"Global Warming Could Make Carbon Dating Impossible"

[krispos42]

Global Warming Could Make Carbon Dating Impossible

The technique is used to determine the age of organic artifacts in fields like archaeology, geology, and ecology. But it could become unreliable within decades.

Adrienne LaFrance
Jul 23, 2015

Since the 1940s, scientists have used carbon dating to determine the age of fossils, identify vintages of wine and whiskey, and explore other organic artifacts like wood and ivory. The technique involves comparing the level of one kind of carbon atom—one that decays over time—with the level of another, more stable kind of carbon atom.

The approach was a sensation when it was introduced. The chemist who developed carbon dating, Willard Libby, won the Nobel Prize for his work. “Seldom has a single discovery in chemistry had such an impact on the thinking in so many fields of human endeavour,” one of Libby's colleagues wrote at the time, according to the Nobel Foundation.

<snip>

By 2050, Graven wrote in a paper published this month in the Proceedings of the National Academy of Sciences, the large amount of carbon dioxide in the atmosphere will make new organic material appear to be 1,000 years old based on today’s carbon-dating models. By the year 2100, the atmosphere will have a radiocarbon age of 2,000 years old.

<more>

http://www.theatlantic.com/technology/archive/2015/07/global-warming-carbon-dating/399398/

[bananas]

1. Nuclear testing had a much larger effect, as evident in Figure 1 in the paper.

The article links to the paper: http://www.pnas.org/content/early/2015/07/15/1504467112.full.pdf

Looking at Figure 1, just a few years of open air testing changed C14 levels almost three times as much as fossil fuels will by 2100: 700 vs 250 (whatever units the graph is in). Carbon dating still has to adjust for nuclear testing. If we hadn't stopped open air testing, carbon dating would have become useless a long time ago.

An excerpt from the paper in the OP:

http://www.pnas.org/content/early/2015/07/15/1504467112.full.pdf

Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century

Heather D. Graven

<snip>

The apparent “aging” of the atmosphere—i.e., the decreasing
trend in the ratio 14C/C of CO2 (reported as ?14CO2) (5)—was
interrupted in the 1950s when nuclear weapons testing produced
an immense amount of “bomb” 14C that approximately doubled
the 14C content of the atmosphere. Direct atmospheric observations
began in the 1950s, capturing the rapid rise of ?14CO2
and its subsequent quasi-exponential decay as the bomb 14C
mixed into oceanic and biospheric reservoirs (6–9) (Fig. 1).

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From its present value of ?20‰ (18), which signifies a 2%
enrichment in 14C/C of CO2 above preindustrial levels, ?14CO2
is certain to cross below the preindustrial level of 0‰by 2030,
but potentially as soon as 2019 (Fig. 1, Table S2). After 2030,
simulated ?14CO2 trends diverge according to the continued
growth, slowing, or reversal of fossil fuel CO2 emissions in the
RCP scenarios.

<snip>

In the low-emission RCP2.6 simulation where fossil fuel
emissions decrease after 2020, ?14CO2 remains nearly constant
around ?15‰ through the end of the century (Fig. 1).
Fossil fuel emissions in the RCP4.5 and RCP6.0 scenarios
continue to rise and then peak later, around 2040 and 2080,
reducing ?14CO2 to a level of ?80‰ (RCP4.5) or ?150‰
(RCP6.0) in 2100.

The business-as-usual emissions in RCP8.5 reduce ?14CO2
more rapidly and more dramatically than the other RCPs:
?14CO2 is less than ?100‰ by 2050 and reaches ?250‰ in
2100, which means that atmospheric CO2 in 2100 is as depleted
in 14C as the “oldest” part of the ocean (19) (Fig. 2).

<snip>

Here's another reference:

http://www.radiocarbon.com/carbon-dating-bomb-carbon.htm

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. The level of bomb carbon in the northern hemisphere reached a peak in 1963, and in the southern hemisphere around 1965.

Implications of the Bomb Effect on Radiocarbon Dating

The change in global radiocarbon levels brought about by human activities necessitated the use of a reference standard for carbon 14 dating. 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. Its radiocarbon content was theoretically the same as a wood sample grown in AD 1950, the zero point of the radiocarbon timescale used in quoting carbon dating results.

Long-term Effects to Radiocarbon Levels

Even after nuclear weapon testing was banned, the bomb effect still remains. According to literature, the excess carbon 14 produced during nuclear weapons testing has already decreased due in part to the global carbon exchange cycle. By the 1990s, the carbon 14 level was only about 20% higher than the theoretical 1950 level as measured by the activity of the oxalic acid reference standard.

[Igel]

2. ?

Bomb C14 created a gap that wouldn't be important for centuries. Suddenly the amount of C14 spiked. You rely on a 1-to-1 mapping between C14/C ratios and age for carbon dating, and while there was a discontinuity there was still a 1-to-1 mapping. There have always been variation in C14 production, a baseline figure + variation.

When I was very young you just really got one C14 date. It was wrong, but it was a good reference point. Later, there were two C14 dates reported. One was the raw # from the lab, the other was the calibrated number from dendrochronology and other proxies. There are nifty graphs showing the calibration factors and C14 formation rate variation (with the amount of C14 being the sum of the area under the curve minus natural decay). I don't recall seeing double dates for quite a while, so I presume that they just use the calibrated C14 dates. This made using Soviet archeology articles a bit sketchy, esp. if they were citing older work--you never knew if they hadn't caught up with calibration or had already moved to entirely calibrated dates. Fortunately the few hundred years' difference seldom mattered.

With a reservoir of old carbon being released the amount of C14 is going to appear to dip. It means that something that's a year old and something that's 400 years old might (in a few years' time) have the same C14/C ratio. In other words, for a portion of the time span there won't be a 1-to-1 mapping. At a certain point in the past that mapping, that 1-to-1 relationship, would still hold but as more old carbon is put in the atmosphere that ambiguous time interval would increase.

Then, if no old carbon is put in the air, the curve will fold back on itself and eventually produce a triple date for any C14/C ratio. It'll make a certain portion of the time interval ambiguous--they'd have to rely on additional information for disambiguation.

It pays to note that there have been periods in the C14-dating range that have been significantly warmer. I don't know that they've (the boffins that specialize in C14 dating) have considered that previous warm periods might have released old carbon reservoirs and caused the same ambiguity in the past, or if they have and concluded that the change in the record is either accounted for by calibration or, if far enough back that dendrochronological calibration is unwieldy, negligible.