Monday, 12 December 2011

Radiometric Dating versus Relative Dating

In palaeontology and archaeology, it becomes necessary to determine the age of an artifact or fossil when it is uncovered. This, of course, is so that it can be properly catalogued, and, if valid, can be related to or associated with other objects from the same era. However, such a task can be quite tricky. Fossils and artifacts don't come with labels attached that clearly state their age. Therefore, scientists need to make use of proper techniques to adequately specify what the age of a fossil or artifact is. Two of the most well-known and most frequently used include radioactive dating and relative dating.

Radiometric Dating

Phrased simply, radioactive dating is the method that uses measurements relating to the radioactivity of the atoms in a fossil or an artifact. How is this done? All things decay. Organic bodies, such as you and me, as well as inanimate objects, such as stone tablets or rocks.
What "decay" means is that the atoms in the object or body become unstable, and, over time, begin to "decompose" by giving off radiation in the forms of subatomic particles (such as electrons and protons). There are different types of radiation: specifically, gamma, alpha and beta radiation. These will be discussed in detail at another time.
This process of radioactive decay eventually leads to the atoms becoming a different element and achieving stability.

For example, in decomposing organic bodies - such as an animal carcass - carbon-14, an isotope of carbon, is present.
Overtime, the C-14 atoms give off radiation, and, eventually, transform into nitrogen-14 atoms.
The term used to define the amount of time it takes for half of the radioactive atoms, such as C-14, in a body or object to decay fully is known as a "half-life."
The half-life of C-14 is approximately 5 730 years. This means that, after 5 730 years, roughly half of the radioactive C-14 atoms in a decomposing organic body will have decayed into nitrogen-14 atoms.
The decay rate, however, is not linear. That is, after two half-lives, 100% of the C-14 atoms will not have decayed into N-14 atoms. The decay rate, rather, is exponential. To put it simply, if one were to draw the decay rate of C-14 on a line chart, it would not be a straight, diagonal line. It would be a curving downward slope.


Scientists can use decay rates to, very roughly, determine the age of a fossil or artifact. If a fossil is found, it means it is organic in nature, and thus has or will have contained C-14 atoms. By using the known decay rate of C-14 as a reference and working out how much of the fossil's composition consists of C-14 and how much of it consists of N-14, they can approximate the age.

But what about inorganic objects, such as, say, stone tablets, or rocks? Some of these objects also contain a radioactive isotope. This time, of the element uranium. This radioactive isotope is uranium-238, and has a half-life of approximately 4.47 billion years (4 470 000 000 years).  This is a very extensive decay rate, but is still useful to scientists. Much in the same way used to approximate the age of organic fossils, scientists use uranium-238's decay rate and the uranium-238 to lead-206 (which it decays into) ratio to approximate an age to assign to the object.

As can be seen, radioactive dating is quite an advanced and sophisticated technique. Unfortunately, though, it is impossible to determine exactly what the age of a fossil or artifact is using it. Why so?
Well, many sources state that a recent test on the accuracy of C-14 dating - and thus, in turn, radioactive dating - attempted to date living penguins. While questionable, it appears as though the living penguins were dated as 8000 years old. Such a massive inaccuracy is inexcusable. There have also been other reported cases.

Aside from these alleged inconsistencies, there is also the assumption that the decay rates of the isotopes is constant, or fixed. If this were not the case, and the decay rate was susceptible to change or was not constant, it would render all ages inaccurate.



Relative Dating

Relative dating is the more conventional of the two. Relative dating is the technique that attempts to roughly determine the age of a fossil using its position or location in relation to other fossils or remains in nearby strata (hence the name, "relative")

In other words, to determine the age of a fossil using relative dating, one would look at the stratum the fossil was found in. Then, one would compare the fossil's position in the stratum to the position of other nearby index fossils or remains. Doing this, one can "map" out where the fossil appeared in the geographic time scale and thus work out a rough estimate of the fossil's age, by comparing it to other fossils (i.e "it came before this fossil, and after this fossil, so therefore it must be an intermediary, and will have appeared in the time period between the two other fossils," or "this fossil was found in the same stratum as this other one, at almost the same depth and in a close radius of each other - therefore, they must have appeared in the same time period.")


Perhaps the most concerning shortcoming of relative dating is how it is incapable of determining an exact or absolute age. In fact, some are of the opinion that its results are actually more of a rough estimate or less trustworthy than the results obtained from radioactive dating. Why is this so? Because the results rely heavily, not necessarily on the position of the fossil or its stratum (which is still an extremely important primary factor), but rather the way in which the scientist interprets it, which means it is vulnerable to bias, miscalculations, and so on. A scientist may present a fossil's position or location in the strata accurately, but then interpret it as only a few thousand years old, whereas another may present it as many millions of years old.

Not only this, but the geological time scale - another fundamental of relative dating - is sketchy and not always linear all over the globe. For example, sometimes the strata of a certain region are in the exact opposite sequence or order to how geologists expect them to be using the geological time scale. Several things may cause this.
A good example would be the eruption at St. Helena, located in the Washington state, U.S.A, in May 1980. The eruption was so intense that many layers of sediment on the volcanic mountain were blown into the air, and settled on the landscape around the volcano. This sediment would form strata. Does the volcanic strata belong where it landed? Of course not! In fact, it may contain coal or fossil fuels - which take, at the very least, thousands of years to form - that now appear near the very top! Such an inconsistency would, logically, confuse geologists in the future if they had no prior knowledge of the St. Helena eruption.
It is known that volcanic eruptions, such as the one at St. Helena, have occurred many times in the past. Not only that, but earthquakes and floods can also sometimes shift and mix strata and sediments.

Relative vs. Radiometric - Which Should Be Used?

It is a gaurantee that different scientists, from different backgrounds, have locked horns over this debate many times, each with their own sets of recorded evidence. But which is truly more efficient?
Each technique has already been discussed in detail above. It is now time to compare the pros and cons.

Radiometric
   Pros
  • Capable of providing a reasonably exact age
  • Makes use of known and predetermined parameters (e.g amount of radioactive isotopes in fossil)
  • Leaves relatively little room for bias
  • Is not usually dependant on dynamic or changing factors (e.g few things can change the amount of radioactive atoms in the fossil)
  Cons
  •  Not failsafe - can present inconsistencies or confusing results (e.g living penguins dated as 8000 years old)
  •  Assumes the rate of radioactive decay is fixed or constant
  • Therefore, does not take into account environmental factors that may have affected decay

Relative
   Pros
  • Easier and quicker to utilize
  • While not exact, does accurately present the fossil's position in the timescale in relation to others
   Cons
  • Does not provide an exact or absolute age
  • Results can be manipulated according to bias and personal opinion
  • Can be interpreted in different ways
  • Relies on dynamic or inconsistent factors (e.g assumes geological time scale is linear)
  • Other index fossils or remains must be present in order to compare positions
From the above list, it can be seen that both techniques have pros and cons. Both are not entirely inaccurate, but neither are both entirely accurate.
However, it must be noted that radiometric dating seems to emerge as superior. Why so? Even though it is fallible, and a small chance holds that it may even be entirely inaccurate, radiometric dating relies more on fixed or solid variables and factors than relative dating does, thus having a smaller margin for error. For example, radiometric dating dates the fossil as it is individually - relative dating compares it to other fossils in an environment (strata and sedimentary layers) that is certainly not linear.

This is not to imply radiometric dating is immediately superior to relative dating and is fully correct. Rather, this entry wishes to point out that radiometric dating, while certainly not infallible, has less of a margin for error, and thus has a higher chance of being correct.

Still, scientists involved in the dating of fossils and artifacts should retain their freedom to date using the techniques they see fit to use.

3 comments:

  1. This was a very informative article, thank you for posting it!

    ReplyDelete
  2. Sometime few educational blogs become very helpful while getting relevant and new information related to your targeted area. As I found this blog and appreciate the information delivered to my database.Meet Thai girls

    ReplyDelete