From: Smigrodzki, Rafal (SmigrodzkiR@msx.upmc.edu)
Date: Fri Mar 29 2002 - 16:09:11 MST
Mike Lorrey [mailto:mlorrey@datamann.com]
quoted me:
>
> ### 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.
and wrote:
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.
### Precisely which reactions do you have in mind (with
equations please).
--------
>
> 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.
### I see you are digging deeper and deeper. Precisely which
volcanoes are not the result of convective movements of magma? - please give
me their names, with quotes from authorities on volcanology. Which are the
chemical reactions driving volcanoes (again, please write out the
reactions, with estimates of the abundance of their energy output, and
references to primary research literature - after all you *are* making very
extraordinary claims here).
--------
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.
### Quote me, or desist.
---------
>
>
> ### 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.
### You elected to use the word "obviously". See below.
------
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.
### Largely? How many % of the continental landmass do you
think is made of limestone? Are you aware of the process of carbonate
weathering and it's role in the carbon cycle? (see below)
--------
To conclude my participation in this thread:
Since so far you did not provide sufficient references for
our discussion, let me correct the situation:
You will find description of significant variations in
climate and carbon dioxide levels, completely at odds with idea of a
long-term trend towards atmospheric depletion in the following article,
available on the SciAm site: Snowball Earth by Paul F. Hoffman and Daniel
P. Schrag
The peer-reviewed work forming the basis of this article is:
A Neoproterozoic Snowball Earth. P. F. Hoffman, A. J. Kaufman, G. P.
Halverson and D. P. Schrag in Science, Vol. 281, pages 1342-1346; August 28,
1998, as well as a large body of research literature which I will not quote
here.
A brief outline of some of the many processes involved in
the long, mid and short term changes in carbon dioxide levels follows here,
showing that so far hardly anything is "obvious" carbon cycle research:
Where does all the carbon come from?
J.C. Varekamp
Wesleyan University, Earth and Environmental Sciences
Carbon fluxes related to chemical weathering make up only a small part of
the short carbon cycle. They are generally assumed to be in steady state,
with a "carbon degassing" flux from the solid earth. I made an inventory of
the amount of CO2 fixed by global weathering based on a generic feldspar
weathering scheme and the riverine bicarbonate flux. The rate of silicate
weathering is based on the riverine Si flux, and the associated CO2
consumption was estimated from the conversion stoichiometry of an
intermediate plagioclase to a mixture of kaolinite and smectite. The
calculated silicate weathering flux and observed bicarbonate flux show
excellent internal agreement, assuming that 60 percent of global weathering
comes from carbonate rock weathering. The final calculations give a fixed
atmospheric CO2 flux of ~20 Tmol CO2/yr for global chemical weathering.
Acidity-generating compounds fuel this global weathering process, which
include the volcanic gases CO2, SO2, HCl and HF. The carbon flux dominates,
and estimates of volcanic CO2 fluxes range from 3-7 Tmol CO2/yr. Another
source of carbon is the "exhumation flux", which liberates organic carbon
from exposed rocks (8 Tmol/yr.). The oxidation of pyrite also fuels silicate
weathering, but when taking all of these sources into account, a shortfall
of 5-8 Tmol CO2/yr. remains. The pre-anthropogenic carbon cycle was in
steady state, given the constant atmospheric CO2 levels of the last 1000
years. The calculated shortfall would deplete the atmosphere in CO2 at a
rate of 3-4.5 ppm/century. Most likely, additional CO2 sources exist (e.g.,
CO2 bubbles in cold springs) or the existing carbon flux estimates are low
by at least 30 percent
----------
The carbon reservoirs are described here, amply demonstrating that the
atmosphere contains only an insignificant amount of carbon dioxide, and the
only reason why it's still there is a rough equilibrium between
sequestration and release, leaving no place for a long-term imbalance
between the two (in the shorter timespan, thousands and millions of years
there are wide fluctuations, both up and down):
The Carbon Cycle's Reservoirs
The World's Carbon Reservoirs Note that:
* the atmosphere's reservoir is smaller than all others except the
forests' (the amount of CO2 in the atmosphere therefore depends strongly on
exchanges between the other components)
* though amounts in the deep oceans and crust are large, the
mechanisms that exchange the carbon there with the atmosphere are slow.
Reservoir Size (Gt C)
Atmosphere 750
Forests 610
Soils 1580
Surface ocean 1020
Deep ocean 38,100
Earth's crust 50 x 106
Fossil fuels (total: yet to be released)
Coal 4,000
Oil 500
Natural gas 500
Total fossil fuel 5,000
------
And finally, the following quote from Daniel H. Rothman, Proc. Natl. Acad.
Sci. USA, Vol. 98, Issue 8, 4305-4310, April 10, 2001, available online:
Unlike the short-term carbon cycle, which is dominated by exchanges of
carbon between the biosphere, atmosphere, oceans, and soils on time scales
ranging from about 100-104 years, the long-term carbon cycle is dominated by
exchanges between rocks and the atmosphere and oceans on time scales of
roughly 105-109 years (14-16, 36, 37). Three slow processes dominate the
long-term evolution of CO2 levels: chemical weathering of silicate minerals,
degassing caused by metamorphic and volcanic processes, and the burial and
weathering of organic carbon. Because the latter acts as an independent
subcycle (14), the former are considered the major controls on atmospheric
and oceanic CO2 levels. They are thought to be in rough balance via
reactions such as (14, 36, 38)
CaSiO3 +CO2 ->CaCO3+SiO2. [ 3 ]
A similar reaction may be written by substituting Mg for Ca. Left to right,
such reactions schematically represent the uptake of CO2 from the
atmosphere, its transformation to dissolved HCO3?during weathering of
silicate rocks, and its eventual precipitation and burial in the oceans as
carbonate minerals. Right to left, the reaction represents metamorphism and
magmatism and the subsequent transfer of CO2 back to the atmosphere and
oceans by volcanism and related processes.
------
If you wish to continue the thread, it behooves you to provide links or
quotes form authoritative sources disagreeing with my claim that long-term
carbon dioxide concentration is not subject to a process of irreversible
depletion.
Rafal
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