According to literature, nuclear weapons testing in the 1950s and 1960s have nearly doubled the atmospheric carbon 14 content as measured in around 1965. The level of bomb carbon was about 100% above normal levels between 1963 and 1965.

1,021 nuclear tests
Nuclear Testing Between 1951 and 1992, the U.S. government conducted a total of 1,021 nuclear tests here. Out of these tests 100 were atmospheric, and 921 were underground.

1945
The history of nuclear testing began early on the morning of 16 July 1945 at a desert test site in Alamogordo, New Mexico when the United States exploded its first atomic bomb.

In this instance, a 1280-feet-in-diameter and 320-feet-deep explosion crater, morphologically similar to an impact crater, was created at the Nevada Test Site. Shot Divider of Operation Julin on 23 September 1992, at the Nevada Test Site, was the last U.S. nuclear test.

On March 1, 1954 the United States tested an H-bomb design on Bikini Atoll that unexpectedly turned out to be the largest U.S. nuclear test ever exploded. By missing an important fusion reaction, the Los Alamos scientists had grossly underestimated the size of the explosion.

#30,000-Year Limit The Lamont-Doherty group says uranium-thorium dating not only is more precise than carbon dating in some cases, but also can be used to date much older objects.

Radiocarbon Dating and Bomb Carbon. One of the assumptions of the radiocarbon dating method is that the global concentration of carbon-14 has not changed over time. Nuclear weapons testing has increased the global radiocarbon levels.

Krane points out that future carbon dating will not be so reliable because of changes in the carbon isotopic mix. Fossil fuels have no carbon-14 content, and the burning of those fuels over the past 100 years has diluted the carbon-14 content.

In 1945, Libby moved to the University of Chicago where he began his work on radiocarbon dating. He published a paper in 1946 in which he proposed that the carbon in living matter might include 14. C as well as non-radioactive carbon.

Radiocarbon dating needed an organic material that was not contaminated with carbon 14 from fossil fuel burning or nuclear weapons testing. Oxalic acid stocked by the U.S. National Bureau of Standards had been adopted as standard for radiocarbon dating.