Dissolution of olivine (potential, side effects) in simulated CO

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Dissolution of olivine (potential, side effects) in simulated CO

₂

removal experiments

—

enhanced weathering, ocean alkalinization, ocean fertilization

Peter Köhler, Judith Hauck Christoph Völker, Dieter A. Wolf-Gladrow

Oxford Greenhouse Gas Removal Conference (30 Sep - 02 Oct 2015)

Peter Köhler 1 October 2015

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(0) Basics: Olivine

foto:HGrobe(AWI)

Olivine is:

• a silicate (Si) containing mineral ((Mg,Fe)₂SiO₄).

• found in dunite, one of the major constituents of the Earth’s upper mantle and accessible at the Earth’s surface.

• highly dissolvable compared to other silicate minerals.

• dissolves within 1-2 yr if grinded to 10–30µm.

• contains a Mg:Fe molar ratio of about 9:1.

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(0) Basics: Silicate (olivine) Weathering

(Ruddiman 2001)

Picture on natural weathering from Textbook W.F. Ruddiman (2001)

“Earth’s Climate, past and future” W H Freeman & Co missing due to copyrights.

Weathering: input of HCO⁻₃ (+DIC, +alkalinity)and ofnutrientsinto ocean.

All C in silicate weathering has its source in atmospheric CO₂. (sum of C in atmosphere-ocean stays constant)

Enhance natural weathering by∼10×: from<0.2 to>1 Pg C yr⁻¹

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(0) Basics: CO

₂

in Seawater

(following Zeebe & Wolf-Gladrow 2001)

2250 2300 2350 2400 2450

surfaceoceanalkalinity[molkg-1 ]

1950 2000 2050 2100 2150

surface ocean DIC [ mol kg^-1] Contour lines of pCO₂[ atm]

100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000

250 250

300 300

350 350

400 400

450 450

500 500

550 550

600 600

650 650

700 700

750 750

800 800

850 850

900 900

950 950

1000 1000

100 100

150 150

200 200

P M

X Z

Y

P: preindustrial M: modern Y: future X: pure weathering input of HCO⁻₃

Z: net weathering no change in DIC

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(1) Potential: Process changes carbonate system

⇒ CO

₂

removal potential ∼ 20% smaller

(Mg,Fe)₂SiO₄+4 CO₂ +4H₂O ⇒2 (Mg,Fe)²⁺+4 HCO⁻₃ +H₄SiO₄ olivine+carbonic acid⇒cations+bicarbonate+silicic acid

Theoretical limit of chemical effect (no enhanced biology):

1 mol olivine removes 4 mol CO₂(1 t olivine = 1.25 t CO₂(0.34 t C)) Realization: about20% smallerdepending on carbonate chemistry

0.8 0.9 1.0 1.1 1.2 1.3 1.4

CO2:Olivine[Pg:Pg]

0.25 0.3 0.35

C:Olivine[Pg:Pg]

0 1 2 3 4 5 6 7 8 9 10

Sequestered CO₂[ atm]

theoretical limit

pCO2= 385 atm pCO₂= 700 atm

(Köhler et al., 2010)

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(2) Dissolution Kinetics: Only particles of 1µm sink slow enough for surface dissolution.

0 10 20 30 40 50 60 70 80 90 100

rdiss(%)

10^-1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 910

dgrain( m)

10^-1 2 3 4 5 6 7 8 91 2 3 4 5 6 7 8 910

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global mean dissolution Dissolution=f(SST, mixed layer depth) as function of grain size Example for grains of∼1µm.

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(3) Chemistry: CO

₂

removal dominated by alkalinization with add-on by silicate fertilization.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

changesoceanicCO2uptake(PgCyr-1 )

0.0 0.05 0.1 0.15 0.2 0.25 0.3

normalized(PgCyr-1 perPgolivine)

2000 2002 2004 2006 2008 2010

Time (yr)

3 Pg olivine dissolution per year

S1 (silicic acid+alkalinity input) S2 (only silicic acid input) S3 (only alkalinity input)

Silicic acid input (ocean fertilization) increases CO₂removal by 8%.

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(4) Marine Biology: Enhanced olivine dissolution is also ocean fertilization, leading to species shifts.

Silicic acid input from olivine dissolution⇒species shift Diatom NPP: + 14%; organic C export: + 1%(Köhler et al., 2013)

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(5) Iron: Iron fertilization (+50% CO

₂

removal) is possible but less feasible.

Already a dissolution and biological availability of 1% of the iron contained in olivine leads to iron saturation.

Iron fertilization is restricted to HNLC areas,

mainly in Eq Pac (model-dependent) and the Southern Ocean.

⇒(up to) 0.55 gC per g olivine (63%alk + 5%Si + 32%Fe)

(Hauck et al., in prep)

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(6) Ships of opportunity: Ballast water of ships has potential to dissolve 0.9 Pg olivine.

dissolution kinetics ship tracks

dissolution (%) of 1µm particles based on NOAA data

Southern Ocean:

particle dissolution slow no ships to go

(Köhler etal., 2013)

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• (1) Potential: Process changes carbonate system⇒CO₂ removal potential∼20% smaller

• (2) Dissolution kinetics:Only particles of 1µm sink slow enough for surface dissolution.

• (3) Chemistry: CO₂removal dominated by alkalinization (∼90%) with add-on by silicate fertilization.

• (4) Marine Biology: Enhanced olivine dissolution is also ocean fertilization, leading to species shifts.

• (5) Iron:Iron fertilization (+50% CO₂removal) is possible, but less feasible.

• (6) Ships of opportunity: Ballast water of commencial ships has potential to dissolve 0.9 Pg olivine.

• (7) Limitation: Local bottleneck might be the saturation concentration of silicic acid H₂SiO₄.

• (8) pH:If distributed on land river pH might rise significantly.

• (9) Time: CO₂removal is not permanent.

• (10) Size of problem:3 Pg yr⁻¹of olivine to remove 1-2 Pg C yr⁻¹(coal production: 8 Pg yr⁻¹).

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References

Hartmann, J.; West, J.; Renforth, P.; Köhler, P.; De La Rocha, C.;

Wolf-Gladrow, D.; Dürr, H. & Scheffran, J. 2013.

Enhanced Chemical Weathering as a Geoengineering Strategy to Reduce Atmospheric Carbon Dioxide, a Nutrient Source and to Mitigate Ocean Acidification.

Reviews of Geophysics, 51, 113 - 149.

Köhler, P.; Abrams, J. F.; Völker, C.; Hauck, J. & Wolf-Gladrow, D. A.

2013. Geoengineering impact of open ocean dissolution of olivine on atmospheric CO₂, surface ocean pH and marine biology.Environmental Research Letters, 8, 014009.

Köhler, P.; Hartmann, J. & Wolf-Gladrow, D. A. 2010. Geoengineering potential of artificially enhanced silicate weathering of olivine.Proceedings of the National Academy of Science, 107, 20228-20233.

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The End

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Open questions

• Dissolution kinetics of olivine not yet clear, our theory needs support from experiments.

• Scavenging and ballast effect might effect how much iron is biological available.