Quantifying Carbon Removal through Enhanced Rock Weathering

October 20, 2022

In this article, our Head of Science, Dr. Elisabete T. Pedrosa shares some of the challenges of quantifying Carbon Removal through Enhanced Rock Weathering in tropical soils. Besides that, she also provides some insights into where the scientific community stands at the moment in the use of rock powders to mitigate climate change.

So, how do we quantify Enhanced Rock Weathering?

Or in other words, what parameter will be used to measure carbon dioxide (CO2) sequestration? Will one be enough or do we have to measure various parameters?  Despite many years of dedicated research, we (the scientific community) still have to find a consensus… but we can already make well-educated guesses.

Here we need to point out that silicate rocks sequester carbon, this is scientific consensus. However, how the weathering processes occur in a natural setting is challenging to clearly measure and understand. Also, the challenge of quantification is not unique to enhanced rock weathering… soil carbon projects, reforestation, and basically all other nature-based systems are in a similar situation. Nature simply is complex.

We have already written a blog post about the difficulty of quantifying enhanced rock weathering before, related to field experiments. Read it here for more background information and to better understand the challenges we encounter.

If this is your first time here…

Weathering is the planet’s way of taking CO2 from the atmosphere by breaking up rocks and transferring the resulting carbon into the groundwaters, rivers, and finally oceans. This happens at a very slow pace and given that there is just too much CO2, we at InPlanet are giving nature a hand.

How Enhanced Rock Weathering works

The CO2 in the air equilibrates with the water in the clouds, and when it rains, rocks and CO2 react. Just like sugar reacts in your coffee, but considerably slower (years to millennia). By adding the rocks as powder, we enhance the process, thus ‚ÄúEnhanced Weathering‚ÄĚ. If you need a metaphor for this, think about the following: Cotton candy melts faster in your mouth than a lollipop.

The CO2 and the rocks react and form many different compounds. Plus, the rocks are made of many different mineral compounds (solids) that can react in many ways, resulting in many different products. See for example the reaction of the mineral anorthite with CO2 and water:

anorthite weathering process

Figure 1: Illustration of the weathering of anorthite, a typical silicate rock mineral.

And this is only for one of the mineral compounds contained in these types of rocks. It also shows that parallel to capturing CO2, other products are formed. These new products add structure to the soil, serve as nutrients to the plants, and are used as food for the living creatures that inhabit the soil, potentially increasing biodiversity.

Measuring carbon sequestration in Enhanced Rock Weathering

Past studies suggested we use a parameter called total alkalinity (TA) for quantifying Enhanced Rock Weathering because it is the typical one used in oceanography and climate studies that are used to work with ocean water samples.

However, TA only works as a proxy for dissolved CO2 in basic conditions (pH>7), and when soils are acidic (pH <7), measuring TA for CO2 drawdown might be misleading. Tropical soils usually show pH values below 6 which leads to the conclusion that TA is not a good proxy for measuring Enhanced Rock Weathering in tropical soils. Click here for additional reading on total alkalinity and its relationship with carbon.

Thermodynamic simulations using PHREEQC1 show that the main products of the reaction above are bicarbonate and aqueous calcium bicarbonate (CaHCO3+):

carbon in anorthite rock weathering

Figure 2: Distribution of carbon species in the products of anorthite weathering simulated using PHREEQC1: Total refers to the sum of the five species indicated in the graph. From left to right: carbon dioxide, bicarbonate, carbonate, calcium bicarbonate, and calcite. 

This seems to indicate that the best way to measure carbon dioxide sequestration is most probably to measure the total dissolved inorganic carbon (DIC) itself. This can be done by acidifying or combusting the soil pore water samples, and subsequently quantifying the extracted CO2 gas by a Coulometer or by an infrared CO2 analyzer2.

We are currently building the first field monitoring station in the tropics. We will measure parameters and compare different methodologies to ensure rigorous monitoring, reporting, and verification for our Enhanced Rock Weathering activities in Brazil. The goal is to quantify the sequestration potential more and more precisely. If you want to find out more, get in touch with us to start your Enhanced Weathering Research Project in Brazil to support the latest climate science.

References:

1 Parkhurst, David L. User’s Guide to PHREEQC: a Computer Program for Speciation, Reaction-Path, Advective-Transport, and Inverse Geochemical Calculations. Lakewood, Colo.: Denver, CO: U.S. Dept. of the Interior, U.S. Geological Survey; Earth Science Information Center, Open-File Reports Section [distributor], 1995.

2 Dickson, A., Goyet, C., 1994. DOE Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water, Version 2.

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“It’s time! If you want to support us on our mission as a farmer, mine or buyer of Carbon Removal Credits, reach out to us and start making a difference today.

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