How do paleontologists know how old fossils are?

Answer 1

Paleontologists deal with two types of dating, absolute and relative. Absolute dating, which estimates the age of a rock or fossil in years, is usually done by measuring the amounts of a radioactive isotope and its decay product; since isotope decay rates are known to be constant, the age can be calculated from the relative amounts of parent isotope to daughter product. Fossils up to about 40,000 years old can be dated using carbon-14 if there is enough organic matter present. Older rocks can be dated using potassium-40, which decays to argon-40, or uranium-235, which decays to lead-207. Many sedimentary rocks cannot be dated directly by these methods; dates usually are obtained from igneous rocks within a sedimentary sequence, such as lava flows or ash beds. Such dates are maximum age estimates for fossils above the dated beds, or minimum estimates for fossils below the beds. Relative dating has been practiced for nearly 200 years, arising from the observation that different layers of sedimentary rock contain different fossils, and that this sequence can be recognized in other rocks at other localities, even those far away. This allows fossil-bearing rocks to be dated relatively; on the basis of its fossils a rock might be placed in, say, the Ordovician Period, which followed the Cambrian Period and was followed by the Silurian Period. This technique does not depend on knowing the actual numerical ages of the rocks. Not all fossils are equally useful for relative dating, or correlation; some are rare, restricted to small geographic areas or to particular environments, difficult to recognize, or have such long ranges as to make precise correlation impossible. Fossils that are the most useful for correlation tend to be widespread, found in many rock types, easily recognizable, and short-lived enough to permit precise placement in the geologic column.

Answer 2

The dating of fossils was originally done by their placement in a geologic column of rock strata. Because of the Law of Superposition, the deeper the stratum, the older it is (in an undisturbed body of rock or sediment). Fossils which were evident in a particular stratum, but missing from other strata were noted as "index" fossils. The presence of these fossils indicated the age of the stratum relative to other layers. The layers of stratum were placed in a geologic column, subdivided, and assigned names based on the presence of different types of fossil organisms.This is the basis of "relative dating" of rock layers and fossils, a dating technique that was in effect until the advent of radiometric techniques in the twentieth century.

Answer 3

Scientists use two types of dating: Relative Dating, and Absolute Dating.

Relative Dating is determining which rock units are older or younger. Basically, the lower down into the Earth you go, the older and older the rocks will get. It provides a sequence of age. Such as 'this fossil was made before this fossil and after this fossil.'

"The proper chronological placement of a feature, object, or happening in the geologic time scale without reference to its absolute age."

Absolute Dating is more accurate. It is the process of determining a specific date for an object. It is usually based on the chemical properties of the object, in this case a fossil. It provides a numerical age. Such as 'this fossil was made in 1923.'

"The determination of the age of an object with reference to a specific time scale, such as a fixed calendar or in years before present (BP), based on measurable physical or chemical qualities or associations with written records."

Information from:

http://www.dictionary.com

RELATIVE DATING

Answer 4

Fossils are dated based on the layer of rock they are found in. They can be dated relatively, by comparing them to the ages of layers that are above and below. The rock layer could be dated by using radiometric dating. Also, index fossils (widespread organisms that existed for a short period of time) can be used to date a rock layer.

Answer 5

answer 2 The age of fossils would ordinarily be determined by looking up the type of fossil in a record system known as Paleontology. Due to various changes in the Earth's climate and environment, most fossils have only a limited age range.

The reasons for the limited age range for various fossils is incompletely known, but the general pattern is well known.

A second approach is based on the idea that the sediment layers near the top of a earth column are younger than those at the bottom. This was the only approach available to early fossil hunters. It has the fancy name of "the principle of superposition" - i.e. things at the top are younger!

A third approach is to consider dating using radioactive techniques. Carbon dating is used for this, but for technical reasons samples older than 60 000 years may not produce reliable results.

So radiocarbon (as it is known) is very useful for Archaeology, but not much older.

Some of the information below (from another contributor) is wrong, as C12 and C13 are naturally occurring items, and are NOT radioactive.

The age of a fossil can be told by a technique called carbon dating. wherein the scientists measure the ratio of the heavy isotope of carbon ( 6C13) to carbon ( 6C12 ) present in the fossil.

Answer 6

Scientists use 'carbon dating' to approximate the age of fossils. Carbon is known to deteriorate at a predictable rate, therefore, examining the carbon degeneration at a microscopic level on any given fossil will give you a pretty good idea of how old said fossil will be.

Answer:
There are several options:

• Carbon dating - the amount of carbon 14 compared to carbon 12 in the specimen changes with time. The ratio can be determined and the date calibrated.
• The geologic strata (rock layer) can be examined. The rocks are laid down atknwn periods and can gie an indication when the fossils were embedded
• The associate fssils with the fossile oua re intersested in can indicate the period of deposition
• Evidence of glacial action can date the rocks
• Magnetic directions in the iron componebts of assoicated rocks can indicate time
  

Answer 7

Any living organism has a carbon content. The life time of carbon is 5700. They will compare with the original (metal) organism. Based on the carbon content,they can determine the age of the fossils.

Answer 8

Paleontologists know how old fossils are because the rule of Nicholaus Steno said that the rocks in the bottom are the oldest one the rocks in the top are the youngest ones so that means that is the fossil is the the very bottom the fossil will be old and the fossil on the top will be young.