Accelerating Ocean Carbon Sequestration: A Race Against Time
Storing Carbon Dioxide under the Ocean
With a flicker, the light switches on; with a roaring spiral, the toilet flushes; and with a brief chortle, the car rises to life. With the simplest of daily actions, carbon dioxide, the contributor of two-thirds of greenhouse effects, is released. Each year, human activities release approximately two billion metric tons of carbon dioxide, increasing the atmospheric concentration of the gas by more than a third since the industrial revolution. The evolved greenhouse gas traps the heat radiating from Earth, causing global climate change. Currently, carbon dioxide concentration has reached an alarming level of 400 ppm, raising existing concerns about the future of carbon dioxide regulation.
The primary source of carbon dioxide is the burning of fossil fuels. While intensive research is being conducted in search for alternate energy sources, our dependency on fossil fuels will likely remain stubbornly strong in the foreseeable future. Subsequently, scientists have turned to a different strategy — carbon sequestration, the removal of carbon dioxide from the atmosphere by its storage in solid or liquid form.
The vast ocean is a major reservoir of greenhouse gases and represents one of the largest potential sinks for carbon dioxide. Currently, one of the methods under active research is direct injection, where carbon dioxide emissions are captured and pumped into deep water, the pressure of which keeps the carbon sequestered, theoretically. Yet the problem persists. This procedure has remained controversial since its proposition due to its potential impacts on sea lives and its unverified payoffs. However, a recent collaborated project between the labs of Caltech and USC may have unearthed a revolutionary key to the solution.
In a natural buffering process, the ocean surface absorbs carbon dioxide (CO2), which reacts with water to form carbonic acid (H2CO3).
H2O + CO2 → H2CO3 → H+ + HCO3-
Carbonic acid readily dissolves to form hydrogen ion (H+) and bicarbonate (HCO3-). This acidifies the ocean water, launching a train of detrimental effects, including the corrosion of coral reefs and disruption of ecosystems. Furthermore, the generated hydrogen ions bind with carbonates, interfering with calcification, which involves the precipitation of dissolved calcium ion into solid carbonates. As calcification is the building process of skeleton and shells, its failure can be fatal for certain species.
Simultaneously, a slower process occurs in the ocean depths. Calcite is a stable crystal form of calcium carbonate. Composed of calcium, carbon, and oxygen, the sedimentary mineral is present in the ocean mostly from shells and skeletons of marine organisms that have died and sunken to the ocean floor. Reaction with calcite neutralizes carbonic acid, giving calcium ion and bicarbonate. This reaction prevents the production of hydrogen ions, maintains a stable water pH, and mitigates the interferences with calcification, all while keeping carbon from the atmosphere.
H2O + CO2 + CaCO3 → Ca2+ + 2HCO3-
As good as that sounds, this reaction can take tens and thousands of years to complete, and our seafloor inhabitants may not have that long.
Fortunately, Subhas, Professor of Geochemistry and Global Environmental Science at Caltech, and Adkins, graduate student at Caltech, decided to investigate things from a different perspective.
To quantify the rate of calcite dissolution, the research team utilized isotopic labelling and measured isotope ratios, in contrast to the more conventional method of monitoring pH level as calcium carbonate dissolves. The team first engineered a calcite sample composed entirely of carbon-13, then dissolved it into seawater. They were able to measure the change in ratio of carbon-12 to carbon-13 over time, giving results 200 times more accurate than the usual approach. Further examination of the process using spectrometers identified the slow step of the reaction — formation of carbonic acid from carbon dioxide and water.
With this newly acquired information, the team directly tackled the problem by adding carbonic anhydrase, a common enzyme that maintains blood pH balance in humans and other animals. This successfully accelerated the reaction by orders of 500.
While scientists still struggle to understand the activities of carbon dioxide governing its atmospheric concentration, this breakthrough may have secured us just the time we need to explore more sustainable options for carbon dioxide regulation across the globe.
