Cosmic rays are emitted continously by the stars, including our sun. Solar flares, novae, and supernovae create wavefronts of cosmic radiation propelled through space. Supernovae waves can take years to pass by the earth, whereas solar flares are measured in hours. Cosmic radiation impacting the earth is also affected by geomagnetic and heliomagnetic forces. The amount of cosmic radiation reaching the earth over time is consistent though showing small hills, valleys, and plateaus.
When cosmic rays collide with certain atoms of the earth and its atmosphere, they can produce radioactive nuclides such as 26Al, 10Be, 14C, 36Cl,and 32Si. Radiocarbon, or 14C, is typically produced in the upper atmosphere when a cosmic ray hits a nitrogen atom. This event transfers mass/energy to a stable nitrogen proton transforming it into an unstable neutron and thus altering the atom's chemical properties to those of carbon which has one less proton than nitrogen. Radiocarbon has also been produced since the 1940's by thermonuclear research, nuclear war, and the nuclear industry.
Radiocarbon quickly combines with oxygen to produce 14carbon monoxide and then into 14carbon dioxide. This molecule diffuses throughout the atmosphere and, more slowly, into the waters, and even into the rocks and soil. Carbon based lifeforms continously incorporate it from their environment while alive. The incorporation of modern 14C stops when the lifeform dies, the water goes underground, the iron is smelted, or air vesicles in lava close. From that point, the 14C concentration diminishes according to its half-life of 5730 ± 40 years. Only contamination by older or newer carbon can alter this time relation. Beyond ten half-lives, or about 57000 years, the 14C concentration (only a trillionth of the total carbon to begin with) has decayed to an almost immeasurable point.
Virtually any carbon showing changes in 14C concentration over time can be measured. Though archaeological, paleontological, and geological studies have likely reaped the greatest benefit, astronomical, environmental, and biomedical applications have also benefited. Atmospheric and tropospheric measures have not only indicated normal 14C concentrations for dating uses, they are also involved in fossil-fuel derived pollution studies, nuclear power plant emissions studies, climate and weather observations, and anything concerning the carbon-exchange cycle. Ice cores and snow drifts record pollution levels, dust storms, climate changes, and volcanic eruptions. When their carbonaceous particle content is high enough, these events can be dated using 14C, and the events themselves being stratified provide an independent calibration for 14C dating and other dating methods.
Water contains carbon derived naturally from atmospheric CO2 exchange and dissolved carbonate rock. Pollution is also a major contributor in many places. 14C concentration of water is therefore diluted by the 'dead' carbon of ancient limestones and fossil-fuel pollutants, and is sometimes greatly increased by nuclear power plant effluents. In order to date plant and animal carbon incorporated within this environment, a 'reservoir age' is determined which reflects these variations. Surface area, depth, currents, and location contribute to reservoir age corrections. Deep aquifers may show changes in 14C concentration that can reflect depletion and concurrent risk of draw-down of younger, possibly polluted, water.
Radiocarbon analysis of soils and sediments provide information about vegetational history, carbon storage and turnover, and volcanic frequency. Total organic matter or various fractions, such as micro- and macro-fossils, charcoal and wood, humic acids, size, and density fractions can be analyzed. Separation is accomplished with hand picking, acid-base treatments, flotation, screening, and magnification. Varves, or yearly sediment deposits, are often used to cross-check 14C and other dating methods. Geologically recent carbonate deposits, such as speleothems, caliche, lime, marl, travertine, and tufa, may have their ages and growth rates estimated with 14C analysis.
Pre-bomb terrestrial vegetation 14C content is primarily reflective of age, atmospheric content, and photosynthetic biochemical pathways. Reservoir effects are large on those growing in water. 14C analysis in plants has contributed to the studies of carbon fixation and release, archaeology, pollution, vegetational history, nuclear emissions, growth rates, and geological history. Its direct comparison to pollen records and tree rings has finely focused our view of prehistory. The tree ring - 14C calibration curve extends unbroken almost 12,000 years.
Faunal 14C content reflects the particular tissue biochemical pathway, age, and diet. Coral radiocarbon/uranium-thorium comparisons reach back to 20000 BC. 14C analysis in animals has been useful to the studies of archaeology, paleontology, climate, extinction, tissue preservation, diet, and proteins. The use of 14C as a tracer in pharmaceutical and biochemical pathways research has been boosted by the production of an AMS unit developed especially for medical applications. Time of death in forensic studies has also been obtained measuring 14C in lipids and hair.
Over the past 50 years, 14C databases have been built detailing changes in the paleolandscape by cataloging human occupations and environmental catastrophes. 14C analysis has contributed much to our understanding of human sociological development, such as, culture types, environmental adaptations, migrations, colonization, arts, and customs. And it has contributed equally well to our understanding of human technological innovation, such as, lithics, agriculture, metallurgy, architecture, ceramics, clothing, writing, and culinary skills.
Contamination by old 14C, such as from fossil fuel fumes, and by new 14C, such as from cigarette smoke, needs to be prevented from sampling thru processing and dating. Sampling and processing vary according to sample type, but are all meant to provide the correct amount of purified component for 14C analysis. Cleaning, crushing, and acid/base treatments are typically used to purify the hydrocarbon specimen, which is then oxidized by combustion producing carbon dioxide and water which is trapped out by freezing. The carbon dioxide is then measured directly, or used to produce a conveniently measurable product such as acetylene for gas proportional counters, benzene for liquid scintillation counters, or graphite for atomic mass spectrometer measurement.
Fractionation is the reduction of isotope content which occurs when an element is processed chemically or physically. 12C, being lighter, survives the processing better than 13C or 14C. The 14C content is reduced twice as far as the 13C content, so part of the carbon dioxide is measured for 13C content to determine the fractionation factor necessary to figure the 14C content measured as grams of original carbon. This is necessary to standardize the results while using various procedures and equipment, and measuring highly varied materials and tissues.
Samples of dead carbon, typically 50000 years old, or older, are processed and analyzed with the original sample to indicate contaminant introduction and the current background level of cosmic radiation. Concurrently, a sample of oxalic acid standard of known 14C content and age is processed in order to produce the factor necessary to convert all measures into the standard 1950 base for exact comparison purposes. The oxalic acid standards are produced by NIST.
The measured 14C content is calibrated into an age determination by factoring in the known variables of fractionation, reservoir age, 1950 standardization, contamination and current background, and known discrepancies in atmospheric content over time. Finally, the error attributed to these factors and the method employed is appended to the age.
The reliability of radiocarbon dating has been ascertained through laboratory intercomparison programs, in which numerous labs determine the 14C content of the same sample - such as a plank of wood, or mammoth tusk - and through comparison with other dating procedures including dendrochronology, varve analysis, U/Th, ice cores, and archaeological, paleontological, and palynological methods.