Today our Head of Science, Dr. Elisabete T. Pedrosa shares some of the challenges of quantifying enhanced rock weathering in tropical soils and where the scientific community stands at the moment. Let’s read through her scientific, yet accessible analysis of the situation we are in right now related to 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 luckily we can already make well-educated guesses. 

Here we need to point out that silicate rocks do 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, 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 the planet 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 “enhanced weathering”. 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:

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 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 weathering in tropical soils. See this page to learn more about total alkalinity.

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

This seems to indicate that the best way to measure carbon dioxide sequestration is most probably to measure 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 project in Brazil or support us to advance 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.