From: Mike Lorrey (mlorrey@datamann.com)
Date: Thu Mar 28 2002 - 13:45:51 MST
> "Smigrodzki, Rafal" wrote:
>
> 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.
The problem is that this is not a clean reaction. With water present, it
will grab some of that O2, then you've got plenty of metals and
nonmetals available that go bonkers for oxygen more than carbon does.
>
> 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.
Depends on the type of volcanic activity. If it were a convection alone,
then the only volcanos we'd see would be at spreading centers and
hotspots like Hawaii. Subduction related eruptions, which are primarily
composed of ash, are, in fact driven by the gasses produced by
subduction chemical reactions. It is just that only a small percent of
the material that subducts ever makes it back up through in eruptions.
So which is it? A few posts ago, you were claiming that all of the
subducted carbon comes back up through volcanic eruptions.
>
> 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.
This is rather obviously not so. The bare fact that we once had 55 times
more atmosphere than we do now demonstrates that this is a process that
is not in equilibrium.
Most carbonates ARE lost to rock. The continents are largely made up of
vast layers of limestone which will never be returned to the atmosphere.
Those coral areas that are not headed for subduction zones for several
hundred million years (i.e. The GBR, Carribean, etc) primarily because
they are on top, will continue to sequester carbon which will be rock
and will never return to the atmosphere.
A couple billion years ago, we still had several times the atmosphere we
have now. it wasn't until the Cambrian period that there was even a
sufficient percent of oxygen available to sustain animal life to an
appreciable degree.
>
> 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).
I'll try to dig it up (I think it's in storage). If anyone else has the
text handy and is willing to look it up, please do.
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