Edouard Bard

Subfossil pines from the banks of the Drouzet River in the southern French Alps provide a unique Late Glacial archive for reconstructing past solar activity. Dendrochronological analyses produced three floating chronologies together spanning ≈ 700 calendar years. High-precision 14C measurements obtained at CEREGE with the AixMICADAS accelerator mass spectrometer, and calibrated by Markov Chain Monte Carlo wiggle-matching against the IntCal20 absolute chronology, reveal a large and abrupt Δ14C increase of ≈ 30‰ at 14 300 cal yr BP. Carbon cycle model simulations forced with the GRIP 10Be flux record are compatible with this spike, confirming a sharp enhancement in cosmogenic isotope production linked to an extreme solar particle event (ESPE). Lower geomagnetic field intensity and reduced atmospheric CO₂ during the Late Glacial contributed to amplifying the ∆14C signal. Recent three-dimensional atmospheric 14C modelling indicates that this event represents the most powerful ESPE evidenced so far, much stronger than the largest ground-level enhancement of cosmic rays observed over the last 80 years of instrumental data. These results extend the ESPE record beyond the Holocene and demonstrate that subfossil wood has the potential to offer annually resolved evidence of ancient solar events, providing new constraints for radiocarbon calibration and solar-terrestrial interaction studies. This article is part of the Theo Murphy meeting issue ‘Radiocarbon and cosmic radiation events’.

Variations in Total Solar Irradiance (TSI) have long been hypothesized to influence global mean surface temperature (GMST) on multidecadal to millennial timescales, but empirical evidence has remained elusive. Here, we explore the link between solar variability and climate over the pre‐industrial Holocene, when anthropogenic influences were minimal, using the causal inference framework of Convergent Cross‐Mapping. We analyze pairings of two temperature targets (a GMST reconstruction and the central Greenland GISP2 record) with two TSI reconstructions across a range of timescales. Our results indicate a discernible influence of TSI on temperature in ∼80% of parameter configurations: stronger but less consistently significant for GISP2, weaker but consistently significant for the global mean. This suggests that solar forcing was a minor–but detectable–driver of global Holocene climate fluctuations, an empirical insight that may serve as a benchmark for evaluating whether climate models correctly capture such subtle, nonlinear dynamics.

Extreme solar particle events (ESPEs) are caused by rare, enormously strong solar eruptions and can produce globally detectable spikes in tree-ring radiocarbon ¹⁴ C, known as Miyake events, which serve as precise chronological tie-points and indicators of extreme solar activity. After production, radiocarbon is subjected to the complex carbon cycle, including large-scale atmospheric transport, which is crucially important for fast and strong Miyake events with highly inhomogeneous ¹⁴ C production. A new 3D dynamical model, SOCOL:14C-Ex, of the radiocarbon atmospheric production and transport is presented here, which can model fast changes in the ¹⁴ C atmospheric concentrations with high temporal and spatial resolution. Precise response curves of Δ ¹⁴ C to a reference ESPE (100×GLE#69) were computed for various event dates. They can be directly applied to analyze Miyake events under different conditions. Seven strong events over the past 14 millennia (AD 993, AD 774, 664 BC, 5260 BC, 5411 BC, 7177 BC, and 12,351 BC) were analyzed by fitting the reference curves to the available annual Δ ¹⁴ C data, identifying the most probable values and confidence intervals of their parameters—strength, event’s date and background level. By applying corrections for the geomagnetic and atmospheric (CO ₂ ) factors, the strengths of the corresponding ESPEs were assessed. The strongest ESPE is confirmed to be that of 12,351 BC, while that of AD 774 remains the strongest event during the Holocene. To conclude, a new tool, based on the radiocarbon atmospheric transport model SOCOL:14C-Ex, is presented to analyze fast changes in the ¹⁴ C production.

Enseignement Cours – Climats extrêmes et analogues actuels : l’Holocène et le Tardiglaciaire Il faut replacer le réchauffement climatique du dernier siècle dans un contexte temporel plus large afin d’en déterminer la singularité et de distinguer les causes sous-jacentes, naturelles et anthropiques. Les variations climatiques allant de la fin de la dernière période glaciaire, le Tardiglaciaire, au début de la période interglaciaire actuelle, l’Holocène, servent à tester de multiples hypothèses...

This study combines a multidisciplinary approach to Pyrenean and Alpine glacial lakes to characterize the sensitivity of Late Glacial to Holocene subaquatic flood deposits in deltaic environments to slope failures triggered either by earthquakes, rockfalls, or snow avalanches. To clarify the possible interactions between environmental changes and these natural hazards in mountain and piedmont lakes, we analyze the lacustrine sedimentary records of key historical events and discuss the recurrence of similar regional events in the past. High-resolution seismic profiles and sediment cores from large perialpine lakes (Bourget, Geneva, and Constance) and from small mountain lakes in the French Alps and the Pyrenees were used to establish a conceptual model linking environmental changes, tributary flood sedimentary processes, subaquatic deltaic depocenters, and potentially tsunamigenic mass-wasting deposits. These findings illustrate the specific signatures of the largest French earthquakes in 1660 CE (northern Pyrenees) and in 1822 CE (western Alps) and suggest their recurrence during the Holocene. In addition, the regional record in the Aiguilles Rouges massif near Mont Blanc of the tsunamigenic 1584 CE Aigle earthquake in Lake Geneva may be used to better document a similar Celtic event ca. 2300 Cal BP at the border between Switzerland and France.

The timing and mechanisms underlying the gradual replacement of Neanderthal populations by Homo sapiens groups have elicited heated debate for decades. The Châtelperronian of France and northern Spain has figured prominently in these discussions. While the Neanderthal authorship of this techno-complex is far less certain than previously thought, the Châtelperronian is now generally accepted as being among the first genuine Upper Paleolithic industries in Eurasia rather than a “transitional” stone tool technology. Here, we report the first association of shell beads with Châtelperronian artifacts from new excavations at the site of La-Roche-à-Pierrot, Saint-Césaire (France), supported by a multiproxy spatial analysis that incorporates site formation processes. Similar types of personal ornaments are unknown from Middle Paleolithic contexts. Comparable examples have, however, been recovered from contexts penecontemporaneous with the Châtelperronian in south-eastern Europe and around the Mediterranean. This hitherto undocumented combination of an early Upper Paleolithic industry and shell beads provides insights into cultural variability in western Europe and raises the question as to whether the makers of the Châtelperronian were influenced by or formed part of the earliest dispersals of H. sapiens into the region.

Ratios of glycerol dialkyl glycerol tetraethers (GDGT), which are membrane lipids of bacteria and archaea, are at the base of several paleoenvironmental proxies. They are frequently applied to soils as well as lake‐ and marine sediments to generate records of past temperature and soil pH. To derive meaningful environmental information from these reconstructions, high analytical reproducibility is required. Based on submitted results by 39 laboratories from across the world, which employ a diverse range of analytical and quantification methods, we explored the reproducibility of brGDGT‐based proxies (MBT′_5ME, IR, and #rings_tetra) measured on four soil samples and four soil lipid extracts. Correct identification and integration of 5‐ and 6‐methyl brGDGTs is a prerequisite for the robust calculation of proxy values, but this can be challenging as indicated by the large inter‐interlaboratory variation. The exclusion of statistical outliers improves the reproducibility, where the remaining uncertainty translates into a temperature offset from median proxy values of 0.3–0.9°C and a pH offset of 0.05–0.3. There is no apparent systematic impact of the extraction method and sample preparation steps on the brGDGT ratios. Although reported GDGT concentrations are generally consistent within laboratories, they vary greatly between laboratories. This large variability in brGDGT quantification may relate to variations in ionization efficiency or specific mass spectrometer settings possibly impacting the response of brGDGTs masses relative to that of the internal standard used. While ratio values of GDGT are generally comparable, quantities can currently not be compared between laboratories.

We present new ¹⁴ C results measured on subfossil Scots Pines recovered in the eroded banks of the Drouzet watercourse in the Southern French Alps. About 400 new ¹⁴ C ages have been analysed on 15 trees sampled at annual resolution. The resulting Δ ¹⁴ C record exhibits an abrupt spike occurring in a single year at 14 300–14 299 cal yr BP and a century-long event between 14 and 13.9 cal kyr BP. In order to identify the causes of these events, we compare the Drouzet Δ ¹⁴ C record with simulations of Δ ¹⁴ C based on the ¹⁰ Be record in Greenland ice used as an input of a carbon cycle model. The correspondence with ¹⁰ Be anomalies allows us to propose the 14.3 cal kyr BP event as a solar energetic particle event. By contrast, the 14 cal kyr BP event lasted about a century and is most probably a common Maunder-type solar minimum linked to the modulation of galactic cosmic particles by the heliomagnetic field. We also discuss and speculate about the synchroneity and the possible causes of the 14 cal kyr BP event with the brief cold phase called Older Dryas, which separates the Bølling and Allerød millennium-long warm phases of the Late Glacial period. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.

Radiocarbon ( ¹⁴ C) is a critical tool for understanding the global carbon cycle. During the Anthropocene, two new processes influenced ¹⁴ C in atmospheric, land and ocean carbon reservoirs. First, ¹⁴ C-free carbon derived from fossil fuel burning has diluted ¹⁴ C, at rates that have accelerated with time. Second, ‘bomb’ ¹⁴ C produced by atmospheric nuclear weapon tests in the mid-twentieth century provided a global isotope tracer that is used to constrain rates of air–sea gas exchange, carbon turnover, large-scale atmospheric and ocean transport, and other key C cycle processes. As we write, the ¹⁴ C/ ¹² C ratio of atmospheric CO ₂ is dropping below pre-industrial levels, and the rate of decline in the future will depend on global fossil fuel use and net exchange of bomb ¹⁴ C between the atmosphere, ocean and land. This milestone coincides with a rapid increase in ¹⁴ C measurement capacity worldwide. Leveraging future ¹⁴ C measurements to understand processes and test models requires coordinated international effort—a ‘decade of radiocarbon’ with multiple goals: (i) filling observational gaps using archives, (ii) building and sustaining observation networks to increase measurement density across carbon reservoirs, (iii) developing databases, synthesis and modelling tools and (iv) establishing metrics for identifying and verifying changes in carbon sources and sinks. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.