Brief introduction: from time to time, I'm going to post a diary based on my own field (organic chemistry), where I explain the chemistry behind something of importance beyond chemistry. My inspiration behind all of these posts is the book "Chemistry in Context", which is a general chemistry text specifically targeted towards people who will never need to take any chemistry again. I was a teaching assistant for several semesters for a course that uses this book.
Anyways, today's topic is carbon isotopes. Among the uses of carbon isotopes are:
Proving that the extra CO2 in the atmosphere is man-made
Carbon NMR (short)
Carbon Dating
Catching athletes who are cheating (pharmaceutically, not Tiger Woods-like)
Join me below the fold for more!
Update: Make sure you catch profh's comment downthread for another major use of carbon isotopes that I completely blanked on.
First, for those who don't know: "What is an isotope"
Atoms with the same number of protons, but differing numbers of neutrons. Isotopes are different forms of a single element.
In chemical reactions, isotopes behave the same.* However, it is easy to detect different isotopes via mass spectrometry (just what it sounds like - finding the mass of atoms or molecules), so chemists can tell how much of each isotope is there, provided that the machine is working properly.
*Different isotopes can affect reaction rates, but if I ever get to posting that, then this series will have gone on for a long time!
So, on to climate change. What do carbon isotopes have to do with climate change, you might ask?
Well, in a nutshell, carbon isotope data gives us the "smoking gun" that reveals that the increase in CO2 since ~1850 is human-derived.
The following three figures I found on skeptical science. Analysis / explanation is mine.
Starting with the theory, then moving on to the evidence:
Plants have a bit more trouble using carbon-13 CO2 than carbon-12 CO2 in photosynthesis, so the burning of any plant-derived material (such as fossil fuels) will incrementally lower the C13 / C12 ratio in the atmosphere and incrementally lower the amount of oxygen in the atmosphere in addition to raising the amount of CO2 in the atmosphere.
As an example of a balanced equation for burning octane (think gasoline):
C8H18 (octane) + 12.5O2 --> 8CO2 + 9H2O (this shows that burning should raise CO2, lower O2, and the carbon-13 in octane or whatever else was burned ends up being carbon-13 CO2)
Dealing with the last point first, as pretty much everyone who's gotten this far knows, atmospheric CO2 has increased dramatically since the advent of the industrial revolution, when mankind started burning tons and tons of fossil fuels:
However, this is correlation, not necessarily causation, since natural sources do emit CO2 (yes, it's a very strong correlation). A second point of conformation is found in examining the amount of oxygen in the atmosphere. As humans burn up CO2, the amount of O2 in the atmosphere should be gradually falling. This is indeed the case; however, we're in no danger of suffocating from lack of O2 - we're talking about ~ 0.1% drop in O2 levels over 160 years (back-of-the-envelope calculation).
Considering there are other ways for O2 to be removed / added to the atmosphere other than human burning of fossil fuels, that graph is another strong correlation, but it isn't the true smoking gun. The following is the true smoking gun - over the last ~ 160 years the C13 / C12 ratio has gradually declined. Direct observation of C13 / C12 ratio is ongoing by the University of Colorado and other researchers, and ice-core data and tree ring data can be used to reconstruct the C13 / C12 ratio for the past.
Graph one: CO2 emissions vs C-13 percentage. Note that the C-13 percentage is plotted inverted (see the right axis), which to me is aggravating, since the graph can easily be misread.
Graph two: From UC-Boulder ; the middle graph shows the decline in C13 over the last 18 years directly.
Taken together, these last two graphs provide the smoking gun answer to the question "How do we know the increase in CO2 isn't naturally caused".
That's one thing that C13 can do for us. Another is to provide structural information in my field (chemistry, especially organic chemistry). C13 NMR is a cousin to MRI, and using proper machines, I can obtain a graph that looks like this:
where each carbon in the molecule I've made shows up as a single vertical line, and it can help me confirm I made what I wanted to make (or, sometimes, I did not make what I wanted to make!).
Carbon-14, the comparatively rare, unstable radioactive isotope of carbon, can be used to date samples in nature. All radiometric dating methods work on the same principle: natural processes keep the amount of any given radioactive isotope the same at any given time. For carbon-14, cosmic radiation restocks carbon-14. As living things die, they stop incorporating this constant amount of C-14, and the ratio of C-14 to C-12 begins to drop. This decay happens independently, and in 5730 years half of the C-14 has decayed. Given another 5730 years, half of that decays (so 1/4 of the original is left), etc. By checking the ratio of C-14 to C-12, people can get a very good approximation of the age of a sample.
Radiometric decay curve:
Yet another use of carbon isotopes is to catch athletes who are doping. The carbon isotope ratio test works on the same principle as the "smoking gun" I talked about above. Naturally occurring testosterone is about 1.1% C-13 labeled, but testosterone from "pharmaceutical sources" is only about 1% C-13. Explained simply, by comparing the ratio of C-13 to C-12 in testosterone in a urine sample to the ratio of C-13 to C-12 in cholesterol or another compound the body makes naturally, doping officials can figure out athletes (such as Floyd Landis) who have been taking unnatural testosterone - aka, doping. See this graphic for a better explanation (won't post here - too big and not part of a larger article / post).
This listing of "uses of carbon isotopes" is not meant to be all-inclusive, it's just what I was able to come up with in the amount of time I allocated myself.
I'll try to find a decent topic another time to bring in more info on radioactivity, etc, with actual equations, etc. At some point I'll also post about ozone and the Montreal Protocol, which is the true international environmental success story.