It's a long time since I did anything in this field, so this is some ad
hoc thinking.
Soil PCO2 is much higher than in the atmosphere. An old paper on this is ...
Drake, J.J. and Wigley, T.M.L., 1975: The effect of climate on the
chemistry of carbonate groundwater. Water Resources Research 11, 958–962.
Adding CaCO3 will add Ca++ and HCO3- to the soil/groundwater. The
dissolution will reduce soil PCO2. So the flux of CO2 from soil to
atmosphere will decrease. But won't the bugs just work harder to keep
the soil PCO2 about the same? The climate/soilPCO2 relationship in the
above paper suggests that this is what will happen.
So the net effect is probably small.
Tom.
+++++++++++++++++++++
On 9/29/2011 5:14 PM, Rau, Greg wrote:
There's a large literature on and practice of crop soil limestoning. In the
context of CO2, my concerns would be added CO2 release from reaction of
limestone with soil or precipt strong acids, and added downstream and
groundwater hardness via dissolved Ca(HCO3)2 addition. The latter can be a
big deal for communities that have to use the water, the potential for CaCO3
reprecipitation and scaling will go up, above that naturally present in the
water, esp in limestone regions. For this reason it would be important to
somehow keep the water below CaCO3 saturation. This goes away with disposal
in seawater, which is why doing this close to a river mouth might be the
best thing. Anyway, we eagerly await the results of your (backyard?)
experiments.
G
On 9/29/11 3:29 PM, "David Zhong"<[email protected]> wrote:
Greg,
This might not be a bad idea.
CO2 emission to the atmosphere through soil respiration is estimated
at about 60 GtC/a. Cutting down 10% of this natural emission will give
us about 6 GtC/a. For comparison, current anthropogenic carbon
emission to the atmosphere is about 8 GtC/a.
To capture 6 GtC/a of carbon emission from soil respiration requires
mining and grinding a minimum of 50 Gt of CaCO3 per year (i.e.,
6/12X100). For comparison, current coal mining is estimated at about
25 Gt/a. Limestone is certainly more abundant and wide spread than
coal. Using your estimate of limestone mining and grinding and
transportation cost of about $5/ton, the annual total cost would be
about $250 billion.
Spreading 50 Gt of limestone to 10% to 20% of the land (or about 15-30
million km2, about 0.3 to 0.15 kg CaCO3 per m2 per year) where soil
respiration is most intensive would probably be sufficient to
“capture” 6 GtC/a.
Compared to “limestoning the ocean” scheme, this “limestoning the
soil" scheme would cut CO2 emission to the atmosphere immediate,
instead of waiting for years or tens or hundreds of years.
As for your two concerns, I am sure that if the soil system is
overloaded with limestone, the end product of limestone dissolution
would be dissolved bicarbonate ions instead of carbon dioxide.
Limestone landscapes are found all over the continents and people have
been living happily in limestone regions for generations. I doubt that
the hardness of the resulting runoff of a “limestoning the soil"
scheme will be any worse than that of the watershed of a limestone
region.
I am sure that there will be some other environmental and/or
ecological “side effects” or risks. But I am also sure that any or
every “solution” to such a planetary scale problem will carry risks.
The question is, are these risks manageable?
What do you think?
David
On Sep 29, 11:14 am, "Rau, Greg"<[email protected]> wrote:
That would work for me a la crop liming IF: 1) soil acids other than
carbonic acid are such that CO2 is not emitted to the air from the acid
limestone reaction, and 2) the hardness of the resulting runoff is within
environmental standards. How about limestoning the mouths of rivers to mop
up excess dissolved CO2 - there are no hardness standards for discharge to
the ocean. Then again, by the time rivers discharge to the ocean they've
probably pretty much degassed and equilibrated with air pCO2.
-Greg
On 9/29/11 10:35 AM, "David Zhong"<[email protected]> wrote:
Would it be more effective (and perhaps simpler) if the limestone is
distributed to a large area of land (ideally in regions with heavy wet
precipitation and with organic matter rich soil) instead of the
upwelling
regions of the ocean?
After all, limestone dissolution in normal rainwater (pH< 5.6, ionic
strength ~ 0) is much faster than in seawater (pH> 7.6, ionic
strength ~ 0.7) and natural CO2 exchange (or flux) between soil
and atmosphere is on the same order as that between ocean and
atmosphere?
Just a thought.
David.
On Sep 29, 9:12 am, "Rau, Greg"<[email protected]> wrote:
Thanks, Tom. I think we all can agree that the volume of CaCO3
undersaturation in the subsurface ocean is vast, it is a very effective
consumer (60-80%) of natural carbonate rain, and hence is a massive
(re)generator of carbonate alkalinity that can in turn consume ocean and
atmospheric CO2. It follows that adding additional CaCO3 to the
undersaturated regions of the ocean, especially particles of high surface
area/volume, will generate additional (new) alkalinity and CO2 consuming
potential. The only question then is can this occur in shallow enough water
(e.g., upwelling areas) such that its communication with and effect on the
atmosphere occurs on a time scale shorter than the usual 1kyr involved in
thermohaline ventilation of deep water. In this regard the subsurface N
Pacific Ocean, being first up for such ventilation, would seem to hold the
most promise. Then there is the CaCO3-challenged Southern Ocean.
Or am I off base?
Anyway, if you don't like the rates afforded by natural seawater carbonate
undersaturation, there is a relatively straightforward way to change
this:http://pubs.acs.org/doi/abs/10.1021/es102671x
Regards,
Greg
On 9/28/11 6:18 PM, "[email protected]"<[email protected]> wrote:
Regardless of possible inhibiters, I think that kinetic limitations make
this an unlikely possibility.
See ...
Plummer, L.N. and Wigley, T.M.L., 1976: The dissolution of calcite in
CO2-saturated solutions at 25°C and 1 atmosphere total pressure.
Geochimica et Cosmochimica Acta 40, 191202.
Plummer, L.N., Wigley, T.M.L. and Parkhurst, D.L., 1978: The kinetics
of calcite dissolution in CO2-water systems at 560°C and 0.01.0 atm
CO2. American Journal of Science 278, 179216.
Tom.
++++++++++++++++++++++++++++
On 9/28/2011 1:44 PM, Rau, Greg wrote:
Thanks, David, for the info. Certainly agree that limestone dissolution
only
works in undersaturated, sub-surface waters, which Harvey goes to some
lengths to locate and model for carbonate dissolution. As for P, I doubt
carbonate rain would have much of a effect on surface ocean P since there
is
precious little there anyway. What happens at depth could be a different
story. Easy enough to test: take some seawater with measurable P, mix in
calcite powder, and see what happens to dissolved P. As for P inhibition
of
calcite dissolution, sample or make calcite undersaturated seawater, add
or
remove P, add calcite, measure differences in resulting alkalinity or DIC
in
the preceding treatments. Even better, let's just rain calcite powder
into
a
likely spot in the ocean and measure vertical profiles of P, DIC,
alkalinity,
etc and compare to Berner et al models (and Harvey's!).
A paleo example: following the PETM event carbonate rain rate went from
zero
to huge numbers while there was not much change in organic C
accumulation,
so
something in surface waters was getting enough P to make the OC despite
high
carbonate rain, if that is your concern.
Another idea: certainly inhibition of carbonate precipitation in the
ocean
is
a major player in setting ocean water column and atmospheric C levels. To
what extent have these inhibitors (P, Mg, organics, etc) varied in the
past,
(how) have they affected C levels, and might we want to investigate
purposely
modulating these inhibitors to manage ocean/air C in the future?
-Greg
________________________________________
From: [email protected] [[email protected]]
On
Behalf Of David Zhong [[email protected]]
Sent: Wednesday, September 28, 2011 11:23 AM
To: geoengineering
Subject: [geo] Re: Monbiot Claims SAI "already tested ... with
catastrophic
results"
Greg,
Phosphate ions are known to have a strong affinity for the reactive
sites of calcite and inhibit the dissolution (BERNER& MORSE, 1974;
MORSE& BERNER, 1979) as well as precipitation (MUCCI, 1986) reactions
of calcite in seawater. It is conceivable that the settling fine
limestone (calcite) particles would scavenge the dissolved phosphate
ions in the upwelling seawater.
Furthermore, let’s not forget that calcite dissolution can only happen
in seawater that is undersaturated with respect to calcite; and most
surface seawaters are in fact supersaturated with respect to calcite.
Adding limestone to a CaCO3-undersaturated upwelling seawater body may
reduce its degree of undersaturation, it could not make it
supersaturated with respect to calcite. Mixing with the CaCO3-
supersaturated surface seawater and/or CO2 degassing and/or primary
productivity (plus temperature and pressure change) will make it
supersaturated with respect to calcite (and aragonite). In view of the
slow calcite dissolution reaction rate in seawater (there are lots of
studies and data on this), I doubt the effectiveness of this scheme.
BERNER R. A. and MORSE J. W. (1974) Dissolution kinetics of calcium
carbonate in seawater. IV. Theory of calcite dissolution. Amer. J.
Sci. 274. 108-134.
MORSE J. W. and BERNER R. A. (1979) The chemistry of calcium carbonate
in the deep oceans. In Chemical Modeling-Speciation, Sorption.
Solubility and Kinetics in Aqueous Systems (ed. E. JENNE), pp.
499-535. ACS Symposium Series 93. American Chemical Society,
Washington, D.C.
MUCCI A. (1986) Growth kinetics and composition of magnesian calcite
overgrowths precipitated from seawater: Quantitative influence of
orthophosphate ions. Gmchimica et Cosmochimica Acta Vol. 50, pp.
2255-2265.
Cheers,
David.
On Sep 27, 1:00 pm, "Rau, Greg"<[email protected]> wrote:
Thanks David. I defer to Harvey's paper as to the particle size and rain
rate needed to effect limestone dissolution at depth. Slow kinetics can
always be countered by increased particle surface area (at a cost). I
wasn't
aware of the P story - reprints? On the other hand elevating pH might
reduce
trace metal solubility - good or bad for phytos? E.g., Cu vs Fe? The
added
alkalinity might save coccoliths, pteropods, etc from an acidic grave.
Let's find out with a mesoscale live ocean test. In contrast to iron
exps,
perhaps Greenpeace will supply the ship and cheering section this time.
No?
Regards,
Greg
________________________________________
From: [email protected] [[email protected]]
On
Behalf Of David Zhong [[email protected]]
Sent: Tuesday, September 27, 2011 11:43 AM
To: geoengineering
Subject: [geo] Re: Monbiot Claims SAI "already tested ... with
catastrophic
results"
Hi Greg,
Two comments here:
Limestone dissolution can be a very slow reaction, even in CaCO3-
undersaturated
upwelling seawaters. (Much slower than the rate of limestone
dissolution in normal
rainwater, for example)
Adding limestone powders to the upwelling seawaters may in fact take
away
a significant portion of phosphorus through adsorption, therefore
reduce the
availability of a critical nutrient for surface ocean primary
production.
David.
On Sep 26, 10:49 am, "Rau, Greg"<[email protected]> wrote:
There is a delay if air capture is the objective - limestone
dissolution
takes place in the subsurface waters and alkalinity is generated, which
can
effect air capture only when upwelling finally brings it in contact
with
air. Gas diffusion rate and CO2 dissolution rate will then also affect
the
air capture rate. Alternatively, I'm suggesting let's use limestone,
silicates, or some other cheap base to mop up some of the excess CO2
naturally present in surface/subsurface upwelling water before it
degasses,
thus reducing ocean CO2 emission to the atmosphere. This at least
avoids
the air-->ocean CO2 uptake rate limitations. It would seem
easier/faster
to
chemically mop up excess CO2 in solution prior to degassing (ocean CO2
emissions reduction) than to chemically enhance CO2 transfer from
...
read more ?
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