Coastal enhanced weathering is a nature-based climate solution in which sand reacts with seawater to store more carbon in the oceans. It works in marine ecosystems by increasing pH and thus pushing the carbonate equilibrium forward. The result is that ocean water becomes less acidic and can absorb more atmospheric CO2. The enhanced weathering reaction is non-stoichiometric, which means that the rate of mineral dissolution, or weathering, is not proportional to the amount of minerals involved at the start of the reaction. A simple rate of change can’t be used to estimate how much carbon is stored per kilogram of sand grains added.
Multiple factors combine to influence the rate of mineral dissolution, and therefore, the amount of CO2 absorbed. A nonprofit company founded in 2019 now known as Vesta aims to sequester 1 gigaton of carbon per year, and aims to eventually remove one trillion tons of carbon from the atmosphere overall using coastal enhanced weathering. Scientists at Vesta are testing this climate solution under real-world conditions in New York and the Dominican Republic to determine which factors interact with each other in complex ways to promote or restrict carbon uptake from enhanced weathering. They are also studying in the impact of coastal enhanced weathering on marine life, and researching other questions which can be found on their website.
Although a variety of rocks can be used for coastal enhanced weathering, olivine appears to be the more popular choice. Olivine is a common and fast-weathering rock which can be used in the process of enhanced weathering for carbon capture. Small particles of the mineral dissolve in the presence of hydrogen, which is released when carbonic acid (H2CO3) becomes bicarbonate (HCO3–).
Interestingly, carbonate precipitation is also believed to occur in a small, statistically insignificant amount during the mineral dissolution of olivine. Exploring the cause of carbonate precipitation during the process (and somehow promoting it) could increase the overall amount of CO2 sequestered from enhanced weathering. In fact, a new carbon removal company called SeaChange appears to be working on exactly this idea of converting dissolved CO2 from the oceans into stable, solid carbonates.
The benefits of this natural climate solution are that it addresses ocean acidification and carbon removal at the same time and does not interfere with other sustainable development goals such as food security and clean drinking water. A downside of enhanced weathering is that it can cause elevated amounts of nickel and chromium in the oceans. Hopefully continued field pilots and laboratory testing can help determine the cause of any unwanted byproducts, and refine the coastal enhanced weathering process.