Rapid soil CO 2 drawdown during incipient weathering of a granular basaltic landscape Joost van...
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Transcript of Rapid soil CO 2 drawdown during incipient weathering of a granular basaltic landscape Joost van...
Rapid soil CO2 drawdown during incipient weathering of a granular
basaltic landscape
Joost van Haren, KaterinaDontsova, Greg Barron-Gafford, Peter Troch, Jon Chorover, Scott Saleska, Stephen Delong, Travis
Huxman, David Breshears, Xubin Xeng, Jon Pelletier, and Joaquin Ruiz
Weathering paradoxLaboratory 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
Seepage TIC flux
CO2 dissolution into pore waterHenry’s law: pCO2
gas= kH*[CO2aq]
Once dissolved:CO2
aq = H2CO3H2CO3 = 2HCO3
- = CO32-
Reactions are pH and T dependentLEO is an in between world: detailed measurements, simplicity and control of the lab experiment and the size of field experiments
Sensor locations0.05-m Soil Depth
-5 -4 -3 -2 -1 0 1 2 3 4 5
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Decagon 5TM Water Content SensorsVaisala GM222 [CO2] Sensors
0.2-m Soil Depth
-5 -4 -3 -2 -1 0 1 2 3 4 5
-5 -4 -3 -2 -1 0 1 2 3 4 5
0.35-m Soil Depth
y-coordinate (m)
-5 -4 -3 -2 -1 0 1 2 3 4 5
-5 -4 -3 -2 -1 0 1 2 3 4 5
0.5-m Soil Depth
-5 -4 -3 -2 -1 0 1 2 3 4 5
-5 -4 -3 -2 -1 0 1 2 3 4 5
0.85-m Soil Depth
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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
• 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• TemperatureHenry’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 valuesThese 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 slope0.92 m2 g-1 524.7x106 g basalt
Basalt weathering based on Na export
Know: • amount of water exported from slope• Na concentration exported
Ca0.44Mg0.3Na0.26K0.06Mn0.01Fe0.38Al0.62Ti0.07(HPO4)0.03Si1.8O5.87
Basalt glass composition
Basaltic glass molecular weight 222.5 (g/mole)
Based on Na export:reacted surface area slope: ~0.04%
Laboratory weathering rate of basalt glass (Stockman et al. 2013)
Weathering rate units:moles m-2 h-1
Multiplied by basalt surface area in slope0.92 m2 g-1 524.7x106 g basalt
How much basalt weathered?
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, KaterinaDontsova, Greg Barron-Gafford, Peter Troch, Jon Chorover, Scott Saleska, Stephen Delong, Travis
Huxman, David Breshears, Xubin Xeng, Jon Pelletier, and Joaquin Ruiz