Limits of Enhanced Weathering potentials – Deductions from column experiments
Geoeng Info
Abstract
In order to elucidate the effects of grain size distribution and soil pCO2 levels, column experiments were set up, simulating humid tropical conditions found in some hotspots of weathering:
- Columns filled with olivine-rich dunite material with three different grain size distributions under fixed atmospheric CO2conditions
- Columns filled with different materials (dunite, basalt, oxisol, and a combination of the latter two) with a fixed grain size comparing the effects of ambient and saturated CO2 conditions
The weathering fluxes under saturated CO2 conditions are about four times higher in all columns except the dunite column, where fluxes were nearly twelve times higher. Grain size distribution is relevant to study further, as highest weathering rates of dunite were observed in columns with coarser grains, which seems counterintuitive at first.
The findings indicate some important issues to be considered in future experiments and a potential rollout as a carbon dioxide removal method: 1) Only in theory do small grainsizes of the spread material yield large CO2 drawdown potentials. The hydrologic conditions, which determine the residence times, i.e., the time available for weathering reactions, are more important than a small grain size. 2) Saturated CO2 column results provide upper limit estimates for weathering rates. 3) As the carbon accounting is still a problem to solve, the observed setup-specific robust relationship between alkalinity and conductivity might provide an easy-to-handle tool in the future to account for carbon uptake from weathering by measuring conductivity in cropland drainage water.
Robert Höglund
Ken Caldeira
Very fine particles on a sandy beach behave very differently from fine particles composing a nearly impermeable homogenous fine grain mud.
There is a large literature among geochemists comparing weathering rates measured in laboratory and (natural) field conditions, and trying to explain why the weathering in field conditions is usually slower.
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Pol Knops
Jens Hartmann (UHH)
Sorry if this comes several times. But it seemed my first two attempts landed in the empty space.
Dear Ken caldera, dear Robert Höglund
a)
Thanks for answering. Yes, there is no black-white answer.
The example with the mud is a good example.
The results confirm that application of EW needs grain size
management and this depends on the original soil material,
climate and how the rock powder is mixed in, etc. .
We achieved high CO2-consumption rates per m^2 land simulated in a column. I will show the data at the Goldschmidt conference.
b)
To answer the previous question "Does this mean that EW has low potential since not even small grains will be able to weather quickly? "
No, the results do not show this. We tested the upper limits
(high rain fall, high CO2 in the soil).
The question of potential: this needs a reference,
high-low relative to what?.
c)
I believe since some time that looking only at EW is not the way to go, but rather figuring out how to optimize the carbon pool per m^2 of land.
This means we have to think about to fusion methods: enhanced weathering, biochar, biological carbon uptake and storage and also the effect of rock powder on possibly stabilizing soil organic carbon, etc.
Some ideas are presented in the figure attached, slightly modified after Amann & Hartmann 2019, Biogeosciences. This could be improved, adding more processes. We should add SOC, e.g..
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-- ========================================== Jens Hartmann (Prof. Dr. rer. nat. habil.) Institute for Geology / Center for Earth System Research and Sustainability Universität Hamburg / Bundesstrasse 55 / D-20146 Hamburg / Germany Phone: +49(0)40 - 42838 6686 / jens.h...@uni-hamburg.de
Robert Höglund
Best regards
Sorry if this comes several times. But it seemed my first two attempts landed in the empty space.
Dear Ken caldera, dear Robert Höglund
a)
Thanks for answering. Yes, there is no black-white answer. The example with the mud is a good example.
The results confirm that application of EW needs grain size management and this depends on the original soil material, climate and how the rock powder is mixed in, etc. .
We achieved high CO2-consumption rates per m^2 land simulated in a column. I will show the data at the Goldschmidt conference.
b)
To answer the previous question "Does this mean that EW has low potential since not even small grains will be able to weather quickly? "
No, the results do not show this. We tested the upper limits (high rain fall, high CO2 in the soil).
The question of potential: this needs a reference, high-low relative to what?.
c)
I believe since some time that looking only at EW is not the way to go, but rather figuring out how to optimize the carbon pool per m^2 of land.
This means we have to think about to fusion methods: enhanced weathering, biochar, biological carbon uptake and storage and also the effect of rock powder on possibly stabilizing soil organic carbon, etc.
Some ideas are presented in the figure attached, slightly modified after Amann & Hartmann 2019, Biogeosciences. This could be improved, adding more processes. We should add SOC, e.g..
All the best, JensOn 2021-06-01 13:00, Geoeng Info wrote:
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Limits of Enhanced Weathering potentials – Deductions from column experiments
Jens Hartmann Thorben Amann
Abstract
Carbon dioxide removal efforts become increasingly important to keep climate change targets. The distribution of rock products into soils to remove CO2 from the atmosphere by weathering (Enhanced Weathering) was identified as a method with high potential [1]. However, the majority of assessments is based on modelling studies, which have to rely on a number of assumptions for parameterization, as real-world measurements are still scarce.In order to elucidate the effects of grain size distribution and soil pCO2 levels, column experiments were set up, simulating humid tropical conditions found in some hotspots of weathering:
- Columns filled with olivine-rich dunite material with three different grain size distributions under fixed atmospheric CO2conditions
- Columns filled with different materials (dunite, basalt, oxisol, and a combination of the latter two) with a fixed grain size comparing the effects of ambient and saturated CO2 conditions
The weathering fluxes under saturated CO2 conditions are about four times higher in all columns except the dunite column, where fluxes were nearly twelve times higher. Grain size distribution is relevant to study further, as highest weathering rates of dunite were observed in columns with coarser grains, which seems counterintuitive at first.
The findings indicate some important issues to be considered in future experiments and a potential rollout as a carbon dioxide removal method: 1) Only in theory do small grainsizes of the spread material yield large CO2 drawdown potentials. The hydrologic conditions, which determine the residence times, i.e., the time available for weathering reactions, are more important than a small grain size. 2) Saturated CO2 column results provide upper limit estimates for weathering rates. 3) As the carbon accounting is still a problem to solve, the observed setup-specific robust relationship between alkalinity and conductivity might provide an easy-to-handle tool in the future to account for carbon uptake from weathering by measuring conductivity in cropland drainage water.
--========================================== Jens Hartmann (Prof. Dr. rer. nat. habil.) Institute for Geology / Center for Earth System Research and Sustainability Universität Hamburg / Bundesstrasse 55 / D-20146 Hamburg / Germany Phone: +49(0)40 - 42838 6686 / jens.h...@uni-hamburg.de
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