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Archaeology

How we broke radiocarbon dating

The paper:

Graven, HD (2015), Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century, PNAS 112 (31) 9542-9545. doi:10.1073/pnas.1504467112

Radiocarbon dating: an essential tool

How do we tell the age of a newly uncovered human skeleton? Or how long it takes organic matter to cycle through the marine ecosystem? How can the dye in a Renaissance painting be distinguished from modern forgeries? In each case, the answer is radiocarbon dating, which has emerged as a vital tool in scientific disciplines ranging from archaeology to marine ecology. However, carbon dioxide (CO2) emissions are changing the chemical composition of the atmosphere so much that by the end of the century, radiocarbon dating won’t work for dating samples younger than 2000 years old.

Radiocarbon dating works because radiocarbon (14C, a heavy isotope of carbon) is generated in the atmosphere, but is unstable and slowly decays, decreasing by half every 5700 years. After it is removed from the atmosphere (usually during photosynthesis), it no longer gets new 14C, but it keeps decaying. The ratio of 14C to total carbon therefore provides a sensitive indicator for how long ago a carbon-containing sample was produced.

Fossil fuel emissions artificially “age” modern carbon

The utility of radiocarbon dating will be severely dampened over the next century due to fossil fuel emissions. Oil and gas represent the remains of organisms (probably mostly algae) that lived millions of years ago, sank to the ocean floor, and were buried in sediments. By now they have lost all of their radiocarbon. When the fuel is burned, the resulting radiocarbon-free CO2 mixes in with the rest of the atmosphere’s CO2 pool and drives down its average ratio of 14C to total carbon, making the atmosphere appear “older”.

Historical and projected radiocarbon content of the atmosphere. 14C spiked around 1960 due to nuclear weapons testing, and has been drifting back toward the baseline as the excess 14C works its way into the ocean, plants, and soils. The trajectory of radiocarbon content for the rest of the century depends on fossil fuel emission scenarios (Representative Concentration Pathways; RCPs). With aggressive action to limit CO2 emissions, atmospheric radiocarbon will only “age” by a hundred years or so (green line) but under “business as usual” policy, it will be appear over 2000 years old by 2100 (grey line), severely limiting the dating of younger materials. From Graves 2015.

Figure 1: Historical and projected radiocarbon content of the atmosphere. 14C spiked around 1960 due to nuclear weapons testing, and has been drifting back toward the baseline as the excess 14C works its way into the ocean, plants, and soils. The trajectory of radiocarbon content for the rest of the century depends on fossil fuel emission scenarios (Representative Concentration Pathways; RCPs). With aggressive action to limit CO2 emissions, atmospheric radiocarbon will only “age” by a hundred years or so (green line) but under “business as usual” policy, it will be appear over 2000 years old by 2100 (grey line), severely limiting the dating of younger materials. From Graves 2015.

Right now, the radiocarbon content of the atmosphere is declining quickly enough to appear about 30 years older every year. The extent to which the atmospheric radiocarbon content will change (like the extent of global warming) depends strongly on how many fossil fuels are burned. This paper used fossil fuel emission scenarios from the latest IPCC report to calculate the range of possible effects on the radiocarbon content of the atmosphere (Figure 1). Under the most optimistic emissions scenario (which requires not only a complete conversion to clean energy but also “negative emissions” in the form of trapping CO2 and storing it underground), the damage is limited and the atmosphere will only appear to be about a hundred years old. But under more realistic scenarios, the change is more drastic: with no policy changes to discourage fossil fuel usage, emissions are predicted to make the atmosphere appear 2000 years old by the end of the century.

Significance and implications

This isn’t the first time human activities have dramatically affected the radiocarbon content of the atmosphere. Nuclear weapons testing in the late 1950s doubled the amount of 14C in the atmosphere, resulting in a huge spike in its radiocarbon content (Figure 1). The spike then slowly petered out as the new 14C found its way into the ocean, plants, and soil (Figure 2). But since weapons testing added radiocarbon to the atmosphere, it actually proved beneficial to dating applications. Enriched samples can still be traced to a specific year, in fact with better precision than from radioactive decay. Diluting the atmosphere with radiocarbon-free CO2 instead makes modern material appear thousands of years old.

Predicted radiocarbon content in the atmosphere, biosphere, and ocean with aggressive action on CO2 emissions (RCP2.6) and with no policy change (RCP8.5). From Graves 2015.

Figure 2: Predicted radiocarbon content in the atmosphere, biosphere, and ocean with aggressive action on CO2 emissions (RCP2.6) and with no policy change (RCP8.5). From Graves 2015.

The “aging” of the atmosphere will badly muddle applications of radiocarbon dating over a large part of human history. Radiocarbon dating will now yield two possible answers: either very young (due to dilution of the atmosphere with radiocarbon-dead CO2) or old (due to radioactive decay). Under the worst emission scenario, scientists working in 2100 will not be able to accurately date anything younger than the Roman era unless they can definitively prove there is no possibility of contamination with modern material—a high bar to clear at complicated field sites. Archaeologists, oceanographers, and scientists in many other fields will need to develop new methods to accurately date anything in this age range. If climate change and ocean acidification weren’t enough, yet another reason to stop burning fossil fuels!

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