Americium-241 and trace elements in peat bogs as markers of the beginning of the Anthropocene
Notice bibliographique
Résumé
Peat bogs are recognized as invaluable archives of atmospheric deposition, preserving records of environmental changes. This thesis investigates the potential of fallout radionuclides and trace elements (TEs) preserved in peat profiles as crucial markers for understanding environmental change, particularly concerning the onset of the proposed new geological time unit, the Anthropocene. While the Anthropocene signifies a period of unprecedented human influence on Earth systems, an unequivocal marker for its commencement remains a subject of ongoing scientific debate. This research explores the suitability of specific anthropogenic markers within peat records to address this challenge. Specifically, it examines the potential of the Americium-241, as a potential global stratigraphic indicator for the Anthropocene. Furthermore, this study investigates the stability of 241Am and TEs preservation in peat bogs over time. Three interconnected studies address these objectives by examining the distribution, preservation, and potential post-depositional movement of these environmental markers, aiming to contribute to a more robust understanding of the Anthropocene's temporal boundaries. This study begins by assessing the reliability of 241Am as a global Anthropocene marker, based on its formation through the decay of 241Pu from nuclear weapons fallout. The study examines 35 peat cores collected from 30 sites across Europe and North America to determine whether 241Am peaks align with the year 1963 (the year of the maximum nuclear bomb tests conducted worldwide), as dated using the 210Pb Constant Rate of Supply model. Results show that 22 of the 35 cores exhibit statistically validated 241Am peaks, with 18 aligning within a decade of AD 1963. Analytical limitations, variations in peat accumulation rates, post-depositional migration, and dating uncertainties contribute to discrepancies. Despite these factors, the widespread detection of 241Am supports its utility as a general Anthropocene marker, albeit with acknowledged uncertainties. This leads to a pertinent question concerning the temporal integrity of fallout radionuclide and TEs records preserved in peat bogs, a topic that formed the basis of analysis and interpretation in the second study. The second study investigates the long-term stability of TE deposition in peat bogs by comparing peat cores collected 20 years apart (2000 vs. 2021–2022) from three ombrotrophic bogs in Southern Ontario. Using 210Pb and 14C bomb-pulse dating, trends in Pb, Cu, Mn, Sr, and Y were analyzed. The results confirm that Pb and Cu remain stable across the studied time period, reinforcing their suitability as markers of historical atmospheric deposition. In contrast, Mn exhibits higher variability, likely due to its redox sensitivity. Fallout radionuclide profiles further validate the stability of the 1963 nuclear testing horizon in two of the three sites, while one of the studied sites displays potential post-depositional movement of 137Cs. These findings underscore the reliability of Pb and Cu as geochemical archives while highlighting the influence of redox conditions on TE mobility. The observed instability of certain TEs over time suggests potential mobility within the peat profile. The suitability of porewater analysis for assessing this mobility is explored as well. The third study evaluates TE mobility in peat bogs by comparing solid-phase and porewater concentrations of 42 elements across five sites in Southern Ontario. The distinction between crustal and excess TE fractions was examined, with Y serving as a stable reference element. Excess Pb and Sb peaked at mid-depths in peat cores, reflecting historical industrial activity, while elements such as Cd and Tl exhibited site-specific variability in their mobility. Correlation analysis between porewater and peat concentrations revealed that Pb and Sb in porewater were more strongly associated with their excess concentrations, particularly in sites closer to industrial sources. In contrast, Rb and Y in porewater showed stronger associations with crustal concentration, indicating different mobilization pathways. The study concludes that the potential mobility of excess Pb and Cd is lower than that of Rb and Tl, emphasizing the importance of distinguishing between crustal and excess contributions when evaluating TE behavior in peat bogs. The results demonstrate the potential use of 241Am as a stratigraphic indicator of the Anthropocene under certain conditions, confirm the long-term stability of Pb and Cu in peat archives, and highlight key factors influencing TE mobility. These findings contribute to the broader understanding of peat bogs as geochemical archives and their role in reconstructing historical atmospheric deposition patterns.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».