13 Input from the Deep: Hot Vents and Cold Seeps
472
direct contributions magmatic volatiles to the
hydrothermal fluids in this setting. Widespread
sulfur-rich fumaroles and low-pH vent fluids,
similar to magmatic-hydrothermal systems in
subaerial arc volcanoes, have been documented at
a number of locations and reflect magmatic
degassing of high-level magma chambers beneath
the summits of the volcanoes (Tsunogai et al.,
1994; de Ronde et al., 2003). A number of other
aspects of this setting also result in dramatically
different deposit types compared to those found on
the mid-ocean ridges and in deeper back-arc
basins. In particular, the shallow water depths
(commonly <1,000 m) result in widespread boiling
and generally lower temperatures of hydrothermal
venting, with potentially significant sub-seafloor
stockwork mineralization. However, the nature of
the hydrothermal activity in these volcanoes is not
yet well characterized. Some clearly involves hightemperature hydrothermal circulation of seawater
and black smoker venting, but many hydrothermal
plumes above the calderas appear to be related
mainly to passive degassing of the volcano rather
than high-temperature hydrothermal activity
(Massoth et al., 2003). Nevertheless, considering
the composition of the arc magmas, the flux of
volatiles from such systems is likely much greater
than that from the mid-ocean ridges.
Deposits on sediment-covered mid-ocean ridges
commonly have lower Cu and Zn contents and
higher Pb contents than bare-ridge sulfides, and
some deposits such as in the Guaymas Basin
contain abundant carbonate. The low metal
contents of deposits in the Guaymas Basin are a
consequence of the higher pHs of the fluids that
arise from chemical buffering by the carbonate in
the sediments, and the high CO 2 in the fluids is
derived directly from the sediments themselves
(Bowers et al. 1985; Von Damm et al. 1985a). A high
ammonium content reflects the thermocatalytic
cracking of buried organic matter (e.g., immature
planktonic carbon; cf., Von Damm et al. 1985a). The
fluids in this environment are also strongly reduced
(in the pyrrhotite stability field) as a consequence
of reaction with organic matter in the sediments
(e.g., C org + 2H 2 O = CO 2 + 2H 2 ), and metal deficient
relative to a volcanic-hosted hydrothermal system
as a result of sulfide precipitation within the
sediments. However, interaction of the hydrothermal fluids with sediments also may result in enrichments in certain trace elements derived from the
sediments (e.g., Pb, Sn, As, Sb, Bi, Se: Koski et al.
1988; Zierenberg et al. 1993). The higher Pb
contents, in particular, reflect the destruction of
feldpars from continentally-derived turbidites, a
process supported by Pb isotope studies (LeHuray
et al. 1988).
13.6 Characteristics of Cold Seep
Fluids at Subduction Zones
In addition to hydrothermal activity at divergent
plate boundaries, low-temperature fluid venting
at both passive and convergent plate margins is
an important global process. Cold seeps have
now been documented at numerous sites along
the circum-Pacific subduction zones and elsewhere on continental shelf, and it is estimated
that this type of oceanic venting is also of major
significance for the chemical budget of seawater.
The most significant fluxes are from fluids that
are expelled from thick organic-rich marine
sediments trapped in accretionary wedges along
the subduction trenches (e.g., Nankai Trench,
Oregon and Alaskan margins). Pore fluids in the
sediment account for as much as 50-70% of their
volume, and this fluid is literally squeezed from
the sediment through diffuse flow at the toe of
the accretionary wedge or by focussed flow
along major fault structures in the accreted sediments.
In order to estimate the mass flux from cold
seeps, flow rates have to be known, and these
have been difficult to measure because of the
large areas of diffuse flow involved. Fluid flow
rates determined from simple advection-diffusion
modelling of temperature and chemical profiles are
often unreliable if applied to sites which are
densely populated with bottom macrofauna.
Calculations by Wallmann et al. (1997), based on a
biogeo-chemical approach using oxygen flux and
vent fluid analyses and taking into account the
high pumping rates of bivalves, have arrived at a
mean value of 5.5 +/- 0.7 L m
-2
d
-1
. Von Huene et al.
(1998) used sediment porosity reduction to
calculate a fluid flow of 0.02 L m
-2
d
-1
. Suess et al.
(1998) calculated an average rate of 0.006 L m
-2
d
-1
based on the occurrence of vent biota at the
seafloor. Although these estimates span several
orders of magnitude, the fluid flux at subduction
zones is thought to be sufficient to cycle the
volume of water in the oceans every 500 Ma. The
total flux may be lower than that of hydrothermal
circulation at mid-ocean ridges, but the fluids
472
direct contributions magmatic volatiles to the
hydrothermal fluids in this setting. Widespread
sulfur-rich fumaroles and low-pH vent fluids,
similar to magmatic-hydrothermal systems in
subaerial arc volcanoes, have been documented at
a number of locations and reflect magmatic
degassing of high-level magma chambers beneath
the summits of the volcanoes (Tsunogai et al.,
1994; de Ronde et al., 2003). A number of other
aspects of this setting also result in dramatically
different deposit types compared to those found on
the mid-ocean ridges and in deeper back-arc
basins. In particular, the shallow water depths
(commonly <1,000 m) result in widespread boiling
and generally lower temperatures of hydrothermal
venting, with potentially significant sub-seafloor
stockwork mineralization. However, the nature of
the hydrothermal activity in these volcanoes is not
yet well characterized. Some clearly involves hightemperature hydrothermal circulation of seawater
and black smoker venting, but many hydrothermal
plumes above the calderas appear to be related
mainly to passive degassing of the volcano rather
than high-temperature hydrothermal activity
(Massoth et al., 2003). Nevertheless, considering
the composition of the arc magmas, the flux of
volatiles from such systems is likely much greater
than that from the mid-ocean ridges.
Deposits on sediment-covered mid-ocean ridges
commonly have lower Cu and Zn contents and
higher Pb contents than bare-ridge sulfides, and
some deposits such as in the Guaymas Basin
contain abundant carbonate. The low metal
contents of deposits in the Guaymas Basin are a
consequence of the higher pHs of the fluids that
arise from chemical buffering by the carbonate in
the sediments, and the high CO 2 in the fluids is
derived directly from the sediments themselves
(Bowers et al. 1985; Von Damm et al. 1985a). A high
ammonium content reflects the thermocatalytic
cracking of buried organic matter (e.g., immature
planktonic carbon; cf., Von Damm et al. 1985a). The
fluids in this environment are also strongly reduced
(in the pyrrhotite stability field) as a consequence
of reaction with organic matter in the sediments
(e.g., C org + 2H 2 O = CO 2 + 2H 2 ), and metal deficient
relative to a volcanic-hosted hydrothermal system
as a result of sulfide precipitation within the
sediments. However, interaction of the hydrothermal fluids with sediments also may result in enrichments in certain trace elements derived from the
sediments (e.g., Pb, Sn, As, Sb, Bi, Se: Koski et al.
1988; Zierenberg et al. 1993). The higher Pb
contents, in particular, reflect the destruction of
feldpars from continentally-derived turbidites, a
process supported by Pb isotope studies (LeHuray
et al. 1988).
13.6 Characteristics of Cold Seep
Fluids at Subduction Zones
In addition to hydrothermal activity at divergent
plate boundaries, low-temperature fluid venting
at both passive and convergent plate margins is
an important global process. Cold seeps have
now been documented at numerous sites along
the circum-Pacific subduction zones and elsewhere on continental shelf, and it is estimated
that this type of oceanic venting is also of major
significance for the chemical budget of seawater.
The most significant fluxes are from fluids that
are expelled from thick organic-rich marine
sediments trapped in accretionary wedges along
the subduction trenches (e.g., Nankai Trench,
Oregon and Alaskan margins). Pore fluids in the
sediment account for as much as 50-70% of their
volume, and this fluid is literally squeezed from
the sediment through diffuse flow at the toe of
the accretionary wedge or by focussed flow
along major fault structures in the accreted sediments.
In order to estimate the mass flux from cold
seeps, flow rates have to be known, and these
have been difficult to measure because of the
large areas of diffuse flow involved. Fluid flow
rates determined from simple advection-diffusion
modelling of temperature and chemical profiles are
often unreliable if applied to sites which are
densely populated with bottom macrofauna.
Calculations by Wallmann et al. (1997), based on a
biogeo-chemical approach using oxygen flux and
vent fluid analyses and taking into account the
high pumping rates of bivalves, have arrived at a
mean value of 5.5 +/- 0.7 L m
-2
d
-1
. Von Huene et al.
(1998) used sediment porosity reduction to
calculate a fluid flow of 0.02 L m
-2
d
-1
. Suess et al.
(1998) calculated an average rate of 0.006 L m
-2
d
-1
based on the occurrence of vent biota at the
seafloor. Although these estimates span several
orders of magnitude, the fluid flux at subduction
zones is thought to be sufficient to cycle the
volume of water in the oceans every 500 Ma. The
total flux may be lower than that of hydrothermal
circulation at mid-ocean ridges, but the fluids