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Carbon dioxide and the end of the Permian

November 4, 2021 - 17:00 -- Admin

We need to start off with a bit of geology for this article, before we move into chemistry. The Permian Period is an interval of geological time which began 298.9 million years ago and ended 251.9 million years ago, so it lasted 47 million years1. 

At the end of the Permian, i.e. at about 251.9 million years ago, the greatest mass extinction in the history of the planet, occurred. This extinction event wiped out an estimated 96% of all marine species, and an estimated 70% of all terrestrial species2.

This event is one of the ‘big five’ extinction events and geologists, geochemists and palaeontologists have been trying to work out the causes of these events. One of the suspects for many, if not all of these extinction events are what are called Large Igneous Provinces (LIPs)3.

The one associated with the end Permian extinction event is called the Siberian Traps. As you would expect, it is mostly located in Siberia in northern Russia, and consisted of intrusive rocks (sills and dykes) and volcanic flows, with the latter covering some 340,000 square kilometres. It is estimated that over 2,000,000 cubic kilometres of basalt were erupted during a period of a couple of million years4. 

Now for the chemistry: the element carbon is the foundation of all life on Earth, as it forms the complex molecules, such as proteins and DNA, that form us. The carbon cycle is the process by which carbon atoms continually travel from the atmosphere to the Earth and back again. Since the planet is, as far as carbon is concerned, a closed system, the amount of carbon in the system does not change. On Earth, most carbon is stored in rocks and sediments, while the rest is located in the ocean, atmosphere, and in living organisms. These are the reservoirs, or sinks, through which carbon cycles. Carbon is released back into the atmosphere mostly as carbon dioxide C02 when organisms die, fires burn, and through a variety of other mechanisms. Humans play a major role in the carbon cycle through activities such as the burning of fossil fuels and land development. As a result, although the total amount of carbon on the planet hasn’t changed, the amount of carbon dioxide in the atmosphere is rapidly rising; it is already considerably greater than at any time in the last 800,000 years5.

All chemical elements have isotopes. Isotopes of an element are those that all have the same number of protons in their nucleus but have different numbers of neutrons. The number of protons in a nucleus determines the element’s atomic number on the Periodic Table. For example, carbon has six protons and its atomic number is 6. Carbon occurs naturally in three isotopes: carbon-12, which has 6 neutrons (plus 6 protons equals 12), carbon-13, which has 7 neutrons, and carbon-14, which has 8 neutrons6.

The addition of even one neutron can dramatically change an isotope’s properties. Carbon-12 is stable, meaning it never undergoes radioactive decay. Carbon-14 is unstable and undergoes radioactive decay with a half-life of about 5,730 years (meaning that half of the atoms of carbon-14 will have decayed after 5,730 years). This decay means the amount of carbon-14 in an object serves as a clock, allowing the object’s age to be determined by a process colloquially called ‘carbon dating’6. It is used extensively in archaeology, as it is only useful to date things younger than about 50,000 years old.

During photosynthesis, the process by which plants turns sunlight into energy, CO2 is fixed into a plant’s structure from the atmosphere. During that process, fractionation of stable carbon isotopes occurs. This means that plants take up one isotope (Carbon-12) more than the other heavier isotopes. As a consequence, plants are generally relatively depleted in the heavier isotope Carbon-137. 

Studying chemical variations within sedimentary rocks is a technique termed chemostratigraphy and has only been used commonly in geology since the early 1980s. Ratios of carbon isotopes in sedimentary rocks can be used to track changes in the carbon cycle over geological history, and can be used to work out whether there have been any perturbations or disruptions in the carbon cycle during major events such as the ‘big five’ extinction events.

At the level of some of these extinction events there are abrupt changes in the ratios of the two stable isotopes and when they are graphed against the level in the rock, they appear as a spike to the right or left of a more or less wobbly vertical line. That spike is termed a ‘carbon isotope excursion’ (CIE), and there are many of these throughout the last 540 million years (the Phanerozoic) of Earth history.

The CIE near the end of the Permian has been found at numerous sites around the world, suggesting a massive quantity of C-13-depleted carbon dioxide was injected into the atmosphere and ocean. However, the exact magnitude and cause of the CIEs, the pace of carbon dioxide emission, and the total quantity of carbon dioxide, remained poorly known until now. New, detailed studies of carbon isotope records from the Finnmark Platform (offshore northern Norway) indicate that a massive injection into the atmosphere and ocean of about 36,000 Gigatonnes of carbon at a rapid rate of about 5 Gigatonnes per annum, mostly, but not entirely, from a volcanic source, would be necessary to drive the observed CIE. This is estimated to have caused an increase in sea-surface temperatures of as much as 10 degrees C and an extreme increase in ocean acidity, which led to the largest mass extinction that life on this planet has ever suffered8.

Currently, we are injecting about 9 Gigatonnes of Carbon into the atmosphere every year9.