Olivine Weathering Reaction For CO2 Removal and Ocean Alkalisation, and its place in the Long-term Carbon Cycle

Weathering is the process of the breakdown of rocks from contact with carbon dioxide (CO2), water, and organisms. When olivine reacts with ocean water, the CO2 dissolved in it, and other ions in the ocean, it undergoes a reaction that binds the carbon dioxide as bicarbonate. That reaction essentially removes CO2 in a semi-permanent way. Eventually, the bicarbonate turns to carbonate, which animals like corals will combine with calcium also dissolved in the water, to form calcium carbonate, which they use to build their skeletons. When the corals and other shelled organisms die, those shells build up in the sediment, eventually turning into limestone and other rock, storing the CO2 in the seafloor for millions of years. Bicarbonate is itself alkaline, and the transfer of CO2 out of carbonic acid and into that form is able to lower the acidity of the water.

TL;DR: Project Vesta is accelerating the natural carbon cycle by speeding up the weathering reaction of the carbon cycle, where ions from rocks cause CO2 to go from dissolved in seawater as an acid, to becoming stored in the material that corals will eventually use to build their shells with, which will eventually lead to the CO2 being stored as rock on the seafloor.

Below is some discussion of the process, a basic introduction to the carbon cycle, a few descriptions, and then a more technical discussion of the chemical reaction and stoichiometry below.

Above is an image of the reactions that eventually lead to CO2 being stored as rock on the seafloor. Taking a step back, it is helpful to recognize that on geological timescales, with no additional energy added, carbon will naturally end up in this solid form (and that includes the same carbon that makes up carbon dioxide). In fact, due to this process, 99.9% of all carbon on Earth is currently found in rock. For example, the White Cliffs of Dover are made up of the fossilized remains of a type of algae whose cell walls are made of calcium carbonate. The white rock making up the cliff below is visible example of rock as a massive store of carbon.

The White Cliffs of Dover are an example of where 99.9% of carbon on Earth is sequestered.

At Project Vesta, we seek to accelerate the steps of the carbon cycle that transfer CO2 from being dissolved in water to it being chemically bound in a form that will eventually turn into rock. To understand Project Vesta and our process, it is helpful to understand the long-term carbonate-silicate cycle. The Earth naturally releases small amounts of CO2 into the atmosphere through volcanic eruptions when friction from tectonic forces causes rock to melt and the carbon in it to be released as CO2 gas. There, the CO2 mixes with water and then falls as acid rain back down onto volcanic rocks like olivine, causing the chemical breakdown of the olivine. Rivers then transport the dissolved ions and molecules of the rock to the ocean. Once in the ocean, a reaction occurs that binds the CO2 into bicarbonate, and once this and additional reactions occur, the CO2 is essentially stored for hundreds of millions of years on the seafloor until maybe it is released from a volcano again.

Long-term carbonate-silicate cycle and where Project Vesta aids in speeding up the cycle

This process is known as the long-term carbonate-silicate cycle, or the inorganic carbon cycle, and is rate limited by the chance exposure of the right rock in the right locations to CO2 infused water. At certain times in our geological history, tectonic forces have exposed large sutures of the type of volcanic rock with these CO2 binding properties in the humid tropics. A robust body of evidence has correlated these chance exposures of carbon sequestering rocks in the tropics, with so much removal of CO2, that it results in global cooling. Notice in the gif below how as volcanic sutures (orange) appear around the humid tropics (green) where it weathers more rapidly, ice coverage then increases at the polls (blue).

For global cooling see times 455 Ma, 365 Ma, 330 Ma, and 30 Ma

The last three major global coolings all correlate with increased weathering of ultramafic (volcanic) rock in the tropics. The ice on our poles right now correlates to the collision of the Himalaya Mountains and the Tibet Plateau that exposed massive quantities of CO2 capturing rock near the equator. In the gif above, this time period correlates with around 50-5 million years ago (Ma), when what would become India collides with the rest of what is now Eurasia.

Project Vesta works to speed up this natural process, which can store billions of tonnes of CO2, by millions of years. We plan to do this by directly exposing the fastest weathering and most effective rock at sequestering CO2 (olivine), in the most optimal location for weathering (high-energy beaches). Once the rocks are in the coastal environment, the wave energy efficiently and constantly grinds the rocks down, accelerating the weathering and breakdown reaction that releases the ions that lead to CO2 becoming sequestered.

The mineral weathering process removes CO2 from the atmosphere by neutralizing carbonic acid (CO2 dissolved in water) to form soluble bicarbonates through the reaction shown below:

The olivine surface absorbs the H+ ions, and releases the magnesium (Mg2+ ions) and increases the ocean’s pH, alkalinity, which results in an increase in the dissolved inorganic carbon content of the ocean. The bicarbonate becomes carbonate and will eventually be transformed by marine calcifying organisms into the calcium carbonate that makes up their shells and skeletons. When they die, those shells go on to settle in the sediment and eventually turn into limestone rock. Small and safe amounts of magnesium and silicate are leftover from the reaction. The silicate is desperately needed by a crucial species of plankton known as diatoms, which themselves, further sink carbon. Watch our lead external scientist, Francesc Montserrat describe the reaction below, from when we presenting our poster on taking olivine weathering from the lab to the beach:

1 tonne of pure olivine can technically sequester up to 1.25 tonnes of CO2. Taking into account the life cycle of the acquisition of the rock, the light milling, transport, purity, and that some olivine that may never weather, we generally settle at a 1:1 ratio of olivine weathered to CO2 removed. 

There are billions to trillions of tonnes of accessible olivine rock sitting just below the surface, waiting to be excavated, crushed and transported to the coastlines.

Last updated on July 19, 2020
Is this article clear and informative?160