Carbon dating is a powerful tool for determining the age of ancient materials, but its applicability to fossils is limited by two major scientific barriers: the stability of the carbon isotope used and the physical process through which fossils form. A fossil is the preserved remnant of a prehistoric organism, but the material making up that remnant is fundamentally different from the living tissue that can be carbon dated. The vast majority of fossils are far too old and composed of the wrong type of material for the carbon dating method to yield meaningful results.
The Time Limit of Carbon Dating
The technique of radiocarbon dating relies on the radioactive isotope carbon-14 ($\text{C-14}$). This isotope is created naturally in the atmosphere and absorbed by all living organisms, maintaining a consistent ratio with stable carbon-12 ($\text{C-12}$). Once an organism dies, it stops taking in new carbon, and the unstable $\text{C-14}$ begins to decay into nitrogen-14 ($\text{N-14}$) at a predictable rate. The time it takes for half of the $\text{C-14}$ to decay is its half-life, which is approximately 5,730 years.
This decay rate means the amount of $\text{C-14}$ shrinks exponentially over time. After about ten half-lives, or roughly 57,300 years, the remaining quantity of $\text{C-14}$ becomes scientifically undetectable. Since most fossils are millions of years old, having formed during the Mesozoic or Paleozoic Eras, they fall far outside this 50,000 to 60,000-year window. Even if a 100-million-year-old dinosaur bone contained its original carbon, the remaining $\text{C-14}$ would be too small to measure reliably.
The Mineral Nature of Fossils
A fundamental material barrier prevents the carbon dating of most fossils. Fossilization typically involves permineralization, where the original organic material is replaced by inorganic minerals. When remains are buried in sediment, mineral-rich groundwater seeps into the porous structures. Over time, minerals such as silica, calcite, or iron compounds precipitate out of the water and fill the empty spaces.
As the original organic material decays, the minerals fully crystallize, replacing the structure cell by cell. This replacement means a fossil is no longer composed of carbon-based tissues but is instead composed of rock. The carbon atoms, including the radioactive $\text{C-14}$ necessary for dating, are entirely removed and replaced by mineral compounds. Since the fossil is a mineral cast or petrified stone, attempting to carbon date it would only yield the age of the mineral that replaced the original tissue.
Dating the Rock Layers Around Fossils
The age of most fossils is determined not by dating the remains directly, but by dating the igneous or volcanic rock layers that surround them. Paleontologists use absolute dating methods, which rely on radioactive isotopes with much longer half-lives than $\text{C-14}$. These techniques, such as Potassium-Argon ($\text{K-Ar}$) dating and Uranium-Lead ($\text{U-Pb}$) dating, are capable of accurately measuring ages in the millions and billions of years.
Potassium-Argon dating measures the decay of potassium-40 ($\text{K-40}$) into argon-40 ($\text{Ar-40}$), a process with a half-life of 1.3 billion years. This method is useful for dating volcanic ash layers, known as tuffs, that are interbedded with fossil-bearing sedimentary rock. Uranium-Lead dating, which has a half-life extending to 4.5 billion years, is used on minerals like zircon found in igneous rocks.
The fossil’s age is established through “bracketing,” using the dated volcanic layers above and below the fossil-bearing strata. For example, if an ash layer dated to 66 million years ago is above the fossil and a layer dated to 68 million years ago is below it, the fossil’s age is constrained between those two dates. This indirect method provides an accurate chronological context for ancient life forms too old for radiocarbon analysis.
Dating Organic Remains Versus Fossils
The distinction between a mineralized fossil and an organic remain is important for understanding the applicability of $\text{C-14}$ dating. While the process cannot be used on a dinosaur fossil from the Mesozoic Era, it is the standard technique for dating organic materials that have not undergone complete mineralization. This includes items like ancient wood, charcoal, textiles, and unmineralized bone collagen from organisms that died relatively recently.
These remains must be younger than the 60,000-year $\text{C-14}$ limit. This makes the technique highly valuable for dating materials from the last Ice Age and archaeological sites tied to human history. For example, a woolly mammoth bone from 20,000 years ago can be dated using $\text{C-14}$, but a 200-million-year-old petrified tree cannot. Carbon dating is a powerful tool for relatively recent prehistory, while other radiometric methods are necessary for the deep time of the fossil record.