How can fossils be accurately dated

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. Scientists use carbon dating when determining the age of fossils that are less than 60, 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. Determining Fossil Ages Paleontology seeks to map out how life evolved across geologic time. There are several different methods for estimating the ages of fossils, including: stratigraphy biostratigraphy carbon dating. Stratigraphy Paleontologists rely on stratigraphy to date fossils.

The deeper layers are older than the layers found at the top, which aids in determining the relative age of fossils found within the strata.

Biostratigraphy Fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called biostratigraphy. These are:. Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.

Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.

A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age. This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples. Uranium is present in many different rocks and minerals, usually in the form of uranium This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split — a process known as fission.

During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral. This damage is in the form of tiny marks called fission tracks. When volcanic rocks and minerals are formed, they do not contain fission tracks. The number of tracks increases over time at a rate that depends on the uranium content. It is possible to calculate the age of a sample by measuring the uranium content and the density of the fission tracks. The age of volcanic rocks and ash can be determined by measuring the proportions of argon in the form of argon and radioactive potassium within them.

Each volcanic eruption produces a new deposit of ash and rock. Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock. An object can be given an approximate date by dating the volcanic layers occurring above and below the object. Argon is gas that gradually builds up within rocks from the decay of radioactive potassium. The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere.

Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption. This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.

The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted. Bones of ancient humans, our hominin ancestors, and other animal species were buried in the sediment, and eventually became fossilized and preserved in the rocks.

In the Koobi Fora formation, bands of sedimentary rock are interspersed with layers of tuff , a sign of times when tectonic and volcanic activity dominated the landscape. Typically, the ash, pumice , and other materials that spew from volcanoes either fall straight back to the earth, or are carried away by air currents or rivers and streams. This volcanic matter eventually settles and over time is compacted to form a special type of sedimentary rock called tuff.

Tectonic activity has had other impacts on research in the Koobi Fora region. During the Pliocene geologic epoch 5. This allowed for erosional forces to expose rock that was buried long ago. These processes also exposed the fossils buried within those layers of rock.

The layers of volcanic rock are extremely important to reconstructing the history of the Turkana Basin because they allow scientists to calculate the age of hominin fossils found in the region.

The volcanic material in tuff is well-suited for radiometric dating , which uses known decay rates for specific unstable isotopes to determine the age of the rock that contains that isotope. Feldspar crystals found in the tuff layers contain an unstable isotope of potassium that can be used for this dating method.

The field of archeology often uses carbon isotopes, which are much more common, but the field of paleontology often uses a potassium- argon dating technique because it can be used to date much older rock material. Over time, the unstable potassium isotope 40 K from the rocks decays into a stable isotope of argon 40 Ar. The ratio of the stable argon isotope formed from decay to the unstable potassium isotopes tells scientists when the tuff layer cooled and solidified into rock.

Knowing the dates of the tuff, scientists can then estimate a date for the fossils. Fossils above a specific layer are inferred to be younger than that layer, and those below are older, in line with the law of superposition, a key scientific principle of stratigraphy. Dating of the fossils contributes to a clearer timeline of evolutionary history. Older methods of dating were more subjective, often an educated hypothesis based on the evidence available.

However, the fossils in the Turkana region can be dated more accurately because they are found in the sedimentary rock between datable layers of tuff. Although radiometric dating of the tuff is scientifically valid, difficulties still exist. Extending the Learning The fossils found in the Turkana Basin support the theory of human evolution and the theory that humans originated in Africa before migrating to other places.

Consider the age and different species of fossils found in the area. Using your knowledge of evolutionary theory, construct an argument that explains these connections. Examine the stratigraphic column diagram.

What relationship seems to exist between the approximate age of the fossils and rocks layers and their depth in the earth? Following the law of superposition in geology, older fossils and rocks are found in lower strata than younger fossils and rock layers. Tectonic activity left some areas of land uplifted , and erosional forces from the lake, nearby rivers, and other forms of weathering exposed rock, even older rock layers, as outcroppings in the landscape.

This made the fossils easier for researchers to find. The volcanic material in tuff layers also makes it possible to get a more accurate date for the fossils. Potassium-argon dating is a form of isotopic dating commonly used in paleontology.

Scientists use the known natural decay rates for isotopes of potassium and argon to find the date of the rocks. The radioactive isotope converts to a more stable isotope over time, in this case decaying from potassium to argon.

If scientists find the ratio of potassium to argon, it tells them how long the rocks have been around by how long the isotopes have been decaying. By understanding the dates of these rocks, scientists can deduce the age of the nearby fossils. What difficulties might paleontologists and archaeologists have when trying to find and date fossils? The amount of carbon 14 in a dead organism decays exponentially, falling to one half of its initial value after about 5, years.

Using an accelerator mass spectrometer, researchers can readily measure the radiocarbon in a sample. The trickier task is estimating how much of it should have been present in the environment when the organism was alive, which can then serve as a baseline for comparison. The Northern Hemisphere, Southern Hemisphere and marine environments all have separate calibration curves, Capriles says.

To achieve the most precise dating, he and other archaeologists also consider factors that cause local variations in atmospheric radiocarbon. Capriles studies the earliest occupants of South America, who arrived from the north and began dispersing throughout the continent around 15, years ago. What was their intensity of occupation?

But at sites older than about 50, years, almost all of the carbon 14 in a dead organism has already decayed, so researchers must turn to longer-lived elements. Over the course of millions of years, uranium and uranium , for example, undergo multistep decays to isotopes of lead, making them ideal for paleontology: researchers can determine the age of a sample by measuring the ratio of lead to uranium isotopes.

But using this technique to date fossils from creatures that lived millions of years ago, such as dinosaurs, is far from straightforward. Cloud State University, who studies fish evolution about million to million years ago, during the early Mesozoic era. Fossils form through various processes, the most common of which is called permineralization.

When a deceased organism is buried, permineralization can preserve its hard parts, such as bones.