Dating of fossils

Fig. 7 Lake Turkana in Kenya where several hominid fossils, such as Turkana Boy, have been discovered

Introduction

Relative Dating

Introduction

Fossils are remains of a plant, animal or human from the past that has been preserved in the earth or sea floor. Some fossils have been found on the surface while others require a significant amount of digging and excavating to reach. Lucy's fossils were found sticking out of a shallow stream bed in the Afar region of Ethiopia. Turkana Boy, discovered near Lake Turkana in Kenya, required careful digging and excavation to unearth. Turkana boy, also called Nariokotome boy, is one of the most complete hominin skeleton that has been found.

There are two ways of dating fossils - relative dating and absolute dating. Relative dating tells how old an object is relative to other objects. It cannot tell the precise date of the fossil. Absolute dating gives the exact (or near exact date allowing for errors) for a fossil. This page will provide an overview of this topic with focus on relative dating and radiometric methods. See the References for more detail.

Relative Dating

Relative dating uses the principle of Stratigraphy (stratum - layer, graphy - writing/description). It is the study of rock layers.

Most fossils are found in sedimentary rocks (like shale, limestone and sandstone) because they form at lower pressures and temperatures that can preserve fossils. Igneous and metamorphic rock form under high pressure and temperature that can destroy fossils. Sedimentary rocks are made of fine pieces of rock that have been transported by rain, wind or water and deposited in layers; hence the name sedimentary. Chert rock (in the form of flint) is a type of sedimentary rock that has been used by hominins to make tools or weapons.

Fig.8 Sedimentary Rock

The lowest layer of a sedimentary rock is the oldest while the surface of the rock is the youngest. In some cases, parts of a sedimentary rock can be broken up by erosion or some other natural phenomenon. Parts of a rock that are separated by a valley but looks similar will of the same age because they were formed at the same time.

In the photo on the left, the reddish layers at the bottom are siltstones. The brown layers at the top are limestone.

Photo courtesy - Mark A. Wilson (Department of Geology, The College of Wooster) via Wikipedia

Geologic Time Scales

Fig.9 Geologic Time Scales

Fossils have been used to date geological periods and vice versa. Briefly, the largest unit of geologic time is an eon. An eon consist of several era. An era is composed of several periods that are themselves composed of epochs. Epochs have several ages. We are currently in the Phanerozoic eon, Cenozoic era, the Quaternary period and the Holocene epoch. The Quaternary period started about 2.58 Mya. The Holocene epoch has been divided by the International Union of Geologic Sciences (IUGS) into the Greenlandian, Northgrippian and Meghalayan ages. The Meghalayan age started 4250 years ago when a mega drought devastated civilizations around the world. There is a theory that the Indus Valley Civilization declined due to this prolonged drought.

For reasons of clarity the figure above shows the scales that are relevant to this discussion. More details are available in Wikipedia.

Biostratigraphy

As the name suggests, biostratigraphy uses a large change in plants and animals fossils to identify a new geologic era, period, epoch or age. The ending of a geologic period is marked by a major change. For example, the Mesozoic era ended 66 Mya in a cataclysmic event caused by an asteroid that crashed into Earth. The crater in Chicxulub is a result of the impact. Called the Cretaceous-Paleogene extinction event, it is supposed to have caused the mass extinction of three-quarters of all plant and animal life, including dinosaurs, on Earth. This event is marked by a thin layer in rocks called the K-Pg boundary. This layer has high Iridium content. Iridium is more commonly found in asteroids than in the Earth's crust. This extinction event marked the end of the Mesozoic era and the beginning of the Cenozoic era. Thus any fossil that is below the K-Pg boundary in rocks belong to the Mesozoic era; those above the boundary belong to the Cenozoic era. The most famous fossils from the Mesozoic era are those of the dinosaurs.

(There is some disagreement if the asteroid impact caused the extinction of dinosaurs or if the dinosaurs were in decline when the event happened. The reason is that dinosaur fossils have been found well below the K-Pg layer. If the extinction happened due to the impact, they must be closer to the iridium layer).

Similarly, the Holocene epoch began with the end of the Last Glacial Period (LGP). The last phase of the LGP was the Younger Dryas where Earth's temperature fell by 3 to 10'c in the Northern Hemisphere. It marked the end of the Pleistocene and the beginning of the Holocene age.

The Meghalayan age started with a worldwide drought about 4250 years ago which devastated several civilizations.

Geologic periods, epochs and ages have fossils of specific species. In addition, index fossils are used. These are fossils of plants and animals that have a specific geologic time range and must be worldwide. For example, mammoths and saber toothed cats are considered index fossils of the Pleistocene epoch. Any fossil found with these index fossils can be dated to the same PeriodEpoch.

When Lucy was discovered, it was dated using biostratigraphy to be between 3.5 and 2.9 Mya. Later, absolute dating put her date at 3.18 Mya.

Magnetostratigraphy

The magnetic North pole points to the geographic North. (Today, the magnetic North pole is not at the geographic North pole but at 86N but that can be ignored for this discussion). There have been instances in the past when the location of the magnetic poles have reversed. The North pole will be near the geographic South pole and vice-versa. The most recent reversal happened about 780,000 years ago.

Iron rich magnetic minerals float in molten rock. These minerals align with the magnetic north. When the molten rock cools and solidifies, the orientation of the iron minerals is locked into place. Geologists found that some volcanic rocks were magnetized opposite to the direction of the Earth's field. Some of the rocks were estimated to be from the Pleistocene age (2.58 Mya - 11,700 years ago) or older.

Scientists study a long sequence of strata and look at the orientation of magnetic materials in the strata. If any layer shows iron minerals aligned to the current geographic south and if the date of that flip is known and there are fossils in that layer, the fossils and the layer can be dated. Australopithecus sediba was dated using this method to 1.98 Mya in the early Plesitocene epoch.

To summarize -

Study the orientation of iron minerals in multiple layers of strata
Note the magnetic reversal of the orientation of the minerals in one or more layers
Compare with the geologic record to obtain a date for the reversal
Fossils in that layer can then be dated to that period

Tephrostratigraphy

Tephra refers to the products of volcanic eruptions. These include lava, ash, volcanic rocks etc. Volcanic ash can be dated using absolute dating. Fossils in layers above the ash are younger than the volcanic ash; those below the ash are older.

Absolute Dating

Absolute dating provides a specific age (or a narrow range) for fossils. Absolute dating is the preferred method to date fossils but not all fossils can be dated using this method.

Absolute dating uses a variety of methods to date fossils. They are of two types - Radiometric and Non-Radiometric dating.

Radiometric dating is based on the principle that certain elements like carbon have isotopes that are radioactive. Measuring the relative abundance of carbon and its isotope can provide a date for the fossil.

The next few sections will explain the concepts of isotopes and radioactivity. Readers who have a background in Physics or Chemistry can skip these sections

Isotopes

Fig.10 Carbon atom

Image courtesy SrKellyOP, CC0, via Wikimedia Commons via Wikipedia

Every element is made of atoms. Atoms have a nucleus with protons and neutrons. Electrons orbit the nucleus. Protons are positively charged. Electrons are negatively charged. Since they are equal in number, at atom is electrically neutral. Neutrons have no charge.

Every element has an atomic number and an atomic weight. The atomic number is the number of protons (or electrons) in an atom. The atomic weight is the sum of the protons and neutrons. A carbon atom has 6 protons and 6 electrons and 6 neutrons. Hence its atomic number is 6 and its atomic weight is 12.

Every element has a symbol. The symbol for carbon is C. The atomic number and atomic weight of an element are shown as a subscript and superscript respectively. Thus carbon is represented as 6C12

Some elements have a variable number of neutrons. Some carbon atoms have 7 neutrons and others have 8 neutrons. Therefore the atomic weight of carbon can be 12 (6 protons + 6 neutrons), 13 (6 protons + 7 neutrons) or 14 (6 protons + 8 neutrons). Carbon with an atomic weight of 13 or 14 are called isotopes of carbon.

The isotopes of carbon are represented as 6C13 and 6C14

6C12 and 6C13 are stable. 6C14 is radioactive. Carbon-12 accounts for ~99.8 % of all carbon atoms, carbon-13 accounts for ~1% of carbon atoms while ~1 in every 1 billion carbon atoms is carbon-14.

Radioactivity

Radioactivity or radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting particles in the form of radiation. The nucleus is then transformed into that of another element.

Three types of radioactive decay are known. Alpha, Beta and Gamma decay. In alpha decay, the atom emits two neutrons and two protons (called an alpha particle). Since the element Helium has 2 protons and 2 neutrons in the nucleus, an alpha particle is the same as Helium nucleus. In beta decay, a neutron is transformed into a proton and an electron. In gamma decay, photons (which are particles of radiation) are emitted by the element. The Wikipedia page on radioactive decay is an excellent source for understanding this topic

Courtesy Inductiveload, Public domain, via Wikimedia Commons
Fig.11 Alpha decay

In alpha decay, the nucleus of the atom emits 2 protons and 2 neutrons. This will reduce the atomic number and atomic weight of the element by 2 and 4 respectively.

Courtesy - NOAA
Fig.12 Beta decay

In Beta decay, a neutron is transformed into a proton, an electron and a anti-neutrino.

Courtesy By Inductiveload - Own work (Original text: self-made), Public Domain, via Wikimedia
Fig.13 Gamma decay

No charged particles are emitted. Instead a high energy electromagnetic radiation (called a photon) is emitted

Radiometric dating

Radiocarbon dating

There is one to two 6C14 atom for every 10^9atoms of 6C12 atoms in the atmosphere. They are formed in the atmosphere by the interaction between cosmic rays (99% of which are protons) and Nitrogen atoms. Cosmic rays knocks a proton out of Nitrogen resulting in 6C14. 6C14 atoms oxidize to form 6C14O2 - carbon dioxide with C-14 (instead of the usual C-12). Plants absorb this carbon dioxide (just like the regular carbon dioxide) during photosynthesis. Animals (and humans) that eat the plants absorb C-14 in this manner. When a living organism dies, the absorption of C-14 stops. C-14 being radioactive decays with a half life of 5730 years. By measuring the amount of C-14 in a fossil, it is possible to determine when the organism died.

Note: A half life of t units (seconds / minutes / years) means that half the atoms will decay into another atom after t units. Since C-14 has a half life of 5730 years, half the C-14 atoms in a sample will decay into Nitrogen by a process called beta decay. See the figure above.

If a sample of carbon has 100 units (atoms or weight) of C-14, after 5730 years, it will reduce to 50 units. After 11,460 years, it will reduce to 25 units. By measuring the amount of C-14 in a fossil, its age can be calculated in terms of years before present.
Radiocarbon dating can be used to date fossils up to 50,000 years old. Beyond that the amount of C-14 in the sample is too small to measure.

Potassium- Argon dating

Potassium-Argon dating (K-Ar dating) can be used to measure the age of rocks more than 100,000 years old. Potassium (symbol K) has 3 isotopes - K39, K40 and K41.. K39 and K41 are stable. K40 is radioactive.

K40 has a half life of 1.248 x 109 years and decays into Calcium-40 via beta decay 89.3% of the time and Argon-40 via electron-capture 10.7% of the time.

In a manner similar to radiocarbon dating, K-Ar dating can be used to get an estimate of the age of a fossil. Measuring the Ar40 in rock layers above and below the fossil will yield an estimate for the age of the fossil.

Argon-Argon dating

Ar-Ar dating is another method for dating. This involves converting stable K39 to radioactive Ar39 using neutrons from a nuclear reactor. Details are available here.

Potassium-Argon dating can be used to date rocks more than 4 billion years old. Argon-Argon dating can be used to measure rocks younger than that. However Argon-Argon dating is more accurate. The disadvantage is the requirement for a neutron source.

This method was used to limit the age range of the Lucy and Turkana boy fossils.

The Smithsonian page on dating has details on the above and other methods of absolute dating.

The figure below shows the various methods for dating fossils

Fig.14 The various methods of dating fossils

References

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