Our carbonated universe

One of the fundamental principles taught to chemistry students is the law of conservation of mass. In any chemical reaction, the amount of mass in equals the amount of mass out.

Put in slightly better terms, it is the law of the conservation of atoms. With the exception of a few nuclear processes, the total number of atoms on the planet stays the same. They just get rearranged in different compounds. It is a bit like having a Lego set where the number of bricks stays the same but there are a number of ways they can be put together.

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The fourth most abundant element in the universe is carbon and, of all the elements, it has the ability to form the widest array of compounds over a number of different oxidation states. A highly versatile element, it can be found everywhere from simple compounds such as carbon dioxide, methane, and carbonate minerals to extremely complex species such as DNA, saxitoxin, and Vitamin B12. To push the analogy, carbon is the standard two-by-four brick of the Lego set, involved in so many different and important structures.

Carbon also moves through the carbon cycle, engaging with the various components of the Earth surface - from water to soil and sediment and back again, from the air to the oceans, lakes, and rivers and back again, and from the air to soil and sediments and back again. We are finally at a point where we have a good understanding of the processes involved.

According to the Deep Carbon Observatory, a 10-year research project which has been amassing data on all aspects of carbon on Earth, there is 1.85 billion giga-tonnes (Gt) of carbon with 99 per cent of that buried deep in the crust and mantle in the form of carbonates.

On the surface or outer layers of the Earth, there are 43,500 Gt of carbon compounds and the majority of that - 38,340 Gt - can be found in the oceans. Not as sea creatures but as carbonate ions formed from the dissolution of carbon dioxide into the waters and dead or decaying organic matter from various organisms. The amount of carbon in ocean biota is estimated to be about three Gt.

The amount of carbonate ions in the ocean is like a chequing account in which carbon is both deposited and withdrawn on a continual basis. Approximately 96 Gt of carbon is sequestered in the ocean each year while 96.9 Gt is released.

This might not seem like a huge amount, being only 0.2 per cent of the total in the ocean, but these fluxes of carbon across the air/water boundary are responsible for stabilizing the atmospheric concentration of carbon dioxide. Further, since more carbon is being released than is being sequestered, the process is presently aiding in the total carbon found in the atmosphere.

One of the other foundations of chemistry is the difference between thermodynamics and kinetics. Thermodynamics tells us where the destination is and kinetics tells us how fast we will get there. Rates matter. In the case of carbon shifting between the various reservoirs in the carbon cycle, the rate of sequestration and release are critical.

The rate at which carbonate is absorbed by the ocean to be converted to calcium carbonate or other species and eventually form new sediment on the bottom of the ocean is only 2 Gt per year from terrestrial sources. On the other hand, we are adding about 9 Gt to the atmosphere through the combustion of fossil fuels, the deforestation of terrestrial ecosystems, from livestock, and other processes. If we think of this in terms of money, we are spending $9 per year and only making $2 Not a recipe for success.

Of course, life also takes up carbon dioxide from the atmosphere. The best estimates we have suggest a further 3 Gt end up in living organisms on the planet - mostly bacteria which dominate life. But the net result, when we consider the equation, is we are putting 9 Gt into the atmosphere and only removing 5 Gt annually. It is easy to see why carbon - in the form of carbon dioxide, carbon monoxide, and methane - is increasing in the atmosphere.

Celina Suarez at the University of Arkansas and with the DCO points out we are destabilizing the carbon cycle with our activities. Throughout the last 500 million years - the period when complex animal and plant life has existed on Earth - the carbon cycle has been in balance for 99 per cent of the time - with inputs into reservoirs matching outputs.

However, four periods are known when the cycle has become unbalanced. "Those occurrences are correlated to mass extinction events" according to Suarez, such as the end-Permian extinction which eliminated 90 per cent of all species.

And with the balance once again out of whack, we seem to be heading towards an extinction of our own making.

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