From: Mike Lorrey (mlorrey@datamann.com)
Date: Thu Mar 28 2002 - 09:08:47 MST
> "Smigrodzki, Rafal" wrote:
>
> Mike Lorrey [mailto:mlorrey@datamann.com]
> wrote:
>
> Rafal's comments about volcanic outgassing are a bit
> overdone. There is
> nowhere near enough volcanic activity around subduction
> zones to allow
> for outgassing of all CO2 (especially since there isn't
> enough O2 for
> all the carbon)
>
> ### Thermal carbonate dissociation does not need oxygen. The
> amount of elemental carbon in sediments is minuscule
> compared to the mass of carbonate.
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.
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) 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.
>
> , although there is a greater proportion than at one
> time. Note that as the Moon orbits further and further from
> the Earth as
> it drains radial momentum from Earth's rotation, its tidal
> influence
> decreases and thus tectonic activities slow down.
>
> ### 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)?
I suggest referring to Fogg's "Terraforming" text for some answers to
this.
>
> Also, lunar tides are not the only source of energy for
> tectonic activity - please remember heat from radioactivity.
Another heat source which is dwindling. Both tidal and radioactive heat
sources are slowly decreasing over time. Radioactivity is the cause of
less than 1/3 of the heat generation. There is far more energy in the
braking effect on earth's rotation (the core turns a little over one
more rotation per year than the mantle) caused by the moon, and won't
stop until either the moon achieves escape velocity or else the Earth
stops rotating. As the moon gets further away, the amount of braking
that occurs decreases over time, reducing the amount of heat being
generated. If the heat drops by half, the crust will thicken up
considerably, greatly reducing the amount of vulcanism.
>
> Nor does subduction do anything about carbonates deposited
> outside of
> subduction areas. The Great Barrier Reef, for one, is not
> subducting and
> is one of the great modern carbon sequestration areas, as is
> the
> Carribean basin. Another sequestration area is the Southern
> Sea around
> Antarctica, which produces a huge amount of phytoplankton
> that
> sequesters billions of tons of carbon each year that settles
> to the
> bottom of the ocean to become hydrates in non-subduction
> areas.
>
> ### As I mentioned above, the amount of carbon sequestered
> in the elemental form or in organic molecules is
> insignificant compared to the mass of carbonate. It is only
> a matter of geological time before any part ocean floor will
> reach a subduction zone, or, conversely, will be elevated
> and weathered.
Yes, geological time. And if enough carbon is sequestered in coral, in
ocean sediments and hydrates fast enough, then geological time just
ain't fast enough. It takes many millions of years for any appreciable
amount of the ocean floor to be subducted (and none occurs in the
Atlantic).
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