INTRODUCTION
The unstable isotope of the element carbon (14C) is formed in the upper atmosphere by nucleus reaction between energetic neutrons and the element nitrogen, due to cosmic particle radiation.
14CO2 is transported within the atmosphere and distributed in natural sinks such as the biosphere and dissolved in the ocean. Any living organism takes up 14C and incorporates it into its tissues, but it ceases exchange with the atmosphere at death. As 14C decays with a half-life of 5730 years, measuring the remaining 14C concentration allows estimation of the age of objects up to 50,000 years old.
During the last 70 years, the natural 14C cycle was disrupted by human activities such as atmospheric nuclear tests (which almost doubled natural atmospheric 14C concentrations by 1963) and emissions of 14C-free fossil carbon (too old to contain measurable 14C) (e.g., Suess Reference Suess1955; Stuiver and Quay Reference Stuiver and Quay1980; Rakowski et al. Reference Rakowski, Nadeau, Nakamura and Pazdur2013; Prăvălie Reference Prăvălie2014). Atmospheric 14C concentrations are expected to return to (or fall below) the reference level of 1 (F14C; Reimer et al. Reference Reimer, Brown and Reimer2004, defined for the year 1950), mainly due to the dilution of atmospheric CO2 by the emission of 14C-free CO2. This level may even have been reached already (Graven Reference Graven2015; see also the discussion section). The ocean, on the other hand, may have become a source of 14C for atmospheric CO2 as suggested by Andrews et al. (Reference Andrews, Siciliano, Potts, Demartini and Covarrubias2016) and Wu et al. (Reference Wu, Fallon, Cantin and Lough2021).
The large concentration differences during the last decades allows the application of radiocarbon measurements in forensic studies, such as the determination of modern carbon contribution in fuels (Reddy et al. Reference Reddy, DeMello, Carmichael, Peacock, Xu and Arey2008; Norton et al. Reference Norton, Cline and Thompson2012) or bio-plastic (Telloli et al. Reference Telloli, Rizzo, Canducci and Bartolomei2019), investigation of the paper age of art and documents (Zavattaro et al. Reference Zavattaro, Quarta, D’Elia and Calcagnile2007; Fedi et al. Reference Fedi, Caforio, Mandò, Petrucci and Taccetti2013; Huels et al. Reference Huels, Pensold and Pigorsch2017), determination of animal-tissue growth age with respect to CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora, e.g., Wild et al. Reference Wild, Kutschera, Meran and Steier2019), or the production date of whiskey (Cook et al. Reference Cook, Dunbar, Tripney and Fabel2020), to name a few.
Here we complement available reference data, such as atmospheric carbon dioxide 14C measurements (e.g., Levin et al 1985, 1997, 2013; Hammer and Levin Reference Hammer and Levin2017) or tree-ring measurements (e.g., Hua et al. Reference Hua, Barbetti and Rakowski2013), with annual radiocarbon measurements on seasonally grown plant tissue (seeds of bread wheat (Triticum aestivum L.), harvested between the years 1946–2020), from the collections of the IPK Gatersleben (Leibniz-Institut für Pflanzengenetik und Kulturpflanzenforschung). An advantage over existing reference data is the possibility to compile a continuous record of plant-tissue 14C data between 1946 until today, and the possibility for continued expansion in future years.
MATERIAL AND METHODS
Material
The plant material was provided by the IPK Gatersleben, which is one of the world’s leading international institutions in the field of plant genetics and crop science. With a total inventory of 150,000 accessions of 3212 plant species and 776 genera, IPK holds one of the most comprehensive collections worldwide. It comprises wild and primitive forms, landraces as well as old and more recent cultivars of mainly cereals but also other crops (Börner et al. Reference Börner, Khlestkina, Chebotar, Nagel, Arif, Neumann, Kobiljski, Lohwasser and Röder2012; Börner and Khlestkina Reference Börner and Khlestkina2019). The institute and its agrarian land is located east of the Harz Mountains in a rural area, dominated by agriculture (Figure 1).
Figure 1 Location of the IPK (labeled as a flag) in Germany.
Wheat (Triticum aestivum L.) was grown in the experimental fields of the IPK in Gatersleben (latitude 51°49'19.74"N, longitude 11°17'11.80"E, 110.5 m.a.s.l., black soil of clayey loamy type) from 1946 onwards. Plot size was 2 m2. Standard agronomic management according to good agricultural practice was applied. It is from this long-lasting continuous regeneration activities that samples of annual harvested wheat seeds where acquired for subsequent radiocarbon measurements.
Seeds were stored in glass tubes as reference samples in the herbarium collection of the IPK. The conditions of the storage place were a temperature of 22.5°C ± 3°C and a relative humidity of 45–50%.
Selected seed samples consist of both winter and spring growth habit. Winter wheat was sown in October and harvested in July whereas spring wheat was sown in March/April (depending on weather conditions) and harvested in August (Opperman et al Reference Oppermann, Weise, Dittmann and Knüpffer2015). Seed tissue thus will contain atmospheric carbon metabolized between April–June and May–July, respectively, for winter and spring wheat. We take the mid of May and mid of June as the current fix-points for defining the dates of the radiocarbon content. A comparison of mean April–June and May–July radiocarbon values of the monthly NH1-dataset (Hua et al. Reference Hua, Barbetti and Rakowski2013) and atmospheric Schauinsland dataset (SIL; Hammer and Levin Reference Hammer and Levin2017) show small differences within measurement uncertainties which do not invalidate constructing a joint time series using both winter-sown and spring-sown wheat 14C measurements.
Methods
Several grains from each season between 1946 to 2020 were available, from which one single grain was selected for further analysis. To remove possible hydrophobic substances such as lipids from handling of seed corns, each grain was washed with acetone for 30 min in an ultrasonic bath, decanted, and dried subsequently at 60°C in an oven over night. Afterwards, the sample material was frozen in liquid nitrogen and powdered, i.e., homogenized, using pestle and mortar. From the powdered material, between 6–7 mg was flame-sealed together with CuO and silver in quartz-ampoules under vacuum, and finally combusted at 900°C. Resulting CO2 was cryogenically purified and graphitized with catalytic Fe and H2 at 600°C.
Radiocarbon measurements were conducted at the Leibniz-Labor with a HVE 3MV Tandetron 4130 accelerator mass spectrometer (AMS). The 14C/12C and 13C/12C isotope ratios were simultaneously measured by AMS and compared to the SRM4990C measurement standards (Oxalic Acid II). The resulting 14C-content is normalized to the δ13C concentration of –25‰ for isotope fractionation correction (Stuiver and Polach Reference Stuiver and Polach1977). Blank correction was performed using the modeled 14C distribution vs. sample size of Alfa graphite, which is also measured regularly in each wheel of sample cathodes (Nadeau et al Reference Nadeau, Grootes, Schleicher, Hasselberg, Rieck and Bitterling1998; Nadeau and Grootes Reference Nadeau and Grootes2013). Measured radiocarbon concentration is given in F14C and Δ14C by correcting for the decay of the measurement standard and samples (Stenström et al. Reference Stenström, Skog, Georgiadou, Genberg and Johansson2011; Stuiver and Polach Reference Stuiver and Polach1977) (
$${\Delta ^{14}}C = (({F^{14}}C*{e^{\lambda \left( {1950 - x} \right)}}) - 1)*1000$$
;
$$\lambda $$
=1/8267, x=year of seed growth). Our reported uncertainty of 14C results takes into account the uncertainty of the measured 14C/12C ratios of sample and measurement standard, the uncertainty of the fractionation correction and the uncertainty of the applied blank correction.
For each season or year, at least two subsamples of prepared single-grain material was measured in different batches. Variance-weighted means were calculated using
$$\overline x = {{\sum\nolimits_{i \,= 1}^n {{{{x_i}} \over {\sigma _i^2}}} } \over {\sum\nolimits_{i \,= 1}^n {{1 \over {\sigma _i^2}}} }}\,\,{\rm{and}}\,\,\sigma _{\overline x}^2 = {1 \over {\sum\nolimits_{i = 1}^n {{1 \over {\sigma _i^2}}} }}$$
with
$${\overline x}$$
weighted mean F14C or Δ14C, and
$${\sigma _x}$$
the error of the weighted mean (Ward and Wilson Reference Ward and Wilson1978). When individual measurements of one sample were judged to be significantly different from each other (i.e., differences are larger than 2σ), an additional measurement was performed, to either improve the agreement of all individual measurements or identify a possible outlier.
RESULTS
The result of the radiocarbon measurements of the wheat samples is visualized in Figure 2 (see also Table S1 of the Supplementary Materials).
Figure 2 Radiocarbon concentration in Gatersleben-wheat tissue, grown between 1946 and 2020. Dashed line corresponds to the summer (May–August) 14C of the NH1 compilation (Hua et al. Reference Hua, Barbetti and Rakowski2013). Lower panel gives the 14C differences between Gatersleben wheat and summer NH1. Error bars shown are calculated based on propagated errors from Gatersleben and NH1 datasets.
Between 1946 and 1955, wheat tissue-14C concentration stayed below the modern standard concentration of Δ14C ≥0‰ (between –28‰ and –48‰) and increased subsequently towards a first maxima in 1959 (285‰) as a result of the increasing number (and energy) of atmospheric nuclear tests (see Figure S1, UNSCEAR 2000; Bergkvist and Ferm Reference Bergkvist and Ferm2000; Prăvălie Reference Prăvălie2014). A pause in atmospheric nuclear test activity in 1959 and only 3 tests with low energy yield in 1960 resulted a first reduction in atmospheric radiocarbon concentration by about 57‰ and 67‰ (1960 and 1961, respectively, see Figure S1). Nuclear test activity resumed in the second half of 1961, causing increased atmospheric 14C activity, as seen in the 1962 spring/early summer biomass 14C concentration. Evolution and 14C concentrations in seeds compare reasonably well to the Trondheim pine tree-ring tissues of the same period (Svarva et al. Reference Svarva, Grootes, Seiler, Stene, Thun, Vaernes and Nadeau2019: see also Figure S1).
Maximum nuclear test activity (and energy) was reached in late 1962, resulting in an increase in 14C formation as seen in wheat biomass from summer 1963 (855‰, see also Figure S1 and Table S1). Maximum atmospheric 14C concentration occurred in late summer 1963 (980‰) as seen in the Northern Hemisphere 1 compilation (Hua et al. Reference Hua, Barbetti and Rakowski2013), although no nuclear test activity was recorded in 1963 (UNSCEAR 2000). An agreement by signing the Limited Test Ban Treaty (LTBT or partial test ban treaty) on August 5, 1963, put an end to above-ground nuclear test activity, except minor amounts of tests conducted by China and France (e.g., Bergkvist and Ferm Reference Bergkvist and Ferm2000). Maximum wheat tissue 14C concentration is measured in summer 1964 sample (908‰; Figure 2, Figure S1).
Since the LTBT, excess atmospheric 14C concentration decreased as 14CO2 was sequestered in the natural sinks such as the ocean and the biosphere. Recorded wheat-tissue 14C decreased rapidly by about 90‰/yr from the maximum concentration until 1968 (550‰; see Figure 2), followed by a plateau with comparable concentrations in 1969 and 1970. Between 1970 and 1977, radiocarbon concentration decreased with a lower mean annual rate of about –25‰/yr. Between 1977 and 2009, annual reduction in wheat-tissue radiocarbon seem to follow an exponential function with a “time-constant” of T1/2 ≈ 11yr.
During the early and maximum phase of the bomb-pulse, seed 14C and summer NH1 show considerably larger variability in differences, varying from ΔΔ(seed_14C-NH1_summer_14C):-26‰ and –25‰ (1957 and 1968, respectively) to ΔΔ: 32‰ and 10‰ (1963 and 1970, respectively). The larger differences between the NH1 compilation and the seed tissue may indicate 14C differences in the atmosphere, i.e. an inhomogeneous isotopic distribution within the Northern Hemisphere air parcels as was recently described for tropical South America (Ancapichún et al. Reference Ancapichún, De Pol-Holz, Christie, Santos, Collado-Fabbri, Garreaud, Lambert, Orfanoz-Cheuquelaf, Rojas, Southon, Turnbull and Creasman2021).
From about 1980 onwards, wheat seed 14C concentrations lie closer to summer NH1 concentrations, but mostly with negative ΔΔ values (differences vary from around –10‰ to 5‰, see Figure 2), indicating on average a generally higher fossil carbon concentration within the wheat tissue than in the NH1 dataset.
In 2010, measured wheat 14C concentration may indicate a larger fossil CO2 contribution, as seen in a considerable drop in Δ14C of about 14.5‰ (see Table S1 and Figure 3), compared to the previous year. Between 2016 and 2018, wheat-tissue 14C concentrations decreased faster than between 2014 and 2016 (see Figure 2 and Figure 3), followed by comparably stable 14C concentrations slightly below the reference standard in 2018 and 2019 (–1.4‰ and –1.1‰, resp.). Recent 2020 wheat tissue gave again a slightly enriched 14C concentration of 1.3‰ (see Table S1).
Figure 3 Wheat tissue 14C (asterisks) between 1986 and 2020. Dashed line represents the atmospheric Jungfraujoch 14C concentration. Black triangles and line in the lower panel give % fossil carbon in seed tissue with respect to calculated mean April–July JFJ 14C (Levin and Kromer Reference Levin and Kromer2004; data from Hammer and Levin Reference Hammer and Levin2017 and Emmenegger et al. Reference Emmenegger, Leuenberger and Steinbacher2020). Error bars shown are calculated based on propagated errors from Gatersleben and JFJ measurements.
DISCUSSION
Overall, radiocarbon concentrations in plant tissue mirror atmospheric radiocarbon concentrations, which result from a complex interplay between radiocarbon production, partitioning into the biosphere and hydrosphere, and dilution with human-made or natural fossil, 14C-free carbon emission. Short-term, inter-annual, seasonal changes in fossil-fuel carbon emission as seen in atmospheric radiocarbon records such as Jungfraujoch (JFJ), Schauinsland (SIL), and Vermunt (VER) (Levin and Kromer Reference Levin and Kromer2004; Levin et al. Reference Levin, Hammer, Kromer and Meinhardt2008; Hammer and Levin Reference Hammer and Levin2017; Emmenegger et al. Reference Emmenegger, Leuenberger and Steinbacher2020), could be caused by cold season heating and energy production. Other contributions to the evolution of atmospheric 14C could be changes in air-sea exchange, for example, variability in ocean upwelling (e.g., Graven et al. Reference Graven, Guilderson and Keeling2012), releasing old, 14C depleted CO2 into the atmosphere. Furthermore, since the early 2000s, the North and South Pacific surface ocean have become a source of 14C, as indicated by 14C measurements of annually precipitated carbonate of sub-surface living corals of Porites sp. (Andrews et al. Reference Andrews, Siciliano, Potts, Demartini and Covarrubias2016; Wu et al. Reference Wu, Fallon, Cantin and Lough2021), influencing the overlying northern and southern hemispheric airmasses. To what extent the North Atlantic also became a source of enriched 14C would need actualized studies such as those from Scourse et al. (Reference Scourse, Wanamaker, Weidman, Heinemeier, Reimer, Butler, Witbaard and Richardson2012).
Using the JFJ radiocarbon record (and calculated April–July mean values) as a reference for a clean-air site (Levin and Kromer Reference Levin and Kromer2004), less influenced by anthropogenic fossil fuel emissions, we estimated the fossil carbon contribution within the wheat tissue by
$$Fossil\_C(\% ) = \left(1 - {{{F^{14}}{C_{seed}}} \over {{F^{14}}{C_{JFJ}}}}\right)*100$$
(Quarta et al. Reference Quarta, Rizzo, D’Elia and Calcagnile2007; Varga et al. Reference Varga, Barnucz, Major, Lisztes-Szabó, Jull, László, László, Pénzes and Molnár2019). Between 1987–2016, fossil C contribution in the wheat tissue is mostly lower than 1%, slightly above 1% in 1990 and 1992, and with stronger excursions in 2010 and 2011 by 1–1.5% fossil C (Fossil_C(%)mean:0.7 ± 0.3; see Figure 3).
The visible pause in the 14C decrease rate in 14C of wheat tissue in 2009 is also observable in the April–July JFJ 14C record. The pause in the 14C decrease rate in wheat tissue 14C in 2008–2009, and also 2019–2020, coincide with the sunspot minima of the ending 23rd and 24th solar cycles, respectively (McIntosh et al. Reference McIntosh, Chapman, Leamon, Egeland and Watkins2020), resulting in a larger 14C production due to less solar particle shielding against cosmic particles (Stuiver Reference Stuiver1961; Stuiver and Braziunas Reference Stuiver and Braziunas1993). In addition, fossil carbon emission also dropped significantly between 2008 and 2009 (Eurostat 2020), largely caused by the Global Financial Crisis in 2008.
The comparably larger radiocarbon decrease in seed tissue in 2010 can also be observed in the atmospheric Schauinsland record, but with a much smaller magnitude, and it has no counterpart in contemporary JFJ record. The atmospheric Schauinsland radiocarbon record is located at lower elevation in the Black Forest and is partially also influenced by industrial fossil fuel contribution from the Rhine valley (Levin and Kromer Reference Levin and Kromer2004). A reduced 14C content with respect to the Jungfraujoch station is also observed in plant-tissue (maize leaves) in the northern Netherlands in 2010 (ΔΔ14CJFJ-leaves of about 6‰; Bozhinova et al. Reference Bozhinova, Palstra, van der Molen, Krol, Meijer and Peters2016) and 2011, and in 2011 samples of Hungarian acacia honey (ΔΔ14CJFJ-honey of about 10‰;Varga et al. Reference Varga, Sajtos, Gajdos, Jull, Molnár and Baranyai2020). The latter study cover an almost continuous period from 1994 to 2018, showing a reasonable good agreement between honey and wheat in measured 14C except a short period in 2006 and 2007 with probably elevated local fossil carbon contribution seen in honey (Varga et al. Reference Varga, Sajtos, Gajdos, Jull, Molnár and Baranyai2020). In comparison to the Gatersleben seed and also the JFJ reference site, the Hungarian honey samples give elevated 14C concentrations since 2014 (ΔΔ14CJFJ-honey of about –4 to –12‰), also visible in two Slovakian wine samples from 2015 (ΔΔ14CJFJ-wine of about –15‰; Povinec et al. Reference Povinec, Kontuľ, Lee, Sýkora, Kaizer and Richtáriková2020; see also Figure S2).
The Gatersleben wheat-tissue 14C record is apparently influenced by fossil carbon at the order of magnitudes up to 1.6%. Nevertheless, we think this record as well as other plant-based 14C records covering the bomb-spike, could be useful in forensic investigations such as the assessment of paper ages (Huels et al. Reference Huels, Pensold and Pigorsch2017; Pigorsch et al. Reference Pigorsch, Kießler, Hüls and Meinl2020) or the estimation of renewable fuels e.g., products such as potato and corn (giving starch, used as a binder within and on the surface of paper), or rapeseed for biofuel production, are grown in agricultural environments, and their 14C content is probably more comparable to the Gatersleben seed record and not to the more remotely located and less fossil-carbon influenced records such as the atmospheric records like Jungfraujoch or Schauinsland.
CONCLUSION
On tissue of seasonal cultivated and archived wheat seeds (Triticum aestivum L.) from the IPK Gatersleben, radiocarbon measurements have been conducted, allowing an overview of the temporal evolution of the plant tissue radiocarbon content during the growth season between 1946 and 2020. Samples consists each of winter or spring type with growth periods between April–July and May–August, respectively.
The course of the resulting radiocarbon concentrations follows published datasets such as the Northern Hemisphere (NH 1) record (e.g., Hua et al. Reference Hua, Barbetti and Rakowski2013) with a sharp increase in atmospheric radiocarbon between 1955 and 1964 as a consequence of atmospheric nuclear testing activities until 1963, followed by a steep reduction in atmospheric radiocarbon concentration due to partitioning into the bio- and hydrosphere of about 90‰/yr. until 1968. The rate of reduction in atmospheric 14C slows down significantly in following decades. For the last 10 years, the reduction in atmospheric 14C concentration appeared discontinuous with sudden drops. The youngest (2018–2020) measured wheat tissue radiocarbon concentrations vary around –1.4‰ (2018) and 1.3‰ (2020). The Gatersleben seed radiocarbon record indicate a fossil carbon contribution in the plant tissue up to 1.6% when compared to the high alpine, clean-air Jungfraujoch record. Nevertheless the Gatersleben radiocarbon record as well as other potential future other agricultural records could be suitable in forensic investigation because of their closer comparability in material and growth situation. It is planned to continue radiocarbon measurements on seeds of coming harvests.
ACKNOWLEDGMENTS
The authors would like to thank the technical staff of the Leibniz-Laboratory for their commitment in sample handling. M H appreciated the constructive comments of two anonymous reviewers as well as from Associate Editor Quan Hua, helping to shape and improve the present article.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/RDC.2021.81
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