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Hydrothermal Plumes Over Spreading-Center Axes: Global Distributions and Geological Inferences

Edward T. Baker

NOAA Pacific Marine Environmental Laboratory, Seattle, Washington, USA

Christopher R. German

Institute of Oceanographic Sciences, Wormley, Surrey, UK

Henry Elderfield

Department of Earth Sciences, Cambridge University, Cambridge, UK

Seafloor Hydrothermal Systems: Physical, Chemical, Biological, and Geological Interactions, Geophysical Monograph 91, AGU, 47-71, 1995.
Copyright 1995 by the American Geophysical Union. Further electronic distribution is not allowed.

ABSTRACT

Hydrothermal plumes, formed by the turbulent mixing of hot vent fluids and ambient seawater, are potent tools for locating, characterizing, and quantifying seafloor hydrothermal discharge. Hydrographic, optical, and chemical tracers have all been used successfully to identify plumes. Observational techniques have progressed from discrete samples collected on vertical casts to continuous, in situ physical and chemical analyses that produce two- and three-dimensional plume maps. We present here a synthesis of available data from spreading centers throughout the world ocean wherever plumes have been mapped on a vent-field, segment, or multisegment scale. About 3% of the global divergent plate margin has been scrutinized for hydrothermal plumes; close to twice that distance has been sampled at least cursorily. Along medium- to superfast-spreading ridges in the eastern Pacific, where the most detailed work has been done, venting is common and plumes overlie 20-60% of the ridge crest length. Plumes are found also wherever careful surveys have been conducted in western Pacific marginal basins. Slow-spreading ridges in the North Atlantic, sampled over greater length scales but in less detail than in the Pacific, appear to have a comparatively low incidence of venting. Little is known of plume distributions over ridges in other oceans. These studies confirm that hydrothermal venting is present across the entire range of spreading rates and that continuous, underway plume surveys are the most efficient means available for locating seafloor discharge sites. Moreover, plume surveys are the only practical approach to mapping hydrothermal discharge patterns over multisegment spatial scales, and to integrating hydrothermal fluxes on a vent-field or larger scale. The plume surveys conducted to date indicate that the incidence of hydrothermal plumes over the ridge axis increases directly with increasing spreading rate. This observation supports models of crustal evolution that predict a direct relationship between the axial hydrothermal heat flux and spreading rate. This conclusion must be tempered, however, by the recognition that most of the global spreading center system remains unexplored for hydrothermal activity.


Introduction
Observational Techniques
Formation of Plumes
Hydrographic and Optical Tracers
Chemical Tracers
Sampling Techniques
Global Distributions
Precursors of Modern Plume Surveys
Eastern Pacific Spreading Centers
Western Pacific Marginal Basins
Mid-Atlantic Ridge
Reykjanes Ridge
Indian Ocean
Discussion
Exploration Implications
Geological Implications
Summary and Acknowledgments
References

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