δ13C

In the fields of geochemistry, archaeology, and paleoclimatology, scientists use a measurement called δ13C. This is pronounced "delta carbon thirteen." It is a normalized ratio of the two stable isotopes of carbon. These isotopes are C12 and C13. The measurement is reported in parts per thousand, which is also called per mille (‰). This ratio is important because it acts like a fingerprint for the Earth. It helps researchers understand how carbon moves through the air, the water, and living things.

To understand how this works, we must look at a process called isotopic fractionation. This occurs when biological or chemical processes prefer one isotope over another. For example, during photosynthesis, plants select specific isotopes to build their tissues. Because of this, different plants can have different isotopic signatures even if they grow in the same place. This fractionation also helps us tell different carbon sources apart. We can distinguish carbon dioxide from modern plants from carbon dioxide made by burning fossil fuels. This specific distinction is known as the "Suess Effect."

Scientists must be very careful to keep their measurements accurate. They do this by comparing their samples to a reference standard. The first major standard was called Pee Dee Belemnite, or PDB. This was a Cretaceous marine fossil named Belemnitella americana. It came from the Peedee Formation in South Carolina. This fossil had a very high C12/C13 ratio of 0.0112372. Because of this specific value, it was set as the zero point for the δ13C scale.

Over time, the supply of the PDB standard was exhausted because it was in such high demand. Scientists had to find new ways to standardize their work. A new standard called VPDB, or "Vienna PDB," was established to replace it. The International Atomic Energy Agency defines the VPDB ratio as 0.011113. Because different standards use different scales, scientists must use subscripts to avoid confusion. They might write δCPDB or δCVPDB to show which scale they used. To save the VPDB supply, labs often use a secondary material called NBS-19. This material has a δCVPDB value of 1.95‰.

Variations in the δ13C ratio can tell us about the history of our atmosphere and oceans. Methane is a great example of this. Methane has a very "light" signature, meaning it has a low value. Biogenic methane has a signature of −60‰, while thermogenic methane is −40‰. Large releases of methane clathrates can change global values. One such event happened during the Paleocene–Eocene Thermal Maximum. Other changes come from primary productivity, which is how much life is being produced in an ecosystem. When more organic carbon is buried in sediments, the δ13C values tend to rise.

We can also use these ratios to study different types of plants from the past. C3 plants and C4 plants have very different signatures. C3 plants typically have a δ13C range of −33 to −24‰. In contrast, C4 plants have a range of −16 to −10‰. By looking at fossils, scientists can detect when C4 grasses became more abundant in the environment. We also see "excursions," which are large swings in the carbon ratio. A positive excursion often means there was more organic carbon being buried in rocks. For example, the evolution of large land plants during the late Devonian caused a rise in δ13C.

History is written in these carbon shifts through many major events. Some of these happened billions of years ago, such as the Lomagundi-Jatuli event. This was a positive excursion between 2,300 and 2,080 million years ago. Other events, like the Shuram-Wonoka excursion, were negative excursions. In the Paleozoic era, we see several positive events like the Ireviken and Mulde events. Even more recent events, like the Cenomanian-Turonian boundary event, show how the Earth's chemistry changes. These numbers help us connect the history of life to the history of the planet itself.