A worldwide meta-analysis ( 1977 – 2020 ) of sediment core dating using fallout radionuclides including 137 Cs and 210 Pb xs

Dating recent sediment archives (<150 years) constitutes a prerequisite for environmental and climatic reconstructions. Radiocaesium (Cs) emitted during thermonuclear bomb testing (~1950 ̶ 1980) and nuclear accidents, as well as the decrease of excess lead-210 (Pbxs) with depth are often combined to establish sediment 10 core chronology. Although these methods have been widely used during the last several decades, there is a lack of structured and comprehensive worldwide synthesis of fallout radionuclide analyses used for dating sediment cores in environmental and Earth sciences. The current literature overview was based on the compilation of 573 articles published between 1977 and 2020, reporting the collection of 1351 individual dating sediment cores (the dataset can be accessed at https://doi.pangaea.de/10.1594/PANGAEA.931493). This review was conducted in order to 15 map the locations where Cs fallout events were detected. These included the thermonuclear bomb testing peak in 1963, the Chernobyl accident in 1986, the Fukushima accident in 2011, and 24 additional events identified in 112 sites that led to local radioactive releases (e.g. Sellafield accidents, Chinese nuclear tests). When Pbxs records were used along with Cs data, detailed information on the Pbxs age depth models were also synthesized. Multiple information including the core collection method, sediment properties, radionuclide analysis 20 techniques and catchment characteristics were also compiled. With the current growing number of studies analyzing sediment cores and the increasing interest in the deployment of sediment fingerprinting techniques including radionuclides as potential discriminant properties, this spatialized synthesis provides a unique worldwide compilation for characterizing fallout radionuclide sources and levels at the global scale. This synthesis provides in particular a referential of Cs peak attribution for improving the 25 sediment core dating and it outlines the main questions that deserve attention in future research as well as the regions where additional Cs fallout investigations should be conducted in priority.


Introduction
Sedimentary sequences have received a growing interest as a support for conducting climatic and environmental reconstructions covering the 20 th century period, which has been highly impacted by socio-environmental changes (Syvitski et al., 2020). These natural archives provide a powerful and unique tool for reconstructing the trajectory, 40 the magnitude and the resilience of terrestrial and aquatic ecosystems facing major environmental changes, climate forcing or contamination pressures (Dearing and Jones, 2003;Jaegler et al., 2019;Sabatier et al., 2014). These recent paleo-reconstructions were carried out in various environments such as lakes, alluvial plains, lagoons, estuaries or even the open ocean. Establishing an age depth model is the first prerequisite of any paleo-investigation based on these sediment archives. Fallout of short-lived radionuclides (caesium-137, lead-210)

characterized by
The type of coring device used for sample collection was usually mentioned (81% of publications). In total, 32 150 different corer types were reported to have been used. Gravity corer is the most commonly used device (39%) followed by hand coring (9.6%), piston corer (8.7%), box-corer (8.2%) and vibra-corer (7.3%). Piston and gravity corers were mostly used in lacustrine studies (in 64 and 76% of the studies, respectively), a box corer was used in 81% of the marine and the coastal/estuarine studies. In contrast, a vibra-corer was mainly used in floodplain/riverine environments (44% of the studies). When the corer type is provided, the corer brand is only 155 reported in 18.8% of the publications (the most cited brands are Uwitec, Kajac and Eijkelkamp in respectively 40, 10 and 6% of the articles reporting this information). Technical characteristics of the corer, as corer diameter or size are detailed in 44% of the studies (their mean diameter amounts to 8.6 cm (SD: 3.2 cm)).
Information about the drainage catchment (e.g. surface area), land use and lake characteristics are only reported in 20, 10 and 17.5% of the studies that investigated continental environments. When they were mentioned, the 160 average catchment and water body (lake, pond, dam reservoir) surface areas reached on average 8330 km² (SD: 55,400 km², range: 0.045 to 525,000 km²) and 260 km² (SD: 1130 km², range: 0.0005 to 10,000 km²). These average values are likely overestimated because few studies provided the catchment metadata. To improve the inter-comparison between study sites in the future, we highly recommend the authors to provide systematically detailed information on the study site description.

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Core subsampling and data availability: Information about the total core length was present in 68% of the publications. This information was given in figures, tables or directly in the text. On average, the core length reached 0.85 m (SD: 1.8 m). The subsampling mode is detailed in 72% of the publications, with an average increment thickness of 2.1 cm (SD: 1.5 cm).

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In 73% of the publications, the analytical method used to analyze 210 Pbxs was provided. Alpha spectrometry was used in 33% of the studies whereas gamma spectrometry was used in the rest of the publications. As shown in Fig.   3, a shift between both analytical techniques was observed around 2005, due to the analytical developments made during this period and the improvement of gamma spectrometry potential at low energies, including at 46 keV, i.e. the gamma emission peak of 210 Pb.

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In 60.5% of the studies, radionuclide data was provided in figures only, while in 30.7% of the articles they were reported in both figures and tables. Tables only were found in 1.3% of the publications. In 7.5% of the studies, radionuclide activities were not presented at all, only the dating results (i.e. estimated year of sediment deposition in the successive layers) were provided. The studies in which the detailed data is not reported could not be included to conduct study comparisons. We highly encourage futures studies to include at least the peak radionuclide 180 activities in the text or, even better, the provide a table with the comprehensive radionuclide analysis results in the sediment archives.

137 Cs detection
The first appearance of 137 Cs in the deepest sediment layers was discussed in 14% of studies. In most of them, this (~75%) decayed since then, the first 137 Cs appearance found in depth in a sediment core likely corresponds to deposition that took place in 1955. It was detected at the average depth of 41 cm (SD:45 cm, range: 2 to 360 cm).
In some studies, conducted in both the northern (e.g. Canada) and Southern Hemispheres (e.g. South-Africa) the 190 deepest fallout appearance in the core was nevertheless attributed to 1957 or 1958, without any further explanation in the text.
Fallout associated with the atmospheric nuclear tests (~1950 to ~1980, with a peak in 1963) was detected in 50.9% of the sediment cores (n=668) at a mean depth of 27.5 cm (SD: 36 cm, range: 1 to 286 cm) with an average activity of 69.8 Bq kg -1 (SD: 91 Bq kg -1 , range: 0.8 to 473 Bq kg -1 ) in the Northern Hemisphere and 11.7 Bq kg -1 (SD: 16

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Bq kg -1 , range: 0.4 to 85 Bq kg -1 ) in the Southern Hemisphere (Fig. 4). The year of maximal fallout was commonly attributed to 1963 in both the northern and Southern Hemispheres although studies conducted in the Southern Hemisphere attributed on several occasions the year 1964 (n=5) or 1965 (n=4) to this peak, to take into account the time needed for a full atmosphere homogenization.
The Chernobyl accident was identified by the publication authors in 20.5% of the sediment cores (n=278 sediment 200 cores). This event was mainly recorded in Europe (77.7%), although it was also reported from cores collected in Asia (17.6%, mostly in the western part of the continent) as well as in Northern Africa (1.4%) - (Fig. 4).
Surprisingly, a peak of radiocaesium was also attributed to this accident in cores from China (n=17) ). The average activity associated with this accidental fallout was 670 Bq kg -1 (SD: 5070 Bq kg -1 , 205 range: 2.3 to 48,000 Bq kg -1 ). The highest activities associated with the Chernobyl accident were detected in the vicinity of the nuclear plant, especially in Ukraine, where the highest activities were recorded, as well as in Sweden, Germany, Poland, Romania and in the Alps. The lowest activities attributed to this accident were mostly located in western Europe (e.g. United Kingdom) and in southeastern Europe (Greece, Turkey). On average, this event was detected at a 30-cm depth (SD: 59 cm, range: 1 to 420 cm) in the sediment cores.

210
The discrimination between fallout that occurred in 1963 and after Chernobyl can be achieved through the analysis of 239+240 Pu and 241 Am. Both radionuclides were respectively searched for in 12.7% and 11% of the sedimentary sequences compiled in this database. Accordingly, 241 Am was found in 30% of studies where the 1963 fallout alone was identified.
Finally, fallout associated with the Fukushima accident was identified in 1.9% of the sediment cores (n=26). This 215 level was mainly detected in mainland Japan or along the Japanese eastern coast (Fig. 4) although it was also surprisingly identified in Mexican and Ghanaian sediment sequences. The average activity associated with this fallout was 1920 Bq kg -1 (SD: 7780 Bq kg -1 , range: 6 to 35,700 Bq kg -1 ). This level was identified at an average depth of 1.8 cm (SD: 3 cm, range: 0.25 and 11.5 cm) in the sediment sequences.

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In addition to fallout attributed to the Fukushima and Chernobyl accidents and the atmospheric nuclear tests, 24 other local events that have released radionuclides into the environment were identified by the authors and listed in the current database (Table 1). Among these local events, emissions following the Sellafield Nuclear Plan accidents were observed in 40 sequences (3% of the sediment cores covered in the database), the fallout associated with the Mayak Production Association (Russia) in 12 sediment cores (0.9%), the dumping of radioactive waste 225 into the Kara Sea (Russia) in 8 sediment cores (0.6%), the Chinese Nuclear test fallout in 5 sediment archives  Table 1).
As the calculation of radionuclide inventories required an estimation of the dry bulk density which is often lacking in publications, 137 Cs inventories were only reported for 26% of the sediment cores. The average inventory in the Northern Hemisphere was 30,890 Bq m -2 (SD: 359,000 Bq m -2 , range: 10 to 6,000,000 Bq m -2 , the highest value 235 was measured in Russia close to the Mayak site) compared to 990 Bq m -2 (SD: 1030 Bq m -2 , range: 205 to 3180 Bq m -2 ) in the Southern Hemisphere. The high standard deviation observed in the Northern Hemisphere is mainly due to the high inventories found in the vicinity of Chernobyl, Krasnoyarsk as well as the Mayak production site ( Fig. 6).
Inventories of 137 Cs at reference locations located nearby the sediment core collection site were provided in 7.7% 240 of the publications. The average reference inventory was 1610 Bq m -2 (SD: 2810 Bq m -2 , range: 268 to 24,000 Bq m -2 ) in the Northern Hemisphere. Only one inventory was reported in the Southern Hemisphere (360 Bq m -2 ).
Information about sedimentation rates based on 137 Cs analyses was provided in 34.6% of the publications. The 1963 peak level was generally used for estimating this average sedimentation rate.

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The maximal activities associated with the atmospheric nuclear tests (activities generally attributed to 1963) were recorded between the latitudinal bands comprised between 40 and 60°N, with an average activity of 90 and 117 Bq kg -1 , respectively. Values were not available for the bands located between 0-10°S, 50-60°S and 80-90°S. This is consistent with the United Nations Scientific Committee of the Effects of Atomic Radiation (UNSCEAR, 2000) compilations recording the highest inventories between 40 and 60°N for this event (inventories reached 250 respectively 1186 and 1383 Bq m²). Distribution of both databases were similar with a strong correlation coefficient between inventories and activities (r²=0.87, p-value <0.05) - (Fig. 7).
Comparison between reference inventories of 137 Cs compiled in the current research and those from UNSCEAR showed a lower correlation (r²=0.4, p-value <0.05). This poorer correlation is mostly due to the absence of inventories reported between 10°N and 30°S and between 30 and 90°S. It can also be explained by the high 255 inventories recorded between 30° and 40°N, which are impacted by the high values measured in the vicinity of the Mayak production site (Fig. 7).

210 Pbxs detection and use
This natural radionuclide was often used in combination with 137 Cs in this database (in 64% of the studies). In these publications, the core chronology based on 210 Pbxs was estimated with 7 different models (Constant Rate of 260 Supply (CRS) (Appleby and Oldfield, 1978), Constant Initial Concentration (CIC) (Pennington et al., 1976), Constant Flux Constant Sedimentation (CFCS) (Krishnaswamy et al., 1971), Periodic Flux model (PF) (Sanchez-Cabeza et al., 2000), Constant Initial Concentration and Constant Sedimentation rate (CICCS) , Constant Rate of Accumulation (CRA) and Constant Initial Activity (CIA) (Robbins and Herche, 1993)).
Among the most employed tools, the CRS model (also referred to as CRSMV model in some publications) was

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According to UNSCEAR (2000), 77% of the total worldwide fallout of 137 Cs is estimated to have occurred in the Northern Hemisphere. Previous studies have identified two main sources of 137 Cs in Europe, including the fallout associated with the thermonuclear weapon tests and that emitted by the Chernobyl accident (Meusburger et al., 2020). In the current database, 137 Cs deposition associated with above-ground nuclear tests was largely detected across the European continent with a peak systematically assigned to 1963 (Fig. 4). This peak was described in 295 the literature as the most reliable time marker worldwide . In addition, no technical problem was reported to detect the occurrence of fallout associated with the Chernobyl accident. This post-accidental fallout was identified in sediment cores across Europe from North to South (from Norway to Spain) and from West to East (from Ireland to Belarus). In certain areas as in western France, south-western England, Southern Spain, Portugal or in Iceland, this post-accidental fallout was not detected (Fig. 4). This observation is in agreement with In some regions (e.g. Alps), the Chernobyl fallout appears to be highly heterogeneous and reflects the radioactive cloud pathway and the heterogeneous rainfall pattern after the catastrophe (Alric et al., 2013). In addition to the peaks associated with the Chernobyl accident and the global fallout, the current research has allowed to identify nine local events in Europe that led to radionuclide releases and peaks in sediment sequences. The
In North America, the 1963 peak was often detected in sediment cores (e.g. Van Metre et al., 2004;Corcoran et al., 2018) - (Fig. 4). Five local events were also identified in addition to this peak with a limited spatial extent as 310 in the vicinity of the Savannah River Site (1964), (Lewis et al., 2000), the Indian Point Energy Center (1971)(1972)(1973)(1974)(1975), (Bopp et al., 1982), the Oak Ridge National Laboratory , the Peach Bottom Atomic Power Station (McLean and Summers, 1990) and the Hanford Plutonium Production Center , (Beasley and Jennings, 1984).

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Asia and more specifically China proved to be the most complex region of the world for identifying the sources of the discrete peaks of 137 Cs. The 137 Cs peaks identified in parts of China (e.g. Xinjiang, Guangdong, Guizhou provinces - (Bai et al., 2002;Chen et al., 2014)) were associated with Chinese above-ground nuclear tests conducted during the 1960-1970s. In addition, several studies conducted in the northwest and the southeast of China (Xinjiang, Yunnan and adjacent areas) proposed that the uppermost 137 Cs peak found in lacustrine sediment 320 cores may be attributed to the Chernobyl fallout in 1986. In Chinese cores, fallout associated with the Chernobyl accident was even detected more frequently than the Chinese nuclear tests. It was detected in the Tibetan plateau (e.g. Lake Ngoring - Zhang et al., 2014), in the headwaters of the Yellow river, as well as in cores collected in lower reaches of this river, in its delta  and along the Yellow Sea coastline . This event was also often identified in several lacustrine deposits of southeastern China (e.g. Yunnan Province) - (Zeng 325 & Wu, 2009;Chen et al., 2014;. On the contrary, it was almost never detected in the central and northeastern parts of China (it was only reported in the Miyun reservoir, north of Beijing; Xia et al., 2015).
However, whether this/these peak(s) should be attributed to the Chinese nuclear tests, Russian tests or the Chernobyl accident remains debated in the scientific community. Nevertheless, the current literature review shows that a 137 Cs peak in a lacustrine sediment core was never attributed to the Chernobyl accident in a vast region 330 comprised between the eastern part of Turkey and China. Accordingly, it is very unlikely that the attribution of a 137 Cs peak to the Chernobyl fallout in Chinese cores is meaningful. As these events took place ~10 years apart, future research should incorporate the use of additional tracers that may unambiguously discriminate both sources of radionuclides in this region. Plutonium ( 240 Pu/ 239 Pu) or caesium ( 135 Cs/ 137 Cs) atom ratios may provide good candidates to achieve this type of discrimination (Bu et al., 2015;Zhu et al., 2020).

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Further to the East, lacustrine and marine sedimentary sequences studied in Japan allowed to identify three main sources of 137 Cs. The first local 137 Cs source detected in Japan was attributed to the atomic bomb peak in 1945 in the Nagasaki area (Mahara and Kudo, 1995). The peak associated with the atmospheric nuclear tests in 1963 was then largely recorded across the entire Japanese peninsula (e.g. Lu & Matsumoto, 2009;Hosono et al., 2016) whereas the fallout associated with the Fukushima accident was mainly identified in the vicinity of the Fukushima According to UNSCEAR (2000), only 23% of the total 137 Cs fallout is estimated to have occurred in the Southern Hemisphere. In contrast to what was reported for the Northern Hemisphere, few studies using both 137 Cs and 210 Pbxs 345 for dating sediment archives were found in this part of the world. This is mainly the case for Africa and Oceania.
Studies available in the Southern Hemisphere reported the occurrence of a single peak attributed to the thermonuclear tests. However, no consensus was shared in the literature to assign a single year to this peak. Several authors have attributed the year 1963 (e.g. Chile, Antarctica - Urrutia et al., 2007;Christ et al., 2015) to this peak as in the Northern Hemisphere, whereas other authors dated this peak to 1964 (e.g. Chile -Elbert et al., 2013) or 350 even 1965 (e.g. in Chile, Argentina, South Africa or in the Kerguelen Island - Arnaud et al., 2006;Kastner et al., 2010;Boardman & Foster, 2011;Ficetola et al., 2018). This shift of 1 to 2 years in the assigned year for the thermonuclear bomb fallout peak between the Northern and Southern Hemispheres can be attributed to the time required for the mixing of air masses from both Hemispheres and the occurrence of significant 137 Cs fallout transfer to the Southern Hemisphere. However, this question remains debated and additional sedimentary sequences should 355 be collected in the future to improve the assignation of this peak (Guevara and Arribére, 2007) and sediment core dating in the Southern Hemisphere.

Limitations of the currently published 137 Cs research
This worldwide database also underlines the unexpected detection of 137 Cs sources in certain regions of the world.
In China, a large number of studies claim to have detected Chernobyl fallout whereas more local sources of 137 Cs 360 attributed to the Chinese and Russian nuclear tests conducted in Lop Nor and Semipalatinsk, respectively, are not considered as a potential origin (Huang et al., 2019;Zhao et al., 2020). The potential impact of these tests should be investigated more thoroughly. On the contrary, the attribution of a peak to the Chernobyl fallout should be avoided without clear isotopic evidence, as the associated fallout was not observed in a vast region that extends from the Ural Mountains/Eastern Turkey to China. For example, in a lacustrine sediment core collected in Mexico, 365 the occurrence of 137 Cs was attributed to both Chernobyl (Méndez-García et al., 2014) and Fukushima accidents (Caballero et al., 2020). This remains to be confirmed as other studies conducted nearby these areas did not record these 137 Cs sources. They only recorded fallout associated with the global bomb peak in 1963 (e.g. Hansen, 2012).
A similar finding is made in Ghana where both fallout sources associated with Fukushima and Chernobyl accidents were recorded in the Amisa and Sakumo River estuaries (Mahu et al., 2016) whereas no single 137 Cs peak could 370 be identified in sediment collected in the nearby Volta and Pra River estuaries (Klubi et al., 2017). In North America, 137 Cs fallout attributed to Chernobyl was identified in Canada, in the Vermillon Lake (Schindler and Kamber, 2013), as well as in the North Lake in the United States (Hardaway et al. 1998) higher 137 Cs levels (Foucher et al., 2019; or in contrast lower levels of 137 Cs in some sediment layers that may be delivered by subsoil sources, sheltered from atmospheric fallout and depleted in this radioisotope (Foster et al., 2005). Furthermore, earlier deposits of 137 Cs on ice and snow were reported to have increased the 137 Cs activity in a sediment sequence of Antarctica after ice melting . On the contrary, certain storm events and the associated flood deposits can lead to the dilution of the 137 Cs signal (Gu et 385 al., 2011). In addition to these natural and anthropogenic processes that may lead to an increase or a decrease of the 137 Cs activity along sedimentary sequences, a large number of studies included in the current database reported the occurrence of post-depositional processes. Among them, bioturbation and post-depositional mobility/diffusion may disturb the temporal profile of radionuclides with depth along the core and dilute their concentrations (Benninger et al., 1998;Larsen et al., 2010;Matter et al., 2010). Finally, in some studies, this intense sediment 390 mixing led to the impossibility to establish a chronology, typically when no discrete peak was clearly detected (Eades et al., 2002) or when inconsistent peaks were observed. The frequency of the occurrence of these mixing processes in sediment sequences is difficult to estimate as the analyses of cores that cannot be dated remain often unpublished.

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Overall, this literature review has demonstrated the frequent lack of metadata in publications dealing with sediment core dating and analysis. Accordingly, we highly recommend the authors of future studies to provide detailed information about the reservoir/coring site characteristics, the catchment properties (when relevant) as well as on the coring methodology. In the current synthesis, raw radionuclide data were only provided in tables or in a reusable format in 30.7% of the publications. To extend the short-lived radionuclide data lifecycle (Bruel and 400 Sabatier, 2020;Courtney Mustaphi et al., 2019;Wilkinson et al., 2016) and allow more frequent inter-comparisons between sites and methodologies, we strongly encourage the authors to systematically provide the detailed radionuclide analysis results in tables associated with the publication or in open access data repositories.
Most of the studies covered in the current compilation were conducted in the Northern Hemisphere. This synthesis therefore highlights a massive lack of studies in the South Hemisphere in general, and in Africa in particular. As 405 these regions of the world have been undergoing major anthropogenic changes and pressures since the mid-20 th century, this should be further explored and investigated through the analysis of sediment cores. The achievement of more studies in the Southern Hemisphere will contribute to improve our knowledge of nuclear fallout-related events in the southern half of the world where no consensus has been reached at this stage regarding the attribution of a single year to the 137 Cs fallout peak.

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Finally, the current meta-analysis suggests the potential occurrence of errors in peak attributions in certain regions of the world. To avoid any dating and subsequent interpretation errors in the reconstructions (e.g. contamination) based on the analysis of these archives, we strongly recommend the analysis of complementary tracers that may discriminate 137 Cs sources unambiguously.

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A unique meta-analysis of the sediment core dating research using radionuclides was conducted for the 1977-2020 period. This literature overview provides a worldwide referential to help the scientific community reaching a https://doi.org/10.5194/essd-2021-168 consensus for dating recent sedimentary archives by identifying the 137 Cs peak(s) that may be found in various regions and environments across the globe. This study documents the occurrence of three main sources of 137 Cs that are the most widely detected in sediment cores (nuclear weapon tests, Chernobyl and Fukushima accidents), 420 as well as 24 additional local releases of 137 Cs, which have been synthetized here. The correct attribution of these sources may improve the establishment of core chronology or the validation of 210 Pbxs age models. Nevertheless, this meta-analysis outlines the complexity to discriminate between some of these 137 Cs sources and the necessity to use additional tools such as plutonium and caesium isotopic ratios to provide an unambiguous distinction between potential sources and avoid any dating errors. This review also highlights the low proportion of paleo-       Freshw. Biol., doi:10.1111/j.1365-2427.1976.tb01617.x, 1976.      43 1957, 1974, 1975, 1981, 1985 Oak