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 gas to
liquid (air capture). A detailed comparison of the two concepts re air CO2
stabilization under realistic ocean physics and starting chemistry would be
an interesting paper. For starters, assuming an air pCO2 of 390 uatms and
upwelling ocean pCO2 of 450 uatms, one would need to chemically drive ocean
pCO2 to below 390 before net air capture is effected. In contrast one has to
only chemically reduce ocean pCO2 to below 450 to reduce some ocean CO2
emissions (over natural) and to 390 to have zero net CO2 emissions from that
ocean parcel.
-G
On 9/26/11 9:25 AM, "Oliver Tickell"<[email protected]> wrote:
Actually this option does not look too bad on first sight - low cost,
low tech, so that's a good start, and the chemistry looks right too.
Biggest problem is the delay of approx 100y before the results come
through, if I read the paper right. That's a long time for us to have
to wait. Also if we change our minds, its a long lead time for
reversal.
Go for Mg silicate weathering on land / intertidal zones, and the CO2
drawdown is immediate, operating on a decadal time scale.
Re the kinetics of Mg silicate, they are unfavourable if carried out
in a chemistry lab. Carried out in nature and enhanced by activity of
fungi, bacteria, roots, digestive systems of worms and higher animals,
etc, it's a great deal faster - the biospheric enhancement factor
speeds it up by several orders of magnitude.
Oliver.
On Sep 26, 4:09 pm, "Rau, Greg"<[email protected]> wrote:
And to round out the options, let¹s not forget Harvey¹s
limestone-rain-in-the-ocean
method:http://iod.ucsd.edu/courses/sio278/documents/harvey_08_co2_mitigation.
..
While billed as (eventual) air capture, I view this as ocean CO2 capture
bomb upwelling areas with limestone to consume the excess CO2(aq) prior to
degassing to air. Don¹t forget that the ocean emits in gross>300 GT CO2/yr.
If we can cut that by 1% it would have a huge effect on air CO2. No?
Humbly,
Greg
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