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

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0.35-m Soil Depth

y-coordinate (m)

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0.5-m Soil Depth

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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