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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2018-398
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-398
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 02 Nov 2018

Research article | 02 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Constraints on Enhanced Weathering and related carbon sequestration – a cropland mesocosm approach

Thorben Amann1, Jens Hartmann1, Eric Struyf2, Wagner de Oliveira Garcia1, Elke K. Fischer3, Ivan Janssens4, Patrick Meire2, and Jonas Schoelynck2 Thorben Amann et al.
  • 1Institute for Geology, Center for Earth System Research and Sustainability, University of Hamburg, Germany
  • 2University of Antwerp, Department of Biology, Ecosystem Management Research Group, Universiteitsplein 1C, B-2610 Wilrijk, Belgium
  • 3Institute of Geography, Center for Earth System Research and Sustainability, University of Hamburg, Germany
  • 4University of Antwerp, Research Centre of Excellence Global Change Ecology, Universiteitsplein 1, B-2610 Wilrijk, Belgium

Abstract. The weathering of silicates is a major control on atmospheric CO2 at geologic time scales. It was proposed to enhance this process to actively remove CO2 from the atmosphere. While there are some studies that propose and theoretically analyze the application of rock powder on agricultural land, results from field experiments are still scarce. In order to evaluate the efficiency and side effects of Enhanced Weathering, a mesocosm experiment was set up and agricultural soil from Belgium was amended with olivine-bearing dunite ground to two different grain sizes, while distinguishing setups with and without crops. Based on measurements of Mg, Si, pH, and DIC, the additional weathering effect of olivine could be confirmed. Calculated weathering rates are up to three orders of magnitude lower than found in other studies. The calculated CO2 consumption by weathering was comparably low with 2.3 to 4.9 CO2 t km−2 a−1. One identified cause was preferential flow leading to a low water-rock interaction time for a significant water volume in the setup, not addressed in previous Enhanced Weathering experiments for CO2 consumption. Correction for preferential flow leads to fluxes about a magnitude higher, confirming that this process and surface runoff in the field must be included in assessments for the CO2 consumption potential of Enhanced Weathering in general. Pore water Mg / Si molar ratios suggest that dissolved Si from the added minerals stays in the system over the observation period, because a cation depleted Si layer forms on the reactive mineral surface of freshly ground rocks. This layer has not reached equilibrium thickness within the first two years. The release of potentially harmful trace elements is an acknowledged side effect of Enhanced Weathering. Primarily Ni and Cr are elevated in soil solution, while Ni concentrations exceed the limits of drinking water quality. The use of olivine, rich in Ni and Cr, is not recommend and alternative rock sources are suggested for the application.

Thorben Amann et al.
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Short summary
Weathering is a major control on atmospheric CO2 at geologic time scales. Enhancement of this process can be used to actively remove CO2 from the atmosphere. Field results are still scarce and with this experiment we try to add some near natural insights into dissolution processes. Results show CO2 sequestration potentials but also highlight the strong variability of outcomes that can be expected in natural environments. Such experiments are of utmost importance to identify key processes.
Weathering is a major control on atmospheric CO2 at geologic time scales. Enhancement of this...
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