Geoscience Reference
In-Depth Information
It is strongly believed that not all submarine hydrothermal vents are hot. The dis-
covery of the Lost City hydrothermal field (LCHF) near the Mid-Atlantic ridge in
the year 2000 characterized by carbonate chimneys that rise 60 m above the ultra-
mafic sea floor
[90]
is highly relevant to the understanding of the origins of life. This
alkaline vent bears H
2
-rich water of about 40
90
C
[91,92]
. This chimney provides
insights into past mantle geochemistry.
14
C radioisotopic dating indicates that hydro-
thermal activity has been ongoing for at
least 30,000 years
[93]
whereas ura-
nium
100,000 years
[94]
.
Such studies indicate that the submarine hydrothermal systems operate today in
much the same manner as they are believed to have functioned in the pre-Cambrian
period. They provide a wide range of closely linked gradients in both physical and
chemical conditions obviously having a direct link with magmatic heat. Thus, the
biological communities (biotic family) occupying vast and relatively stable soft bot-
tom habitats of the sea are characterized by low population densities, high species
diversity, and low biomass, compared to those inhabiting the unstable conditions of
submarine hydrothermal ecosystems which exhibit high densities and biomass, low
species diversity, rapid growth rates, and high metabolic rates.
Despite such an understanding of the hydrothermal ecosystem, there are many
unclear aspects like period of formation and depth. As mentioned earlier, the avail-
able evidences indicate that the ancient ocean was a chemically reducing environ-
ment, probably warm or hot (
thorium dating indicates that venting has been active for
B
100
C)
[95]
, shallow (maximum depth of
1000
2
2000 m), and considerably more active, tectonically and hydrothermally,
than it is today
[96]
. The model proposed by Abbott and Hoffman
[97]
suggests
that, at the time of formation of the ocean (4.2 Ga), it is likely that the whole earth
was covered with water and that hydrothermal activity was at least five times
greater than at present. The overall rate of sea floor spreading and subduction has
declined steadily over the evolution of the earth.
Figure 1.4
shows the geological
and geochemical features of the hydrothermal circulation of seawater through the
oceanic crust at mid-ocean ridges
[98]
. Rona et al.
[99]
have found two types of
vents occurring along spreading ridge crests of the East Pacific Rise and Juan de
Fuca Ridge in the Pacific. The fluids emanating from these vents are enriched with
magmatically derived elements and complexes. Depending upon the degree of mix-
ing, we can expect a variety of chemical components in different quantities which
serve as the primary energy source for the extensive microbial communities as
represented in
Figures 1.5 and 1.6
. All the biologically important trace elements
are also known to be associated with submarine hydrothermal vents. However, the
actual reactions involved in the production of various chemical components are not
clearly known. It is well known that both H
2
and CO are common gases associated
with volcanic activity. The major source of H
2
is believed to be the oxidation
of magnetite to hematite, while CO is believed to originate within the magma
from C
30
Cto
.
.
a
a
a
S equilibrium
[101]
. Similarly, the concentration of CH
4
and
helium gases has been discussed in the submarine hydrothermal vents. Oxygen and
other electron acceptors
O
H
NO
3
2
;
SO
2
4
and PO
3
4
and a variety of many unknown
chemical components have allowed for the abundant growth of diverse microorgan-
isms in the modern oceans.
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