examples:

Determine the decimal fraction of C-14 remaining:

6.00 / 13.6 = 0.4411765

How to solve the equation:

D = D0 + D* Therefore, D = D0 + N (e λ t – 1) or, for small λ t, D = D0 + N λ t , This is the basic radioactive decay equation used for determining ages of rocks, minerals and the isotopes themselves. D and N can be measured and λ has been experimentally determined for nearly all known unstable nuclides.

Radiometric dating calculates an age in years for geologic materials by measuring the presence of a short-life radioactive element, e.g., carbon-14, or a long-life radioactive element plus its decay product, e.g., potassium-14/argon-40.

this link shows a sample range chart.

How are fossils formed?

Fossils can form in several ways, but the basic method can be summarized in three steps. 1: the animal or plant must die. Unfortunately organisms do not live forever, and once they are dead they have the opportunity to become a fossil. Not everything that dies becomes a fossil, otherwise we would have dead animals and fossils over our heads!! Some of the dead organisms decompose and are recycled back into the environment. If a dead animal or plant is to become a fossil it must 2: become buried in sediment. Sometimes this happens fast (land/mudslide) and sometimes it happens slow (lake bottom). Not every dead animal or plant that becomes buried in sediment becomes a fossil though. Some are decomposed by bacteria, insects and worms and are recycled throughout the ecosystem. In order to become a fossil, buried animals must be protected from scavengers and decomposers. If they are protected, 3: lots of time has to pass before an animal can become fossilized. The soft parts of the animal usually rot away, leaving the hard bones that react with minerals in groundwater, turning the bones into stone.

What happens to an animal or plant that does not become a fossil?

The majority of animals do not become fossils. It is quite rare that an animal might be fossilized, which makes the fossils we do find very special. Several things may prevent an animal from becoming a fossil. Scavengers may consume the body of dead organisms. This could lead to the complete loss of the animal, or to scattered fossil bones. Some animals and plants rot away in oxygen rich environments where bacteria and fungus thrive. Animals that do not become fossils are still a part of the paleo-environment, and their nutrients are recycled throughout the ecosystem. Other plants and animals benefit from the source of food and nutrients dead animals provide. If every animal and plant became a fossil, there would be no more nutrients to cycle through the environment!

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Preservation Bias in the Fossil Record

What is a bias?

A bias occurs when we make a judgment about something based on our own personal experience. We may favor one state over another because we grew up in one and/or had a bad experience in the other. We may say our favorite mineral is quartz, so we have a bias for quartz when viewing or learning about other minerals. A bias is not always a bad thing, and in science it can help explain why some things occur more often than others

How does this relate to fossils?

Animals themselves do not have a bias, meaning they do not have a preference of becoming a fossil or not. However, the environmental conditions and the physical features of the animal/plant can increase or decrease the chance of an organism becoming a fossil. We call this the preservation bias. Certain environmental conditions (aquatic, low oxygen, quick burial) favor the preservation of organisms. Body features such as hard parts also help with preservation of organisms. Therefore, the fossil record has a bias toward animals that lived in a low oxygen environment and contained hard parts. There are more of them preserved than animals that lived on land and had only soft parts.

There are several biases that can help inform our research of the fossil record. We will discuss three biases that are most evident in the fossil record (and relate to Fossil Lake!)

Land or water? Animals that live in water are more likely to be fossilized because they are more easily buried in sediment.

Hard or soft parts? Organisms with soft parts are rarely preserved because the soft tissue rapidly decomposes. Hard parts such as bones and shells are more readily fossilized.

Observer bias. The paleontologist may favor one type of fossil over another, and looks exclusively for that type of fossil. The other fossils are present but are not collected.

How can we use the preservation bias to learn more about fossils?

In Fossil Lake, we know the depositional environment favored the preservation of aquatic animals like fish. Fish are the most common fossil in Fossil Lake (millions have been found!). We have also found several terrestrial (land) animals such as snakes, lizards and a horse (among others). These fossils are quite rare because they lived on land. But it is safe to assume that during the time of Fossil Lake, many of these animals roamed the shore and were not fossilized after they died. The preservation bias of a fossil location can help us better understand the abundance of ancient animals.

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Comparing radiocarbon dating methods

Dr Christine Prior is Team Leader of the Rafter Radiocarbon Laboratory at GNS Science. In this video, she compares conventional and accelerator mass spectrometry (AMS) radiocarbon dating. AMS is faster and needs a much smaller sample, but is more expensive. Also shown are views of bone preparation at the Waikato Radiocarbon Dating Laboratory.

Radiometric dating

Most absolute dates for rocks are obtained with radiometric methods. These use radioactive minerals in rocks as geological clocks.

The atoms of some chemical elements have different forms, called isotopes. These break down over time in a process scientists call radioactive decay. Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter. Each isotope is identified with what is called a ‘mass number’. When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.

Absolute dating methods

Absolute dating methods give rocks an actual date or date range in numbers of years. This interactive explores four different methods used in absolute dating.

Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element. These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.

Because of their unique decay rates, different elements are used for dating different age ranges. For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.

Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14. Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old. Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.

The table below shows characteristics of some common radiometric dating methods. Geologists choose a dating method that suits the materials available in their rocks. There are over 30 radiometric methods available.

Dating method

Material dated

Age range dated

Carbon-14 to nitrogen-14 (radiocarbon)

Organic remains, archaeological artefacts

Up to 60,000 years ago

Luminescence

Tephra, loess, lake sediments

Up to 100,000 years ago

Fission track

Tephra

10,000 to 400 million years ago

Potassium-40 to argon-40

Volcanic rocks

20,000 to 4.5 billion years ago

Uranium-238 to lead-206

Volcanic rocks

1 million to 4.5 billion years ago

Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock. Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in. However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.

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What is an isotope?

Dr Fiona Petchey explains what an isotope is, and then focuses on the isotopes of carbon and explains how the radioactive isotope carbon-14 is used in dating artefacts of historical importance.

All radiometric dating methods measure isotopes in some way. Most directly measure the amount of isotopes in rocks, using a mass spectrometer. Others measure the subatomic particles that are emitted as an isotope decays. Some measure the decay of isotopes more indirectly. For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes. Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.

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Dating rocks near Whanganui

Some of the relative and absolute methods of dating rocks near Whanganui are outlined by Dr Alan Beu of GNS Science. Fossil correlation is important as is the counting of climate cycles represented in the rocks. Fission track dating and paleomagnetism both provide absolute dates to tie the relative dating to.