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Fading of the 14C bomb peak – students’ project to observe the Suess effect

Published online by Cambridge University Press:  14 November 2025

Irka Hajdas Open the ORCID record for Irka Hajdas [Opens in a new window] ,
André Albrecht ,
Danuta Michalska Open the ORCID record for Danuta Michalska [Opens in a new window] ,
Céline Mikosch ,
Urs Ramsperger Open the ORCID record for Urs Ramsperger [Opens in a new window]  and
Lucyna Wasowicz
Irka Hajdas*
Affiliation:
Laboratory of Ion Beam Physics, ETH Zurich, Switzerland
André Albrecht
Affiliation:
Laboratory of Ion Beam Physics, ETH Zurich, Switzerland
Danuta Michalska
Affiliation:
Geochronology Research Unit, Institute of Geology, Faculty of Geographical and Geological Sciences, Adam Mickiewicz University, ul. Bogumiła Krygowskiego 12, 61-680 Poznań, Poland
Céline Mikosch
Affiliation:
Laboratory of Ion Beam Physics, ETH Zurich, Switzerland
Urs Ramsperger
Affiliation:
Laboratory of Ion Beam Physics, ETH Zurich, Switzerland
Lucyna Wasowicz
Affiliation:
Laboratory of Ion Beam Physics, ETH Zurich, Switzerland
*
Corresponding author: Irka Hajdas; Email: hajdas@phys.ethz.ch
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Abstract

The natural variability of atmospheric 14C has been significantly altered by anthropogenic activities linked to technological advancements and energy consumption over the past two and a half centuries. The Suess effect, a consequence of the combustion of old carbon (fossil fuels) since the mid-18th century and the bomb peak from the mid-20th century’s thermonuclear tests, has obscured the natural 14C signal in the atmosphere. This study presents a 14C analysis of leaves, flowers, and grass collected from various locations worldwide. Over the last 10 years, more than 150 samples have been collected and used as materials for experiments conducted by students in physics lab classes (Department of Physics, ETH Zurich) or as part of school projects. Short-lived vegetal fragments are ideal material for teaching radiocarbon dating and demonstrating our research. The collection of data presented here underscores the sensitivity of radiocarbon analysis for detecting fossil carbon components. Trees from urban sites worldwide demonstrate a dilution of the atmospheric 14C concentration of 2–3%. Trees growing close to busy roads and traffic show a dilution of up to 10%. Moreover, the data show a fading trend of the bomb peak observed from 2015 to the present, as well as the direct impact of fossil CO2 on the 14C concentration of the living biota around us.

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Conference Paper
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Radiocarbon , First View , pp. 1 - 9
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
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© The Author(s), 2025. Published by Cambridge University Press on behalf of University of Arizona

Introduction

About the bomb peak and the Suess effect

The natural variability of atmospheric 14C necessitates calibration and can affect the precision of the calendar ages. The radiocarbon time scale is punctuated by periods of such an imprecise 14C clock (for example, the Younger Dryas period). However, natural changes have no comparison to the anthropogenic impact on the atmospheric 14C during the last 250 years. Although humans have shaped their surrounding environment for millennia, the unprecedented changes to the Earth System (Syvitski et al. Reference Syvitski, Waters, Day, Milliman, Summerhayes, Steffen, Zalasiewicz, Cearreta, Gałuszka, Hajdas, Head, Leinfelder, McNeill, Poirier, Rose, Shotyk, Wagreich and Williams2020) have their roots in the Industrial Revolution. The shift to fossil carbon as the source of energy has disturbed the atmospheric 14C content. In 1955, Suess documented this by measuring the annual rings from the last 300 years (Suess Reference Suess1955). His observation was possible thanks to the tree rings, which recorded the annual conditions over millennia. The second anthropogenic impact on atmospheric 14C was underway during his study. The series of above-ground thermonuclear explosion tests followed the Ivy Mike detonation on Elugelab (“Flora”) Island, Enewetak Atoll, in the morning (local time) on November 1, 1952 (Zander and Araskog Reference Zander and Araskog1973; Bergkvist and Ferm Reference Bergkvist and Ferm2000). The intensity of tests reached its highest point in 1963, shortly before the partial ban was signed in November of that year. Interestingly, Rafter and Fergusson, who established a radiocarbon laboratory in New Zealand (see Turnbull et al. Reference Turnbull, Lowe, Manning and Sparks2021), were the first to observe the bomb peak 14C in the Southern Hemisphere (Rafter and Fergusson, Reference Rafter and Fergusson1957). The follow-up measurements and new applications of the 14C spike have been established in various research fields. As summarized by Kutschera (Kutschera Reference Kutschera2022) and others (e.g., Nydal and Gislefoss Reference Nydal and Gislefoss1996; Horvitz and Sternberg Reference Horvitz and Sternberg1999; Levin and Hesshaimer Reference Levin and Hesshaimer2000; Wild et al. Reference Wild, Arlamovsky, Golser, Kutschera, Priller, Puchegger, Rom, Steier and Vycudilik2000; Geyh Reference Geyh2001; Fedi et al. Reference Fedi, Caforio, Mando, Petrucci and Taccetti2013; Handlos et al. Reference Handlos, Svetlik, Horackova, Fejgl, Kotik, Brychova, Megisova and Marecova2018; Hajdas et al. Reference Hajdas, Koutouzis, Tai, Hendriks, Maurer and Rottig2019, Reference Hajdas, Ascough, Garnett, Fallon, Pearson, Quarta, Spalding, Yamaguchi and Yoneda2021, Reference Hajdas, Calcagnile, Molnar, Varga and Quarta2022) the bomb peak as a tracer was a “silver lining” of the controversial and tragic weapon testing. The combined data sets of the bomb peak 14C measurements around the globe (Nydal and Lovseth Reference Nydal and Lovseth1965; Nydal and Lövseth Reference Nydal and Lövseth1983, Reference Nydal and Lövseth1996; Hua et al. Reference Horvitz and Sternberg1999, Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, De Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022) helped to achieve calendar ages for measured values of F14C (Reimer et al. Reference Reimer, Brown and Reimer2004).

Following its discovery, the fate of the bomb peak 14C, its geographic distribution, and its transfer from the atmosphere to other carbon reservoirs were intensively studied (Nydal and Lovseth Reference Nydal and Lovseth1965, Reference Nydal and Lovseth1970). The ocean absorbed a large portion of the bomb peak; however, fossil fuel CO2 emissions, which continue to rise, led to a lower atmospheric 14C/12C ratio. The observations were made in highly polluted (high CO2 emissions) urban areas (Levin et al. Reference Levin, Kromer, Schmidt and Sartorius2003, Reference Levin, Hammer, Kromer and Meinhardt2008, Reference Levin, Kromer and Hammer2013), but it was also predicted that the Suess effect would eventually overprint the global bomb peak signal (Levin et al. Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010). The case of the fading bomb peak gained a wider audience after publication by Graven (Reference Graven2015) and even received some media attention. Many users and collaborators asked questions about the future of radiocarbon dating. Indeed, the predicted change of atmospheric 14C is closely related to the current and near-future emissions of fossil fuel CO2. If the emissions remain high, the green plants and animals living at the end of this century will be depleted in 14C to a magnitude that measured F14C will be around 0.8, compared to the pre-anthropogenic value F14C = 1. Such development poses problems for future research in forensics, art, and archeology.

Monitoring of the atmospheric 14C in the atmosphere around the globe has been performed since the 1960s), and decades of efforts (Reference Turnbull, Lowe, Manning and SparksTurnbull et al. Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017; Seiler et al. Reference Seiler, Grootes, Svarva and Nadeau2023) have resulted in a network of instrumental data collection (Wang et al. Reference Wang, Broquet, Ciais, Chevallier, Vogel, Wu, Yin, Wang and Tao2018). The collection of atmospheric CO2 for the subsequent 14C analysis has undergone technical development (Gautschi Reference Gautschi2017), but it still requires organizational efforts. In contrast, as demonstrated by studies on trees (Hua et al. Reference Hua, Barbetti, Zoppi, Chapman and Thomson2003) and recent research on leaves and grass (Varga et al. Reference Varga, Barnucz, Major, Lisztes-Szabó, Jull, László, Pénzes and Molnár2019, Reference Varga, Jull, Lisztes-Szabó and Molnár2020), vegetation provides a straightforward method for collecting data, even in the most remote regions.

The publication by Graven (Reference Graven2015) coincided with an ongoing school project that focused on the bomb peak 14C signal in leaves of trees from the surroundings. So far, this education project has been going on for more than 10 years and has involved high school and ETH students (physics lab). The primary goal is to observe the local changes in 14C (ETH, Zurich) and at remote geographic locations. The second goal is to allow laboratory involvement in teaching. Additionally, these data can support research projects where the local 14C signal is relevant. The dataset documents the transition from the bomb peak elevated atmospheric 14C to the 14C-depleted atmosphere.

Methods

Sites and samples

The choice of samples was arbitrary and related to opportunities to collect the leaves (home location, travel). Students and colleagues were asked to document the location of the samples. Over the years, one of the authors (IH) has collected leaves from the same trees. A map, prepared to the best of our knowledge (Figure 1), shows the locations of the sites. Three locations in Zurich and nearby regions have the highest number of samples. A couple of trees were sampled annually or biannually. The sites included trees at (1) an urban Zurich (409 m a.s.l.) with different intensities of traffic (2) a campus ETH Hönggerberg (520 m a.s.l.) located slightly above the urban Zurich and on the edge of a forest; (3) a village Boppelsen country site near Zurich (521 m a.s.l.). Locations of sites worldwide are shown on a map (Figure 1) and listed in Table S1 (supplementary). Most samples originated from Europe, but a few were also collected from other continents. For example, samples from Accra Airport (Ghana) and Faleme Valley (Senegal) were among the first to be collected for this study.

Figure 1. Google Maps shows sites where samples were collected (see the link Leaves sampling sites)

Fresh annual leaves were mainly collected during their growing season. A few samples were prepared from dry leaves collected on the ground, possibly from previous growing seasons. Table 1 summarizes all the sites; details are included in the supplementary section. Figure 2 shows the two trees repeatedly sampled: Persian ironwood (Parrotia persica) at the campus ETH Hönggerberg and a poplar tree (Populus nigra ‘Italica’) in Boppelsen (near Zurich).

Table 1. Overview of the number of samples from various locations (Figure 1)

Figure 2. Pictures of two trees that were sampled multiple times in the last 10 years: (a) Persian ironwood (Parrotia persica) at the ETH Hoenggerberg Campus (HPM building), (b) Poplar (Populus) tree in Boppelsen. Leaves of corn (on the right) were also sampled and analyzed.

14C analysis

Only the first acid step from the standard ABA treatment was applied to remove possible carbonates that might be present in dust and potentially contaminate the leaf’s surface. The simplified treatment allows one to complete the treatment in one day, which is convenient when working on short (a few days) school projects, as well as for teaching physics labs. After soaking in 0.5 M HCl, 60°C for an hour, and multiple washes in MilliQ water, the leaves were freeze-dried (overnight). Dry leaves are weighed into Al – boats for combustion in an Elemental Analyzer. Samples of one milligram of carbon require approximately 3 milligrams of dry leaves to be combusted. The subsequent graphitization was completed using the AGE graphitization system (Nemec et al. Reference Nemec, Wacker and Gaggeler2010). The graphite was then pressed into the Al cathodes for analysis using the AMS MICADAS or LEA (Ramsperger et al. Reference Ramsperger, De Maria, Gautschi, Maxeiner, Müller, Synal and Wacker2023; Synal et al. Reference Synal, Stocker and Suter2007; Wacker et al. Reference Wacker, Bonani, Friedrich, Hajdas, Kromer, Nemec, Ruff, Suter, Synal and Vockenhuber2010) systems. The measured 14C concentration is given in F14C (Reimer et al. Reference Reimer, Brown and Reimer2004).

The fossil carbon Fraction (%fF) or dilution for the different samples was estimated using the equation shown below (after Quarta et al. Reference Quarta, Rizzo, D'Elia and Calcagnile2007; Varga et al. Reference Varga, Barnucz, Major, Lisztes-Szabó, Jull, László, Pénzes and Molnár2019). Due to the lack of information about additional sources of carbon dioxide, such as the decomposition of organic matter, no corrections were applied.

%fF=(1-F14Csample/F14Cclean)x100%

Here, the F14C sample is measured in collected leaves, grasses, and flowers, and the F14C clean is the annual mean value estimated by Jungfraujoch (JFJ) (Leuenberger et al. Reference Leuenberger, Levin and Hammer2024). For simplicity, the JFJ site was chosen as the background site for all the samples.

Results and Discussion

Results of the AMS analysis (F14C, with a standard precision of approximately 0.003) are reported in Table S1 (Supplementary Material). Figure 3 presents all the results in comparison to the NH1 bomb peak data (Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, De Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022) and supplemented by the data set from the JFJ (Leuenberger et al. Reference Leuenberger, Levin and Hammer2024). The JFJ data represent a clean environment that is not polluted by local fossil carbon emission sources.

Figure 3. Results of 14C analysis of leaves compared to the values for “clean air” NH1 (Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, De Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022) and JFJ data (Leuenberger et al. Reference Leuenberger, Levin and Hammer2024): (a) ETH Zurich Campus Hoenggerberg, (b) Boppelsen (near Zurich), (c) other locations in Switzerland (CH), (d) sites around the world (see Figure 1)

At all locations, the atmospheric 14C concentration was declining, and at some locations, it reached the F14C < 1 earlier than at others. Apparent differences can be observed between clean and urban locations, even within the same area, such as the Campus Science City ETH Hönggerberg (Figure 3a) and the rural region of Boppelsen (Figure 3b). Our other sites in Switzerland (Figure 3c) and around the world (Figure 3d) have a wide range of environmental cleanness from natural remote regions (natural parks and sanctuaries) to urban areas of mega cities (Bangkok, Tokyo). Nevertheless, the observed trend of declining 14C levels is present in all sites.

ETH Science City Campus Hönggerberg Zurich

The F14C values measured at the campus indicate the presence of fossil carbon CO2 in the atmosphere. The campus is situated on a forested hill (520 m a.s.l.) away from traffic, resulting in relatively clean air. However, sources such as the extensive underground parking lot and the frequent bus connection are potential sources of fossil CO2. In addition, a contribution from the Zurich urban areas surrounding the hill contributes to the observed low atmospheric F14C.

The values of %fF for the HPM tree fluctuate between 1 and 2%, which is comparable to other observations from urban areas (Varga et al. Reference Varga, Barnucz, Major, Lisztes-Szabó, Jull, László, Pénzes and Molnár2019). The grasses and flowers show a higher %fF, with the highest of 2.5% observed for a buttercup flower collected in spring 2020. The only ‘negative’ dilution values (i.e., F14C higher than clean JFJ air) are observed for a dry leaf (2018) collected from the ground. Other locations at the campus show a similar degree of dilution, with the lowest values at locations close to the forest (HPK building) and “Biennenkomission” the location of bee hives at the campus (Figure 4a).

Figure 4. Fossil fuel dilution effect (%fF) based on F14C measured in leaves: (a) ETH Zurich Campus Hoenggerberg, (b) Boppelsen (near Zurich), (c) other locations in Switzerland (CH), (d) sites around the world (see Figure 1)

Boppelsen and other locations in Switzerland

Despite its proximity to Zurich’s urban areas and surrounding industrial and traffic infrastructure, the trees and shrubs growing in the forest and fields surrounding the village exhibit a relatively low fossil fuel dilution effect. The values of %fF fluctuate between −0.4 and 1.4% (Figure 4 b). There was however one exception, leaves collected in November 2023 at the “clean” location exhibit higher dilution than in previous years.

Other locations in Switzerland show a wider range of dilution. The highest level of 2.9–3.1% was observed at a gas station on the highway (Sample “Wuerenlos, 2017”) and a tree growing close to a busy street in Zurich (Sample “Altstetten Lindenplatz, 2024”). Interestingly, the nearby location of Bachwissenpark in Altstetten shows a low %fF of 1.1±0.3%, indicating that the dilution is strongest close to the source, i.e., traffic.

High dilution is observed for a rowan berry (part of a wedding bouquet, 2008), which was most probably assembled by a local floral shop in the village of Muri (Switzerland). The 14C values suggest that the rowan tree was growing close to the road or a parking space, which is, in fact, quite a common practice.

Other countries

The spectrum of samples collected worldwide is quite broad. Nevertheless, most samples were collected in Europe. The %fF values were calculated only using the continental values from Europe, which is only an estimate for the samples from the southern hemisphere. Except for two samples, Bangkok (10.5%) and Trieste (5.6%), the values of %fF fall within the range of –0.7% to 4% (Figure 4d). The very high F14C measured for leaves collected close to Canary Wharf (London) requires additional observations because the offset from the NHZ1 values is much higher than reported for barley from Rothamsted (Dunbar et al. Reference Dunbar, Scott and Tripney2024) or Gatersleben wheat and soybean (Hüls et al. submitted Reference Hüls, Pawlyta, Rakowski, Börner and Hamann2024). Generally, the lowest %fF values were observed in rural regions that were more distant from urban centers. These observations align with previous studies (Quarta et al. Reference Quarta, Rizzo, D'Elia and Calcagnile2007; Varga et al. Reference Varga, Barnucz, Major, Lisztes-Szabó, Jull, László, Pénzes and Molnár2019).

Records of the contemporary atmospheric 14C record

The analysis of atmospheric 14C captures the global and local effects of fossil fuel combustion. Data collected by the worldwide network confirms the growing dilution of the atmospheric 14C signal with a fossil component (Graven Reference Graven2015; Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, De Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022). Our samples were collected in the spring and summer, and reflect values for the growing season. Therefore, most of the %fF values estimated for the leaves are attributed to pollution with fossil carbon-bearing CO2 from the exhausts of combustion engines. As expected, urban areas with heavy traffic show high %fF values, which were also observed in other studies (Battipaglia et al. Reference Battipaglia, Marzaioli, Lubritto, Altieri, Strumia, Cherubini and Cotrufo2010; Heskel et al. Reference Heskel, Hrycyna, Gomes, Mergenthal, Noor and Xu2024; Ndeye et al. Reference Ndeye, Sène, Diop and Saliège2017; Piotrowska et al. Reference Piotrowska, Pazdur, Pawełczyk, Rakowski, Sensuła and Tudyka2020; Rakowski et al. Reference Rakowski, Nakamura, Pazdur, Charro, Villanueva and Piotrowska2010). Green spaces such as urban parks help to significantly reduce the effect (e.g., Zurich Altstetten), showing that the life comfort for the population affected can be improved by consequent reduction in the use of combustion of fossil fuels and an extensive greening of the urban space.

The potential of 14C analysis in leaves and flowers

The results of this ad hoc project demonstrate the considerable potential of analyzing short-lived vegetal fragments of terrestrial plants. The simplicity of sample collection allows data to be gathered from most remote locations around the globe. Moreover, archives and collections of leaves can be utilized in future studies to expand regional data sets (Hüls et al. Reference Hüls, Börner and Hamann2021; Hüls et al. submitted Reference Hüls, Pawlyta, Rakowski, Börner and Hamann2024). The collection of leaves and flowers can also be part of future Citizen Science projects that might help encourage communities to reduce fossil fuel combustion. In addition, the simplicity of sample preparation, combined with the modern AMS technique, allows for the flexible design of short-term projects planned according to the participant’s age and knowledge. The following short description can be used when collecting leaves.

A short guide to collecting leaves for a record of atmospheric 14C

Collecting leaves or flowers for 14C analysis requires simple documentation of:

  • • sample name

  • • the datum of collection,

  • • the location, the geographic coordinates,

  • • the estimated height of the sample (optimally, 1–1.5 m above ground),

  • • the type (species) of the chosen tree

  • • (optional) a photograph of the tree and the collected leaf (leaves)

Conclusions

Radiocarbon analysis in short-lived plants and leaves has the potential to determine atmospheric 14C levels anywhere around the globe. The remote locations, such as Svalbard and Faleme Valley (Senegal), of samples collected in our project illustrate this potential. The sample collection and preparation are simple and can be completed as part of school projects that involve the local community and raise awareness about the fossil carbon dioxide released into the atmosphere. Our results align with systematic studies conducted at other locations, which demonstrate the degree of 14C concentration dilution in urban areas. Moreover, data collected regionally benefits the radiocarbon laboratories working in the region. Potential requests to estimate the degree of dilution in various samples or to provide the age of the most recent organic matter require knowledge about the local atmospheric F14C.

Supplementary material

To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2025.10140

Acknowledgments

The project was completed with the support of the LIP team. IH thanks Mantana Maurer and Maria Belen Röttig for years of support and sample preparation. Students of ETH, the University of Basel, the University of Zurich, and the University of Bern were involved, as were pupils of several high and primary schools in Zurich and other regions of Switzerland. Many thanks to all the colleagues who collected the samples.

Footnotes

Selected Papers from the 4th Radiocarbon in the Environment Conference, Lecce, Italy, 23–27 Sept. 2024

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Figure 0

Figure 1. Google Maps shows sites where samples were collected (see the link Leaves sampling sites)

Figure 1

Table 1. Overview of the number of samples from various locations (Figure 1)

Figure 2

Figure 2. Pictures of two trees that were sampled multiple times in the last 10 years: (a) Persian ironwood (Parrotia persica) at the ETH Hoenggerberg Campus (HPM building), (b) Poplar (Populus) tree in Boppelsen. Leaves of corn (on the right) were also sampled and analyzed.

Figure 3

Figure 3. Results of 14C analysis of leaves compared to the values for “clean air” NH1 (Hua et al. 2022) and JFJ data (Leuenberger et al. 2024): (a) ETH Zurich Campus Hoenggerberg, (b) Boppelsen (near Zurich), (c) other locations in Switzerland (CH), (d) sites around the world (see Figure 1)

Figure 4

Figure 4. Fossil fuel dilution effect (%fF) based on F14C measured in leaves: (a) ETH Zurich Campus Hoenggerberg, (b) Boppelsen (near Zurich), (c) other locations in Switzerland (CH), (d) sites around the world (see Figure 1)

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