Introduction to C14-Dating Method

A method by using which scientists can tell the age of a fossil. It estimates the age from radio active decay of C14 Carbon atoms. C14 is an unstable carbon isotope.

BY: THOMAS HIGHAM

"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure. We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 1989:71)].
Nyerup's words illustrate poignantly the critical power and importance of dating; to order time. Radiocarbon dating has been one of the most significant discoveries in 20th century science. Renfrew (1973) called it 'the radiocarbon revolution' in describing its impact upon the human sciences. Oakley (1979) suggested its development meant an almost complete re-writing of the evolution and cultural emergence of the human species. Desmond Clark (1979) wrote that were it not for radiocarbon dating, "we would still be foundering in a sea of imprecisions sometime bred of inspired guesswork but more often of imaginative speculation" (Clark, 1979:7). Writing of the European Upper Palaeolithic, Movius (1960) concluded that "time alone is the lens that can throw it into focus".

The radiocarbon method was developed
by a team of scientists led by the late Professor
Willard F. Libby of the University of Chicago in immediate post-WW2 years.
Libby later received the Nobel Prize in Chemistry in 1960:

"for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics, and other branches of science."
According to one of the scientists who nominated Libby as a candidate for this honour;
"Seldom has a single discovery in chemistry had such an impact on the thinking of so many fields of human endeavour. Seldom has a single discovery generated such wide public interest."
(From Taylor, 1987).

Today, there are over 130 radiocarbon dating laboratories around the world producing radiocarbon assays for the scientific community. The C14 technique has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine.




The 14C Method
There are three principal isotopes of carbon which occur naturally - C12, C13 (both stable) and C14 (unstable or radioactive). These isotopes are present in the following amounts C12 - 98.89%, C13 - 1.11% and C14 - 0.00000000010%. Thus, one carbon 14 atom exists in nature for every 1,000,000,000,000 C12 atoms in living material. The radiocarbon method is based on the rate of decay of the radioactive or unstable carbon isotope 14 (14C), which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen 14. The reaction is:


14N + n => 14C + p
(Where n is a neutron and p is a proton).
The 14C formed is rapidly oxidised to 14CO2 and enters the earth's plant and animal lifeways through photosynthesis and the food chain. The rapidity of the dispersal of C14 into the atmosphere has been demonstrated by measurements of radioactive carbon produced from thermonuclear bomb testing. 14C also enters the Earth's oceans in an atmospheric exchange and as dissolved carbonate (the entire 14C inventory is termed the carbon exchange reservoir (Aitken, 1990)). Plants and animals which utilise carbon in biological foodchains take up 14C during their lifetimes. They exist in equilibrium with the C14 concentration of the atmosphere, that is, the numbers of C14 atoms and non-radioactive carbon atoms stays approximately the same over time. As soon as a plant or animal dies, they cease the metabolic function of carbon uptake; there is no replenishment of radioactive carbon, only decay. There is a useful diagrammatic representation of this process given here

Libby, Anderson and Arnold (1949) were the first to measure the rate of this decay. They found that after 5568 years, half the C14 in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on (see figure 1 below). The half-life (t 1/2) is the name given to this value which Libby measured at 5568±30 years. This became known as the Libby half-life. After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 - 60 000 years, then, the limit of the technique is reached (beyond this time, other radiometric techniques must be used for dating). By measuring the C14 concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the countrate or number of decay events per gram of Carbon. By comparing this with modern levels of activity (1890 wood corrected for decay to 1950 AD) and using the measured half-life it becomes possible to calculate a date for the death of the sample.

As 14C decays it emits a weak beta particle (b ), or electron, which possesses an average energy of 160keV. The decay can be shown:


14C => 14N + b
Thus, the 14C decays back to 14N. There is a quantitative relationship between the decay of 14C and the production of a beta particle. The decay is constant but spontaneous. That is, the probability of decay for an atom of 14C in a discrete sample is constant, thereby requiring the application of statistical methods for the analysis of counting data.

It follows from this that any material which is composed of carbon may be dated.Herein lies the true advantage of the radiocarbon method, it is able to be uniformly applied throughout the world. Included below is an impressive list of some of the types of carbonaceous samples that have been commonly radiocarbon dated in the years since the inception of the method:


Charcoal, wood, twigs and seeds.
Bone.
Marine, estuarine and riverine shell.
Leather.
Peat
Coprolites.
Lake muds (gyttja) and sediments.
Soil.
Ice cores.
Pollen.
Hair.
Pottery.
Metal casting ores.
Wall paintings and rock art works.
Iron and meteorites.
Avian eggshell.
Corals and foraminifera.
Speleothems.
Tufa.
Blood residues.
Textiles and fabrics.
Paper and parchment.
Fish remains.
Insect remains.
Resins and glues.
Antler and horn.
Water.


The historical perspective on the development of radiocarbon dating is well outlined in Taylor's (1987) book "Radiocarbon Dating: An archaeological perspective". Libby and his team intially tested the radiocarbon method on samples from prehistoric Egypt. They chose samples whose age could be independently determined. A sample of acacia wood from the tomb of the pharoah Zoser (or Djoser; 3rd Dynasty, ca. 2700-2600 BC) was obtained and dated. Libby reasoned that since the half-life of C14 was 5568 years, they should obtain a C14 concentration of about 50% that which was found in living wood (see Libby, 1949 for further details). The results they obtained indicated this was the case. Other analyses were conducted on samples of known age wood (dendrochronologically aged). Again, the fit was within the value predicted at ±10%. The tests suggested that the half-life they had measured was accurate, and, quite reasonably, suggested further that atmospheric radiocarbon concentration had remained constant throughout the recent past. In 1949, Arnold and Libby (1949) published their paper "Age determinations by radiocarbon content: Checks with samples of known age" in the journal Science. In this paper they presented the first results of the C14 method, including the "Curve of Knowns" in which radiocarbon dates were compared with the known age historical dates (see figure 1). All of the points fitted within statistical range. Within a few years, other laboratories had been built. By the early 1950's there were 8, and by the end of the decade there were more than 20.





--------------------------------------------------------------------------------
Figure 1: The "Curve of Knowns" after Libby and Arnold (1949). 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.
In the 1950s, further measurements on Mediterranean samples, in particular those from Egypt whose age was known through other means, pointed to radiocarbon dates which were younger than expected. The debate regarding this is outlined extensively in Renfrew (1972). Briefly, opinion was divided between those who thought the radiocarbon dates were correct (ie, that radiocarbon years equated more or less to solar or calendar years) and those who felt they were flawed and the historical data was more accurate. In the late 1950's and early 1960's, researchers measuring the radioactivity of known age tree rings found fluctuations in C14 concentration up to a maximum of ±5% over the last 1500 years. In addition to long term fluctuations, smaller 'wiggles' were identified by the Dutch scholar Hessel de Vries (1958). This suggested there were temporal fluctuations in C14 concentration which would neccessitate the calibration of radiocarbon dates to other historically aged material. Radiocarbon dates of sequential dendrochronologically aged trees primarily of US bristlecone pine and German and Irish oak have been measured over the past 10 years to produce a calendrical / radiocarbon calibration curve which now extends back over 10 000 years (more on Calibration). This enables radiocarbon dates to be calibrated to solar or calendar dates.

Later measurements of the Libby half-life indicated the figure was ca. 3% too low and a more accurate half-life was 5730±40 years. This is known as the Cambridge half-life. (To convert a "Libby" age to an age using the Cambridge half-life, one must multiply by 1.03).

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 (Gove, 1994). 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 (Kromer, 1994) 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.




INDEX Introduction Measurement Applications WWW Links k12 Publication Corrections
Age calculation Calibration Pretreatment References Awards Credits What's new? Email

Amateur Science--Strong Tradition, Bright Future

Amateur Science--Strong Tradition, Bright FutureForrest M. Mims III

Forrest M. Mims III is a writer, teacher, and amateur scientist. He received a Rolex Award for developing a miniature instrument that measures the ozone layer and has contributed projects to "The Amateur Scientist" column in Scientific American. His scientific publications have appeared in Nature and other scholarly journals.

CREDIT: ALLAN BURCH

Contemporary science has its roots in the achievements of amateur scientists of centuries past. Although they lacked what we would define as formal scientific training, they deciphered the basic laws of physics and principles of chemistry. They invented instruments. And they discovered, documented, sketched, and painted planets, comets, fossils, and species.
An editorial in a leading science journal once proclaimed an end to amateur science: "Modern science can no longer be done by gifted amateurs with a magnifying glass, copper wires, and jars filled with alcohol" (1). I grinned as I read these words. For then as now there's a 10× magnifier in my pocket, spools of copper wire on my work bench, and a nearby jar of methanol for cleaning the ultraviolet filters in my homemade solar ultraviolet and ozone spectroradiometers. Yes, modern science uses considerably more sophisticated methods and instruments than in the past. And so do we amateurs. When we cannot afford the newest scientific instrument, we wait to buy it on the surplus market or we build our own. Sometimes the capabilities of our homemade instruments rival or even exceed those of their professional counterparts.
The term amateur can have a pejorative ring. But in science it retains the meaning of its French root amour, love, for amateurs do science because it's what they love to do. Without remuneration or reward, enthusiastic amateurs survey birds, tag butterflies, measure sunlight, and study transient solar eclipse phenomena. Others count sunspots, discover comets, monitor variable stars, and invent instruments.
Many amateurs have contributed observations and data that have been incorporated into papers and books. Some are accepted as colleagues by their professional counterparts. They present their findings at conferences and publish papers in peer-reviewed journals. For each of these, hundreds more devote their spare time to making observations, measurements, sketches, photographs, and reports without receiving direct recognition. Although some are retired, others are taxi drivers, photographers, civil servants, pilots, or missionaries, the latter group having an especially impressive record of achievement. And some, like my grandmother Leitha Mims, do not even think of themselves as amateur scientists. Yet through years of careful gardening she cultured a new variety of amaryllis.
Then there are the student scientists. Each year more than half a million science fair projects are prepared by students in the United States. Although most projects are required learning assignments, a surprising number of students do original work, and some even make discoveries. Many alumni of the International Science and Engineering Fair (ISEF) and the Science Talent Search are now working scientists, and at least five are Nobel laureates.
Amateur scientists identify with student scientists, perhaps because we often don't realize that some of our experiments are not supposed to work. When my son Eric wanted to build a novel optical fiber seismometer, a professional seismologist said it would not succeed because our Texas house rests on soil and not bedrock. Eric proceeded anyway, and his supersensitive seismometer detected many earthquakes and two underground nuclear tests in Nevada, an achievement that won him college scholarships, science fair awards, and trips to the ISEF and the annual meeting of the American Association for the Advancement of Science. An atmospheric scientist said my daughter Vicki's attempt to detect solar x-ray flares with a Geiger counter would not work. Remembering Eric's experience, I excitedly told her this meant her project would succeed! And succeed it did, for Vicki detected six X-class x-ray flares. Her project won science fair awards and was recently published in a book.
The journal Science itself was begun by a famous amateur scientist and inventor. Although his methods were sometimes ridiculed by some scholarly scientists, their names are long forgotten. But everyone remembers Thomas Edison, who began Science as a private venture in 1880. Expelled from school at the age of seven for being "retarded," Edison was taught at home by his mother. His life changed forever when he found an old copy of Michael Faraday's Experimental Researches in Electricity and promptly built every project in the book. Thus the self-taught English amateur scientist, who was also schooled at home, passed the torch to the young American.
Astronomy has traditionally been among the most fertile fields for serious amateurs. Clyde Tombaugh's discovery of Pluto ranks among the best known of their comparatively recent achievements. In recent years, hundreds of other amateur astronomers have filled a wide range of niches left behind when many professionals graduated to fully automated observatories dedicated to a limited range of tasks. They discover new supernovae, comets, and time occultations; patiently count sunspots; photograph meteor trails; and measure the fluctuations of variable stars. More than 100 members of the American Association of Variable Star Observers have logged from 10,000 to more than 100,000 observations each. The record is held by South African Danie Overbeek, who has logged more than 188,000 variable stars in some 40 years of observing.
Many serious amateur astronomers have worked closely with professionals, even coauthoring books and papers with them. A paper on massive storms on Saturn that appeared in Science (2) was coauthored by Donald Parker, who discovered the storms and who is famous for his detailed planetary images. Although astronomy is his passion, Parker earns a living as an anesthesiologist for Mercy Hospital in Miami, Florida.
Although thousands of amateurs observe the solar system and beyond, many more thousands monitor Earth. More than 10,000 citizen volunteers make daily observations for the U.S. National Weather Service. Several years ago, the Weather Service honored Earl Stewart, who in 75 years provided nearly 28,000 daily readings from his station in Cottage Grove, Oregon.
Thousands of amateur naturalists participate in the Audubon Society's Christmas bird count, providing vital data for studies of bird migration and population trends. Cornell University's Laboratory of Ornithology has collaborated with thousands of amateurs to survey bird populations and identify food preferences.
Many other amateurs pursue science on their own. French taxi driver Pierre Morvan is a self-taught entomologist who for more than 20 years has spent his vacations collecting, drawing, and studying Asian ground beetles, especially those of the Himalayas.
Johan Gjefsen Reinhard used his own funds to finance a 2-year investigation of ancient ceremonial centers in the Andes, the highest archaeological ruins on Earth. An important aspect of Reinhard's work is diving in high-altitude lakes once viewed as significant by the Incas.
Roger Baker has contributed projects to "The Amateur Scientist" column in Scientific American and to the amateur science magazine Science Probe. He used one of these projects, a simple means of measuring ground-level ozone, to compare his results to those from a government instrument. Baker found that the latter was malfunctioning, a fact acknowledged by the responsible agency. Among his many scientific pursuits, Baker grinds lenses from window glass and has made instruments that measure the oxygen in water and the turbidity of what appear to be perfectly clear fluids.
Although many prizes, awards, and honors are given to student and professional scientists, there are only a few major prizes for which amateurs are eligible. Among these few is the Rolex Award for Enterprise, a prize that has been received by several amateur scientists, including beetle collector Morvan and Inca researcher Reinhard. Aside from occasional commendation letters, most amateur scientists are never recognized for their achievements.
For some amateur scientists, the most important recognition is the opportunity to work alongside their professional colleagues or to be sent by them on field assignments. NASA has sent my instruments and me on field trips to measure various atmospheric and ecological effects of smoke from biomass burning, twice to Brazil and three times to major forest fires in the western United States. Several publications have come from this work, which has been the most fulfilling of my experiences as an amateur scientist. Among the findings is that the survivability of nonpigmented, potentially pathogenic, airborne bacteria is enhanced during the burning season in Brazil, a phenomenon that is highly correlated with diminished ultraviolet B (UV-B) caused by thick smoke.
A few scientists refuse to take the work of their amateur counterparts seriously. In 1990, Jerry McDonald, who was working on a Ph.D. in sociology, found hundreds of beautifully preserved tracks of reptiles, amphibians, and insects in Permian sandstone in southern New Mexico. In 1 year alone, McDonald carried on his back more than 18,000 kilograms of footprint-bearing slabs on 240 trips along the 1-kilometer trail between the excavation and his jeep.
Professional paleontologists were unimpressed by McDonald's claims, because Permian trackways had never been found in southern New Mexico. Undaunted, McDonald drove some of his specimens to the Smithsonian Institution's National Museum of Natural History and the Carnegie Museum of Natural History. The paleontologists at these museums were so impressed that they asked for samples to display. Nevertheless, some paleontologists continued to reject McDonald's find. "Scientist wins world acclaim but is snubbed in New Mexico," read a page-one headline in the El Paso Times (3). When New Mexico politicians learned that McDonald's trackways were being acquired by major museums back east, they came to his rescue. Soon thereafter the U.S. Congress authorized a study of McDonald's discoveries and how to protect them.
Fortunately McDonald's experience is unusual. In this era of big science, the most important lesson to be learned from his discovery and the achievements of countless other amateurs is that scientific observations and discoveries don't necessarily require giant government grants and huge teams of researchers with specialized degrees. Small science still works, and it often works during off hours, weekends, and holidays when professionals are generally at home or on vacation.
As we enter the next millennium, the future of amateur science has never looked better. Amateurs built some of the first home computers, and today many us own systems that far outclass what was available to our professional colleagues only a few years ago. It no longer matters that I can't do a nonlinear regression with a calculator, because economical software does it automatically, and an inexpensive printer then produces plots as crisp as any published in Science.
Computers have greatly expanded the capabilities of professionals and amateurs alike, but the Internet has become the great equalizer. Several years ago I measured record low ozone over central Texas. Thanks to e-mail, I quickly notified scientists at NASA, the National Oceanic and Atmospheric Administration, and the Environmental Protection Agency and then organized a quick paper for Eos with them as coauthors. No one asked if I had a degree in the field; all that mattered was the significance of the event and the quality of the data. When I measured large spikes in UV radiation caused by the scattering from cumulus clouds over Hawaii's Mauna Loa Observatory, I e-mailed the results to UV specialist John Frederick. I then incorporated Frederick's comments in a communication we jointly sent to Nature. Frederick, the editors at Nature, and the peer reviewers never asked to see my credentials. Instead, they judged the work on its merits.
I could write much more about amateur science, but the allowed space has run out. Besides, the data logger connected to the UV-B radiometer in the field outside my window is beeping to be downloaded. I always enjoy writing about science, but doing science is much more exciting. At noon, the ozone layer measured a thinner-than-normal 240 Dobson units. I wonder if today's hazy sky reduced the UV-B enough to balance the increase expected from the reduced ozone?
References

A. Sanchez-Lavega et al., Science 271, 631 (1996).
D. E. Koshland, ibid. 257, 1607 (1992).
El Paso Times, 12 March 1990, p. 1.
The author is at 433 Twin Oak Road, Seguin, TX 78155, USA. E-mail: fmims@aol.com

Strategies for Successful Engineering Management

I visited this site today and found a very interesting article about the role and skills of technical managers. The web address being: http://www.todaysengineer.org/2007/Nov/management.asp

Five Strategies for Successful Engineering Management
By Gary C. Hinkle

If you read surveys about the highest paying jobs in the United States, you'll notice that the median salary for engineering managers ranks not too far behind doctors, attorneys and CEOs (typical CEO base salary, not the extreme high end of the scale that often makes the news). Good engineering managers are well paid because they have a very difficult job that not many people with engineering backgrounds want — and even fewer are highly qualified to do.
Michael Aucoin, author of From Engineer to Manager: Mastering the Transition, found that most engineers are unhappy with the "promotion" to manager, saying, "Much of this frustration is the result of lack of preparation and training."
Outside of the technical challenges engineering managers face, three major issues make engineering management a difficult profession:

Read More at: http://www.todaysengineer.org/2007/Nov/management.asp

پاکستان زندہ باد

پاکستان اللہ تعالی کی ایک عظیم نعمت ہے یہاں ان صفحات میں میری کوشش ہو گی کہ وطن عزیز کی خوبیوں کو اجاگر کیا چاے۔

1۔ پاکستان دنیا کی واحد اور دنیا کی ساتویں ایٹمی طاقت ہے
2۔ پاکستان کی زبان اردو کا شمار دنیا کی بڑی زبانوں میں شمار ہوتا ہے
3۔ پاکستان کو اللہ تعالی نے قدرتی گیس کی دولت سے نوازا ہے
4۔ وطن عزیز میں فیملی سسٹم مضبوط ہے اور والدین کی خدمت کو باعث افتخار سمجھا جاتا ہے

الیکشن 2008

آج 19 فروری ہے۔ آج اور کل کا دن پاکستان کی تاریخ میں نمایاں اہمیت کے حامل ہیں۔ آج کا جنگ اخبار میرے سامنے ہے۔ سب سے اہم خبر یہ ہے کہ بہت سے بڑے بڑے سیاست دان جو کہ ق لیگ سے وابستہ تھے'شکست کھا گۓ ہیں۔ ان میں سپیکر قومی اسمبلی امیر حسین، شیخ رشید احمد، شجاعت حسین، پرویزالہی، ہمایوں اختر خاں، اعجازالحق، زبیدہ جلال، وصی ظفر، میاں اظھر، ریاض فتیانہ، دانیال عزیز اور دیگر شامل ہیں۔ اگرچہ ان میں سے اکثر لیگ اپنے حلقوں میں برادری ازم، خاندانی اثرورسوخ، نام نہاد ترقیاتی کاموں کے زعم میں اور دیہی عوام کی اکثریت کے زیادہ تعلیم یافتہ نہ ہونے کے باعث اپنی کامیابی کے ضرورت سے زیادہ پر اعتماد تھے۔ مجھے خوشی ہے کہ پاکستان کے باشعور عوام نے ان عناصر کو حالیہ انتخابات میں مسترد کر کے اپنی بیدار مغزی کا ثبوت فراہم کیا ہے۔ اخبار کے مطابق اب تک مسلم لیگ ن اور پیپلزپارٹی نے بالترتیب 55 اور 35 نشسپیں حاصل کر رکھیں ہیں۔ اللہ تعالی سے دعا ہے کہ آنے والی حکومت ملک و ملت کی خیر خواہ ثابت ہو اور پیارے وطن کو پرویزی دور کے پیداکردہ مسایل سے نکال سکے۔ ہم بجا طور پر آنے والی حکومت سے یہ توقع رکھتے ہیں کہ وہ محسن پاکستان ڈاکٹر عبدالقدیر اورچیف جسٹس افتخارصاحب اور ان کے تمام ساتھیوں کو ان کا جایز مقام دے گی-آمین