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Rapid soil CO2 drawdown during
incipient weathering of a granular
basaltic landscape
Joost van Haren, Katerina
Dontsova, Greg Barron-Gafford, Peter Troch, Jon
Chorover, Scott Saleska, Stephen Delong, Travis
Huxman, David Breshears, Xubin Xeng, Jon
Pelletier, and Joaquin Ruiz
Weathering paradox
Laboratory weathering rates orders of
magnitude faster than field weathering rates.
Why?
• preferential flow paths,
• lower reactive mineral-water interface in
structured soils,
• passivation of primary mineral surfaces by
secondary mineral coatings,
• localized (pore-scale) variation in chemical affinity
Field weathering rates derived from
riverine bicarbonate
Only carbon component measured:
River bicarbonate (HCO3-)
Why use LEO for Carbon Cycle research?
Atmospheric CO2 flux
CO2 dissolution into pore water
Henry’s law: pCO2gas= kH*[CO2aq]
Once dissolved:
Seepage
aq
CO2 = H2CO3
TIC flux
2H2CO3 = 2HCO3 = CO3
Reactions are pH and T dependent
LEO is an in between world: detailed measurements, simplicity and control of the lab
experiment and the size of field experiments
Sensor locations
0.05-m Soil Depth
0.2-m Soil Depth
0.35-m Soil Depth
0.5-m Soil Depth
0.85-m Soil Depth
x-coordinate (m)
-5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5
30
30
28
28
26
26
24
24
22
22
20
20
18
18
16
16
14
14
12
12
10
10
8
8
6
6
4
4
2
2
0
0
-5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5 -5 -4 -3 -2 -1 0 1 2 3 4 5
Decagon 5TM Water Content Sensors
Vaisala GM222 [CO2] Sensors
y-coordinate (m)
CO2 soil gas measurements and fluxes
Precipitation events cause:
- Increase in soil moisture
content
- Decrease in soil CO2
concentrations
- Decrease CO2 gas flux,
due to waterlogged
pores
CO2 soil gas measurements and fluxes
Precipitation events cause:
- Increase in soil moisture
content
- Decrease in soil CO2
concentrations
- Decrease CO2 gas flux,
due to waterlogged
pores
Diffusion limitation due to water filled pore spaces immediately following the rains
Slower recovery of soil CO2 concentrations at depth further indicates diffusion limitation
to weathering
LEO weathering rate relative to field
weathering rate
Location
pH
Annual riverine uptake
Iceland bare basalt
Iceland bare basalt+soil
Weathering rind uptake
7.03
6.78
Laboratory experiments
8.3-9.5
HCO3(mmol/l)
188
113
CO2 uptake rate
-2 -1
(mg-C m h )
0.8 (0.4-8.8)
0.50
0.48
3.3
Reference
661±117
4
1
2
2
3
LEO-Jan 2013
9.3
10000
4.1-5.1
This study
LEO-DOY 49, 2013
9.4
5000
0.6-0.7
This study
LEO-May 2013
9.6
7000
3.4-4.9
This study
1
2
3
References: Desert et al. 2003, Moulton et al. 2000, Navarre-Stichler and Brantley 2007,
4
Stockman et al. 2013.
• LEO rates are very similar to the global average of field weathering rates
• they are much lower than laboratory rates.
CO2 soil gas measurements and fluxes
Precipitation events cause:
- Increase in soil moisture
content
- Decrease in soil CO2
concentrations
- Decrease CO2 gas flux,
due to waterlogged
pores
Precipitation events cause:
- Rapid response at each profile
- faster and greater response at shallow
depths
Validation of CO2 gas concentrations
pCO2 based on:
• DIC
• pH
• Temperature
Henry’s law and reaction
constants
- Vertical error bars
denote ± SE
- Bar along Y-axis
denotes instrument
error ± SE
- Bars along the X-axis
represent a ± 0.1 pH
difference at each PCO2
Soil solution and gas phase samples yield comparable low gas phase CO2 values
These results were further confirmed by manual analysis of soil air grab samples
Soil gas concentration implications
• Weathering rates are similar to measured field rates, but not
laboratory rates
• CO2 supply from the atmosphere into the basalt appears to limit
weathering in LEO
Soil gas concentration implications
• Weathering rates are similar to measured field rates, but not
laboratory rates
• CO2 supply from the atmosphere into the basalt appears to limit
weathering in LEO
Could diffusion limitation represent a new
explanation why field and laboratory weathering
rates are so different?
How much basalt weathered?
Laboratory weathering
rate of basalt glass
(Stockman et al. 2013)
Weathering rate units:
moles m-2 h-1
Multiplied by basalt
surface area in slope
0.92 m2 g-1
524.7x106 g basalt
Basalt weathering
based on Na export
Know:
• amount of water exported from slope
• Na concentration exported
Basalt glass composition
Ca0.44Mg0.3Na0.26K0.06Mn0.01Fe0.38Al0.62Ti0.07(HPO4)0.03Si1.8O5.87
Basaltic glass molecular weight 222.5 (g/mole)
Based on Na export:
reacted surface area slope: ~0.04%
How much basalt weathered?
Laboratory weathering
rate of basalt glass
(Stockman et al. 2013)
Weathering rate units:
moles m-2 h-1
Multiplied by basalt
surface area in slope
0.92 m2 g-1
524.7x106 g basalt
Two independent measures show that less than 0.1% of the landscape
particle surface weathered.
Leo landscape carbon balance
 Carbon in slope solution
Line: modeled carbon in
solution based on:
Cslope = Cslopei + Catm Cseepage
Grey lines rain events
Very good carbon balance =>
river bicarbonate good indicator weathering rate.
Conclusions
Carbon dynamics in LEO landscape has revealed unexplored
reason for weathering paradox
Riverine bicarbonate indeed good indication landscape
weathering
1) Field scale laboratory experiment reveals novel
resolution to the weathering scale paradox.
2) Basalt weathering limited by CO2 diffusion?
3) CO2 diffusion limited basalt weathering.
Joost van Haren, Katerina
Dontsova, Greg Barron-Gafford, Peter Troch, Jon
Chorover, Scott Saleska, Stephen Delong, Travis
Huxman, David Breshears, Xubin Xeng, Jon
Pelletier, and Joaquin Ruiz