Dissolution of olivine

Dissolution of olivine (potential, side effects)
in simulated CO2 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
Olivine is:
foto: H Grobe (AWI)
(0) Basics: Olivine
• a silicate (Si) containing mineral ((Mg,Fe)2 SiO4 ).
• 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 10–30 µm.
• contains a Mg:Fe molar ratio of about 9:1.
Peter Köhler
2
October 2015
(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 of nutrients into 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
Peter Köhler
3
October 2015
(0) Basics: CO2 in Seawater (following Zeebe & Wolf-Gladrow 2001)
Contour lines of pCO2 [ atm]
150
1000
300
950
-1
surface ocean alkalinity [ mol kg ]
2450
400
900
850
500
2400
Z
X
600
700
800
650
P: preindustrial
M: modern
Y: future
600
550
Y
P M
750
700
900
1000
2350 100
800
500
450
400
X: pure weathering
input of HCO−
3
350
2300
300
250
200
150
200
2250 250
1950
Z: net weathering
no change in DIC
350
100
450 550 650750850950
2000
2050
2100
2150
-1
surface ocean DIC [ mol kg ]
Peter Köhler
4
October 2015
(1) Potential: Process changes carbonate system
⇒ CO2 removal potential ∼ 20% smaller
(Mg,Fe)2 SiO4 + 4 CO2 + 4H2 O ⇒ 2 (Mg,Fe)2+ + 4 HCO−
3 + H4 SiO4
olivine + carbonic acid ⇒ cations + bicarbonate + silicic acid
1.4
theoretical limit
1.3
0.35
1.2
1.1
0.3
1.0
pCO2 = 385 atm
pCO2 = 700 atm
0.9
0.8
0
1
2
3
4
0.25
5
6
7
8
9
C : Olivine [Pg:Pg]
CO2 : Olivine [Pg:Pg]
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: about 20% smaller depending on carbonate chemistry
10
Sequestered CO2 [ atm]
Peter Köhler
(Köhler et al., 2010)
5
October 2015
(2) Dissolution Kinetics: Only particles of 1µm sink
slow enough for surface dissolution.
100
90
80
(c)
rdiss (%)
70
60
50
40
30
20
10
0
-1
10
2
3
4
5
6 7 8 9
1
2
3
4
5
6 7 8 910
dgrain ( m)
global mean dissolution
as function of grain size
Dissolution = f (SST, mixed layer depth)
Example for grains of ∼ 1µm.
(Köhler et al., 2013)
Peter Köhler
6
October 2015
3 Pg olivine dissolution per year
0.3
0.9
0.8
0.25
0.7
0.6
S1 (silicic acid+alkalinity input) 0.2
S2 (only silicic acid input)
0.15
S3 (only alkalinity input)
0.5
0.4
0.3
0.1
0.2
0.05
0.1
0.0
2000
2002
2004
2006
2008
0.0
2010
-1
1.0
normalized (Pg C yr per Pg olivine)
-1
changes oceanic CO2 uptake (Pg C yr )
(3) Chemistry: CO2 removal dominated by
alkalinization with add-on by silicate fertilization.
Time (yr)
Silicic acid input (ocean fertilization) increases CO2 removal by 8%.
(Köhler et al., 2013)
Peter Köhler
7
October 2015
(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)
Peter Köhler
8
October 2015
(5) Iron: Iron fertilization (+50% CO2 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)
Peter Köhler
9
October 2015
(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)
Peter Köhler
10
October 2015
• (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: CO2 removal 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% CO2 removal) 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 H2 SiO4 .
(8) pH: If distributed on land river pH might rise significantly.
(9) Time: CO2 removal is not permanent.
(10) Size of problem: 3 Pg yr−1 of olivine to remove
1-2 Pg C yr−1 (coal production: 8 Pg yr−1 ).
Peter Köhler
11
October 2015
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.
Peter Köhler
12
October 2015
The End
Peter Köhler
13
October 2015
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.
Peter Köhler
14
October 2015