REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
83
Fig. 4.2. High-temperature water flows from sealed conduits in the crustal basalts that allow fluids to emerge nearly undiluted. Laden with
dissolved minerals and sulphide (converted from seawater sulphates), precipitation of mineral sulphide complexes occurs at the seafloor to
form chimneys and “black smokers”. Diffuse, lower-temperature venting can also occur as sea water percolates through cracked basalts to
dilute the “end-member” hydrothermal (“hi-T”) fluids. The extent of this venting depends largely on the nature of the basalts and cracks.
Low-temperature sites are usually colonized – sometimes only by microbial mats. The scenario represented here might be found on ridges
of the eastern Pacific Ocean.
mineral deposits, which may build up over thousands
of years in a stable site like TAG on the Mid-Atlantic
Ridge (Lalou et al., 1993); some chimneys may last
only a few years as the subsurface conduits shift or clog
(Hannington et al., 1995).
Mineralized chimneys form at temperatures over
250ºC. Lateral percolation of cooler fluid around and
through chimney walls provides extensive habitat for
vent animals. A second major habitat forms around
cooler, less focused flows where fluid diffuses through
cracks in the basalts (Fig. 4.2). Mixing with primary
hydrothermal fluid has occurred below the surface, and
emerging fluids may range in temperature from 2º to
about 100ºC. The composition of vent fluids varies
markedly from site to site and can change over time.
Dissolved oxygen is rapidly scavenged by reduced
chemical compounds, and is present in trace amounts
only. Chemical composition changes with dilution and
temperature. Johnson et al. (1988) have demonstrated
both spatial and temporal variability in micromolar
concentrations of oxygen and sulphide which co-vary
inversely over scales of centimetres and seconds.
Distribution of hydrothermal vent sites (Fig. 4.1)
is sparse and uneven – a pattern that is mirrored on
smaller scales. The global ridge is about 50 000 km
long but underlying heat sources are not continuous.
Regional ridges are divided into separate segments
usually 30 to 100 km long. Magmatic activity beneath
them can be quite different (Batiza, 1995). Figure 4.3
is an oblique view of the Juan de Fuca Ridge in the
northeast Pacific. Seven segments of this ridge sponsor
quite different venting characters: small eruptions have
occurred on two segments in the last 15 years, while
two more are relatively quiescent with little venting.
The central volcano lies over an active mantle hot
spot that erupted in January, 1998. To the north, deep
tectonic faults feed relatively stable vent fields with
large polymetallic sulphide accumulations.
A single vent field may contain from a few to
hundreds of openings that create a mosaic of colonies.
Fields vary in size from metres to hundreds of metres
across. Ultimately, field vigour and extent are governed
by the nature of the underlying magma chamber and
by the porosity and stability of the intervening rocks
(Alt, 1995). The overall nature of hydrothermalism in a
ridge segment is dictated by the geophysical processes
83
Fig. 4.2. High-temperature water flows from sealed conduits in the crustal basalts that allow fluids to emerge nearly undiluted. Laden with
dissolved minerals and sulphide (converted from seawater sulphates), precipitation of mineral sulphide complexes occurs at the seafloor to
form chimneys and “black smokers”. Diffuse, lower-temperature venting can also occur as sea water percolates through cracked basalts to
dilute the “end-member” hydrothermal (“hi-T”) fluids. The extent of this venting depends largely on the nature of the basalts and cracks.
Low-temperature sites are usually colonized – sometimes only by microbial mats. The scenario represented here might be found on ridges
of the eastern Pacific Ocean.
mineral deposits, which may build up over thousands
of years in a stable site like TAG on the Mid-Atlantic
Ridge (Lalou et al., 1993); some chimneys may last
only a few years as the subsurface conduits shift or clog
(Hannington et al., 1995).
Mineralized chimneys form at temperatures over
250ºC. Lateral percolation of cooler fluid around and
through chimney walls provides extensive habitat for
vent animals. A second major habitat forms around
cooler, less focused flows where fluid diffuses through
cracks in the basalts (Fig. 4.2). Mixing with primary
hydrothermal fluid has occurred below the surface, and
emerging fluids may range in temperature from 2º to
about 100ºC. The composition of vent fluids varies
markedly from site to site and can change over time.
Dissolved oxygen is rapidly scavenged by reduced
chemical compounds, and is present in trace amounts
only. Chemical composition changes with dilution and
temperature. Johnson et al. (1988) have demonstrated
both spatial and temporal variability in micromolar
concentrations of oxygen and sulphide which co-vary
inversely over scales of centimetres and seconds.
Distribution of hydrothermal vent sites (Fig. 4.1)
is sparse and uneven – a pattern that is mirrored on
smaller scales. The global ridge is about 50 000 km
long but underlying heat sources are not continuous.
Regional ridges are divided into separate segments
usually 30 to 100 km long. Magmatic activity beneath
them can be quite different (Batiza, 1995). Figure 4.3
is an oblique view of the Juan de Fuca Ridge in the
northeast Pacific. Seven segments of this ridge sponsor
quite different venting characters: small eruptions have
occurred on two segments in the last 15 years, while
two more are relatively quiescent with little venting.
The central volcano lies over an active mantle hot
spot that erupted in January, 1998. To the north, deep
tectonic faults feed relatively stable vent fields with
large polymetallic sulphide accumulations.
A single vent field may contain from a few to
hundreds of openings that create a mosaic of colonies.
Fields vary in size from metres to hundreds of metres
across. Ultimately, field vigour and extent are governed
by the nature of the underlying magma chamber and
by the porosity and stability of the intervening rocks
(Alt, 1995). The overall nature of hydrothermalism in a
ridge segment is dictated by the geophysical processes