Radiocarbon calibration is vital for coming up with chronologies in archaeological and palaeoenvironmental studies. Radiocarbon dating measurements produce radiocarbon ages that you convert to calendar years through calibration. The measurements are presented in terms of years, Before Present (BP).
The calibration is essential as the atmospheric ratio used in calculating the ages hasn't been consistent.
Here, we'll look at the agreed international radiocarbon curves and features of the curves.
Carbon dating refers to the method of determining the age of an object that contains organic material. The dating method also referred to as carbon-14 dating or carbon dating, uses properties of radiocarbon or a radioactive isotope of carbon.
Since the 1960s, there has been researching to determine the proportion of radiocarbon (14C) in the atmosphere for the last fifty years. In the form of a radiocarbon curve, the results from the research are used to convert radiocarbon measurements into calendar years.
Picture source: https://pixabay.com/photos/tree-year-annual-rings-wood-ring-73363/
You can determine the ages of a carbonaceous sample by comparing the carbon content to tree rings with known calendar years. If the carbonaceous sample has the same radiocarbon content as the tree rings, you can conclude that they are the same age - fortunately, there is calibration software you can use to help such as Halcon, Kalibr, and PhotoModeler.
Many trees lay down a ring every year. Once the ring is laid down, it is not changed during the life of the tree. The rings are helpful as records of the radiocarbon concentration in the past. If the tree is a hundred years old, you can measure the radiocarbon in the rings and determine the concentration corresponding to each calendar year.
However, the tree-ring calibration may not be very straightforward for various reasons. For instance, the individual measurements on the tree rings may not be precise, which results in a range of calendar ages being obtained.
As such, the results of the radiocarbon calibration are presented as a range instead of an absolute value. The ranges are calculated using the probability of intercept method, both of which require a calibration curve.
The intercept method uses intercepts on a graph, while the probability method may require a computer as the calculations are complicated.
The results from measurements of the tree rings are compiled into calibration curves. The calibration curves by IntCal Group form the basis of the calibrations by programs such as OxCal and CALIB.
The first calibration curve used in determining the dates stretches back 8000 years, and it was based on a continuous tree sequence.
Hans Suess used the tree-ring sequence by Wesley Furguson in the 1960s to develop the first useful calibration curve. The curve by Has Suess was based on bristlecone pine, and it used tree rings as the calendar axis.
After the Suess'curve, other calibration curves have been developed but didn't receive as much proliferation. The dating and the use of accelerator mass spectrometers have also been used to generate calibration curves.
A laboratory published a high-precision calibration curve in Belfast, Northern Island, based on an Irish oak. Today, internationally accepted calendar calibration curves stretch back about 48,000 BC (Reimer et al., INTCAL13 and Marine13 radiocarbon age calibration curves 0 – 50000 yrs cal BP, Radiocarbon 55(4), 2013).
A lot of data is available about atmospheric 14C concentration available after 1950. The post-modern data is vital in determining the calendar age of very young materials (Hua, et al., Atmospheric Radiocarbon for the period 1950-2010, Radiocarbon, 55(4), 2013).
A calibration curve contains a calendar or dendral timescale on the x-axis and the radiocarbon age represented on the y-axis.
The concentration of radiocarbon varies between the atmosphere and the ocean. The radiocarbon samples from the marine environment need a marine-specific curve.
The dating results should include the uncalibrated results, the calibration method used, and the corrections made to the original result before the calibration. You must also include the confidence level of the calibrated ranges.
Dendrochronologically calibrated radiocarbon results are cited using cal BP, cal BC, or cal AD. The dating results should be clear and should not be reported simply as BP, BC, or AD. BP refers to how many years before 1950, while BC and AD correspond to historical years.
Carbon is available in three isotopic forms, including carbon-12, carbon-13, and carbon-14. The numbers refer to the atomic weight, meaning carbon-12 has 6 protons and 6 neutrons, carbon-13 has 6 protons and 7 neutrons, while carbon-14 has 6 protons and 8 neutrons. The neutrons in the carbon-14 make it radioactive, hence the name radiocarbon.
Carbon-14 is produced in the upper atmosphere after Nitrogen-14 isotopes have been impacted by cosmetic radiation. It is then taken by plants through photosynthesis, while the plants are in turn taken by organisms.
As such, all living things contain an amount of carbon-14 within them. When the organism dies, the radiocarbon reduces through decay. The process is referred to as the half-life of the isotope. It takes about 5730 years for half the atoms in a radioactive isotope to decay.
Half-lives differ depending on the isotope. For instance, nitrogen has a half-life of 4,173 seconds while Uranium-238 has 4,500 years.
Measuring the age of corals is essential in paleoclimatology studies. You can use a combination of uranium-thorium and radiocarbon dating for deep-sea corals. The radiocarbon dates represent the age of the water and the corals, while uranium-thorium represents the coral itself.
The various ages can offer information regarding circulation rates in the deep sea.
Coral polyps are organisms that feature a calcium carbonate skeleton. The organisms multiply to form the coral leaf structures, which are colorless. Isotopic studies on corals can also provide variations in ocean temperatures.
The carbon-14 dating and isotopic study of corals are vital in understanding corals' growth in different climatic conditions.
Radiocarbon calibration is essential in archeology and palaeoenvironmental studies. The calibrations help in coming up with radiocarbon ages that you can convert into calendar ages.
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