86
Verena TUNNICLIFFE et al.
Martin et al., 1996). The carbon sources are organic:
methane, petroleum, other hydrocarbon gases, or from
solid gas hydrates (Simoneit et al., 1990; Aharon, 1994;
MacDonald et al., 1994). These compounds derive
from accumulated sedimentary organic carbon and thus
are photosynthetic in origin. At several sites, petroleum
products are abundant enough to coat collected biota
with oil. Ice-bound methane (or methane hydrates)
can slowly release enough gas to support a dependent
community (Suess et al., 1999). This commentary
addresses only the seeps of the deep sea. Numerous
shallow-water settings of methane and natural gas
release are known where a variety of organisms and
microbial activity is reported. Dando et al. (1995) and
Jensen et al. (1992) have provided shallow examples.
The extent of seepage communities in the deep ocean
is unknown, as margin environments are incompletely
explored. Areal extent can be large: Olu et al. (1996a)
have described cold-seep communities dispersed along
a 40 km transect from a depth of 2600 to 5300 m
on the Peruvian margin. The major clam field has
a total surface area of 1000 m
2 with clusters of
1000 individuals m
−2 . Observations of demographic
features, trophic complexity and high species richness
suggest sustained seepage at several sites (Carney,
1994; Olu et al., 1996b). Carbon-14 ages of mussel
shells from the Barbados prism yield dates of 10 000
to 20 000 years ago (Gonthier et al., 1994). Aharon
et al. (1997) have proposed that the current seeps of
the Gulf of Mexico have been active since the last
glaciation period. Indeed, ages of extinct shallow sites
indicate that the area has been seeping for at least
200 000 years.
Organic remains
Windfalls of organic origin on the ocean floor can
attract a wide array of deep-sea animals to this
sudden bounty. However, sometimes the nature of the
windfall is such that it generates an anoxic habitat
inaccessible to many animals. Very often, the same
chemoautotrophic microbial production occurs as is
found at vents and seeps, albeit on a smaller and more
ephemeral scale. Perhaps the best known is that of large
carcasses. The fortuitous discovery of whale skeletons
led to descriptions of a community of invertebrates with
many features similar to those of vent communities
(Smith et al., 1989; Bennett et al., 1994; Naganuma
et al., 1996). This community relies on autotrophic
microbes using reduced compounds generated from the
slow release and decay of organics from the lipid-rich
bones. The high (pre-whaling) abundance of marine
mammals has formed a reliable enough organic source
to develop a distinct ‘following’ in the deep sea of
organisms apparently adapted entirely to subsistence on
carcasses. Martill et al. (1995) have speculated that the
abundant marine reptiles of the Jurassic and Triassic
may have provided an even older source of scattered
reducing habitats.
Wood has been around since the upper Paleozoic.
While the biomass sequestered in a single tree is insufficient to develop an extensive reducing environment,
most animals adapted to submerged wood are allied
to those known at other reducing sites. The organisms
involved are mostly molluscs (Turner, 1973; War´ en
and Bouchet, 1993) and polychaetes (Wolff, 1979;
Desbruy` eres and Laubier, 1988). One interesting find
is that of a vestimentiferan (Pogonophora, Obturata)
tube worm and mussel in the wreck of a cargo ship
(Dando et al., 1992). Both genera reported are known
from seeps and vents, and contain symbiotic microbes.
The animals were found among the cargo of beans,
sunflower seeds and sisal – all organic-rich and liable
to produce sulphide upon decay.
Dysaerobic basins
There are several places in the ocean in which oxygen
concentrations are low and occasionally are zero (see
Chapter 2); dysaerobia (or a hypoxic condition) occurs
below 2 ml °
−1 oxygen. Larger basins occur mostly in
marginal areas where organic input is relatively high.
Microbial oxidation depletes dissolved oxygen, and
hydrogen sulphide can build up in bottom waters and
sediments. The Peruvian Trench, Californian Basin,
Black Sea and numerous deep fjords are well-published
examples; Diaz and Rosenberg (1995) have presented a
comprehensive list and reviewed the effects of depleted
oxygen on benthic communities. Often these basins
experience oxygen levels fluctuating on annual to
centenary scales. Work on long cores of sediment
has identified the nature of such variability in bottom
oxygen from basins on the west coast of North
America (i.e. Kennett and Ingram, 1995). The lack of
macrofauna in the sediments allows annual lamination
to develop, reflecting seasonal diatom production.
However, periods of delamination and bioturbation
indicate development of benthic communities during
prolonged periods of oxygenation.
Benthos of hypoxic basins is specialized (Bacesco,
Verena TUNNICLIFFE et al.
Martin et al., 1996). The carbon sources are organic:
methane, petroleum, other hydrocarbon gases, or from
solid gas hydrates (Simoneit et al., 1990; Aharon, 1994;
MacDonald et al., 1994). These compounds derive
from accumulated sedimentary organic carbon and thus
are photosynthetic in origin. At several sites, petroleum
products are abundant enough to coat collected biota
with oil. Ice-bound methane (or methane hydrates)
can slowly release enough gas to support a dependent
community (Suess et al., 1999). This commentary
addresses only the seeps of the deep sea. Numerous
shallow-water settings of methane and natural gas
release are known where a variety of organisms and
microbial activity is reported. Dando et al. (1995) and
Jensen et al. (1992) have provided shallow examples.
The extent of seepage communities in the deep ocean
is unknown, as margin environments are incompletely
explored. Areal extent can be large: Olu et al. (1996a)
have described cold-seep communities dispersed along
a 40 km transect from a depth of 2600 to 5300 m
on the Peruvian margin. The major clam field has
a total surface area of 1000 m
2 with clusters of
1000 individuals m
−2 . Observations of demographic
features, trophic complexity and high species richness
suggest sustained seepage at several sites (Carney,
1994; Olu et al., 1996b). Carbon-14 ages of mussel
shells from the Barbados prism yield dates of 10 000
to 20 000 years ago (Gonthier et al., 1994). Aharon
et al. (1997) have proposed that the current seeps of
the Gulf of Mexico have been active since the last
glaciation period. Indeed, ages of extinct shallow sites
indicate that the area has been seeping for at least
200 000 years.
Organic remains
Windfalls of organic origin on the ocean floor can
attract a wide array of deep-sea animals to this
sudden bounty. However, sometimes the nature of the
windfall is such that it generates an anoxic habitat
inaccessible to many animals. Very often, the same
chemoautotrophic microbial production occurs as is
found at vents and seeps, albeit on a smaller and more
ephemeral scale. Perhaps the best known is that of large
carcasses. The fortuitous discovery of whale skeletons
led to descriptions of a community of invertebrates with
many features similar to those of vent communities
(Smith et al., 1989; Bennett et al., 1994; Naganuma
et al., 1996). This community relies on autotrophic
microbes using reduced compounds generated from the
slow release and decay of organics from the lipid-rich
bones. The high (pre-whaling) abundance of marine
mammals has formed a reliable enough organic source
to develop a distinct ‘following’ in the deep sea of
organisms apparently adapted entirely to subsistence on
carcasses. Martill et al. (1995) have speculated that the
abundant marine reptiles of the Jurassic and Triassic
may have provided an even older source of scattered
reducing habitats.
Wood has been around since the upper Paleozoic.
While the biomass sequestered in a single tree is insufficient to develop an extensive reducing environment,
most animals adapted to submerged wood are allied
to those known at other reducing sites. The organisms
involved are mostly molluscs (Turner, 1973; War´ en
and Bouchet, 1993) and polychaetes (Wolff, 1979;
Desbruy` eres and Laubier, 1988). One interesting find
is that of a vestimentiferan (Pogonophora, Obturata)
tube worm and mussel in the wreck of a cargo ship
(Dando et al., 1992). Both genera reported are known
from seeps and vents, and contain symbiotic microbes.
The animals were found among the cargo of beans,
sunflower seeds and sisal – all organic-rich and liable
to produce sulphide upon decay.
Dysaerobic basins
There are several places in the ocean in which oxygen
concentrations are low and occasionally are zero (see
Chapter 2); dysaerobia (or a hypoxic condition) occurs
below 2 ml °
−1 oxygen. Larger basins occur mostly in
marginal areas where organic input is relatively high.
Microbial oxidation depletes dissolved oxygen, and
hydrogen sulphide can build up in bottom waters and
sediments. The Peruvian Trench, Californian Basin,
Black Sea and numerous deep fjords are well-published
examples; Diaz and Rosenberg (1995) have presented a
comprehensive list and reviewed the effects of depleted
oxygen on benthic communities. Often these basins
experience oxygen levels fluctuating on annual to
centenary scales. Work on long cores of sediment
has identified the nature of such variability in bottom
oxygen from basins on the west coast of North
America (i.e. Kennett and Ingram, 1995). The lack of
macrofauna in the sediments allows annual lamination
to develop, reflecting seasonal diatom production.
However, periods of delamination and bioturbation
indicate development of benthic communities during
prolonged periods of oxygenation.
Benthos of hypoxic basins is specialized (Bacesco,