Geoscience Reference
In-Depth Information
In the longer term (thousands to millions of years), certain
conditions can lead to a significant change in the greenhouse effect.
This was the case for episodes of the “Snowball Earth”, similar to the
Varanger glaciation [HOF 98] which occurred between 900 and 540
million years ago, that is to say, before the Cambrian Period.
Numerous authors consider that such episodes were not total
glaciations, but that they preserved the hydrological cycle and the life
that is associated with it across a peri-equatorial unfrozen belt that
became larger or smaller according to the seasons. These phenomena
could have been caused by phases of fragmentation of super
continents such as Rodinia, the most ancient of the super continents
that would have been formed more than a billion years ago and that
would have fragmented 750 million years ago [DON 04]. The
volcanic phenomena that would then be produced would have been
similar, but much more extended, to those of the current large East
African rift and would have greatly extended the basalt surface
exposed to chemical weathering, favoring the following sequence of
transformations:
- chemical weathering of aluminosilicates (the example of
anorthite is used here), which consumes two molecules of atmospheric
CO
2
to dissolve each molecule of aluminosilicate, and transport of the
dissolved products by rivers:
[CaAl
2
Si
2
O
8
]
solid
+ 3H
2
O + 2CO
2
→
[Al
2
Si
2
O
5
(OH)
4
]
solid
+
[Ca
2+
]
dissolved
+ 2[HCO
3
-
]
dissolved
[1.5]
- adjustment of the equilibriums of inorganic carbon dissolved in the
ocean, restitution of one molecule of CO
2
to the atmosphere and
creation of a supersaturation of the ocean in CO
3
2-
:
2[HCO
3
-
]
dissolved
→ CO
2
+ [CO
3
2-
]
dissolved
+ H
2
O
[1.6]
- precipitation of solid carbonate in the ocean and its stocking in
the sedimentary reservoir:
[CO
3
2-
]
dissolved
+ [Ca
2+
]
dissolved
→ [CaCO
3
]
solid
[1.7]
The equilibrium of inorganic carbon dissolved in the ocean having
been adjusted, the final result of the alteration of aluminosilicates
