Rapid soil CO 2 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 (HCO 3 -)

Why use LEO for Carbon Cycle research? Atmospheric CO 2 flux Seepage TIC flux CO 2 dissolution into pore water Henry’s law: pCO 2 gas = k H *[CO 2 aq ] Once dissolved: CO 2 aq = H 2 CO 3 H 2 CO 3 = 2HCO 3 - = CO 3 2- 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

CO 2 soil gas measurements and fluxes Precipitation events cause: -Increase in soil moisture content -Decrease in soil CO 2 concentrations -Decrease CO 2 gas flux, due to waterlogged pores

CO 2 soil gas measurements and fluxes Precipitation events cause: -Increase in soil moisture content -Decrease in soil CO 2 concentrations -Decrease CO 2 gas flux, due to waterlogged pores Diffusion limitation due to water filled pore spaces immediately following the rains Slower recovery of soil CO 2 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.

CO 2 soil gas measurements and fluxes Precipitation events cause: -Increase in soil moisture content -Decrease in soil CO 2 concentrations -Decrease CO 2 gas flux, due to waterlogged pores Precipitation events cause: -Rapid response at each profile -faster and greater response at shallow depths

Validation of CO 2 gas concentrations pCO 2 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 PCO 2 Soil solution and gas phase samples yield comparable low gas phase CO 2 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 CO 2 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 CO 2 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 m 2 g x10 6 g basalt

Basalt weathering based on Na export Know: amount of water exported from slope Na concentration exported Ca 0.44 Mg 0.3 Na 0.26 K 0.06 Mn 0.01 Fe 0.38 Al 0.62 Ti 0.07 (HPO 4 ) 0.03 Si 1.8 O 5.87 Basalt glass composition Basaltic glass molecular weight (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 slope 0.92 m 2 g x10 6 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: C slope = C slope i + C atm - C seepage 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 CO 2 diffusion? 3) CO 2 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