Presentation on theme: "Radiometric and Trapped-Charge Dating R.A. Varney Paleoresearch Institute."— Presentation transcript:
Radiometric and Trapped-Charge Dating R.A. Varney Paleoresearch Institute
Presentation Format 1.Basic Chemistry for Radiometric Dating 2.Types of Radiometric Dating for Archaeology 3.Primer on Radiocarbon and Radiocarbon Dating 4.Potassium-Argon Dating 5.Argon-Argon Dating 6.Uranium Disequilibrium Dating 7.Lead 210 Dating 8.Intro to Trapped-Charge Dating 9.Thermo-Luminescence and Optically-Stimulated Luminescence Dating. 10.Question/Discussion Session.
Relative dating is great for determining the sequence of events within a culture, but to compare cultures to other cultures or to place events within calendar time you need to have a method of absolutely dating an archaeological context. This calls for absolute dating.
Basic Chemistry for Radiometric Dating Chemistry refresher: Elements, Isotopes, Radioactive Decay, and Half-life Carbon Isotopes: 14 C formation and decay, Brief history of Radiocarbon Dating, Radiometric Dating, Accelerator Mass Spectrometry Dating Daughter products: Uranium series and Potassium/ Argon, Argon/Argon Dating
Basic Terms Atom – a group of particles composed of two main parts: electrons and the nucleus Electrons – negatively charged particles which form the outer, mostly empty, portion of an atom. Different numbers of electrons change the charge of the atom and are responsible for most chemical reactions. Nucleus – relatively densely packed inner portion of the atom. Composed of positively charged protons and generally uncharged neutrons. Protons – Positively charged particles, the number of which, in an atom, defines that element. Neutrons – Generally uncharged particles in the nucleus, different numbers of neutrons in the nucleus define the different isotopes of an element. Element – A single type of matter, gold is an element, as is lead, copper, carbon, argon and hydrogen for example.
An element is defined by the number of protons in the nucleus. Hydrogen ALWAYS has one proton and Carbon ALWAYS has 6 protons. If there is a change in the number of protons is a different element. The number of electrons and neutrons can and does change. A change in the number of electrons changes the electrical charge of the atom. A change in the number of neutrons makes a different isotope of the same element.
All of the nuclei pictured here are the element Hydrogen, because they all contain a single proton, but each is a different isotope of Hydrogen, because they contain different numbers of neutrons.
0H0H 1 14 C 6 Number of Protons This is how the element if defined Number of Neutrons and Protons This is how the isotope if identified Element and Isotope Nomenclature
Different isotopes of an element have different properties. For purposes of this discussion, different isotopes of an element have different levels of stability. A stable isotope will not change into another isotope or element under normal circumstances. An unstable isotope will change into another isotope of the same element or into another element within a statistically determined time. There are various levels of stability found in isotopes, resulting in different periods of time that the unstable isotope will exist. Isotopes that change into other isotopes or different elements by emitting radiation are called radioisotopes.
Radioisotopes give off radiation as they change in a process called decay. There are three main types of radioactive decay: Alpha radiation is two protons and two neutrons. When an radioisotope decays by Alpha radiation a different element is created. This is the fallout that many of us grew up worrying about. Alpha radiation can be shielded by a piece of paper. Gamma radiation is an electromagnetic wave just as FM radio or light is a wave, but at a higher frequency. It is very difficult to block gamma radiation. This is the most dangerous, but shortest-lived component of a nuclear explosion. Gamma radiation causes changes to DNA. Beta radiation emits beta particles which can be either either high speed electrons or positrons. Beta radiation can be stopped by a sheet of aluminum. Carbon 14 decays by beta emission.
Radioactive decay does not happen all at once. A certain percentage of the quantity of the radioisotope will decay over a period of time. The time that it takes for half of the original mass of the radioisotope to decay is called the half-life of the isotope. If the original mass is 1000 grams, at one half-life in time there will be 500 grams of the radioactive material remaining. At two half-lives, there will be 250 grams remaining.
Selected Types of Radiometric dating. Radiocarbon: Used for things that were once alive, including organic decay products like Humates and carbonates. Age range: pre-aboveground atomic bomb test (1952) to ~45000 year ago and post-atomic bomb (1952) to modern. Argon dating (both Potassium/Argon and Argon/Argon): Used to date igneous rocks (rocks that formed directly from the molten state) as well as some metamorphic rocks (rocks that have been changed by heat and/or pressure) Used to great effectiveness in bracket dating pre-humans in volcanic regions. Olduvai for example. Age range 100,000 to ~4 billion years Uranium lead dating: Generally used on the element Zircon though other elements can be used. Age range: 10 million years ago to 4.5 billion years. Not used for archaeology. Commonly used in dating geologic events. Uranium disequilibrium dating: Measures the amount of uranium absorbed by buried porous materials such as teeth, eggshell, coral, etc. 1 to 400,000 or 500,000 years. Not technically a radiometric method, but uses radioisotopes. Lead 210 dating: Measures the decay of Radon xx into lead 210. generally used for ecological studies in lakes and marine sediments, but can be used to date sediments less than 200 years old
Radiocarbon There are 15 known isotopes of Carbon, from 8 C with only 2 neutrons, to 22 C which contains 16 neutrons in the nucleus. Only three of the Carbon isotopes occur in the natural world: 12 C, 13 C, and 14 C Carbon 12 And Carbon 13 are stable, with nearly 99 percent of the carbon in the world being the Carbon 12 isotope and nearly 1 percent being Carbon 13. There is about one Carbon 14 atom for every 850 billion Carbon 12 atoms in the atmosphere.
Carbon 14 is formed when cosmic radiation in the upper atmosphere excites a neutron, causing the neutron to impact a Nitrogen 14 atom and dislodges a proton forming carbon 14. This is an ongoing process, generating a relatively stable percentage of Carbon 14 atoms in the atmosphere. All living things are composed of this same fraction of the isotopes of carbon. When an organism dies, it is no longer taking in the carbon and the decay clock on the radiocarbon begins. The longer the time that has past since an organism has died, the smaller the percentage of radioactive carbon will remain in whatever is left of the organism. The half-life of Carbon 14 is 5730 years, therefore when there is half of the atmospheric percentage of carbon 14 remaining, the organism died 5730 years ago. We are able to measure the relative quantity of Carbon 14 very precisely.
Carbon 14 is created most intensely at altitudes from 30,000 to 50, 000 feet and at high latitudes. This is a plasma model of the magnet fields of the earth showing the relatively less protected polar areas where gamma radiation is able to most easily interact with the Nitrogen in the atmosphere and change the Nitrogen 14 to Carbon 14.
Carbon 14 Radiocarbon dating was developed in 1949 by Willard Libby, one of the developers of the atomic bomb who wanted to explore peaceful uses of radiation after his involvement in the bomb. In 1960, Dr. Libby received the Nobel prize for his radiocarbon research. Originally, radiocarbon dating was conducted by counting the radioactive emissions over a period of time and calculating the quantity of Carbon 14 remaining in the sample, thus determining the age of the sample. This method requires a large sample and returns fairly broad time periods for an age of the sample (plus or minus around 90 to 150 years). This method is called proportional counting and works on the same principal as the Geiger counter. Liquid scintillation counting is conducted by dissolving the sample in either benzene or toluene with fluors ( material which emits light when excited by the beta emissions) the light emitted is then multiplied and provides the data for the age of the sample. More light corresponds to more beta emissions and a younger age. Proportional and liquid scintillation counting are relatively insensitive, and yield larger uncertainties than AMS dating. These methods generally require over a gram of carbon. Accelerator Mass Spectrometry (AMS) counts the actual numbers of atoms of each isotope, requires only very small samples (>0.0002 grams of processed carbon, or about 0.001 grams of field sample), and returns very precise ages for the sample (plus or minus 15 to 30 years). Paleoresearch Institute only utilizes the AMS method of Radiocarbon dating.
Carbon 14 The Graphite sample is placed on a sample wheel, where a Cesium ion beam turns the sample into atoms. Powerful magnets pull the atoms into the accelerator, then strip all of the non- carbon atoms from the plasma, and send a beam of carbon atoms through a series of bends. Due to the different atomic weights of the isotopes, they will deflect at different angles and be received at different detectors. The detectors count the impact of the different isotopes and establish the ratio of each. The age of the sample is then straightforward math.
Radiocarbon Sample Pretreatment Radiocarbon samples collected in the field contain various contaminants that must be removed in order to get an accurate date. Contaminations sources include old carbon from percolation of water through sediments containing coal, limestone or other sources of carbon 14 depleted sources. These have the risk of making the age that you get too old. The sample can also be contaminated with young carbon that is richer in carbon 14 from sources such as decaying organic matter. Organic matter decays into a rich soup of carbon compounds including humins, humates and foelvic acids. If no particulate carbon is found, humate dates can usually be processed to get a rough age for the sample. These humate or soil organic matter dates are problematic because they are high affected by water soluable carbon percolating through the sediments. Pretreatment of charcoal samples require the use of an acid/base/acid process to remove contaminates. Pretreatment of bone samples requires removal of the mineral component of the bone to enrich the carbon content of the sample. There are many methods of pre-treating samples to ensure an accurate date, all of them need to be conducted in a specialized lab.
Radiocarbon date calibration Radiocarbon dates returned from the lab need to be calibrated to calendar years. Radiocarbon creation in the atmosphere has varied a little through time, due to changes in the levels of gamma radiation based variations in the earths magnetic field and the frequency and intensity of solar storms. These variations in the quantity of radiocarbon in the atmosphere are called Seuss Squiggles. Radiocarbon mixing in the atmosphere and old carbon influx into the atmosphere has varied a little through time primarily due to changes in climate ie. Rock weathering and exposure after glacial periods
Carbon 14 Statistical Introduction This is a normal curve, the curve is the same shape on either side of the mean or center point of the distribution. Mean
Standard Deviation Radiocarbon dates from the lab are usually returned in one standard deviation.
Carbon 14 This is read as zero plus or minus one half (0 to two standard deviations
Carbon 14 The result from the counter or accelerator will be reported as a number( the mean:7630) and a range (Standard Deviation or the sigma: ±15). The first part, the mean, tells you the middle of the distribution of the probability of the number of radiocarbon years ago that the organism died. Present is agreed to be AD 1950 to prevent confusion as time goes on.
This is the current radiocarbon calibration curve going back to about 14800 radiocarbon years.
This is INTCAL 2004 calibration curve for the past ~14000 Radiocarbon years Radiocarbon age calibration is the process of drawing a line from the 14 C determination to the curve and then dropping a line to the calendar year. Sounds simple right? Example: 14 C determination of 12500 RCYBP becomes calendar year 15000 BP.
Potential Complicating Factors for Radiocarbon Dating You must remember that you are dating the death of the organism. This can create an old wood problem where archaeological people collect wood or other organic materials that have been dead for hundreds or thousands of years. Water percolation either up or down through the sediment column can carry soluable carbons of an older or younger age than the item you are dating. These soluable carbons can contaminate your sample and skew the age. Coal contamination can skew the radiocarbon age to an older date. For an example of this problem read about Meadowcroft rockshelter in Pennsylvannia.
Radiocarbon Review Carbon 14 is produced in the atmosphere by gamma ray exciting a neutron which displaces a proton in the Nitrogen 14 atom creating carbon 14. Carbon 14 mixes in the atmosphere and becomes incorporated in organisms through respiration and eating. Carbon 14 is an unstable isotope and decays by beta radiation to form nitrogen 14. Radiocarbon dates can be returned for things that used to be alive. Radiocarbon can be used for objects as old as about 45000 years old. The amount of carbon 14 in the atmosphere has been nearly, but not completely, stable through time. Radiocarbon determinations returned from the lab need to be calibrated to provide calendar years before present. You must remember that you are dating the death of the organism.
Potassium/Argon dating Potassium (K) is the seventh most common element in the earths crust and makes up 1.5% of the weight of the crust. There are three naturally occurring isotopes of Potassium: 39 K the most common form occurs 93.3% of the time 40 Kthe unstable form of potassium (0.0117%) 41KStable occurs 6.7% of the time 40 K decays into stable Argon 40 and stable Calcium 40 Calcium 40 is too common to be used for dating 40K has a half-life of 1.25 x 10 9 years and is useful for dating rocks older than 100,000 years.
What can be dated with Potassium/Argon dating? Volcanic rocks high in potassium, particularly the micas (muscovite and biotite), volcanic feldspar, and plutonic or highly metamorphic horneblend While the rock is liquid, the gas argon can escape, but when the rock solidifies, the argon is trapped and accumulates as the potassium 40 decays into argon. Measuring the relative quantities of potassium and argon 40 gives the age of the rock. This makes the assumption that there was no argon in the rock when it was liquid. The sample is split into two aliquots The Argon aliquot is melted and measured on a mass spectrometer, and the potassium aliquot is measured using a an atomic absorption spectrometer or flame photometry. The ratio of the two returns the age of the rock. You are actually measuring the time when the rock solidified and argon was no longer able to escape.
Difficulties with Potassium/argon dating When the sample is prepared for dating the rock is split in a potassium and an argon sub-sample. If the rock is not completely homogeneous the ratios of argon and potassium in the two sub- samples may not be the same and the date will not be accurate. Argon/ Argon dating prevents this problem by measuring the same sample for both isotopes.
Argon / Argon Dating Argon / Argon dating eliminates the potential for heterogeneous mixing of elements in rocks by sampling the same crystal. As in Potassium / Argon dating the argon 40 gas can escape when the rock is molten, so you are dating the time that the rock solidified. A single crystal of a potassium bearing mineral is selected and irradiated in a nuclear reactor to turn a known portion of the potassium 40 into Argon 39. The crystal is then degassed under heat and in high vacuum and the ratio of argon 40 to Argon 39 is measured in a mass spectrometer. The conversion rate from Potassium 40 to Argon 39 must be precisely controlled to obtain an accurate date. This method has been effectively used to date the Pompeii volcanic eruption that buried the Roman city of Herculaneum. Argon / Argon dating measures the last time that the rock solidified and may not record all of the events that the rock has undergone. Discrete sampling is needed to ensure that you are dating the event in which you are interested.
Uranium / Lead Dating in Brief The Uranium Lead decay series is the oldest and most reliable radiometric aging method. This series is useful for dating geological events, not archaeological events, as the entire series has a half-life of 4.6 billion years. Used for rock older than 10 million years Some of the daughter products have short enough half-lives to be used in archaeological contexts. These are called the uranium disequilibrium dating and will be discussed next. An assumption in Uranium / lead dating is that there was no lead in the rock when it was formed. For this reason, zircon crystal are the first choice for dating because they do not bond with lead, but preferentially bond with uranium. The ratio of Uranium 238 to lead 206 establishes the age of the rock. This method dates the formation of the rock.
This is the complex Uranium Lead series. All of these daughter products are radioactive except Lead 206 (Pb 206) in the lower left corner. The half-life of each isotope is given in the number below the isotope. Several components of this decay series are used for dating.
Uranium Disequilibrium Dating One of the properties of Uranium that helps in dating absorption-prone archaeological materials is its solubility in water. These materials are items such as bone, eggshell and other porous materials. These materials, when buried, absorb water containing uranium and the uranium is preferentially deposited in the porous material. This creates a disequilibrium in the quantity of uranium in the archaeological material relative to the sediment in which it is buried and by measuring the difference between the quantity of uranium in the buried object and the sediment the length of time that the item has been buried can be determined. This method measures the time that the porous object has been buried. This assumes that the object has only been buried once. If an abject has been buried more than once the date will reflect a combination of the uranium absorptions. This method is used for objects that have been buried from 1 to 4 or 500,000 years.
Lead 210 Dating Lead 210 is part of the uranium series. Radon 222 is a gas released from sediments and has a half-life of 3.8 days. It decays into Polonium 218 with a half- life of 3.05 minutes, which decays into lead 214 with a half-life of 26.8 minutes, which decays into Bismuth 214 with a half-life of 19.8 minutes, which decays into Polonium 214 with a half-life of 0.16 milliseconds, which decays into Lead 210 with a half-life of 22 years. Lead 210 forms in the atmosphere and is deposited along with other fine particles onto the land and surface of bodies of water where it is incorporated in sediments. Given the relatively short half-life of lead 210, it is used to date the age of sediments back to about 200 years ago. This method is primarily used in studies of recent ecological change and can be used for archaeology to date the sediments in which historical archaeological material has been buried. Lead 210 dates the time since the sediments were deposited by measuring the amount of lead 210 remaining in the sediment.
Non-Carbon 14 Radiometric Dating Review Potassium /Argon dating: Dates the time since the rock was molten, Useful for dating rock older than 100,000 years, used at Olduvai Gorge for dating human ancestors. The rock must be homogenous for Potassium and Argon for the date to be accurate. Method works on Potassium rich volcanic or high-grade metamorphic rocks. Argon / Argon dating: Dates the time since the rock was molten, Useful for dating rocks older than 2000 years. Used on volcanic flows at Pompeii. Method works on same types of rocks as Potassium / Argon dating Uranium /Lead dating: Not used for archaeology, Dates the formation of the rock. Useful for rocks older than 1 million year to 4.5 billion years. Parts of the series are useful to archaeologists. Uranium disequilibrium dating: Dates the amount of soluble uranium the porous material has absorbed. Useful for material buried from 1 to 400,000 or 500,000 years. Method works on porous material like bone or eggshell. This method is not technically a radiometric dating method, but it uses a radioisotope, so it is included here. Lead 210 dating: Dates the amount of time since the sediment was deposited. Primarily used for ecology, but can be used to date sediments for archaeology.
Concepts for Trapped-Charge Dating All natural crystalline minerals contain Imperfections with the crystal lattice. For example, quartz crystals. Natural radiation exists in all sediments from the radioisotopes contained in the sediment. Radiation frees electron from atoms within the crystal lattice and these electron become trapped in the areas of imperfection. The longer that an crystal has been exposed to radiation, and not exposed to heat (over about 350 degrees C) or light, the greater number of trapped electron will exist in the crystal. Exposure to light or heat mobilizes these electron and allows them to either leave the crystal entirely or become captured by another atom reducing to zero the trapped charges in the crystal. When the electron becomes free to move from its trapped position, it releases a photon, which, conceptually is a tiny amount of light. This tiny bit of light can be multiplied and quantified on very sensitive equipment. The quantity of radiation in the sediment around the crystal must be known for the radiation dose over time to be assessed.
Optically Stimulated and Thermo Luminescence (OSL and TL) Dating Optically Stimulated Luminescence can be conducted on quartz or feldspar crystals. Both are very common in sand and generally in the crust of the earth. On quartz, the crystal is exposed to blue or green light and the luminescence is measured in the near ultraviolet. On feldspar, the crystal is exposed to near infrared light and the luminescence is measured in violet light. Thermo luminescence can be conducted on heated flint or chert, or other quartz rich rocks, calcite from caves, quartz or feldspars in sediments that have been buried. But the sample is heated to release the photons. Both methods require that some event has set the electron clock to zero. This can be exposure to either heat or intense sunlight. For archaeological dating both methods can be applied to heated flint knapping stone, buried pottery with a sand matrix or can be applied to sediments in the archaeological context for a bracketing date. Samples for dating can not be exposed to heat or light before being submitted for dating. Thermo Luminescence on a single grain of crystal (selected from a large sample by the dating lab) is the most accurate of these methods.
Trapped charge dating review Optically stimulated and Thermo luminescence dating are very similar techniques. Optically stimulated luminescence uses light and Thermo luminescence uses heat to release the photons. Both methods require that the item dated has not been exposed to heat or sunlight since the object was used by people. Either method is used on quartz or feldspar crystal which are common in sand, flint knapping stone, and as matrix in pottery, as well as bracket dating sediments for archaeological contexts. Artifacts found in caves are ideal candidates for both methods.
The Big Concepts Absolute dating places the culture or cultural event in calendar time rather than in a floating chronology. There are many methods of placing archaeological materials in time, all of them date different events, and all of them have limitations and assumptions that must be taken into account when considering the results that you receive from the lab. All absolute dates must be evaluated for feasibility and appropriateness for your particular situation. There are many more methods of obtaining absolute dates than have been presented here, but these are the main methods that are commonly used in archaeology.