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

Graven, H. D. 2015. Proceedings of the National Academy of Sciences,  Vol. 112, pp. 9542-9545.

Radiocarbon analyses are commonly used in a broad range of fields, including earth science, archaeology, forgery detection, isotope forensics, and physiology. Many applications are sensitive to the radiocarbon (14C) content of atmospheric CO2, which has varied since 1890 as a result of nuclear weapons testing, fossil fuel emissions, and CO2 cycling between atmospheric, oceanic, and terrestrial carbon reservoirs. Over this century, the ratio 14C/C in atmospheric CO2 (Δ14CO2) will be determined by the amount of fossil fuel combustion, which decreases Δ14CO2 because fossil fuels have lost all 14C from radioactive decay. Simulations of Δ14CO2 using the emission scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report, the Representative Concentration Pathways, indicate that ambitious emission reductions could sustain Δ14CO2 near the preindustrial level of 0‰ through 2100, whereas “business-as-usual” emissions will reduce Δ14CO2 to −250‰, equivalent to the depletion expected from over 2,000 y of radioactive decay. Given current emissions trends, fossil fuel emission-driven artificial “aging” of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected. This finding has strong and as yet unrecognized implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.

One of the themes that I have explored on this blog is the intersection of past, present, and future in the Anthropocene.  Specifically, I’m interested in the ways  that the past is still with us, and how we (as a species), our decisions, and our planetary system are hybrids, if you will, of different times. Admittedly, this concept can become abstract to the point of losing intelligibility. So, I’m always on the lookout for case materials where this hybridity is concretely materialized.  Graven’s paper fits the bill.  In it, she examines the impact of current and projected fossil fuel emissions on the abundance of radiocarbon in the atmosphere. Radiocarbon is a naturally occurring isotope of carbon, and is incorporated into the tissue of living organisms. Graven’s paper is relevant not only to sciences that exploit radiocarbon’s properties for a range of studies, but also more broadly it also illustrates how very ancient carbon (from fossil fuels) is incorporated into contemporary organisms.

By way of introduction, let me briefly describe why radiocarbon is so important today.  Chronologies are the lifeblood of historical sciences (e.g. archaeology, geology). Absolute chronologies, in particular, are the scaffolding upon which we are able to reconstruct when (in years before present) particular events occurred, or over what stretches of time processes may have occurred. When combined with environmental or social contexts, we can make informed narratives about the past, and relate how the past may advise contemporary and future practices. In this regard, one of the most important developments in historical sciences is radiocarbon (14C) dating. This method is based on the existence of naturally occurring stable (12C, 13C) and unstable (14C) isotopes of carbon. Carbon isotopes are incorporated into the tissues of living organisms throughout their lives. Once the organism dies, the proportion of 14C relative to stable isotopes decreases at a known rate (the half-life of 14C is 5,700 ± 30 years).  The age of death of preserved tissues can be determined by comparing the ratio of stable to unstable isotopes.  Radiocarbon dating can be used to derive an absolute age estimate of organic materials, such as seeds, bone, wood, or fibers, that are up to 50,000 years old or so.  After 50,000 years there is very little 14C to detect, and soon after the remainder of 14C will have decayed.

Importantly, three primary factors can impact atmospheric 14C’s frequency. Radiocarbon is naturally produced through the bombardment of cosmic rays of the atmosphere.  Natural production has varied through time, but we can calibrate radiocarbon ages to approximate calendar ages in a variety of ways.  However, humans have recently changed the amount of 14C that is produced.  Atomic bomb detonations in the middle of the last century increased the amount of neutrons in the atmosphere, resulting in a higher-than-natural abundance of 14C.  After reaching its climax, the bomb effect has trailed off.  Another impact, and the subject of Graven’s paper, is the introduction of carbon into the atmosphere during the combustion of fossil fuels.  All things being equal, 14C has been depleted from fossil fuels because they are millions of years old.  The “Suess effect” describes the dilution of 14C, in the form of CO2, in the atmosphere by the introduction of stable carbon isotopes.

In this paper, Graven examines how the Suess effect will impact radiocarbon studies in the future through modeling. To skip to the point, she notes that the dilution of 14C due to fossil fuel combustion makes it appear that the atmosphere is “ageing” ca. 30 years per year.  By extension, organisms also appear to be “ageing,” at least as far as radiocarbon dating techniques are concerned.  Graven argues that “Continued business-as-usual growth in emissions will cause the atmosphere to approach a 1,000-y-old radiocarbon age by 2050 and a 2,000-y-old age by 2100.”   So, in the year 2100, organic matter that is created will potentially be of the same radiocarbon age as an organism that died 2000 years ago!

Graven’s study highlights the potential impact of the Anthropocene on future scientific practice without further developments in precision and accuracy.  Although historical sciences will be affected, Graven notes that many other sciences rely on radiocarbon for study (e.g. isotope forensics, ecology, and physiology). More interestingly, this study provides yet another example of the hybridity of the Anthropocene.  Here the carbon that we are releasing as fossil fuel emissions will be measurable in the future. Our excesses are being incorporated into our very fabric, and that of the world, in a way that bends time.

2 thoughts on ““Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century”

  1. Fascinating! So… we will need a different formula for things deposited since the 19th C, and we will need ways to distinguish pre- and post-Industrial Revolution deposits. Context is one (obvious) way, but are there any telltale molecular signs in the remains of organisms that lived before/after the Industrial Revolution?

    • Great question! Off the top of my head I don’t know of any one in particular, but I suspect there are a suite of stable isotopes that could be used. In addition, there are chemical relative dating techniques that when combined with context could be indicators. So too looking at concentrations of particular elements that may occur in higher than pre-industrial concentrations (Mercury, for one) may help.

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