21.4.4: Carbon Dating and Estimating Fossil Age
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Learning Objectives
- Summarize the available methods for dating fossils
Determining Fossil Ages
Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages. There are several different methods for estimating the ages of fossils, including:
- stratigraphy
- biostratigraphy
- carbon dating
Stratigraphy
Paleontologists rely on stratigraphy to date fossils. Stratigraphy is the science of understanding the strata, or layers, that form the sedimentary record. Strata are differentiated from each other by their different colors or compositions and are exposed in cliffs, quarries, and river banks. These rocks normally form relatively horizontal, parallel layers, with younger layers forming on top.
If a fossil is found between two layers of rock whose ages are known, the fossil’s age is thought to be between those two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is difficult to match up rock beds that are not directly adjacent.
Biostratigraphy
Fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called biostratigraphy. For instance, the extinct chordate Eoplacognathus pseudoplanus is thought to have existed during a short range in the Middle Ordovician period. If rocks of unknown age have traces of E. pseudoplanus , they have a mid-Ordovician age. Such index fossils must be distinctive, globally distributed, and occupy a short time range to be useful. Misleading results can occur if the index fossils are incorrectly dated.
Relative Dating
Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying evolution. This is difficult for some time periods, however, because of the barriers involved in matching rocks of the same age across continents. Family-tree relationships can help to narrow down the date when lineages first appeared. For example, if fossils of B date to X million years ago and the calculated “family tree” says A was an ancestor of B, then A must have evolved earlier.
It is also possible to estimate how long ago two living branches of a family tree diverged by assuming that DNA mutations accumulate at a constant rate. However, these “molecular clocks” are sometimes inaccurate and provide only approximate timing. For example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different approaches to this method may vary as well.
Carbon Dating
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating (” radiocarbon dating ” or simply “carbon dating”). The principle of radiocarbon dating is simple: the rates at which various radioactive elements decay are known, and the ratio of the radioactive element to its decay products shows how long the radioactive element has existed in the rock. This rate is represented by the half-life, which is the time it takes for half of a sample to decay.
The half-life of carbon-14 is 5,730 years, so carbon dating is only relevant for dating fossils less than 60,000 years old. Radioactive elements are common only in rocks with a volcanic origin, so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers. Carbon dating uses the decay of carbon-14 to estimate the age of organic materials, such as wood and leather.
Key Points
- Determining the ages of fossils is an important step in mapping out how life evolved across geologic time.
- The study of stratigraphy enables scientists to determine the age of a fossil if they know the age of layers of rock that surround it.
- Biostratigraphy enables scientists to match rocks with particular fossils to other rocks with those fossils to determine age.
- Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle is the difficulty of working out fossil ages.
- Scientists use carbon dating when determining the age of fossils that are less than 60,000 years old, and that are composed of organic materials such as wood or leather.
Key Terms
- half-life : The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.
- stratigraphy : The study of rock layers and the layering process.
- radiocarbon dating : A method of estimating the age of an artifact or biological vestige based on the relative amounts of various isotopes of carbon present in a sample.