)
is calculated as U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
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.
Seafloor hydrothermal circulation is the principal agent of energy and mass exchange between the ocean and the earth's crust. Discharging fluids cool hot rock, construct mineral deposits, nurture biological communities, alter deep-sea mixing and circulation patterns, and profoundly influence ocean chemistry and biology. Although the active discharge orifices themselves cover only a minuscule percentage of the ridge-axis seafloor, the investigation and quantification of their effects is enhanced as a consequence of the mixing process that forms hydrothermal plumes. Hydrothermal fluids discharged from vents are rapidly diluted with ambient seawater by factors of 104-105 [Lupton et al., 1985]. During dilution, the mixture rises tens to hundreds of meters to a level of neutral buoyancy, eventually spreading laterally as a distinct hydrographic and chemical layer with a spatial scale of tens to thousands of kilometers [e.g., Lupton and Craig, 1981; Baker and Massoth, 1987; Speer and Rona, 1989].
Early investigations focused on plumes simply as indicators of nearby discharge sources. More comprehensive studies, however, quickly demonstrated that plumes integrate hydrothermal heat and mass flux [Baker and Massoth, 1987; Rosenberg et al., 1988; Rudnicki and Elderfield, 1992], provide natural laboratories for measuring the chemical reactions that control the ultimate dispersal of various hydrothermal species [Klinkhammer et al., 1983; Mottl and McConachy, 1990; Feely et al., 1991; German et al., 1991; Metz and Trefry, 1993; Rudnicki and Elderfield, 1993; Kadko, 1994], disperse vent-specific biological populations [Mullineaux et al., 1995], track deep-ocean circulation patterns [Lupton and Craig, 1981; Reid, 1982], and, in general, provide opportunities for identifying and quantifying the effects of hydrothermal activity. In this paper we focus narrowly on the use of plumes as a tool for hydrothermal exploration and geological interpretation. We begin by outlining commonly used techniques for producing detailed maps of the hydrographic and chemical anomalies produced by plumes along oceanic spreading centers. We then review those ridge-crest areas in the Pacific (including the western Pacific marginal basins), Atlantic, and Indian Oceans where sufficient data exist to map, or at least detect, plumes at vent-field, segment, or multisegment scales. Many of these data are new or formerly available only in fragmented form. Finally, we offer some geological interpretations of the data, using as an example the emerging coherence between hydrothermal discharge patterns and ridge-crest spreading rate.
Hydrothermal plumes form above sites of venting because of the buoyancy of the hot hydrothermal fluids that rise, entraining ambient sea water with a consequent continuous increase in plume volume, until neutral buoyancy is achieved and the plume disperses laterally. Models describing the physical and chemical properties of hydrothermal plumes have been presented by several workers [e.g., Middleton and Thomson, 1986; Little et al., 1987; Speer and Rona, 1989; McDougall, 1990; Rudnicki and Elderfield, 1992; Lavelle, 1994], all based upon the principles for turbulent entrainment in plumes described by Morton et al. [1956] and Turner [1973]. Using these models it is possible to calculate heat fluxes from plume and local hydrographic data, and to predict the ratio of entrained seawater to hydrothermal fluid as a function of plume height. During plume rise, ambient seawater is entrained from a range of depths such that the concentration of a property in the neutrally buoyant plume is a function of both the concentration and flux of that property in the vent fluid, and of the background profile of the property integrated over the whole height of rise of the buoyant plume. These complexities are discussed more fully in the following sections, and especially in accompanying papers by McDuff [this volume], Lupton [this volume], and Helfrich and Speer [this volume].
Quantification of hydrothermal temperature anomalies is complicated by the variable
background profiles of temperature and salinity found in the world ocean. In the deep
Pacific, the ambient salinity gradient increases with depth and so the neutrally buoyant
plume is warmer and saltier than its surroundings. An estimate of the hydrothermal
temperature anomaly ()
is calculated as
(1)
where k and b are the slope and intercept, respectively, of the generally
linear relationship between potential temperature () and
potential density (
) in hydrothermally unaffected
water immediately above the neutrally buoyant plume. To properly compare this value in the neutrally buoyant plume with the hydrothermal
heat input at the seafloor source, however, the plume values must be corrected for the
effect of local hydrography and vent fluid salinity.
In the Atlantic and wherever else the salinity gradient is negative above the ridge crest, the neutrally buoyant plume is cooler and fresher than the surrounding seawater. Equation (1) gives a negative value in this case, and hydrographic mapping of the plume is problematic at best. Evaluation of temperature anomalies must be considered within the framework of a plume model [e.g., Speer and Rona, 1989; McDougall, 1990; Rudnicki and Elderfield, 1992]. Thus most plume mapping in the Atlantic has depended on optical or chemical indices of hydrothermal discharge.
Both light attenuation and light scattering have been widely used to define hydrothermal plumes. Unlike conservative hydrographic tracers, optical tracers are nonconservative and depend on an ever-changing balance between particle production by precipitation and biological growth, and particle loss by deposition, dissolution, and ingestion. Hydrothermal fluids with a very low level of precipitable species, such as the vapor phase of a phase-separated fluid, or a fluid discharging through a thick layer of sediments, may yield a plume with a tenuous optical signal, but optically invisible plumes are rare. The only documented case we are aware of is a CH4 plume with no nephelometer or total dissolved Mn signal near 15°N on the Mid-Atlantic Ridge (MAR), a plume hypothesized to arise from fluid circulation in ultrabasic (low silica) rocks rather than basalt [Charlou et al., 1991a]. Moreover, optical tracers are often more sensitive and simpler to interpret than hydrographic tracers, since their background profiles are roughly vertically uniform. Their nonconservative nature can add to their usefulness, since an optical signal decreases more sharply than temperature with distance from its source.
Hydrothermal fluids are enriched by up to a factor of 107 in several key tracers (e.g., Mn, Fe, CH4, H2, 3He, and many other trace species) relative to typical oceanic deep waters, so chemical anomalies associated with hydrothermal plumes can often be detected at significant distances away from hydrothermal vent sites. (The chemistry of hydrothermal fluids and plumes is more fully discussed by Lilley et al. [this volume], Lupton [this volume], Kadko et al. [this volume], and Von Damm [this volume].) Dissolved manganese (Mn) and methane (CH4) are commonly used to define hydrothermal plumes for three reasons. First, both are enriched ~106-fold in high-temperature vent fluids relative to ambient seawater [e.g., Welhan and Craig, 1983; Von Damm, 1990], so their concentrations in neutrally buoyant hydrothermal plumes directly above hydrothermal vent sites are ~102-fold higher than typical oceanic deep water. Second, both can be analyzed precisely at sea. Third, the contrasting quasiconservative behavior of Mn and distinctly nonconservative behavior of CH4 provide dynamical information about plumes. Because neutrally buoyant plumes overlie a much greater area of the midocean ridge crest than is occupied by active hydrothermal chimneys and mounds, these water column enrichments present geochemists a magnifying glass with which to prospect for new hydrothermal vent sites.
Fig. 1. Schematic comparison of three methods of shipboard plume mapping and sampling.
Hydrographic and optical measurements are commonly combined in the form of a
conductivity/temperature/depth (CTD) system interfaced with a transmissometer or
nephelometer (or both). A basic sampling strategy is simply vertical casts widely spaced
along the ridge axis (Figure 1). Because hydrothermal
plumes are dynamic features with steep horizontal and vertical gradients, however, it is
useful to adopt a sampling scheme that will reduce the problem of temporal and spatial
aliasing as much as possible. Data return should be maximized from the plume horizon and
minimized from waters above the plume where little useful information is present. An
efficient strategy in mapping and sampling neutrally buoyant plumes is thus to conduct
lengthy "tow-yos," continual raisings and lowerings of the instrumentation
through the plume while slowly steaming (Figure 1).
Experience shows that a typical CTD package can be towed with standard electromechanical
cable at a rate of about
The majority of geochemical studies associated with hydrothermal plume surveys up to about 1990, and which continue in widespread use today, required conventional collection of seawater samples using CTD-rosette systems followed by ship-board or shore-based analyses for geochemical tracers. To accommodate the enlarged data sets required to address more complex geochemical problems, much investment has been made recently in novel instrumentation which can achieve greater sampling coverage or even produce real-time in situ geochemical analyses. This instrumentation will generate data in a form comparable to that readily obtainable from hydrographic and optical sensors, thus facilitating improved understanding of the biogeochemical processes active in plumes.
The first advance in geochemical studies of hydrothermal plumes came from the
IFREMER laboratory's development of a continuous underway towed water sampler--the
"Palanquée Dynamique" or "Dynamic Hydrocast" [Bougault
et al., 1990]. This system comprises four sets of water sampling rosettes,
each with a CTD sensor package and data logger, that are clamped to a cable
and towed at different, known heights above the seafloor (Figure
1). Each set of water sampling rosettes comprises ten water-sampling bottles
which fill progressively, and sequentially, as the rosette package is towed
through the water column. The gearing on the rosettes can be adjusted so that
each bottle fills progressively over
Although the Dynamic Hydrocast system enables systematic and continuous water-sampling,
only a limited number of samples can be collected and these must be analyzed
onboard or ashore. An alternative approach was taken by Johnson and co-workers
at the Moss Landing Marine Laboratories. They developed a novel instrument package,
the Submersible Chemical Analyzer (SCANNER), that could
carry out analyses of nutrients, sulfide, Mn and Fe in situ [Johnson
et al., 1986a,b;
Coale
et al., 1991; Chin
et al., 1994]. The system is a deep-towed spectrophotometer that measures
dissolved Mn and total dissolved Fe (Fe(II) and Fe(III)) every 5 seconds in
seawater that has previously been passed through a 10 
m
mesh prefilter. The system connects routinely to a conventional CTD package
augmented with a nephelometer and/or transmissometer. Coale
et al. [1991] and Chin
et al. [1994] used the SCANNER to map dissolved Fe and Mn anomalies
in the range
More recently, SUAVE (Submersible System Used to Assess
Vented Emissions), an enhanced version of the SCANNER system,
has been developed by NOAA's Pacific Marine Environmental Laboratory. The SUAVE
system comprises a total of six on-line colorimetric chemical detectors for
Mn, Fe(II), Fe(II+III), Si, H2S, and one of PO4 or Cl,
currently under development for plume and diffuse-flow studies, respectively
[Massoth
et al., 1991] (G. J. Massoth, personal communication, 1994). The system
can currently be deployed for at least
Sensitivity of both the SCANNER and SUAVE systems are limited by the colorimetric
techniques available for dissolved Mn and Fe analyses, originally
To overcome this potential problem, Klinkhammer
[1994] has developed an even more sensitive dissolved Mn analyzer, ZAPS
(Zero Angle Photon Spectrophotometer), that can detect dissolved Mn to ambient
seawater concentrations 
1 nmol l-1),
Even before the discovery of submarine hydrothermal vents in 1977 [Corliss et al., 1979] and 1979 [RISE Project Group, 1980], a few investigators had recognized large-scale physical and chemical anomalies that were rightly attributed to hydrothermal discharge. Over 30 years ago, Knauss [1962] suggested that abnormally warm water between 3000 and 3500 m over the East Pacific Rise (EPR) near 20°S was probably related to a recently mapped zone of high seafloor heat flow in the same area [Von Herzen, 1959]. Ten years later, Warren [1973] noticed the same feature in data from the SCORPIO expedition at 28°S and 43°S, although he was reluctant to attribute the anomaly to geothermal effects since purely oceanographic effects might also lead to the same temperature distribution. Lonsdale [1976] made perhaps the earliest attempt to quantify the temperature anomaly of a hydrothermal plume when he plotted temperature-salinity curves of profiles west, east, and directly over the EPR at 8°S. The axial stations revealed near-bottom temperature increases of several hundredths of a degree Celsius relative to off-axis water of the same salinity (Figure 2).
Fig. 2. Plot of potential temperature vs. salinity for discrete samples near the East Pacific Rise at 8°S showing hydrothermal heating at the rise crest (reprinted from Lonsdale [1976]).
Measurements of the helium isotopes 3He and 4He in the Kermadec Trench [Clarke et al., 1969] and across the EPR [Craig et al., 1975] provided early evidence that hydrothermal venting could produce widely dispersed chemical anomalies. Shortly thereafter, sampling near the Galapagos Ridge by Klinkhammer et al. [1977] and Bolger et al. [1978] showed that hydrothermal plumes also contain significant anomalies of dissolved and particulate Mn. Since then, plume surveys have contributed to the discovery and characterization of hydrothermal discharge sites on ridges and hotspots throughout the oceans and marginal seas.
Hydrothermal plumes have been mapped in more detail over eastern Pacific ridge
crests than anywhere else in the ocean. Virtually continuous surveys of plume
hydrographic, optical, and/or chemical anomalies have been conducted along much
of the Explorer-Juan de Fuca-Gorda Ridge complex in the northeast Pacific, in
the Gulf of California, and along the EPR from about 9° to 13°N and 13° to 22°S.
Full-rate spreading speeds at these sites range from a medium rate of
Fig. 3. Location map for northeast Pacific spreading centers.
The Explorer-Juan de Fuca-Gorda Ridge complex is an isolated set of spreading
ridges that extend for almost
Plumes emanating from the JDFR have been mapped more intensively and extensively
than those over any other group of ridge-crest segments in the ocean. The first
indication of JDFR hydrothermal plumes came in 1980 from observations of dissolved
Mn [Jones
et al., 1981] and 3He [Lupton,
1990] at five widely spaced stations stretching from 44°31'N to 47°47'N.
In 1982, Crane
et al. [1985] conducted the first large scale, continuous survey of
hydrothermal plumes anywhere on the global midocean ridge (MOR). They attempted
to measure hydrothermally induced temperature anomalies in plumes by hanging
a 50-m-long thermistor chain from a side-scan sonar vehicle while towing it
along the entire JDFR. Their measurements were successful in identifying a few
major thermal anomalies but did not fully reveal the actual plume distribution
because of calibration problems and the very slow
Plume mapping on the JDFR has focused on three areas: the Endeavour segment,
Axial Volcano, and the Cleft segment (Figure 3).
Along the Endeavour segment, Baker
and Massoth [1987] and Thomson
et al. [1992] used transects of temperature and light attenuation anomalies
to characterize the plume and discover vent locations. They combined these anomalies
with concomitantly collected records of plume advection from moored current
meters to estimate mean heat fluxes of 1700 ± 800 MW
Fig. 4. CTD tow-yo transects of hydrothermal temperature anomaly (°C) along
the Cleft segment axial valley, 1986-1991. Triangles show location of known
venting sites. The lower half of event plume EP86 ("megaplume") is
apparent in the 1986 transect. The anomaly at
The plume distribution over the entire Cleft segment has been mapped one or
more times every year since 1986 [Baker
and Massoth, 1986; Baker
and Massoth, 1987; Baker,
1994], making it a unique source of information about the temporal variability
of hydrothermal venting. Discharge from the Cleft segment is concentrated in
two areas centered at 44°38'-44°43'N and 44°54'-45°N (Figure
4). The first plume survey to estimate hydrothermal fluxes was conducted
at the southern site in 1985 [Baker
and Massoth, 1986; 1987],
finding a heat flux of 580 ±
Fig. 5. (A) Cross-section of EP86 and underlying chronic plume (reprinted
from Baker
et al. [1989]). Contours are isolines of hydrothermal temperature anomaly
(°C). Potential density
Fig. 6. (A) Cross-sections of two event plumes mapped over the CoAxial segment in July, 1993 [Baker et al., 1995]. Both plumes were smaller and had lower temperature anomalies than EP86 or EP87. (B) Plan view of all three CoAxial event plumes showing their relative locations when first discovered. EP93A and EP93B were found directly over the lava eruption mound (solid bead) and warm-water fissure (heavy line) near 46°30'N. EP93C was found two weeks after EP93A and EP93B; its discharge site is unknown.
The Cleft segment time series is proving especially useful in revealing the
linkage between hydrothermal and magmatic activity, because it began simultaneously
with the observation of a new class of plume phenomena: "event plumes."
In 1986, Baker
et al. [1987b] discovered a "megaplume" at the northern end
of the Cleft segment (Figure 5). Analysis of
this and a second megaplume found over the Vance segment in 1987 [Baker
et al., 1989; Gendron
et al., 1993] demonstrate that event plumes are near-instantaneous releases
of enormous volumes of hydrothermal fluid; about
Fig. 7. Summary of hydrothermal temperature anomalies observed along the
crest of the Juan de Fuca Ridge, including Axial Volcano, from 1985 to 1989.
For clarity, segments and the overlying plumes are displayed individually, without
overlap. Triangles show location of known venting sites. Anomalies at the northern
end of the Cobb segment, 47.4°-47.6°N, are believed to mark an intrusion of
geothermally heated bottom water from Cascadia Basin to the east, rather than
axial hydrothermal heating (reprinted from Baker
and Hammond [1992]). Significant hydrothermal plumes
0.02°C)
In addition to intensive work along the Cleft segment and at other specific sites, the NOAA/VENTS Program also undertook to survey, at least once, as much of the entire JDFR as feasible. As of 1989, about 90% of the ridge from the Cleft to Endeavour segments had been surveyed at least once, and more than 50% at least twice, by a continuous tow-yo transect. The goal of this program was to test the hypothesis that the distribution and intensity of hydrothermal activity are directly related to the variable magmatic budget along the ridge. Baker and Hammond [1992] reported that hydrothermal discharge is strongest on those segments, or portions of segments, where the apparent magmatic budget is highest, as indicated by the degree of along-axis inflation or other morphological characteristics (Figure 7).
Fig. 8. Location map for East Pacific Rise spreading centers. Solid circles indicate locations of axial discontinuities.
Three areas on the EPR, one medium (Gulf of California,
Fig. 9. A 
(3He) section through
the Gulf of California showing evidence of hydrothermal venting in the Guaymas
Basin (reprinted from Lupton
[1979]). Triangle shows location of known venting sites in the Guaymas Basin.
The Gulf of California is a series of en echelon spreading centers opening
at 50-60 mm yr-1 and creating a string of deep, semi-enclosed basins
(Figure 9). Helium isotope profiles in each of
the six major basins [Lupton,
1979] imply that only Guaymas Basin, with
(3He) values of 65-70% ((3He)
=
Plumes over the fast-spreading section of the EPR between 8° and 13°N have been systematically studied since at least 1983. Crane et al. [1988] used the same techniques employed by Crane et al. [1985] on the JDFR to identify perturbations in the temperature field between 8°20' and 13°10'N along the ridge crest, but the results were similarly compromised by the equipment used. This same area has since been resurveyed in detail, by Dynamic Hydrocast and CTD casts from 12°08' to 13°12'N in 1986 [Bougault et al., 1990; Charlou et al., 1991b], and by CTD/transmissometer tows and casts from 8°40' to 11°50'N in 1991 [Lupton et al., 1993; Baker et al., 1994; Feely et al., 1994; Mottl et al., 1995]. These studies precisely mapped the distribution of hydrothermal plumes along the ridge crest, revealing a fine-scale agreement between hydrothermal activity and the apparent magmatic budget, as inferred from ridge morphology and subsurface structure [Bougault et al., 1990; Baker et al., 1994].
Fig. 10. Transects of methane and dissolved manganese along the East Pacific Rise determined from three Dynamic Hydrocast tows [after Bougault et al., 1990]. Connected triangles show area of known venting sites.
Fig. 11. Transects of light-attenuation anomaly (continuous tow-yo) [Baker
et al., 1994], dissolved Mn, and CH4 (both from discrete
samples) [Mottl
et al., 1995] obtained in November 1991 along the East Pacific Rise
south and north of the Clipperton Transform Zone. Contour intervals are
Between 12°08' and 13°12'N, Bougault
et al. [1990] and Charlou
et al. [1991b] found plumes with dissolved Mn >10 nmol kg-1and
CH4
Plume sampling on the southern EPR has been concentrated along the superfast-spreading
Fig. 12. Tow-yo transects of light attenuation along the East Pacific Rise
just south of the Garrett Transform Zone [Baker
and Urabe, 1994]. Nephelometer data are combined from five tow-yos,
comprising >1000 vertical profiles. Contour interval is 
0.02 m-1
The first large-scale plume survey in this area was completed in 1993 by the
Japanese-NOAA Ridgeflux Expedition using detailed chemical sampling and nearly
continuous CTD/SUAVE/optical tow-yos between 13°50' and 18°40'S [Urabe
et al., 1994; Baker
and Urabe, 1994; Massoth
et al., 1994; Ishibashi
et al., 1994; Feely
et al., 1994; Lupton
et al., 1994]. Preliminary analyses show that hydrothermal plumes covered
~60% of the entire study area (Figure 12). Plumes
were most common south of ~16°30'S, becoming virtually continuous between 17°20'
and 18°40'S. The southern portion of the study area was also characterized by
CH4/Mn ratios as high as 3.9, particulate S/Fe ratios >1, and
Fe/Mn ratios >8. Similarly high CH4/Mn and S/Fe ratios were found
in 1991 in plumes at 9°50'N on the EPR [Lupton
et al., 1993],
The western boundary of the Pacific Ocean is a broad expanse of marginal basins and troughs fragmented by trench-arc systems that mark subduction zones of several tectonic plates [Taylor and Karner, 1983] (Figure 13). Many of these basins contain active spreading centers that likely host hydrothermal activity. Systematic though limited plume sampling has been conducted in the Harve Trough, Lau Basin, North Fiji Basin, Woodlark Basin (Solomon Sea), Manus Basin (Bismark Sea), Mariana Trough, and Okinawa Trough.
Fig. 13. Location map for western Pacific marginal basins. Thin lines are fracture zones, thick lines are spreading centers, and triangled lines are convergent plate edges. 1, Harve Trough; 2, Lau Basin/Valu Fa Ridge; 3, North Fiji Basin; 4, Woodlark Basin; 5, Manus Basin; 6, Mariana Trough; 7, Okinawa Trough.
The 1986 Papatua Expedition [Craig and Poreda, 1987] occupied four hydrocast stations in the Harve Trough, five in the Lau Basin, and three in the Woodlark Basin. Only weak to negligible hydrothermal indications were found in each basin. Subsequent seafloor explorations in the Lau [Fouquet et al., 1991] and Woodlark [Binns et al., 1993] basins, however, have discovered active hydrothermal sites, indicating that sampling during the Papatua Expedition was too sparse to adequately survey these large basins.
Fig. 14. Transects of methane and dissolved Mn along the central spreading axis in
the North Fiji Basin. Data compiled from published and unpublished sources (see text)
spanning several years. Note that the data base is not identical for each tracer. Contour
intervals are
Plumes in the North Fiji Basin have been sampled more intensively than in any
other back-arc basin. Geophysical surveys [Auzende
et al., 1994] indicate two north-south spreading axes: a medium-rate
(3He)
maximum of 71%, confirming its magmatic origin [Craig
et al., 1987]. Nojiri
et al. [1989] interpreted as an event plume a strong plume
Fig. 15. Transects of methane and manganese collected in 1990 along the spreading center in the eastern Manus Basin (reprinted from Gamo et al. [1993] with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 16B, UK). The Papatua Expedition [Craig and Poreda, 1987] collected samples near stations 37 and 29. Triangle shows location of known venting sites.
The Manus Basin is a fast-spreading (3He) maximum of 58%. The
Papatua Expedition also found CH4 anomalies over the southern ridge,
which was sampled more extensively by Gamo
et al. [1993] in 1990. Gamo
et al. [1993] mapped multiply layered CH4 and Mn plumes between
1000 and
Farther to the north, plumes have been mapped in the Okinawa and Mariana Troughs.
In the Okinawa Trough, a relatively shallow basin behind the deep Ryukyu Trench,
strong CH4, Mn, and 3He (up to a (3He)
of 65%) anomalies at depths between 1300 and 1600 m were found in 1987 [Ishibashi
et al., 1988] and 1988 (J. Ishibashi, personal communication, 1994).
Horibe
et al. [1986] occupied a series of hydrocasts between 18°11'N and 18°15'N
in 1982 in the Mariana Trough, mapping CH4 plumes near-bottom and
700-
Unlike fast-spreading ridge axes such as the EPR, the neovolcanic zone of the
slow-spreading MAR (Figure 16) sits not at a
narrow axial high, but within a broad (5-
Fig. 16. Location map for the northern Mid-Atlantic Ridge and Reykjanes Ridge. Known and suspected hydrothermal sites identified in Figure 19.
Fig. 17. Along-axis maxima in total reactive (~dissolved) Mn [Klinkhammer et al., 1985] and CH4 [Charlou and Donval, 1993] from vertical casts on the Mid-Atlantic Ridge. High values in parentheses at 26°N and 29°N are representative samples over the TAG [Klinkhammer et al., 1986] and Broken Spur [Elderfield et al., 1993] hydrothermal fields, respectively. Thin dashed line on each plot marks the regional background value of Mn and CH4.
Until 1984 it was widely predicted that hydrothermal activity might be restricted to fast-spreading ridges and that at slow-spreading ridges, such as the MAR, heat flow would be insufficient to support active high-temperature black smoker vent fields. It is now clear that this is not the case. In 1984, Klinkhammer et al. [1985] carried out systematic discrete sampling and shipboard Mn analysis of seawater samples from stations occupied throughout the MAR rift valley between 11°N (the Vema Fracture Zone) and 26°N (Figure 17). Nine discrete segments were occupied during this survey and dissolved Mn anomalies--interpreted as the product of active hydrothermal venting--were found at every station. As a conservative estimate, Klinkhammer et al. [1985] concluded that a minimum of at least five discrete hydrothermal fields must characterize this section of the MAR.
Fig. 18. Geochemical plume transect for total reactive Mn at the TAG hydrothermal site in 1985 (reprinted from Klinkhammer et al. [1986] with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kiddington 0X5 16B, UK). Solid circles indicate sample positions.
These predictions were confirmed in 1985 when a series of CTD-nephelometer hydrocasts (Figure 18) and deep-tow video camera deployments confirmed the first active black-smoker hydrothermal field at the northernmost station occupied by Klinkhammer et al. [1985], the TAG hydrothermal field at 26°N [Rona et al., 1986]. In 1986, black-smoker hydrothermal venting was also found at the Snakepit hydrothermal field at 23°N [Ocean Drilling Program Leg 106 Scientific Party, 1986], close to where Klinkhammer et al. [1985] had reported water column dissolved Mn anomalies.
Between 1985 and 1988 another series of cruises mapped the distribution of CH4 between 12° and 26°N [Charlou and Donval, 1993] (Figure 17). High CH4 concentrations were found over the TAG and Snakepit sites, as well as the 15°20' fracture zone. Because of very low Mn in the plumes near the 15°20' fracture zone, and the occurrence of local outcrops of serpentinized ultrabasic diapirs, Charlou and Donval [1993] hypothesized that CH4 anomalies there arise from fluid circulation in ultrabasic rocks rather than basalt-seawater interactions typical of faster-spreading ridges. The changes of mechanical properties and density caused by the serpentinization of deep crustal rocks, as inferred from these CH4 anomalies, may play an influential role in the construction of slow-spreading ridges [Charlou et al., 1991a; Charlou and Donval, 1993; Bougault et al., 1993].
Fig. 19. Summary of evidence for hydrothermal venting along the Mid-Atlantic Ridge compiled from water-column investigations between 11° and 40°N [German et al., 1995]. Only four segments or segment portions have confirmed active venting.
In 1992, as part of a joint FARA (French American Ridge Atlantic) cruise aboard
the R/V Atlantis II, Project FAZAR, systematic water-column sampling was carried
out along a more northerly section of the Mid-Atlantic Ridge between 32°N and
the Kurchatov Fracture Zone close to 40°N [e.g.,
Wilson et al., in press; Klinkhammer
et al., in press]. Stations were occupied approximately every
German
et al. [1994] carried out a systematic survey of the Reykjanes Ridge
between 57°45'N and 63°09'N. Approximately 120 stations were occupied at
Fig. 20. 38 kHz echo-sounder trace from a transect along the axis of the Reykjanes Ridge between 63°06.04'N and 63°06.10'N, indicating the presence of gas-rich hydrothermal plumes rising from the seabed close to 63°06.06'N (reprinted from German et al. [1994] with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 1GB, UK).
Very few studies of hydrothermal activity have been carried out in the Indian
Ocean, even though sediments exhibiting hydrothermal metal enrichments along
the Indian Ocean ridge system have been known for almost thirty years [Boström
et al., 1969]. To date, however, discovery of evidence for hydrothermal
activity along the ridges of the Indian Ocean has been limited to three specific
studies. The Central Indian Ridge between 21°S and 24°S, close to the Rodriguez
Triple Junction, was investigated between 1983 and 1988 by cruises of the R/V
Sonne. A series of CTD hydrocast stations occupied between 21°15'S and
21°30'S in 1986 found distinct plumes of dissolved Mn and CH4 anomalies
at several stations, with concentrations up to
In 1993, a group of Japanese investigators conducted water-column observations
around the Rodriguez Triple Junction [Gamo
et al., 1994]. Hydrothermal optical and chemical (CH4 and
dissolved Mn and Fe) anomalies were found centered around the
The studies described above document that about 10% of the
In the heady early days of hydrothermal discovery, many workers believed that the chemistry of hydrothermal solutions was simply a function of exit temperature, and that this function could be established empirically by careful sampling at a few sites [e.g., Edmond et al., 1982]. Further discoveries demonstrated a much more complex universe of chemical variations [e.g., Von Damm, 1990], emphasizing the need for continued exploration and the compilation of as large a hydrothermal data base as possible. (We leave unwritten the many biological, physical, and geological reasons for vent field exploration.)
The search for new vent fields has historically followed an inefficient path: A tectonic segment, or segment portion, is chosen on geological considerations. A remote or manned search for geologic or biologic visual clues of active venting is then mounted to pinpoint discharge locations. Finally, vent fluids are sampled from a submarine and plume waters from a surface ship. Consideration of the several studies reviewed in this paper demonstrates that a more efficient approach would postpone detailed seafloor exploration until after systematic plume mapping had revealed the relative magnitude and approximate location of discharge sources. Moreover, chemical and physical observations of plume waters can provide first-order information on the composition of the source fluids, which are likely indicative of the present magmatic and hydrothermal conditions of the vent field.
The efficacy of plume reconnaissance can be documented from several recent
examples. On the EPR near 10°N, an expensive and exhaustive
While the above example is an after-the-fact plume success story, instances of plume mapping directing the discovery of seafloor discharge sources are common. On the CoAxial segment of the JDFR, plume surveying immediately after the remote detection of seismic activity in 1993 directed seafloor imagery mapping by a remotely operated vehicle towards the eventual discovery and sampling of a new lava eruption [Embley et al., 1995]. On the MAR, systematic plume surveying between 27° and 30°N in 1993 discovered optical and chemical evidence for three possible hydrothermal sites, including a strong signal at 29°10.1'N, 43°10.3'W [Murton et al., 1994]. The Broken Spur vent field was confirmed at this site three months later during an Alvin dive series [Murton et al., 1994].
Fig. 21. Comparison of the global extent of plume studies and seafloor vent fluid sampling. Shaded areas on spreading centers mark the locations of multi-segment plume mapping investigations on the Reykjanes Ridge, Mid-Atlantic Ridge, northeast Pacific ridges, Gulf of California, northern and southern East Pacific Rise, and the north Fiji Basin. Numbers refer to Table 1 and mark sites of vent fluid collection and analysis along spreading centers; fluid discharge sites noted only from visual observations are not included.
The number of fluid discharge sites sampled by submersible or remote vehicle is steadily growing but remains only a tiny fraction of all global sources (Table 1, Figure 21). Plume mapping and sampling has allowed us to expand our knowledge of the distribution and geologic setting of hydrothermal venting beyond the limits imposed by camera and submersible operations. Figure 21 graphically compares the relative coverage of vent and plume data sets. The expansion of hydrothermal information afforded by plume studies is essential for regional-scale issues such as the effect of hydrothermal cooling on heat transport within the crust [Phipps Morgan and Chen, 1993], the movement of melt within the crust [Purdy et al., 1992], magmatic differentiation [Sinton et al., 1991], and the effect of hydrothermal heating on abyssal waters [Hautala and Riser, 1993].
TABLE 1. Sites of Hydrothermal Vent Fluid Collection
As an example of the kind of linkage between geological and hydrothermal data
made possible by plume studies, we examine a simple but crucial hypothesis:
A functional relationship exists between axial hydrothermal heat flux and ridge-crest
spreading rate, a convenient analogue of the time-averaged magmatic budget.
Incorporation of hydrothermal cooling into models of the genesis of oceanic
crust [e.g.,
Phipps Morgan and Chen, 1993] requires generalizations of this kind.
To use plume distributions to test this hypothesis, we assume that hydrothermal
heat output is proportional to plume incidence, the percentage of ridge-axis
length overlain by a significant hydrothermal plume. Simply put, more hydrothermal
cooling produces more plumes. (We operationally define a "significant"
plume as one with plume tracer values greater than three standard deviations
above the background (non-plume) mean.) Our assumption of the equivalence of
heat flux and plume incidence is reasonable over the narrow crest and shallow
axial depressions characteristic of ridges spreading faster than
Fig. 22. (a) Full-rate seafloor spreading vs. axial magma chamber depth
from seismic observations [Purdy
et al., 1992] (solid circles; LAU, Valu Fa Ridge in the eastern Lau
Basin; JDFR, Cobb segment on the Juan de Fuca Ridge; NEPR, northern East Pacific
Rise, 9°-13°N; SEPR, southern EPR, 14°-20°S) and from the Phipps
Morgan and Chen [1993] crustal thermal model (solid line). (b) Full-rate
seafloor spreading vs. plume incidence (solid squares) and hydrothermal cooling
in the neovolcanic zone according to a crustal thermal model (Y. J. Chen and
J. Phipps Morgan, The effect of magma emplacement geometry, spreading rate,
and crustal thickness on hydrothermal heat flux at mid-ocean ridges, submitted
to Geophys. Res. Lett., 1994) (dashed line). Plume incidence data available
for the Reykjanes Ridge (RR) [German
et al., 1995], Mid-Atlantic Ridge (MAR) (11°-40°N) [German
et al., 1994], JDFR [Baker
and Hammond, 1992], NEPR (8°40-13°10'N) [Bougault
et al., 1990; Baker
et al., 1994], and SEPR (13°50'-18°40'S) [Baker
and Urabe, 1994]. "Plume incidence" is the percentage of axis
length overlain by a significant (plume tracer values greater than background
mean plus three standard deviations) plume. The heavy solid line is a least-squares
fit forced through the origin:
Continuous surveys from the JDFR (Figure 7) [Baker and Hammond, 1992], the northern EPR (Figures 10, 11) [Bougault et al., 1990; Baker et al., 1994], and the southern EPR (Figure 12) [Baker and Urabe, 1994] have yielded direct measures of plume incidence. On slow-spreading ridges, in the current absence of conclusive continuous surveys, we approximate plume incidence from estimates of vent field frequency derived from discontinuous plume surveys along the MAR and Reykjanes Ridge [German et al., 1994; German et al., 1995]. If we assign a reasonable plume size of 10 km to each of the 15 suspected sites of venting on the MAR between 11° and 40°N [German et al., 1995], we calculate a 6% plume incidence. Similarly, the one vent field found on the Reykjanes Ridge [German et al., 1994] yields a 2% plume incidence. When data from all five regions are plotted together, a strong linear relationship between plume incidence and spreading rate emerges (Figure 22).
It is vital to realize that Figure 22 is valid only over
long time or space scales. Vigorous hydrothermal activity can occur on segments of any
spreading rate, so an instantaneous observation at a particular location may not be
representative of its long-term hydrothermal history. We obtain a representative
hydrothermal history of a particular ridge-crest location only by observing it through
geologic time, which is impractical, or by simultaneously observing many individual
segments spreading at the same rate. Figure 22 thus implies
that, over long time scales, (1) one-third of the global ridge axis spreading at
How does the plume vs. spreading rate relationship compare to other observed
and modeled parameters? Axial magma chamber (AMC) depth from seismic observations
[Purdy
et al., 1992] shows a rapid decrease from 60 to
Still at issue is whether our meager coverage of the global ridge crest is
representative and the trends described above are general. Over
Hydrothermal plumes formed by mixing of seafloor vent fluids and ambient seawater are
easily detectable by physical and chemical tracers. New in situ instrumentation is
providing the capability of continuous measurement of certain chemical species (e.g.,
dissolved Mn and Fe) to complement the continuous measurement of hydrographic and optical
tracers. Hydrothermal plume evidence of seafloor venting has been recognized for over 30
years, though only in the last decade have systematic surveys been conducted. Most
detailed surveys have been conducted along medium- to superfast-spreading eastern Pacific
spreading centers because the narrow crest of these ridges permits a largely
two-dimensional mapping effort. Carefully studied areas include the Juan de Fuca Ridge
(44°30'-48°30'N) and 9°-13°N and 13°40'-18°40'S on the East Pacific Rise. Plumes
have been detected in several western Pacific marginal basins, but only parts of the North
Fiji (15°-20°S) and Manus Basins have been surveyed in detail. In the North Atlantic,
plume surveys have visited a longer length of ridge than in the Pacific, but surveys
typically have been less detailed because of the wide and deep axial valleys. Work along
the Reykjanes Ridge (58°-63°N) and the Mid-Atlantic Ridge (11°-40°N) indicates a lower
incidence of venting than in the Pacific. Plumes have been sampled near the Rodriguez
Triple Junction in the Indian Ocean, but no systematic mapping has been accomplished
there. About 10% of the
Two primary conclusions are apparent from this review. First, careful plume surveys can efficiently locate seafloor discharge sites to within a few kilometers or less. Plume mapping can substantially diminish the search time required to locate vents precisely by submersible or remote imagery, and should normally precede seafloor exploration efforts to improve their efficiency. Second, while seafloor venting occurs on ridge segments of spreading rates from slow to superfast, hydrothermal plume production along any multisegment ridge portion is directly related to the spreading rate.
Acknowledgments. This review was supported by the NOAA VENTS Program (ETB), by IOSDL and NERC grant BRIDGE 15 (CRG), and by NERC grant GR3/8596 and a grant from the NERC BRIDGE Program (HE). The impetus for this paper was discussions at the 1993 RIDGE Theoretical Institute at Big Sky, MT, USA. We thank the conveners and participants for channeling our energies into this effort. Cambridge Earth Sciences contribution number ES 4086. IOSDL contribution number 95003. PMEL contribution number 1538.
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