Radiocarbon Content Research Articles

Extraction of Dissolved Organic Carbon from Glacier Ice for Radiocarbon Analysis

ABSTRACTAlpine glaciers are valuable archives for the reconstruction of human impact on the environment. Besides dating purposes, measurement of radiocarbon (14C) content provides a powerful tool for long-term source apportionment studies on the carbonaceous aerosols incorporated in ice cores. In this work, we present an extraction system for14C analyses of dissolved organic carbon (DOC) in ice cores. The setup can process ice samples of up to 350 g mass and offers ultra-clean working conditions for all extraction steps. A photo-oxidation method is applied by means of external UV irradiation of the sample. For an irradiation time of 30 min with catalyzation by addition of Fe2+and H2O2, we achieve an efficiency of 96 ± 6% on average. Inert gas working conditions and stringent decontamination procedures enable a low overall blank of 1.9 ± 1.6 μg C with a F14C value of 0.68 ± 0.13. This makes it possible to analyze the DOC in ice samples with a carbon content of as low as 25 μg C kg−1ice. For a first validation, the new method was applied to ice core samples from the Swiss Alps. The average DOC concentration and F14C values for the Fiescherhorn ice core samples show good agreement with previously reported data for the investigated period of 1925–1936 AD.

Mass flowering of Fagus crenata does not depend on the amount of stored carbohydrates in trees

Stored carbohydrates in trees do not act as key requirements for mass flowering in a masting species, Fagus crenata. Many tree species exhibit high interannual variability in flower production at population level, called mass flowering. These species may need to store a certain amount of carbohydrate resources in trees for flowering. Based on this assumption, we hypothesized that masting tree species would blossom heavily when carbohydrates were sufficiently stored up in some specific organs. To test this hypothesis, we studied the relationship between stored carbohydrates in various organs just before the season of flower bud initiation and the number of flowers in the following year in a typical masting species, Fagus crenata (Fagaceae), over 10 years. In addition, we estimated the carbon accumulation period required for flower production by radiocarbon (14C) contents of inflorescences in this species. During the study period, the individuals studied flowered synchronously about every 2 years, but levels of the flowering magnitude were different from each other. Total nonstructural carbohydrate (TNC) concentrations also exhibited large interannual variations, while the long-term trend of TNC over the 10-year observation period showed a general increase in all the investigated tree individuals. There was no significant relationship between the TNC concentrations in each organ and the number of flowers produced in the following year, including the massive flowering event in 2006 when all the selected trees flowered heavily. Fagus crenata produced inflorescences using carbon resources assimilated mainly in the year when the flower buds were formed. These results suggest that the storage of carbohydrate resources might not be a limiting factor in the occurrence or frequency of flowering in this species.

Radiocarbon Tracers in Toxicology and Medicine: Recent Advances in Technology and Science.

This review summarizes recent developments in radiocarbon tracer technology and applications. Technologies covered include accelerator mass spectrometry (AMS), including conversion of samples to graphite, and rapid combustion to carbon dioxide to enable direct liquid sample analysis, coupling to HPLC for real-time AMS analysis, and combined molecular mass spectrometry and AMS for analyte identification and quantitation. Laser-based alternatives, such as cavity ring down spectrometry, are emerging to enable lower cost, higher throughput measurements of biological samples. Applications covered include radiocarbon dating, use of environmental atomic bomb pulse radiocarbon content for cell and protein age determination and turnover studies, and carbon source identification. Low dose toxicology applications reviewed include studies of naphthalene-DNA adduct formation, benzo[a]pyrene pharmacokinetics in humans, and triclocarban exposure and risk assessment. Cancer-related studies covered include the use of radiocarbon-labeled cells for better defining mechanisms of metastasis and the use of drug-DNA adducts as predictive biomarkers of response to chemotherapy.

Soil gas monitoring at the Illinois Basin – Decatur Project carbon sequestration site

An extensive monitoring, verification, and accounting (MVA) program was carried out from 2011 to 2016 as part of the Illinois Basin – Decatur Project (IBDP), a large-scale carbon capture and storage (CCS) demonstration project in Decatur, Illinois, USA. As part of the monitoring program, a near-surface monitoring network for soil gas was used to characterize the natural variability of CO2 and identify anomalies that could be associated with CO2 leakage in the vadose zone. In the fall of 2011, 21 permanent soil gas sampling nests were installed on-site and three nests were installed off-site as reference locations. Soil gas was collected from three depth intervals (15–30, 47–61, 107–122 cm) at each nest. Fixed gas (CO2, N2, O2, and light hydrocarbon) concentrations, stable carbon isotopic composition (δ13C), and radiocarbon (14C) content of CO2 in soil gas were monitored during the pre-injection, injection, and post-injection phases from 2011 to 2016. Both CO2 concentrations and carbon isotopic compositions showed large spatial and temporal variations; however, the relationships among CO2, N2, and O2 concentrations in soil gas indicated that these variations should be attributed to natural processes and not CO2 leakage. Soil CO2 concentrations responded to temperature and precipitation variations and increased with increasing soil depth. While 14C cannot be used to determine the occurrence of CO2 leakage at IBDP site due to the indistinctive signature between injected CO2 and soil gas CO2, experiences at IBDP validated that δ13C can serve as an effective natural tracer at this site. Results from this project have value for future industrial CCS projects that plan to use soil gas monitoring techniques for site characterization and leakage detection.

Annual radiocarbon measurements in a century-old European beech tree (Fagus sylvatica) from coastal northeastern North America

Radiocarbon (14C) concentrations of annual tree rings from an European beech tree (Fagus sylvatica) from Woods Hole, MA, USA were analyzed at National Ocean Sciences Accelerator Mass Spectrometry facility (NOSAMS) to construct a 14C bomb curve record from northeastern North America. The 14C concentration rises from a pre-bomb (1895–1955) average of Fraction Modern (F14C) = 0.9764 to a peak of 1.8639 in 1964. After 1964, F14C gradually decreases to 1.0611 in 2014. The annual tree-ring radiocarbon content agrees with the atmospheric radiocarbon content of the Northern Hemisphere and is very similar to the radiocarbon concentration of a red oak (Quercus rubra) tree located in Bear Mountain State Park in New York, USA. Intra-annual tree-ring pairs did not produce evidence of a seasonal effect on the radiocarbon concentration, but there were few samples and more study is warranted.

East Asian Monsoon Variability Since the Sixteenth Century

AbstractThe East Asian Monsoon (EAM) impacts storms, freshwater availability, wind energy production, coal consumption, and subsequent air quality for billions of people across Asia. Despite its importance, the EAM's long‐term behavior is poorly understood. Here we present an annually resolved record of EAM variance from 1584 to 1950 based on radiocarbon content in a coral from the coast of Vietnam. The coral record reveals previously undocumented centennial scale changes in EAM variance during both the summer and winter seasons, with an overall decline from 1600 to the present. Such long‐term variations in monsoon variance appear to reflect independent seasonal mechanisms that are a combination of changes in continental temperature, the strength of the Siberian High, and El Niño–Southern Oscillation behavior. We conclude that the EAM is an important conduit for propagating climate signals from the tropics to higher latitudes.

The Technological and Chronological Implication of 14C Concentrations in Carbon Samples Extracted from Mongolian Cast Iron Artifacts

ABSTRACTCast iron objects recovered primarily in eastern Mongolia, spanning the Xiongnu through the Early Historic periods (ca. 3rd BC–AD 17th century), were examined for their radiocarbon (14C) concentration and microstructure. Most of the samples examined were found to have originated from charcoal-based smelting with a few exceptions that were made using a mineral coal-based technique. A comparison of 14C dates with dates derived from artifact typology allowed the charcoal-smelted objects to be classified into two groups, based on whether the radiometric and typological periodization are in agreement or not. In addition, those with differing 14C and typological dates can be divided into two subgroups with and without evidence for a melt treatment applied after original casting. These conflicting dating results are confusing and would seem to provoke skepticism about the use of 14C measurements for dating iron artifacts. We demonstrate however that 14C analysis, when combined with metallographic examination and other lines of chronological evidence, can clarify the history of a given iron object and its multiple users, often separated in time by more than a millennium.

Position-dependent radiocarbon content of the macroalgae Undaria pinnatifida as an indicator of oceanographic conditions during algal growth

The radiocarbon content (∆14C) of the large brown macroalga Undaria pinnatifida was investigated as an indicator of oceanographic conditions during algal growth in Otsuchi Bay on the Pacific coast of northeastern Japan. In this region, warm boundary currents characterized by high dissolved inorganic carbon (DIC) ∆14C and cold subarctic Oyashio Current (low ∆14C-DIC) flow into the bay seasonally. We hypothesized that ∆14C values in U. pinnatifida pinnae (leaflet-like structures arising sequentially along the vertical axis of the blade) reflect ∆14C-DIC values in bay water during photosynthetic production. During the U. pinnatifida cultivation period (November 2013–April 2014), the Oyashio intrusion occurred from early March to early April, accompanied by a decrease in bay water ∆14C-DIC, from 15.7 ± 8.6‰ to − 57.1 ± 20.7‰. The ∆14C of the pinnae reflected ∆14C-DIC, as pinnae that formed in early March had significantly higher ∆14C (3.0 ± 5.4‰) than those that formed in early April (− 23.6 ± 7.5‰). These results indicate that the ∆14C of U. pinnatifida pinnae can serve as a record of Oyashio intrusion into the bay during algal growth.

Modelling the physical states, element stoichiometries and residence times of topsoil organic matter

Soil organic matter (SOM) is a major ecosystem component, central to soil fertility, carbon balance and other soil functions. To advance SOM modelling, we devised a steady‐state model of topsoil SOM, with explicit descriptions of physical states and properties, and used it to simulate SOM concentration, carbon:nitrogen:phosphorus (C:N:P) stoichiometry, bulk density and radiocarbon content. The model classifies SOM by element stoichiometry (αSOM is poor in N and P, βSOM is rich), mean residence times (1–2000 years) and physical state (free, occluded, adsorbed, hypoxic). The most stable SOM is either βSOM preferentially adsorbed by mineral matter, or αSOM in strongly hypoxic zones. Soil properties were simulated for random combinations of plant litter input (amount and C:N:P stoichiometry), mineral sorption capacity, propensity for hypoxia, and bulk density of non‐adsorbed αSOM. To optimize model parameters, outputs from 5000 simulations were used to construct bivariate relations among soil variables, which were compared with those found in data for 835 survey sites, covering all common land uses. The bivariate relations, and patterns of data scatter, were reproduced, and also variations in soil radiocarbon with soil type, suggesting that apparent scatter in measured data might reflect SOM diversity. The temporal acquisition by soil of ‘bomb 14C’ could also be simulated. The steady‐state model is the basis for a dynamic version, suitable for simulating changes in SOM through time. It provides insight into the possible manipulation of soil organic carbon (SOC) sequestration; for example, increasing litter inputs might only increase moderately‐stable SOC pools, whereas encouraging the creation of βSOM by adsorption to mineral matter from deeper soil could lead to long‐term stabilization.Highlights Models of SOM should include explicit descriptions of physical states and properties. Our new topsoil SOM model is constrained by C:N:P stoichiometry, SO14C, and physical fractionation data. Simulated soil properties, randomly generated, account for measured trends and patterns of scatter in SOM data. SOM properties depend upon litter input, interactions with mineral matter, hypoxia, and bulk density.

A detailed investigation of pretreatment protocols for high precision radiocarbon measurements of annual tree-rings

Much discussion has revolved around proper sample types and pretreatment methods to generate a calibration dataset that would allow for the most accurate representation of an unknown sample’s calendar age. Since the development of radiocarbon dating, tree-rings have played a vital role in the creation of global calibration datasets, with these samples being subjected to varying degrees of both physical and chemical pretreatments. With an international move away from multi-ring composite measurements and toward higher resolution annual and/or sub-annual measurement of radiocarbon content, and with the on-set of a major new research project using, primarily the highly resinous North American Bristlecone Pine, we set out to review sample preparation procedures. The question became; which fraction, and therefore how much pretreatment is really necessary in order to extract the most representative and replicable C14 determination in a tree-ring for any given growth year, and could it be both specimen as well as species dependent? For this study, a total of four samples were chosen. These samples consisted of two rings from two different years from Bristlecone Pine and two rings from these same years from Irish Oak. These rings were each treated with a series of progressively more aggressive chemical treatments allowing for the extraction of different fractions from each ring. An analysis of the different pretreatment methods, as well as interspecies variance, is discussed.

Interannual Coral Δ14C Records of Surface Water Exchange Across the Luzon Strait

AbstractThe Luzon Strait (LS) hosts the largest transport of water between the Western Pacific Ocean (WPO) and the South China Sea (SCS). The transport through the strait, dominated by the westward propagation of the Kuroshio Intrusion, influences the climate and circulation of the SCS. While numerical models have investigated the interannual variability of the transport and subsequent water exchange across the LS, a lack of long‐term on‐site records prevents a general consensus on the transport rates, variability, and drivers. Corals offer high‐resolution, continuous histories of radiocarbon (Δ14C) content of the seawater dissolved inorganic carbon, allowing us to track changes in ocean transport and circulation through time. Seasonal and annual Δ14C samples from Houbihu, Taiwan, and Palaui, Philippines, located on either side of the strait, are compared to the Western Pacific Ocean and SCS Δ14C records to examine the spatial and temporal Δ14C variability in the region. We calculated the mean transport across the strait using a five‐box mixing model and identified its potential drivers. The mean amount of water exchanged across the strait from 1970 to 1999 was 2.2 Sv, ranging from −13.4 to 16 Sv, where a positive (negative) value indicates net flow into (out of) the SCS. A weaker East Asian Winter Monsoon increases the contribution of the SCS outflow on the Kuroshio Intrusion‐dominated LS, while the El Niño–Southern Oscillation primarily drives the intrusion into the SCS. These results provide support to the dominant control of El Niño–Southern Oscillation on the long‐term ocean circulation variability in this region.

Radiocarbon Age Offsets Between Two Surface Dwelling Planktonic Foraminifera Species During Abrupt Climate Events in the SW Iberian Margin.

This study identifies temporal biases in the radiocarbon ages of the planktonic foraminifera species Globigerina bulloides and Globigerinoides ruber (white) in a sediment core from the SW Iberian margin (so‐called Shackleton site). Leaching of the outer shell and measurement of the radiocarbon content of both the leachate and leached sample enabled us to identify surface contamination of the tests and its impact on their 14C ages. Incorporation of younger radiocarbon on the outer shell affected both species and had a larger impact downcore. Interspecies comparison of the 14C ages of the leached samples reveal systematic offsets with 14C ages for G. ruber being younger than G. bulloides ages during the last deglaciation and part of the Early and mid‐Holocene. The greatest offsets (up to 1,030 years) were found during Heinrich Stadial 1, the Younger Dryas, and part of the Holocene. The potential factors differentially affecting these two planktonic species were assessed by complementary 14C, oxygen and carbon isotopes, and species abundance determinations. The coupled effect of bioturbation with changes in the abundance of G. ruber is invoked to account for the large age offsets. Our results highlight that 14C ages of planktonic foraminifera might be largely compromised even in settings characterized by high sediment accumulation rates. Thus, a careful assessment of potential temporal biases must be performed prior to using 14C ages for paleoclimate investigations or radiocarbon calibrations (e.g., marine calibration curve Marine13, Reimer et al., 2013, https://doi.org/10.2458/azu_js_rc.55.16947).

Solar activity cycles and grand minima occurrence

Sunspot number reconstructions (SNRs) based on dendrochronologically dated radiocarbon concentrations are analyzed through the Empirical Mode Decomposition (EMD) to provide a deeper characterization of the solar activity long-term periodicities and to investigate the role of the Gleissberg and Suess cycles in the grand minima occurrence. The Gleissberg and Suess cycles, with timescales of 60-120 yr and 200-300 yr respectively, represent the most energetic contribution in SNR records. The EMD approach reveals that these cycles are characterized by multiple scales of variation and that the grand minima sequence is produced by the coupling between Gleissberg and Suess cycles, the latter being responsible for the most intense and longest Sporer-like minima, with typical duration longer than 80 yr.

Reconstruction of the Wolf Numbers Based on Radiocarbon Data from the Early 11th Century until the Middle of the 19th Century with Respect to Climate Changes

The results of the reconstruction of Wolf numbers from the 11th century until the middle of the 19th century A.D. based on radiocarbon data are presented. This time span includes the Oort, Wolf, Sporer, Maunder, and Dalton minima of solar activity, as well as the Medieval Solar Maximum (the 12th century and the first half of the 13th century A.D.) and the Late Medieval Maximum (the second half of the 14th century). It is known that the climate changed appreciably over the studied period: there were changes in the global temperature and in the concentration of carbon dioxide in the Earth’s atmosphere; more precisely, there was the Little Ice Age. This work differs from previous reconstructions of other authors in taking into account the influence of climate changes on the radiocarbon content in examined samples at that time span. During the reconstruction, changes in carbon dioxide content and changes in the regimen of carbon dioxide exchange between the ocean and the atmosphere are considered. It is especially important to consider changes in these parameters for the Little Ice Age, which was accompanied by significant changes in global temperature and the carbon dioxide content in the Earth’s atmosphere.

14C and Other Radionuclides in Impermeable Graphite Material Waste form Long Term Behavior

ABSTRACTThe radiocarbon (14C) content of irradiated graphite is the most important problem for the management of Spanish irradiated graphite (Vandellós I NPP) as L&ILW, due to this material exceeding the maximum14C inventory for the C.A. El Cabril repository. Therefore, the encapsulation of graphite in an impermeable matrix and making an appropriate waste form are indicated as potential management options to be studied. The conversion of the graphite to a long-term stable glass matrix, called IGM (impermeable graphite matrix), uses a long-term stable inorganic binder which additionally encloses the graphite pore system. The world’s first IGM samples made with irradiated graphite have been manufactured in CIEMAT facilities. The durability of the matrix is investigated in leaching experiments in deionized water and granitic bentonite water. The results show that ∼0.05% of14C is leached. A species of organic carbon was found as formate and oxalate (∼10–1mg/L). CO was detected as volatile specie in both media in the first leaching steps; for deionized water (∼3.101mg/L) and in granitic bentonite water (ranging 1.101–3.101mg/L). These low values demonstrated the durability of the IGM glass matrix for final disposal.

Smoke radiocarbon measurements from Indonesian fires provide evidence for burning of millennia-aged peat

In response to a strong El Niño, fires in Indonesia during September and October 2015 released a large amount of carbon dioxide and created a massive regional smoke cloud that severely degraded air quality in many urban centers across Southeast Asia. Although several lines of evidence indicate that peat burning was a dominant contributor to emissions in the region, El Niño-induced drought is also known to increase deforestation fires and agricultural waste burning in plantations. As a result, uncertainties remain with respect to partitioning emissions among different ecosystem and fire types. Here we measured the radiocarbon content (14C) of carbonaceous aerosol samples collected in Singapore from September 2014 through October 2015, with the aim of identifying the age and origin of fire-emitted fine particulate matter (particulate matter with an aerodynamic diameter less than or equal to 2.5 μm). The Δ14C of fire-emitted aerosol was -76 ± 51‰, corresponding to a carbon pool of combusted organic matter with a mean turnover time of 800 ± 420 y. Our observations indicated that smoke plumes reaching Singapore originated primarily from peat burning (∼85%), and not from deforestation fires or waste burning. Atmospheric transport modeling confirmed that fires in Sumatra and Borneo were dominant contributors to elevated PM2.5 in Singapore during the fire season. The mean age of the carbonaceous aerosol, which predates the Industrial Revolution, highlights the importance of improving peatland fire management during future El Niño events for meeting climate mitigation and air quality commitments.

Influence of Different Acid Treatments on the Radiocarbon Content Spectrum of Sedimentary Organic Matter Determined by RPO/Accelerator Mass Spectrometry

ABSTRACTIn practice, obtaining radiocarbon (14C) composition of organic matter (OM) in sediments requires first removing inorganic carbon (IC) by acid-treatment. Two common treatments are acid rinsing and fumigation. Resulting14C content obtained by different methods can differ, but underlying causes of these differences remain elusive. To assess the influence of different acid-treatments on14C content of sedimentary OM, we examine the variability in14C content for a range of marine and river sediments. By comparing results for unacidified and acidified sediments [HCl rinsing (RinseHCl) and HCl fumigation (FumeHCl)], we demonstrate that the two acid-treatments can affect14C content differentially. Our findings suggest that, for low-carbonate samples, RinseHClaffects the Fm values due to loss of young labile organic carbon (OC). FumeHClmakes the Fm values for labile OC decrease, leaving the residual OC older. High-carbonate samples can lose relatively old organic components during RinseHCl, causing the Fm values of remaining OC to increase. FumeHClcan remove thermally labile, usually young, OC and reduce the Fm values. We suggest three factors should be taken into account when using acid to remove carbonate from sediments: IC abundance, proportions of labile and refractory OC, and environmental matrix.

Ventilation of Northern and Southern Sources of Aged Carbon in the Eastern Equatorial Pacific During the Younger Dryas Rise in Atmospheric CO2

AbstractAtmospheric carbon dioxide (CO2) levels rose by ~90 ppmv during the last deglaciation, but the source of this carbon remains unknown. One popular hypothesis suggests carbon accumulated in the deep Southern Ocean, becoming increasingly radiocarbon (14C) depleted during the last glacial period, and was released into Antarctic Intermediate Water (AAIW) and, subsequently, the atmosphere during deglaciation. Detection of extremely 14C depleted carbon in the intermediate‐depth tropical oceans during periods of atmospheric CO2 rise was initially considered the smoking gun for the hypothesis, but attempts to reproduce the anomalies closer to the Southern Ocean source have largely failed. Here we present new 14C records from four cores recovered at intermediate depths in the eastern equatorial Pacific (EEP). In the context of additional geochemical records, including benthic foraminiferal stable isotopes and sedimentary nitrogen isotopes, we demonstrate that the extreme 14C anomalies observed during the last deglaciation do not reflect the radiocarbon content of AAIW. We show that although AAIW likely transported modestly 14C depleted carbon to the EEP subsurface, the extreme 14C signatures might reflect a distinct source of aged carbon arriving from the north, suggesting the North Pacific helped transport deep ocean carbon to the atmosphere during the last deglaciation. In the EEP, additional local hydrothermal inputs of 14C‐free carbon near the Galapagos Islands magnified the already low‐14C signatures. Finally, regardless of arrival route (North/South Pacific or hydrothermal), 14C‐depleted carbon was released from the EEP subsurface during a late deglacial pulse of renewed upwelling, likely contributing to the Younger Dryas rise in atmospheric CO2.

Radiocarbon measurements of ecosystem respiration and soil pore-space CO<sub>2</sub> in Utqiaġvik (Barrow), Alaska

Abstract. Radiocarbon measurements of ecosystem respiration and soil pore space CO2 are useful for determining the sources of ecosystem respiration, identifying environmental controls on soil carbon cycling rates, and parameterizing and evaluating models of the carbon cycle. We measured flux rates and radiocarbon content of ecosystem respiration, as well as radiocarbon in soil profile CO2 in Utqiaġvik (Barrow), Alaska, during the summers of 2012, 2013, and 2014. We found that radiocarbon in ecosystem respiration (Δ14CReco) ranged from +60.5 to −160 ‰ with a median value of +23.3 ‰. Ecosystem respiration became more depleted in radiocarbon from summer to autumn, indicating increased decomposition of old soil organic carbon and/or decreased CO2 production from fast-cycling carbon pools. Across permafrost features, ecosystem respiration from high-centered polygons was depleted in radiocarbon relative to other polygon types. Radiocarbon content in soil pore-space CO2 varied between −7.1 and −280 ‰, becoming more negative with depth in individual soil profiles. These pore-space radiocarbon values correspond to CO2 mean ages of 410 to 3350 years, based on a steady-state, one-pool model. Together, these data indicate that deep soil respiration was derived primarily from old, slow-cycling carbon, but that total CO2 fluxes depended largely on autotrophic respiration and heterotrophic decomposition of fast-cycling carbon within the shallowest soil layers. The relative contributions of these different CO2 sources were highly variable across microtopographic features and time in the sampling season. The highly negative Δ14C values in soil pore-space CO2 and autumn ecosystem respiration indicate that when it is not frozen, very old soil carbon is vulnerable to decomposition. Radiocarbon data and associated CO2 flux and temperature data are stored in the data repository of the Next Generation Ecosystem Experiments (NGEE-Arctic) at http://dx.doi.org/10.5440/1364062 and https://doi.org/10.5440/1418853.

Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE

Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.