δ13C

Everything has tiny bits of carbon. Some bits are a little heavier than others. Scientists look at these bits to learn about our world. They can tell if carbon comes from plants or from fuel. Plants pick out the lighter bits when they grow. This helps us see how much life was in the past. We can even see how much carbon is buried in rocks. These tiny bits tell a big story about the Earth.

Carbon is found in almost everything. It comes in two main types. One type is light. The other is a bit heavier. We call these types isotopes.

Scientists study the ratio of these isotopes. They use a scale called delta carbon thirteen. This scale helps them see where carbon comes from. For example, it can show if carbon came from plants. It can also show if it came from fossil fuels.

Plants use a way called isotopic fractionation. This means they pick one type of carbon over the other. This makes plants easy to spot in old rocks.

Scientists also use special standards to stay accurate. One old standard was a fossil from South Carolina. Now, they often use a standard called VPDB.

These tiny bits tell a big story. They can show how much life lived long ago. They can even show when many plants were buried in the ground. This helps us learn about the history of our Earth.

Scientists use a special tool to study the history of our Earth. This tool is called delta carbon thirteen, or δ13C. It measures the ratio between two types of carbon. These two types are called isotopes. One isotope is C12 and is lighter. The other is C13 and is a bit heavier. By looking at these ratios, experts can learn a lot about the past.

This measurement works because different things have different amounts of each isotope. For example, plants use a process called isotopic fractionation. This means they pick one isotope over the other as they grow. Because of this, plants have a different signature than other things. We can even tell the difference between plants and fossil fuels. This happens because the carbon in fuels comes from very old organic matter.

In the past, scientists used a very specific fossil to set their scale. This fossil was called Pee Dee Belemnite, or PDB. It came from a marine creature named Belemnitella americana. This creature lived in the Cretaceous period in South Carolina. The PDB was used as a standard to make sure measurements were right. Eventually, the supply of these fossils ran out. Now, scientists use a new standard called VPDB.

There are many important numbers in this science. The ratio for the VPDB standard is 0.011113. Scientists also use a material called NBS-19 to help their work. This material has a δCVPDB value of 1.95‰. Different types of methane also have their own numbers. Biogenic methane has a signature of −60‰. Thermogenic methane has a signature of −40‰.

These tiny numbers tell a huge story about our planet. Changes in these ratios show when life was very busy. For example, the Lomagundi-Jatuli event happened long ago. It was a positive excursion between 2,300 and 2,080 million years ago. We also see a negative excursion called the Shuram-Wonoka event. These events show how the Earth's carbon moved through the air and sea.

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.