Radiometric dating as practiced is reliable.

1. Radiocarbon Dating Radiocarbon dating was discovered by Willard Libby, a nuclear chemist at the University of Chicago. He announced his discovery of carbon dating in 1949. Scientists immediately wondered if the method was reliable. The tests they applied included many test of ancient artefacts having known dates. The method worked so well and proved so reliable that Libby was awarded the Nobel Prize in Chemistry in 1960. [1. http://en.wikipedia.org...] Carbon dating only works to date things that were once living. Radioactive carbon 14 is continually generated by cosmic rays in the upper atmosphere. Plants take in the carbon as carbon dioxide, and animals eat the plants. Once carbon 14 is taken out of exchange with the atmosphere, the radioactive carbon starts to decade to stable carbon 12, and the proportion of the radioactive isotope decreases. Measuring the ratio of the isotopes implies the time since the exchange with the atmosphere stopped. Scientists tested the reliability of the method using blind tests of archaeological artefacts having known dates. The first sample was from Zoser’s tomb at Saqqara, Egypt, dated by archaeologists to be 4,650 years old. The radiocarbon date was within 1%. The second sample, supposedly from the the time of Ptolemy, produced a radiocarbon date of zero. It turned out the ample was from a "reputable dealer" in Cairo, and was a modern fake. In the early 50s hundreds of samples were dated and the reliability of the method established. [2. Macdougall, Doug (2008-05-31). Nature’s Clocks: How Scientists Measure the Age of Almost Everything, University of California Press. Kindle Edition locations 1016-1017]. The many checks of the method showed that the dates being obtained were averaging a few percent too young. Libby did not have the precise half-life of carbon-14. That was corrected., with 5,568 years updated to 5,730 years. At a finer level, dating tree rings showed that the amount of carbon-14 in the atmosphere varied by a few percent due to variation in the level of cosmic radiation. By using overlapping patterns of tree rings, calibration back 11,000 years is achieved. Approximate methods go back to the useful extent of carbon dating, about 40,000 years. “Carbon-14 dating is now so widespread that there is an entire scientific journal, published three times each year, dedicated exclusively to the results of radiocarbon research. It is called simply Radiocarbon, and a typical issue may contain articles about applications in areas as diverse as archaeology, geology, oceanography, and climate change.” [2. op cit 1207-1209, also http://www.radiocarbon.org...] 2. Uranium-Lead dating The decay of naturally occurring isotopes is used for dating for dating rocks. “For uranium-lead dating... it is virtually impossible to know the sample’s original content of the radioactive isotope used for dating. So, instead of determining how much of the isotope has decayed away, the important measurement is of the amount of daughter isotope that has accumulated—the product of the radioactive decay. By plugging this value into the radioactive decay equation, together with the present-day content of the radioactive parent (another value that can be measured directly), an age can be calculated.” [2. op cit 1659-1664]. Ultimately it turned out that four quantities should be measured: two isotopes of uranium and their lead decay products. This “provides a kind of built-in cross-check for dating. If ages calculated from each of the two decay paths agree, it is likely that the date is correct. This feature makes uranium-lead dating the most widely used technique for high-precision age measurement.” [2. op cit 2359-2360] The half-life of uranium is 704 million years, so very old rocks can be dated. One of the practical problems in uranium-lead dating was achieving isolation from environmental lead. By about 1960, the problems had been worked out and reliable dates were obtained. The way that reliability is ensured is by cross-checking among dating methods, and by showing consistent results from different samples. 3. Multiple techniques “The utility of using multiple techniques was shown dramatically in work by H. Baadsgaard, a professor at the University of Alberta, Canada, and his colleagues in 1993. These researchers used all three methods ... to date samples from a volcanic ash layer in sedimentary rocks from western Canada. By separating different minerals from the ash layer, Baadsgaard was able to find material suitable for each of the dating techniques—zircon crystals for uranium-lead dating, potassium-rich mica for potassium-argon dating, and feldspar crystals for the rubidium-strontium method. When the researchers compared their results, they found no discernible difference among the dates. Three completely independent techniques gave ages of 72.5, 72.5, and 72.4 million years, in each case with a measurement uncertainty of just a few tenths of a million years.” [2. op cit 2369-2372] As scientists work to get higher precision in the dating they discovered small errors. The zircon crystals used uranium lead dating can “leak” very small amounts of lead near the surface of the crystal. Etching away the surface removes the depleted zircon. Crystals previously dated at 251 million years were re-dated to 252.6 million years. The different minerals were processed at different labs." [3. Mike Walker. Quaternary Dating Methods (p. 5)] Potassium-Argon and Argon-Argon dating “When in a molten state, volcanic rocks such as lavas or the crystal components of volcanic ashes will release any 40Ar that is generated by the decay of 40K. Once the rocks or minerals begin to cool, however, argon can no longer escape and is trapped within the mineral crystal lattices where its abundance increases over time. In the laboratory, the 40Ar concentrations in rock samples can be determined by heating them to the melting point in a furnace. ... A second sample is used to determine 40K content using an atomic absorption spectrophotometer or a flame photometer. The ratio between 40K and radiogenic 40Ar then gives an indication of the time that has elapsed since the volcanic event occurred. [3. op cit pp. 58-59] The Ar/Ar method voids the problem of measuring the amount of potassium in the sample. Ar/Ar dating is a complex process that involves incremental heating and compensation for atmospheric oxygen. When properly performed, the method is reliable. A test was done on sanidine (feldspar) crystals from Pompeii formed during the eruption of Mount Vesuvius: "... an isochron age of 1925±94 years was obtained. This is in excellent agreement with the Gregorian calendar-based age (1918 cal. years ago) of the eruption. Although excess argon … was present in the sample, this could be detected by careful laboratory analysis of the sanidine and corrected for. This example shows that a sample less than 2000 years old can be dated with better than 5% precision, and validates the 40Ar/39Ar technique as a reliable geochronometer into the Late Holocene." [3. p. 65] The Late Holocene is the last few thousand years. Checks with non-radiometric methods We have mentioned tree rings. Ice-cores from Greenland and Antarctica provide dating back over 100,000 years. There are also varves (sediment layers in lakes and estuaries laid down seasonally) and coral growth rings. Recently developed methods include optically stimulated luminescence [4. http://en.wikipedia.org...] that measures the time since a mineral crystal was last exposed to sunlight. Cosmogenic nuclide dating measures how long a surface has been exposed to cosmic rays. [5. http://en.wikipedia.org...] We know radiometric dating is reliable because it has been checked against dated archaeological objects, bcause different radiometric methods agree, and becaue the radiometric dates agree with independent methods.