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a very peculiar type of sediment formation; they produce limestone bodies in
sediment where limestone is usually absent. They also contain a fauna dif-
ferent from that in the surrounding sediment.
Present-day hydrothermal vents and cold seeps are characterized by lo-
calized concentrations of hydrogen sulfide and/or methane-rich fluid that are
generated and then expelled onto the sea floor. At these sites, high densities
of chemoautotrophic bacteria that live on methane occur in suspended vent-
fluid emissions, or as bacterial symbionts enclosed within invertebrate tis-
sues, or even as free-living surface mats. The chemoautotrophic bacteria ox-
idize the vent-seep fluids to produce metabolites and energy at the base of a
chemosynthetic food chain. These vent fluids mix with sea water to form lo-
calized and often anomalous sedimentary precipitates that include metallic
sulfides and barite, and isotopically distinctive carbonates. Many of the or-
ganisms adapted to these areas live in environmental conditions that would
prove toxic to most marine organisms.
The recognition of vent and cold-seep localities in the ocean floor
caused paleontologists to re-evaluate the origin of numerous anomalous rock
and faunal associations. Most troubling had been the presence of isolated
limestone bodies in deep-water sandstone or turbidites. These had long been
interpreted as shallow shoals or fossiliferous blocks transported to deep-water
settings. The fossil fauna within these limestone bodies was quite different
from faunal elements found in the surrounding matrix, just as it is in modern
cold-seep areas. The final peculiar characteristic of these limestone bodies
was their isotopic composition. They yielded isotopic readings typical of very
warm water, when in fact they formed in water near the freezing point. Lime-
stones in modern seep deposits, just like the shells of the ancient inoce-
ramids, give a false picture of their temperature of formation.
The fauna of such cold seeps are now known to be highly distinctive in
both their recent incarnations and the distant past. Most common and
prominent members of extant cold-seep communities are large clams and
mussels with bacteria in their flesh that enable the clams to live off methane
in the surrounding sea water rather than off plankton, like most other clams.
