Basalt Weathering and the Carbon Cycle

Basalt weathering removes carbon dioxide from the atmosphere and sequesters it for extremely long timescales. In order to understand how basalt weathering is a key player for carbon capture and storage, we must first appreciate that this is a natural process that has been regulating climate for billions of years.  

The geologic carbon cycle is responsible for controlling Earth’s natural climate. In short, volcanoes emit CO2 into the atmosphere, chemical weathering sequesters it into rivers, and carbonate precipitation in oceans traps it as solid rock. Carbonate rocks on the ocean floor are recycled back down into the mantle during tectonic activity at plate boundaries, and this carbon is then rereleased to the atmosphere through volcanism. This process is referred to as the “geologic carbon cycle” or the “long term carbon cycle” because it happens over million year timescales.

How exactly do rocks pull CO2 out of the air?

Picture something so old and rusty that parts of what was once metal has withered away. Similar to how rust is a chemical reaction between gas in the atmosphere (in this case, oxygen) and solid metal, rocks also break down and “weather” over time. Rocks are made up of minerals that in this context are made of silica, hence the term silicate minerals. When silicate minerals come in contact with carbon dioxide (CO2), water they chemically weather or dissolve. CO2 and water combine to form carbonic acid, and this slightly acidic solution rains down onto rocks and very slowly dissolves the silicate minerals within them. The dissolution of silicate minerals transforms carbonic acid to is other dissolved form known as bicarbonate, or HCO3 , which is more stable in surface waters (e.g., rivers) where these reactions take place. When CO2 in the atmosphere is transformed into HCO3, the carbon no longer plays a role in controlling climate. The dissolution of silicate minerals also releases metals such as Ca2+ and Mg2+. This is chemical weathering, which we’ll call Part 1 of the long-term carbon cycle.

Chemical weathering is the most essential part of the carbon cycle, because it is a negative feedback that stabilizes climate over long timescales.  The more CO2 added into the atmosphere, the faster chemical weathering happens, which then lowers CO2 levels back down again. This is why weathering is sometimes referred to as “Earth’s thermostat”.

Rates of silicate chemical weathering can be quantified by analyzing the chemistry of rivers draining silicate bedrock, and this information can help us understand controls on the long term climate. Rivers are essentially a snapshot of the long term carbon cycle because they contain all the dissolved constituents that participate. Rivers transport dissolved carbon and metals to oceans, where Part 2 of the long-term carbon cycle happens. Here, the dissolved carbon, in the form of CO32-, combines with the dissolved metals from mineral weathering (Ca2+ and Mg2+) to make calcium carbonate (CaCO3). This is called carbonate precipitation or “mineralization”, and can happen biogenically, as small organisms build calcium carbonate shells, or inorganically. The calcium carbonate eventually settles onto the ocean floor and becomes a rock (lithifies) commonly known as limestone. The limestone that you see on continents now was once under the ocean, and is made up of carbon that was once controlling climate in the atmosphere. Like most geologic processes, this is incredibly slow, as it takes about 1 million years for a molecule of atmospheric carbon dioxide to end up as a solid rock on the ocean floor, 

To summarize, chemical weathering (Part 1) transforms carbon from gas in the atmosphere to dissolved in water, and carbonate precipitation or mineralization (Part 2) transforms dissolved carbon into a solid carbonate mineral. Part 1 and 2 combined sequester carbon over long timescales and control Earth’s climate.

Why basalt?

Basalt is a volcanic rock, such as that which makes up Iceland or Hawaii. Basalt is mafic, meaning low amounts of silica and lots of divalent metal cations (meaning with a 2+ charge) such as Ca2+, Mg2+ and Fe2+. Calcium and magnesium are two key components of the long-term carbon cycle because they are required for carbon mineralization in Part 2. Only Ca2+ and Mg2+ sourced from the weathering of silicate minerals regulates climate. This is because the alternative source of riverine Ca2+ and Mg2+ is carbonate mineral weathering, which has no net effect on long-term climate because as carbonate minerals dissolve, they release carbon.

The weathering of mafic rocks is thought to have a disproportionate role in climate regulation compared to other silicate rocks because theres lots of Ca2+ and Mg2+ (necessary for Part 2) and it has been suggested to weather faster than other rock types. Studies reporting high solute fluxes in rivers draining basalt have interpreted this as rapid chemical weathering and thus rapid drawdown of carbon from the atmosphere by basalt weathering. Additionally, mafic minerals in rocks like basalt (such as olivine) form at high temperatures characteristic of the Earth’s mantle. Because olivine forms at high temperatures (1000 or more degrees C), it is less thermodynamically stable at lower temperatures characteristic of the surface Earth. Moreover, mafic minerals in basalt like olivine generally show faster dissolution rates in laboratory experiments relative to other silicate minerals, like those common in granites, for example. Thus, the chemical weathering of silicate minerals within basalt, like olivine, sequester carbon dioxide from the atmosphere and control climate. 

 

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