HYDROTHERMAL PLUMES
Edward T. Baker
Joint Institute for the Study of Atmosphere and Ocean –
NOAA/PMEL, University of Washington, Seattle,
WA, USA
Definition
Hydrothermal plumes. Near-bottom oceanic layers, with
distinct chemical and physical signatures, formed by the
mixing of ambient ocean water and discharging hydrothermal vent fluids.
Introduction
Hydrothermal circulation is a primary agent for the
exchange of heat and chemicals between the Earth’s crust
and ocean (see “Hydrothermalism”). Hydrothermal discharge occurs on the seafloor where crustal fluids have
access to magmatic heat sources, such as along
mid-ocean ridges on divergent plate boundaries, volcanic
arcs at convergent plate boundaries, and intraplate hot
spots (see “Mid-ocean Ridge Magmatism and Volcanism,” “Hot Spots and Mantle Plumes”). These hydrothermal vents occupy only a tiny fraction of the seafloor, but
the hydrothermal plumes formed by the mixing of vent
fluids and seawater can be traced even across ocean
basins. Mapping and sampling hydrothermal plumes
locates vent fields, describes the dispersal of chemicals
from vent fields, estimates hydrothermal fluxes of heat
and chemicals, traces ocean circulation, and provides
information on the transfer of heat and chemicals to the
ocean during eruptions that construct the Earth’s crust.
Hydrothermal plume characteristics
Hydrothermal plumes form above the sites of venting as
hot hydrothermal fluids rise, entraining ambient seawater
with a consequent continuous increase in plume volume,
until nonbuoyancy is achieved and the plume disperses
laterally. Models of plume formation are based on observations of atmospheric plumes and the principles of turbulent entrainment described by Morton et al. (1956). Plume
rise height, which depends on the buoyancy flux of the
discharge (a function of temperature and volume flux)
and the ambient vertical density gradient, typically ranges
from 10s to 100s of meters. Entrainment ultimately dilutes
high-temperature (~150–400
C) vent fluids by roughly
a factor of 10
4 or more. At the level of lateral spreading,
hydrothermal plumes have different hydrographic characteristics depending on the local salinity profile (Speer and
Rona, 1989). In the deep Pacific, where salinity increases
with depth, the plume is warm and salty compared to
ambient seawater. In the deep Atlantic, where salinity
decreases with depth, the plume is relatively cool and
fresh. Using these models and local hydrographic profiles,
it is possible to calculate the true hydrothermal temperature anomaly within the laterally spreading plume. Several
detailed papers on plume formation can be found in
Humphris et al. (1995).
Hydrothermal fluids are enriched, relative to typical
oceanic deep waters, by up to a factor of ~10
7 in several
key chemical tracers found in vent fluids (see “Hydrothermal Vent Fluids (Seafloor)”) (e.g.,
3
He, Mn, Fe, CH 4 , H 2 ,
H 2 S, suspended particles, and other trace species). These
tracers allow hydrothermal plumes to be detected at significant distances away from hydrothermal vent sites
(Figure 1). Hydrothermal species can be classified as conservative or nonconservative. Conservative species
include heat and
3 He, and their concentration in plumes
changes only by dilution. Nonconservative species, such
as suspended hydrothermal precipitates, Mn, Fe, CH 4 ,
H 2 , and H 2 S, are also lost by chemical and biological degradation or by deposition. Their residence time in the
plume varies widely, from several years for dissolved
Mn and Fe and Mn oxyhydroxides to months or days for
suspended particles and CH 4 and to days or hours for H 2
and H 2 S. These differences in residence time make it possible to roughly estimate the age of a plume and thus its
distance from the source.
Hydrothermal plumes can be categorized on the basis
of their discharge and temporal characteristics. Discrete
vent sources are individual orifices that discharge hightemperature fluids (~150–400
C). These include the
familiar “black smokers” emitting opaque smokestacklike plumes with high concentrations of precipitated
hydrothermal metal sulfides and oxyhydroxides (see
“Black and White Smokers”). Diffuse vent sources leak
low-temperature (a few 10s of
C) fluids over a broadly
distributed area. Plumes from diffuse vent sources have
few suspended particles and have insufficient rise height
to form a distinct above-bottom layer. Plumes from stable
vents that discharge for years are termed chronic plumes.
Plumes formed in association with a magma eruption or
intrusion are termed event (or mega-) plumes. Event
plume discharge lasts only minutes to days yet forms
large, eddy-like plume features extending up to
a kilometer above the seafloor (Baker et al., 2011).
Hydrothermal plume applications
Exploration
The earliest hydrothermal plume sampling occurred during a near-bottom sensor tow that detected a temperature
spike (Weiss et al., 1977) and
3
He anomalies (Lupton
et al., 1977) on the Galapagos Rift. This observation led
directly to the discovery of seafloor vents at the same location, the first ever found on a mid-ocean ridge (Corliss
et al., 1979; see “Mid-ocean Ridge Magmatism and Volcanism”). This success demonstrated the power of using
hydrothermal plumes as a tool for finding the vents themselves. Researchers quickly recognized that “tow-yoing”
a sensor package through near-bottom water to construct
a two-dimensional transect of plumes is an efficient and
high-resolution search and sampling strategy (Figure 1;
Baker et al., 1985).
HYDROTHERMAL PLUMES
335
Edward T. Baker
Joint Institute for the Study of Atmosphere and Ocean –
NOAA/PMEL, University of Washington, Seattle,
WA, USA
Definition
Hydrothermal plumes. Near-bottom oceanic layers, with
distinct chemical and physical signatures, formed by the
mixing of ambient ocean water and discharging hydrothermal vent fluids.
Introduction
Hydrothermal circulation is a primary agent for the
exchange of heat and chemicals between the Earth’s crust
and ocean (see “Hydrothermalism”). Hydrothermal discharge occurs on the seafloor where crustal fluids have
access to magmatic heat sources, such as along
mid-ocean ridges on divergent plate boundaries, volcanic
arcs at convergent plate boundaries, and intraplate hot
spots (see “Mid-ocean Ridge Magmatism and Volcanism,” “Hot Spots and Mantle Plumes”). These hydrothermal vents occupy only a tiny fraction of the seafloor, but
the hydrothermal plumes formed by the mixing of vent
fluids and seawater can be traced even across ocean
basins. Mapping and sampling hydrothermal plumes
locates vent fields, describes the dispersal of chemicals
from vent fields, estimates hydrothermal fluxes of heat
and chemicals, traces ocean circulation, and provides
information on the transfer of heat and chemicals to the
ocean during eruptions that construct the Earth’s crust.
Hydrothermal plume characteristics
Hydrothermal plumes form above the sites of venting as
hot hydrothermal fluids rise, entraining ambient seawater
with a consequent continuous increase in plume volume,
until nonbuoyancy is achieved and the plume disperses
laterally. Models of plume formation are based on observations of atmospheric plumes and the principles of turbulent entrainment described by Morton et al. (1956). Plume
rise height, which depends on the buoyancy flux of the
discharge (a function of temperature and volume flux)
and the ambient vertical density gradient, typically ranges
from 10s to 100s of meters. Entrainment ultimately dilutes
high-temperature (~150–400
C) vent fluids by roughly
a factor of 10
4 or more. At the level of lateral spreading,
hydrothermal plumes have different hydrographic characteristics depending on the local salinity profile (Speer and
Rona, 1989). In the deep Pacific, where salinity increases
with depth, the plume is warm and salty compared to
ambient seawater. In the deep Atlantic, where salinity
decreases with depth, the plume is relatively cool and
fresh. Using these models and local hydrographic profiles,
it is possible to calculate the true hydrothermal temperature anomaly within the laterally spreading plume. Several
detailed papers on plume formation can be found in
Humphris et al. (1995).
Hydrothermal fluids are enriched, relative to typical
oceanic deep waters, by up to a factor of ~10
7 in several
key chemical tracers found in vent fluids (see “Hydrothermal Vent Fluids (Seafloor)”) (e.g.,
3
He, Mn, Fe, CH 4 , H 2 ,
H 2 S, suspended particles, and other trace species). These
tracers allow hydrothermal plumes to be detected at significant distances away from hydrothermal vent sites
(Figure 1). Hydrothermal species can be classified as conservative or nonconservative. Conservative species
include heat and
3 He, and their concentration in plumes
changes only by dilution. Nonconservative species, such
as suspended hydrothermal precipitates, Mn, Fe, CH 4 ,
H 2 , and H 2 S, are also lost by chemical and biological degradation or by deposition. Their residence time in the
plume varies widely, from several years for dissolved
Mn and Fe and Mn oxyhydroxides to months or days for
suspended particles and CH 4 and to days or hours for H 2
and H 2 S. These differences in residence time make it possible to roughly estimate the age of a plume and thus its
distance from the source.
Hydrothermal plumes can be categorized on the basis
of their discharge and temporal characteristics. Discrete
vent sources are individual orifices that discharge hightemperature fluids (~150–400
C). These include the
familiar “black smokers” emitting opaque smokestacklike plumes with high concentrations of precipitated
hydrothermal metal sulfides and oxyhydroxides (see
“Black and White Smokers”). Diffuse vent sources leak
low-temperature (a few 10s of
C) fluids over a broadly
distributed area. Plumes from diffuse vent sources have
few suspended particles and have insufficient rise height
to form a distinct above-bottom layer. Plumes from stable
vents that discharge for years are termed chronic plumes.
Plumes formed in association with a magma eruption or
intrusion are termed event (or mega-) plumes. Event
plume discharge lasts only minutes to days yet forms
large, eddy-like plume features extending up to
a kilometer above the seafloor (Baker et al., 2011).
Hydrothermal plume applications
Exploration
The earliest hydrothermal plume sampling occurred during a near-bottom sensor tow that detected a temperature
spike (Weiss et al., 1977) and
3
He anomalies (Lupton
et al., 1977) on the Galapagos Rift. This observation led
directly to the discovery of seafloor vents at the same location, the first ever found on a mid-ocean ridge (Corliss
et al., 1979; see “Mid-ocean Ridge Magmatism and Volcanism”). This success demonstrated the power of using
hydrothermal plumes as a tool for finding the vents themselves. Researchers quickly recognized that “tow-yoing”
a sensor package through near-bottom water to construct
a two-dimensional transect of plumes is an efficient and
high-resolution search and sampling strategy (Figure 1;
Baker et al., 1985).
HYDROTHERMAL PLUMES
335