THE VERSATILE USES OF THE 14C BOMB PEAK

ABSTRACT A concise review is presented of applying 14C produced in the atmospheric nuclear weapons testing program (1950 to 1963), subsequently forming the so-called 14C bomb peak. In order to convey the versatility of this unique isotope signal, selected applications from a variety of different fields are discussed touching on environmental, archaeological, biological, and forensic issues. A comprehensive list of references is supplied for in-depth studies of the respective fields.


INTRODUCTION
It is well known that two anthropogenic contributions have influenced the natural 14 CO 2 content in the atmosphere since the beginning of the 20th century. The emission of 14 C-free CO 2 from fossil fuel burning dilutes the 14 C content (Suess 1955), leading also to temporal variations of the interhemispheric 14 C offsets early in the 20th century (McCormac et al. 1998;Stuiver and Braziunas 1998). In contrast, atmospheric nuclear weapons testing after the second World War increased the 14 C content considerably (Figure 1).
In the late 1950s, a noticable increase of 14 C was detected in the atmosphere and biosphere (Rafter and Ferguson 1957;De Vries 1958;Münnich and Vogel 1958;Broecker and Walton 1959;. Finally, the massive nuclear weapons testing in the early 1960s led to a sharp increase of 14 C, which peaked in 1963 (Nydal and Lövseth 1965) when the limited Nuclear Test Ban Treaty ended this program. Since then, the anthropogenic excess of 14 C is gradually distributed from the atmosphere into the biosphere and hydrosphere. By now (2022) atmospheric 14 CO 2 levels have reached almost pre-nuclear levels. This 14 C excess is called the " 14 C bomb peak" and provides a distinct and rapidly changing 14 C signal for the past ∼60 years (Levin and Hesshaimer 2000;Hua et al. 2013;Levin et al. 2022). Within this period 14 C "dating" with a time resolution of 1 to 2 years is possible. Since CO 2 emissions from fossil fuel burning will continue-albeit with hopefully decreasing intensity-a substantial depression below the natural level of the atmospheric 14 CO 2 is predicted towards the end of the 21st century, shown in Figure 2 (Graven 2015).
The 14 C bomb peak was once colloquially called "The mushroom cloud's silver lining" (Grimm 2008). This points out a positive side effect of the atmospheric nuclear weapons testing program. In essence, every carbon pool on Earth which was in exchange with atmospheric CO 2 since the late 1950s has been labeled by bomb 14 C (e.g., Bada et al. 1987). Combined with the extraordinary sensitivity of 14 C detection by AMS, allowing one to perform 14 C measurements in carbon samples down to the microgram range Steier et al. 2017), has led to a large variety of applications. In the following, a limited number of representative examples will be discussed, demonstrating the versatile uses of the 14 C bomb peak: (i) The uptake of anthropogenic atmospheric CO 2 into ocean and land. (ii) The age of groundwater systems. (iii) The study of annual tree ring growth in olive trees. (iv) The age of cells in the human body. (v) Other uses of the 14 C bomb peak in the biosphere. (vi) The evaluation of artwork with respect to its authenticity. (vii) The question of fraudulent whisky making. Although many uses of the 14 C bomb peak lead to a better understanding of basic processes in the environment at large (i-iv), it is also being used for forensic investigations elucidating the dark side of human beings (v-vii).

APPLICATIONS OF THE 14 C BOMB PEAK
The Uptake of Anthropogenic CO 2 into Ocean and Land An important question for the development of the future climate on Earth is how much of the anthropogenic CO 2 released into the atmosphere can be taken up by oceans and land (plants and soil). We know that ∼45% of it stays in the atmosphere, being monitored since more than 50 years at the Mauna Loa Observatory in Hawaii (Keeling 2008). The rest is taken up by the ocean (∼25%) and land (∼30%). However, to verify reported anthropogenic CO 2 emissions with independent Earth system observations, a better understanding of both contributions from natural variations in the CO 2 cycle and of the uptake processes is needed (Peters et al. 2017). These complex questions are being investigated from different angles for the oceans (Worden et al. 2015;Gruber et al. 2019), plants (Bastin et al. 2019;Slotta et al. 2021), and soil (Trumbore 2009;Shi et al. 2020 Figure 1 Schematic presentation of 14 C production from cosmic rays and nuclear explosions in the atmosphere and its pathway into terrestrial archives (Wild et al. 2019). to study decadal dynamics of CO 2 , whereas cosmogenic 14 C is useful at the millennium time scale.
As an interesting example, we show the surprising penetration of bomb 14 C into the deepest regions of the oceans (Wang et al. 2019), depicted in Figure 3.

The Age of Groundwater Systems
Groundwater is the largest freshwater resource on earth. Since our life depends on the availability of freshwater, it is important to understand the residence time of a groundwater system (Seltzer et al. 2021), and how fast it is being recharged by precipitation. The age of groundwater systems can provide a first estimate of this information.
Very old groundwater system such as the Great Artesian Basin in Australia have ages around several hundred thousand years (Collon et al. 2000). They are being continuously depleted by human use, with expected very long recharging times.
Here we give an example of a likely short recharging time established by 14 C bomb peak dating of young speleothems in a groundwater system about 100 km from Sydney (Hodge et al. 2011).
Calendar Year (A.D.) RCP = RepresentaƟve Greenhouse Gas ConcentraƟon Pathways 2.6 -8.5 = RadiaƟve Forcing (W/m 2 ) Figure 2 Modeling the impact of fossil fuel emissions in the 21st century on the anthropogenic "aging" of the atmosphere, depending on different scenarios of CO 2 reduction (Graven 2015). In the fifth assessment on climate change, the International Panel on Climate Change (IPCC 2013) used so-called Representative (greenhouse) Concentration Pathways (van Vuuren et al. 2011) to predict different radiative forcing values for 2100. Compared to an equilibrium between incoming radiation from space and outgoing radiation into space, positive radiation forcing values mean an increase of the temperature on Earth. A continuation of "business-as-usual" results in a radiative forcing of 8.5 W/m 2 in 2100 and the most negative Δ 14 CO 2 values (gray curve).

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Only a 5-year shift occurs between the atmospheric 14 C bomb signal and the one recorded in a fast growing stalagmite ( Figure 4). Thus, a short recharging time is likely to be expected.
In general, however, in a cave system there are different water pathways for different stalagmites, depending on their locations in the cave and the overlaying rocks and soil. It is therefore a complex undertaking to come up with good estimates of the groundwater recharging times for different cave system (Markowska et al. 2019).

The Study of Annual Tree Ring Growth in Olive Trees
An exact date of the Late Bronze Age eruption (∼1600 BCE) on the Greek island of Santorini in the Aegean Sea is of great interest to archaeologists and historians studying the interaction of different civilizations in the Eastern Mediterranean during that time (Warren 2006;Kutschera 2020). Great efforts have been made to determine this date by 14 C dating. A key object was a branch of an olive tree supposedly buried alive in the tephra from the volcanic eruption (Friedrich et al. 2006). A detailed tree-ring study of a modern olive tree with the help of the 14 C bomb peak (Ehrlich et al. 2018(Ehrlich et al. , 2021) revealed a complex tree ring growth pattern, raising doubts on the reliability of using olive trees for 14 C dating ( Figure 5). In addition, a new year-to-year 14 C calibration (Pearson et al. 2018) established a critical shift in the "plateau" region between 1600 and 1500 BCE, which does not allow one to obtain a precise date within this time range from 14 C measurements. It seems that other dating methods need to be used to determine an accurate date for this important time beacon (Kutschera 2020).    One of the most fascinating applications of the 14 C bomb peak was developed by the Department of Cell and Molecular Biology of the Karolinska Institute in Stockholm, based on the following hypothesis, quoted from their seminal paper (Spalding et al. 2005): "Most molecules in a cell are in constant flux, with the unique exception of genomic DNA, which is not exchanged after a cell has gone through its last division. The level of 14 C integrated into genomic DNA should thus reflect the level in the atmosphere at any given point, and we hypothesized that determination of 14 C levels in genomic DNA could be used to retrospectively establish the birth date of cells in the human body." All humans who lived during the time of the bomb peak are thus potential objects for these studies ( Figure 6).
The formation of new neurons after birth is of particular interest for the human brain. Here we show one striking example. Evidence for the formation of new neurons after birth were found in the hippocampus, an important part of the human brain where memory forming, organizing, and storing takes place (Spalding et al. 2013). The result of this investigation is displayed in Figure 7. In contrast, essentially no renewal of neurons after birth was found in other sections of the human brain, the neocortex (Bhardwaj et al. 2006) and the olfactory bulb of the human brain (Bergmann et al. 2012). The age and genomic integrity of neurons after cortical strokes has also been investigated (Huttner et al. 2014). In general, the ability of the human brain to adapt to changing conditions (socalled "neuroplasticity") is a field of intense studies (Yeung et al. 2014;Frisén 2016).
The method has also been extended to study cells in the human heart (Bergmann et al. 2009), delivering information about constancy and renewal rates of different cells, important to a

Calendar year
Life span

C bomb peak
Cells from intestine Death Figure 6 Visualization of how the birth date of different cells in an individual of a known life span can be determined retrospectively by comparing the measured 14 C signal from DNA extracts with the 14 C bomb peak calibration curve (Spalding et al. 2005).
reliable functioning of the heart (Bergmann et al. 2015). Another application of considerable health impact is the study of the turnover of fat cells (adipocytes) since obesity increases in many countries around the world (Spalding et al. 2008;Arner et al. 2011;Hyvönen and Spalding 2014;Arner et al. 2019).

Timing of Birth and Death
The time of birth of humans can be determined by measuring bomb 14 C in parts of the body where carbon is not exchanged after birth. Examples are the enamel of teeth (Spalding et al. 2005;Buchholz and Spalding 2010), and in the eye lens crystalline (Lynnerup et al. 2008;Kjeldsen et al. 2010). In a refined 14 C bomb-peak study, the separation of water-soluble and insoluble proteins of the human lens showed that the latter forming the crystalline core of the lens exhibited very low level of new carbon incorporation/exchange/turnover (Stewart et al. 2013). A particularly detailed study of the age distribution of tissue in the human eye was performed by the AMS group in Debrecen (Rinyu et al. 2020).
The time of death of a human can be of forensic interest. It can be determined from measuring 14 C in material that has been renewed as close to the death of the person as possible, i.e., lipid Versatile Uses of 14 C Bomb Peak 1301 fraction of bones, or hair (Wild et al. 1998). In a court case, where two old women were found dead in an apartment, a time difference of ∼1 year for the time of death for the two individuals could be established (Wild et al. 1998).

Longevity of Sharks
Sharks are iconic species in the animal world of the oceans, and the age of some great white sharks (Carcharodon carcharias) has been determined with 14 C bomb peak dating of banded vertebrae to be as high as 70 years (Hamady et al. 2014). In a similar study of banded vertebrae in whale sharks (Rhincodon typus), the largest fish in the world, ages of up to 50 years were determined (Ong et al. 2020). An even greater age has been found for Greenland sharks (Somniosus microcephalus), with a maximum age of 400 years (Nielsen et al. 2016). The latter was established with the eye-lens dating method mentioned above.

African Elephants and Ivory Trade
A comprehensive description of the use of the 14 C bomb peak for the measurement of recent biological tissues and application to wildlife forensics can be found by Uno et al. (2013). In the land animal world, African elephants can also be considered iconic species. Their killing has been prohibited since 1989 by CITES (The Convention on International Trade in Endangered Species of Wild Fauna and Flora). However, since their tusks provide valuable ivory, they are still being hunted after the ban. In a big effort to uncover illegal trade of ivory, the ages of large ivory seizures were determined with 14 C bomb peak dating (Cerling et al. 2016). In the abstract they state: "Carbon-14 measurements on 231 elephant ivory specimens from 14 large ivory seizures (≥0.5 ton) made between 2002 and 2014 show that most ivory (ca. 90%) was derived from animals that had died less than 3 years before ivory was confiscated" (Figure 8). This is in clear violation of the CITES convention and revealed the relentless poaching of elephants in Africa (Wasser et al. 2015;Biggs et al. 2017).
Another study related to ivory and forensics was performed by the AMS group at the University of Salento, Italy, where also tusks from mammoths were investigated (Quarta et al. 2019). In a more positive application of the 14 C bomb peak, six privately owned tusks from African elephants, supposedly from before the ban from CITES, were clearly confirmed to belong to elephants hunted in the 1960s (Wild et al. 2019).

The Authenticity of Artwork
It is probably not an exaggeration to say that the preservation of cultural heritage is a key to understand the development of different cultures from the past to their present state, and that it may contribute to a more respectful interaction between cultures. Part of this includes artwork, and often they gain considerable value if an authenticity can be proven by scientific methods. The development of 14 C dating with AMS allows one to make such measurements on artwork with very small samples. However, great care needs to be taken to identify the origin of the object to be 14 C dated, because, unfortunately, looting and forging is not uncommon on the art market (Huysecom et al. 2017;Hajdas et al. 2019).
Here, we give an example where the authenticity of a painting by a French artist was checked by applying 14 C bomb peak dating (Caforio et al. 2014). It turned out that the painting, which supposedly was produced by the artist early in the 20th century, actually originated from a time period when the artist was already dead. This was proven by finding a 14 C signal in a small sample from the canvas of the painting, which clearly showed the 14 C excess from the 14 C bomb peak (Figure 9). In this case, it was fairly easy to show that the painting must have been a fake.

The Question of Fraudulent Whisky-Making
Scotch whiskies are world-renown products whose original year of production is of great importance for connoisseurs and collectors. In an elaborate effort at the SUERC AMS facility near Glasgow, the Northern Hemisphere 14 C bomb peak calibration curve NH1 (Hua et al. 2013) was verified closely by measuring 14 C in the ethanol component of 221 single malt whisky samples of known distillation year ( Figure 10A). The observed ca. 1-year offset is consistent with the time between barley harvest and distillation (Cook et al. 2020). This "whisky" calibration curve from 1950 to 2015 was then used to examine a range of whiskies supposedly distilled over the last 150 years. Some were found to be genuine, but others were clearly identified to be fakes (Cook et al. 2020). For these conclusions it was also important to measure δ 13 C values for the samples, since the slow decrease with time due to the atmospheric CO 2 contribution from fossil fuel burning provided an additional signature to fix the year of the distillation ( Figure 10B).

CONCLUSION
The current review of the versatile uses of the 14 C bomb peak could only touch an a few specific examples. Nevertheless, it should have made it clear that 14 C from the atmospheric nuclear weapons testing program entered the entire world through the CO 2 cycle in the second half of the 20th century. In particular, the entire biosphere on Earth including humans, animals, and plants who lived through this time period is labeled with an excess of 14 C. This allows unique studies of dynamic processes on decadal time scales, including also the important question of how much of the anthropogenic CO 2 is taken up in various reservoirs on Earth. It is likely that many more studies will be conducted benefitting from the 14 C bomb signal which, however, will be "washed out" in the 21st century. There, the continued emission of "dead" CO 2 from fossil fuel burning will lead to a marked suppression of the 14 C signal below the natural level (Graven 2015).

ACKNOWLEDGMENT
The author acknowledges helpful discussions with colleagues around the world who had particular expertise in the respective fields described in this review. Detailed comments from two reviewers on the original manuscript are also gratefully acknowledged.
Figure 10 14 C calibration curve from known-age whiskies closely resembles the atmospheric 14 C bomb curve (A). The lower part of the figure displays the δ 13 C values of the ethanol from these whisky samples (B). Together, these two carbon isotope signatures allowed one to determine the true age of valuable whiskies, and to distinguish between genuine products and fakes (Cook et al. 2020).
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