Dating Lucy
Archaeologists make use of various dating techniques to establish the age of an artifact or fossil. Some of these techniques provide so-called absolute dates, others only relative dates.
Relative dating technique
Typically an excavation results in a square or rectangular pit. The walls of the pit often will show differently colored layers of soil. These layers, or “strata” were deposited on top of each other in a sequence. In an ideal situation, we have a layer cake effect, with the oldest layer at the bottom of the pit and the most recent on top. Objects found in these layers can be dated in comparison (or in relation) to each other, thus the name “relative dating technique". An object retrieved from the top layer is considered to be younger than an object found in the bottom layer.
This technique is neither useful nor applicable to hominid fossils. For starters, we do not get an absolute date, and fossils tend to be found on the surface, rather than as the result of digging an excavation pit.
Absolute dating techniques
An absolute dating technique uses a fixed point in time to indicate the number of years that separate an object from that fixed point in time. Most Western societies refer to the birth of Christ as their preferred fixed point in time. We express this with “A.D.” or “B.C”, or – more recently – “C.E.” and B.C.E.” Either way, these abbreviations refer to the birth of Christ. The eruption of Mount Vesuvius occurred in 79 A.D.; Lucy was found in 1974 A.D.
There are several absolute dating techniques used by archaeologists. None of these techniques is a magical bullet, all have limitations and restrictions.
Tree ring dating (dendrochronology)
Scientists establish a sequence of tree rings using overlapping wood samples.
In the American Southwest bristlecone pine chronologies now extend 8,500 years. Work done in Germany and Northern Ireland has expanded the European oak and pine chronologies to over 11,000 years ago.
- All samples that are compared need to be from the same region.
- Does not allow dating fossils.
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Radiocarbon dating (C14 dating)
Radio-carbon dating is a method of obtaining age estimates on organic materials. It has been used to date samples as old as 50,000 years. The method was developed following WW II by Willard F. Libby and coworkers. It has provided age determinations in archaeology, geology, geophysics and other branches of science. Radiocarbon determinations can be obtained on wood; charcoal; marine and fresh-water shell; bone and antler; peat and organic-bearing sediments, carbonate deposits such as tufa, caliche, and marl, and dissolved carbon dioxide and carbonates in water.
Radioactive carbon, produced when nitrogen 14 is bombarded by cosmic rays in the atmosphere, drifts down to earth and is absorbed from the air by plants. Animals eat the plants and take C14 into their bodies. Humans in turn take carbon 14 into their bodies by eating both plants and animals. When a living organism dies, it stops absorbing C14 and the C14 that is already in the object begins to disintegrate. Scientists can use this fact to measure how much C14 has disintegrated and how much is left in the object. Carbon 14 decays at a slow but steady rate and reverts to nitrogen 14. The rate at which Carbon decays (Half-life) is known: C14 has a half-life of 5730 years. This means that half of the original amount of C14 in organic matter will have disintegrated 5730 years after the organism’s death; half of the remaining C14 will have disintegrated after another 5730 years and so forth. After about 50,000 years, the amount of C14 remaining will be so small that the fossil can't be dated reliably.
To discover how long an organism has been dead (to determine how much C14 is left in the organism and therefore how old it is), we count the number of beta radiations given off per minute per gram of material. Modern C14 emits about 15 beta radiations per minute per gram of material, but C14 that is 5730 years old will only emit half that amount, (the half-life of C14) per minute. If a sample taken from an organism emits 7.5 radiations per minute in a gram of material, then the organism must be 5730 years old. The accuracy of radiocoarbon dating was tested on objects with dates that were already known through historical records such as parts of the Dead Sea scrolls and some wood from an Egyptian tomb. Based on the results of the Carbon 14 test the analysis showed that C14 agreed very closely with the historical information.
Each sample type has specific problems associated with its use for dating purposes, including contamination and special environmental effects. While the impact of radiocarbon dating has been most profound in archaeological research and particularly in prehistoric studies, extremely significant contributions have also been made in hydrology and oceanography.
Most of mankind’s evolution occurred prior to 50,000 years ago. Radiocarbon dating does not apply to Lucy.
(Courtesy: emuseum at Minnesota State University, Mankato.)
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Potassium Argon dating (K-Ar dating)
This technique is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40), decays to the gas Argon as Argon-40 (Ar-40). By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.
Geologists have used this method to date rocks as much as 4 billion years old. Potassium-argon dating is accurate from 4.3 billion years (the age of the Earth) to about 100,000 years before the present.
Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass). One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40). These each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon-40 (Ar-40), an inert gas. For every 100 K-40 atoms that decay, 11 become Ar-40.
- The technique works well for almost any igneous or volcanic rock, provided that the rock gives no evidence of having gone through a heating-recrystallization process after its initial formation. For this reason, only trained geologists should collect the samples in the field.
- This technique is most useful to archaeologists and paleoanthropologists when layers with evidence of human presence are sandwiched between layers of lava or volcanic tuffs. This provides dates bracketing the evidence of human presence. We do not directly date the artifacts or fossil bones. Because of this, it is critical that the association between the igneous/volcanic beds being dated and the layers containing human evidence is very carefully established.
- The resolution of the K-Ar technique is not a fine as radio carbon.
(Courtesy University of California, Santa Barbara.)
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Argon – Argon dating (40Ar-39Ar)
A variant of the K-Ar method gives better data by making the overall measurement process simpler. The key is to put the sample in a neutron beam, which converts potassium-39 into argon-39. Because 39Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, thus it makes a clean indicator of the potassium content. The advantage is that all the information needed for dating the sample comes from the same argon measurement. Accuracy is greater and errors are lower. This method is commonly called "argon-argon dating."
The analysis of the data is more complex than in the K-Ar method, because the irradiation creates argon atoms from other isotopes beside 40K. These effects must be corrected, and the process is intricate enough to require computers.
The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method. Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.
These dating methods have been under constant improvement for more than 50 years. The learning curve has been long and is far from over today. With each increment in quality, more subtle sources of error have been found and taken into account. Good materials and skilled hands can yield ages that are certain to within 1 percent, even in rocks only 10,000 years old, in which quantities of 40Ar are vanishingly small.
Source:http://geology.about.com/od/geotime_dating/a/K_argon_dating.htm
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