These days whenever you turn on the TV, radio or check out the news online and someone mentions carbon, they are most likely referring to the ongoing crisis of human caused climate change. Which is fair enough but can be a little political, which we are trying to avoid on this website. So this short article is actually about one of the many useful properties of carbon, dating organic matter. OK, a bit odd for a garden wildlife site, but we’ll explain at the end.
Lets start with a bit of science, carbon dating. This is a term we will all have heard but if you’re anything like me, not fully understood. However in principle its remarkably simple. We all know that a molecule, say carbon dioxide (CO2), is made up of atoms. In this case one Carbon atom and two Oxygen atoms. So carbon, for example, can exist in an atomic form, i.e. just carbon, or a molecular form for example in carbon dioxide or glucose.
Carbon dating makes reference initially to the atomic form, because whilst carbon is carbon it has what are called isotopes. So an atom, if you remember science class, is made up of a nucleus of protons (positively charged) and Neutrons (Neutral) and with a bunch of Electrons (negatively charged) whizzing around it. Lets forget about the Electrons for now and focus on the nucleus of protons and neutrons. All forms of carbon have 6 protons, so that’s simple. And about 99% of carbon out there has 6 neutrons as well. This half a dozen of one and half a dozen of the other gives this 99% of carbon a ‘atomic weight’ of 12, and is known as Carbon 12 (C-12), which is one Isotope of Carbon. Fab. But high in the atmosphere, where radiation from the sun bombards carbon we sometimes end up with Carbon 14 (C-14), a different isotope, which still has 6 protons (meaning it is still carbon and acts like it) but now has 8 Neutrons. C-14 therefore has an atomic weight of 14. You can also get carbon 13 with 7 neutrons (see below) but for now lets focus on Carbon 14.
The thing about carbon 14 is that it is unstable and decays at a very specific rate. The useful thing about C-14 is its decay globally matches it production, in other words the total amount of the stuff is fairly constant. So when lifeforms absorb carbon they take in both C-12 and C-14 at this constant ratio. However once these life forms die this exchange ceases, but the C-14 decay doesn’t. It decays at a known rate (a half life of 5570 years, the time it takes for half of it to decay), so by using clever lab techniques it is quiet possible to determine somethings age reasonable accurately by measuring the relative amounts of C-12 to C-14, the lower the amount of C-14 the older the thing is you are studying.
So carbon dating can be used to date all organic matter because it will have absorbed carbon whilst it was living matter. It can also date some inorganic materials produces by life, for example shells.
Dating the existence of life
Now if you thought C-14 dating was useful, C-13 may ultimately answer a far larger question altogether. C-14 as noted above is reasonably constant, but is only ever found in trace amounts, tiny but certainly measurable. The truth is 98.89% of the carbon anywhere on the planet is in the form of C-12. Nearly all the remaining 1.11% is C-13. Unlike C-14, both these lighter forms of atomic carbon are very stable, they don’t decay.
However just because they are constantly found in the ratio 98.89% / 1.11%, this doesn’t mean they are found in this ratio everywhere. The basic thing is this, its easier for lifeforms to incorporate C-12 than C-13. What this means is that when life forms die, and get buried in the ground in one organic form or another, fossil fuels for example, then they take a disproportionate amount of C-12 with them when compared to C-13. The flip side is, because the total amount of C-12 and C-13 remains stable at 98.89% / 1.11%, then that remaining above ground is disproportionately rich in C-13.
Now for a bit of geology. Certain rocks, such as limestone, absorb C-12 and C-13 at whatever proportion it is found in the environment when the rock is formed. So the upshot is that layers of rock (which we can date by various means), will also have more or less C-13 and this is a very good indicator of how much ‘life’ was present on the planet at the time the rock was formed. The more life the higher the C-13 because more C-12 has been removed by buried life forms.
This is useful for one very important reason, it helps us to some extent, make a case for how far back life on the planet actually existed. And it seems that could have been as far back as 3.8 Billion Years (remembering the planet itself is only 4.6 Billion Years old).
Interesting don’t you think. But what’s that got to do with garden wildlife. Well, to be honest not a great deal other than our next article will be about Nostoc, and that’s a very old form of life, just thought it be fun to partially describe how we know this….
DC – 24/09/2021