From: Smigrodzki, Rafal (SmigrodzkiR@msx.upmc.edu)
Date: Thu Mar 28 2002 - 12:43:43 MST
Mike Lorrey [mailto:mlorrey@datamann.com]
Wrote:
> for outgassing of all CO2 (especially since
there isn't
> enough O2 for
> all the carbon)
> I answered:
> ### Thermal carbonate dissociation does not need
oxygen. The
> amount of elemental carbon in sediments is
minuscule
> compared to the mass of carbonate.
Mike:
I'm not talking about elemental carbon. Carbonate does not
contain
enough oxygen within it for a proper 2 to 1, or even one to
one,
relationship to produce CO2 from this cracking process
without air
present. It could get some from water that seeps into
subduction zones
from the ocean, but there are plenty of elements that oxygen
prefers
more than carbon.
### Let's write out the reaction :
CaCO3 ->(high temp)-> CaO + CO2
More comments?
This results in the formation of alkaline oxides, which
usually react with SiO2 present almost everywhere, to form relatively stable
silicates.
Mike:
Volcanic activity in subduction areas is generally due to
the pressure
of superheated water with some CO2 and CO present (a sort of
natural
producer gas)
Me:
### Volcanic activity is not due to water pressure. It is
caused by convection of magma within Earth's mantle.
Mike:
but is never in quantities similar to the amount of carbon
subducted. If it were, there would be far more volcanoes
(with more
frequent eruptions) in subduction zones than there are.
Lets take, for example, the subduction zone on the west
coast of North
America. You've got a 4000-6000 km of subduction zone,
subducting at
several cm a year along the length of it. This means that
you have a
minimum of a few cubic kilometers of material: hydrates,
carbonates, and
other organic compounds, along with rock, etc, subducting
every year.
Yet how much material is ejected in volcanic eruptions along
that zone?
A cubic kilometer or so every couple decades. At best, less
than 10% of
subducted materials are erupted, much of it being rock or
metamorphosed
carbonates which retain their rock form.
### Spectacular eruptions are just a part of the story.
There is constant outgassing from volcanic activity areas. Also, if indeed
only 10% of subducted materials were replaced by volcanism, the surface of
the Earth would have long since disappeared (into the 5th dimension?). The
imbalances you are implying are incompatible with the existence of an
atmosphere, or indeed Earth's crust itself. If most of carbonate was
irretrievably lost in subduction, without a constant, steady, and
approximately equal replenishment from volcanism, all carbonate would have
been scrubbed out of the atmosphere a couple of billion years ago.
Me:
> ### Do you know precisely how much volcanic
activity is
> there, and how much CO2 is being released, in
relationship
> to the amount sequestered in sediments? Can you
point me to
> any authoritative text implying that there is a
long-term
> imbalance between the two processes, impacting
on Earth's
> ability to sustain life (in the time until the
Sun becomes a
> red giant)?
Mike:
I suggest referring to Fogg's "Terraforming" text for some
answers to
this.
### Please give me the page numbers which specifically state
that a long-term imbalance in CO2 release and sequestration is responsible
for a steady loss of atmospheric CO2, with implications for sustainability
of life on Earth (quote would be nice).
Rafal
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