Wednesday, November 26, 2008

RadioActive Carbon Dating

The radiocarbon dating method was developed in the 1940's by Willard F. Libby and a team of scientists at the University of Chicago. It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artifacts and geologic events up to about 50,000 years in age.

Carbon Dating Carbon has unique properties that are essential for life on earth. Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes. One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or 14C, or radiocarbon. Carbon-14 is made when cosmic rays knock neutrons out of atomic nuclei in the upper atmosphere. These displaced neutrons, now moving fast, hit ordinary nitrogen (14N) at lower altitudes, converting it into 14C. Unlike common carbon (12C), 14C is unstable and slowly decays, changing it back to nitrogen and releasing energy. This instability makes it radioactive.

We can take a sample of air, count how many 12C atoms there are for every 14C atom, and calculate the 14C/12C ratio. Because 14C is so well mixed up with 12C, we expect to find that this ratio is the same if we sample a leaf from a tree, or a part of your body. The C-14 within an organism is continually decaying into stable carbon isotopes, but since the organism is absorbing more C-14 during its life, the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere. When the organism dies, the ratio of C-14 within its carcass begins to gradually decrease. The rate of decrease is 1/2 the quantity at death every 5,730 years. That is the half-life of C-14. Obviously, this works only for things which were once living. It cannot be used to date volcanic rocks, for example. The rate of decay of 14C is such that half of an amount will convert back to 14N in 5,730 years (plus or minus 40 years). This is the “half-life.” So, in two half-lives, or 11,460 years, only one-quarter of that in living organisms at present, then it has a theoretical age of 11,460 years. Anything over about 50,000 years old, should theoretically have no detectable 14C left. That is why radiocarbon dating cannot give millions of years. In fact, if a sample contains 14C, it is good evidence that it is not millions of years old.

Because the decay rate is logarithmic, radiocarbon dating has significant upper and lower limits. It is not very accurate for fairly recent deposits. In recent deposits so little decay has occurred that the error factor (the standard deviation) may be larger than the date obtained. The practical upper limit is about 50,000 years, because so little C-14 remains after almost 9 half-lives that it may be hard to detect and obtain an accurate reading, regardless of the size of the sample.

The ratio of C-14 to C-12 in the atmosphere is not constant. Although it was originally thought that there has always been about the same ratio, radiocarbon samples taken and cross dated using other techniques like dendrochronology have shown that the ratio of C-14 to C-12 has varied significantly during the history of the Earth.

This variation is due to changes in the intensity of the cosmic radiation bombardment of the Earth, and changes in the effectiveness of the Van Allen belts and the upper atmosphere to deflect that bombardment. For example, because of the recent depletion of the ozone layer in the stratosphere, we can expect there to be more C-14 in the atmosphere today than there was 20-30 years ago. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years.

When reading archaeological reports, be sure to check if the carbon-14 dates reported have been calibrated or not.

The major developments in the radiocarbon method up to the present day involve improvements in measurement techniques and research into the dating of different materials. Briefly, the initial solid carbon method developed by Libby and his collaborators was replaced with the Gas counting method in the 1950's. Liquid scintillation counting, utilising benzene, acetylene, ethanol, methanol etc, was developed at about the same time.

Today the vast majority of radiocarbon laboratories utilise these two methods of radiocarbon dating. Of major recent interest is the development of the Accelerator Mass Spectrometry method of direct C14 isotope counting.

In 1977, the first AMS measurements were conducted by teams at Rochester/Toronto and the General Ionex Corporation and soon after at the Universities of Simon Fraser and McMaster. The crucial advantage of the AMS method is that milligram sized samples are required for dating. Of great public interest has been the AMS dating of carbonacous material from prehistoric rock art sites, the Shroud of Turin and the Dead Sea Scrolls in the last few years.

The development of high-precision dating (up to ±2.0 per mille or ±16 yr) in a number of gas and liquid scintillation facilities has been of similar importance (laboratories at Belfast (N.Ireland), Seattle (US), Heidelberg (Ger), Pretoria (S.Africa), Groningen (Netherlands), La Jolla (US), Waikato (NZ) and Arizona (US) are generally accepted to have demonstrated radiocarbon measurements at high levels of precision).

The calibration research undertaken primarily at the Belfast and Seattle labs required that high levels of precision be obtained which has now resulted in the extensive calibration data now available. The development of small sample capabilities for LSC and Gas labs has likewise been an important development - samples as small as 100 mg are able to be dated to moderate precision on minigas counters with similar sample sizes needed using minivial technology in Liquid Scintillation Counting.

The radiocarbon dating method remains arguably the most dependable and widely applied dating technique for the late Pleistocene and Holocene periods.

Figure: The "Curve of Knowns"

The first acid test of the new method was based upon radiocarbon dating of known age samples primarily from Egypt (the dates are shown in the diagram by the red lines, each with a ±1 standard deviation included). The Egyptian King's name is given next to the date obtained. The theoretical curve was constructed using the half-life of 5568 years. The activity ratio relates to the carbon 14 activity ratio between the ancient samples and the modern activity. Each result was within the statistical range of the true historic date of each sample.

Other forms of Radioactive Dating

There are various other radiometric dating methods used today to give ages of millions or billions of years for rocks. These techniques, unlike carbon dating, mostly use the relative concentrations of parent and daughter products in radioactive decay chains. For example, potassium-40 decays to argon-40; uranium-238 decays to lead-206 via other elements like radium; uranium-235 decays to lead-207; rubidium-87 decays to strontium-87; etc. These techniques are applied to igneous rocks, and are normally seen as giving the time since solidification.

Rubidium occurs in nature as two isotopes: radioactive Rb-87 and stable Rb-85. Rb-87 decays with a half-life of 48.8 billion years to Sr-87. This half-life is so long that the Rb-Sr method is normally only used to date rocks that are older than about 100 million years.

Tuesday, November 25, 2008

Neutral Evolution and the Shape of Our Genome

Finding: There is a growing body of evidence which shows that many of the genetic bits and pieces that drive evolutionary changes do not confer any advantages or disadvantages to humans or other animals.

The conventional view
The basic belief of evolution was that all random genetic changes that manage to stick around have some selective advantage on the species.

But a study concludes that we are what we are largely due to random changes that are completely neutral. This study reinforces and highlights the equal, and in some cases greater, importance of neutral genetic drift.

Repeat Elements
Repeat elements are fragments of DNA containing the same repetitive sequence of chemical base pairs several hundred times. Experiments demonstrate that repeat elements rose to prominence without offering any benefits to the organism it inhabits. They are one of the major architectural markers of the human genome, and they make up over 40 percent of our genome,

Numts or Copies of mitochondrial sequences found in DNA portions

One type of repeat element was found while looking at genes associated with Bardet Biedl syndrome, a rare disorder. Researchers found portions of DNA that had been copied from the mitochondria, the energy-making apparatus of human cells that has its own small genome. These mitochondrial sequences are known as numts.

More Numts as the species gets more sophisticated
The whole human genome, has more than 1200 such pieces of mitochondrial DNA of various lengths embedded into chromosomes. While chimps have a comparable number, mice and rats only have around 600 numts. Since they increase in frequency as species advance, it suggested there was some evolutionary purpose to keeping them around.

But none of these numts contained an actual gene to make a protein that does anything, nor did they seem to control the function of any nearby genes. These numts are a neutral part of our genome. If anything, they may be mildly negative since long repeat sequences can be unstable or get inserted inside genes and disrupt them.

The researchers believe they have uncovered a possible reason why these potentially damaging but mostly neutral bits of DNA accumulate over time by comparing the sequences of human numts with those in different animals. How closely the different species' sequences match can provide an estimate of when that particular sequence got inserted into the ancestor of the human genome.

Numts became embedded roughly when primates emerged: 54 Million years ago
Calculations made about the location and structure of the numts revealed that most numts became embedded in our genome over a 10-million-year period centered roughly 54 million years ago -- right around the time when the first primates emerged. When new species emerge, their numbers and therefore their genetic differences are very small. The consequences are that this creates a genetic bottleneck during which any changes in the genome will either get eliminated quickly or spread to the whole population quickly.

Numts expanded because they were not eliminated - they were not detrimental

Numts, being "neutral," were generally at low levels in ancient mammals, but during the primate emergence 54 million years ago, they accumulated and spread through the small early primate populations precisely because they were not detrimental enough to be eliminated. Then, as these populations expanded, numts reached stable but higher frequencies.

Saturday, November 22, 2008

Radioactive Dating: Rubidium-Strontium

Rubidium-Strontium dating:The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87. Strontium-87 is a stable element; it does not undergo further radioactive decay. (Do not confuse with the highly radioactive isotope, strontium-90.) Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium-86.

If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides behave the same chemically. (Note that this does not mean that the ratios are the same everywhere on earth. It merely means that the ratios are the same in the particular magma from which the test sample was later taken.) As strontium-87 forms, its ratio to strontium-86 will increase. Strontium-86 is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process. The amount of strontium-86 in a given mineral sample will not change. Therefore the relative amounts of rubidium-87 and strontium-87 can be determined by expressing their ratios to strontium-86: Rb-87/Sr-86 and Sr87/Sr-86 We measure the amounts of rubidium-87 and strontium-87 as ratios to an unchanging content of strontium-86.

Because of radioactivity, the fraction of rubidium-87 decreases from an initial value of 100% at the time of formation of the mineral, and approaches zero with increasing number of half lives. At the same time, the fraction of strontium-87 formed increases from zero and approaches 100% with increasing number of half-lives. The two curves cross each other at half life = 1.00. At this point the fraction of Rb87 = Sr87 = 0.500. At half life = 2.00, Rb87 = 25% and Sr87 = 75%, and so on.

Wednesday, November 12, 2008

Radioactive Dating: Uranium-Lead

The uranium-lead method is the longest-used dating method. It was first used in 1907, about a century ago. The uranium-lead system is more complicated than other parent-daughter systems; it is actually several dating methods put together.

Natural uranium consists primarily of two isotopes, U-235 and U-238, and these isotopes decay with different half-lives to produce lead-207 and lead-206, respectively. In addition, lead-208 is produced by thorium-232. Only one isotope of lead, lead-204, is not radiogenic.

The uranium-lead system has an interesting complication: none of the lead isotopes is produced directly from the uranium and thorium. Each decays through a series of relatively short-lived radioactive elements that each decay to a lighter element, finally ending up at lead. Since these half-lives are so short compared to U-238, U-235, and thorium-232, they generally do not affect the overall dating scheme. The result is that one can obtain three independent estimates of the age of a rock by measuring the lead isotopes and their parent isotopes. Long-term dating based on the U-238, U-235, and thorium-232 will be discussed briefly here; dating based on some of the shorter-lived intermediate isotopes is discussed later.

The uranium-lead system in its simpler forms, using U-238, U-235, and thorium-232, has proved to be less reliable than many of the other dating systems. This is because both uranium and lead are less easily retained in many of the minerals in which they are found. Yet the fact that there are three dating systems all in one allows scientists to easily determine whether the system has been disturbed or not.

Using slightly more complicated mathematics, different combinations of the lead isotopes and parent isotopes can be plotted in such a way as to minimize the effects of lead loss. One of these techniques is called the lead-lead technique because it determines the ages from the lead isotopes alone. Some of these techniques allow scientists to chart at what points in time metamorphic heating events have occurred, which is also of significant interest to geologists

Monday, November 10, 2008

Intelligent Design ... by the numbers

The most endearing element of Intelligent Design is that the universe is so fine tuned that only a designer with a purpose could have made it so.

If the structure of the atom was changed just so...If the energy property of the quarks was altered just so slightly...every thing would be different. The Universe as we know it would not exist. And life would not exist.

OK...Let's see.

The Dimensions of the Game
Let's talk about baseball. Many baseball purists think that the game is perfect. It is the perfect team sport and the perfect individual sport. Defense vs. Offense. And if you change the dimensions of the game, it would be different.

Think of it. The bases are 90 ' ft from each other. This accounts for the batting averages being what they are. The majority of baseball players have a batting average between 250 and 275. The really good batters will have an average above 280 and up to 340. Very few if any will have a yearly batting average above 350. In the last 60 years only one player has had an average above 400. It is very hard to get a hit given the dimensions of the game.

Now suppose you changed the dimensions only a little bit. Instead of the base path at 90', you changed them to 89' and 10". That's only two inches. But that would be enough to change the batting averages. Not much but you would affect the close calls. Now instead of missing the close call base hit by 2" you make it by 2". Batting averages would go up from 250 to 275; the outstanding hitters will have averages above 350 to 380.

Changing the dimensions of the game make a change in the game. But it is still baseball. Ohter adjustments may be made to favor the picture. 5 balls will equal a walk; 2 stikes equal a strike out. Change the dimensions to favor the batter, and you change other rules to favor the picture.

But it is still baseball. The game has changed, because the rules have changed. But it is baseball.

The Structure of the Universe
Well that's what the universe is like. If you change the rules so that this universe will produce a set of atoms with a certain atomic structure, you can have different molecular structures that would adjust to the different chemical rules, and hence different physical rules. The table of periodic elements would look different, but you would still have a table of periodic elements.

Fine Tuning
So the fine tune tinkering would not necessarily be unique. In fact, if you have a universe with the basic laws of F=MA and Shroedingers equation.

You have this universe. Now you can take this universe and change it by tinkering with the rules. But you would get a different universe. However, there is nothing to say that it couldn't evolve life and consciousness. If it did that, if it allowed that, then you would have a universe like ours. It would permit life and consciousness. But it would not be unique.

The Arguement for Design does it show a necessary universe?
The result is that if any universe can create the conditions of life and consciousness, it undermines the argument of design. This is a finely tuned universe, but so is any universe that can create the conditions of life and consciousness. There is nothing special; this is a condition of nature and not necessarily unique. If it is not necessarily unique then the uniqueness factor is not the necessary factor to make you believe in a designer. That is unless you believe that the designer is nature, in the sense that God is nature via Spinosa.

A necessary universe is a unique universe. A universe which can create life and consciousness is necessary only if there is no other way to create those elements; only if life and consciousness can only be created in one universe. If any universe or a great number of possible universes can create life and consciousness then those conditions are ubiquitous; they are pan-universal. So the universe is not unique among universes. It is not unique, and if it is not unique, it is not necessary.