N area is 1.55 to 2.45 s; the total range in the 13°N area is 1.60 to 2.05 s. The variation is systematic, indicating thinner crust locally associated with overlapping spreading centers (OSCs) and, in the 9°N area, segment‐scale variation along crustal isochrons. Crustal travel time is found to be a valid proxy for oceanic crustal thickness. Outside of the axial low‐velocity volume, thickness can be calculated from time to ∼500 m. Even in the axial region thickness can be calculated to <1 km, if low‐velocity zone position is known. Crustal thicknesses calculated from travel times vary by 2.6 km in the 9°N area, and by 1.5 km in the 13°N area. The majority of this variation is attributed to seismic layer 3 (the lower crust). Segment‐scale variation of ∼1.8 km (∼5.5 to 7.3 km thickness) is observed in the 9°N area, with thinnest crust formed between ∼9°40′ and 9°50′N and thickest formed between ∼9°15 and 9°20′N. Results imply a three‐dimensional pattern of magma supply to the 9°N segment. The OSC at 9°03′N is associated with major disruptions of the segment‐scale pattern, in the form of local thin areas within the discordant zone; the smaller OSC at 12°54′N is not associated with dramatic changes in thickness of the surrounding crust. In the absence of OSCs, the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment. Thicker crust does not always correlate with shallower ridge bathymetry, broader axial cross section, or more negative mantle Bouguer or subcrustal gravity anomaly. Variable thickness of the crust‐mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result. We hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick Moho transition zone and a subcrustal gravity low. Such a diapiric upwelling center appears to be now located beneath the East Pacific Rise near 9°40′ to 9°50′N.
The depth to the axial magma chamber at three oceanic spreading centers, as indicated by seismic reflection surveys, is greater than that predicted by conductive cooling thermal models of ridge crests. The additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust. A theoretical model for the temperature distribution at spreading centers, which includes distributed heat sources and sinks, is presented. By representing the hydrothermal heat loss as a series of heat sinks, the depths to the axial magma chamber at the East Pacific Rise at latitude 9øN., the southern Juan de Fuca Ridge, and the Lau Basin spreading center are modeled. Heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys. The thermal modeling indicates that the axial hydrothermal heat flux is between 7.1 x 10 6 and 13.8 x 10 6 MJ/m 2, or 10 to 20% of the total missing heat at the ridge axis, requiring extensive low-temperature circulation off axis. High-temperature vents that are spaced 1 km apart and are active between 4 and 10% of the time would account for the axial hydrothermal heat loss.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.