Geoscience Reference
In-Depth Information
has been added; and, in many places, CO
2
has also been added, typically as
calcium carbonate precipitated in the pores and fissures through which the water
flowed. In the nineteenth century, geologists attempted to calculate the age of the
Earth by measuring the amount of salt in the sea and dividing it by the amount of
salt annually brought down by rivers from the land. We now know that that early
calculation was invalid, because hydrothermal circulation is one of the principal
controls on sodium in sea water.
The characteristic feature of a hydrothermal vent field on a mid-ocean ridge is
the vent chimneys. The rising hydrothermal fluid is suddenly cooled as it re-enters
the ocean (which is at about 4
◦
C, just above freezing) and minerals immediately
precipitate from the fluid. Chimneys build up as the minerals precipitate. When
the fluids venting from the chimneys have temperatures over
330
◦
C, clouds
of dark minerals are precipitated, giving rise to the name
black smokers
.
White
smokers
are similar chimneys formed by somewhat cooler (200-330
◦
C) milky-
white venting water. Warm (5-60
◦
C) clear, shimmering water is discharged as a
diffuse flow over surrounding areas several hundreds of metres square. Chimneys
can grow very fast (over 2 m in five days has been documented on the Juan de
Fuca Ridge), but they are fragile. Hydrothermal vent fields are dependent upon
the vagaries of the underlying magmatic heat source, so fluid-circulation paths can
change rapidly. However, long-lived hydrothermal systems have been discovered:
U-series geochronology has shown that the TAG field at 26
◦
Nonthe Mid-Atlantic
Ridge has been episodically active for over 50 000 years.
Exotic communities of life have been discovered around these hydrothermal
vents: giant tubeworms up to 3 cm in diameter and 3 m long, giant clams, crabs
and unusual bacteria. These communities of unusual organisms were discovered
by geophysicists in 1977, much to the surprise of biologists. The lives of these
organisms do not depend directly on sunlight (as other life does) but on chemical
energy from the hydrothermal system. They are not wholly disconnected from
the rest of the biosphere, however, since they depend on the oxidation state of the
environment, which is set by photosynthetic life. The bacteria live by exploiting
the contrast between the relatively oxidized sea water (a product of surface photo-
synthesis) and the reduced fluids in the volcanic plumbing system - this process
is known as
chemosynthesis
. The bacteria grow in abundance, covering surfaces
of rock and fauna and are the first life to colonize any new vent. Many of the
organisms have a symbiotic relationship with the bacteria, some even do without
a digestive system - the bacteria live within them and meet all their energy
requirements. Microbial life deep within the oceanic crust could prove to be
an important aspect of hydrothermal circulation. Heat-tolerant
hyperthermophile
bacteria can withstand temperatures up to 113
◦
C and have been discovered in
material from 3 km depth.
Interestingly, the gross features of the biogeographic distribution of the deep-
sea hydrothermal vent fauna are controlled by plate tectonics. Of roughly 500
species and about 250 genera catalogued, over 90% of the species and half of
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