Comment: Variations in the isotope composition of mercury in a freshwater sediment sequence and food web
Notice bibliographique
Résumé
Discussion 2311 In a recent paper, Jackson (2001) presented data on variations of the Hg isotope composition in sediments and associated food webs in Lake Ontario. The author claimed that the results demonstrated fractionation of Hg isotopes along a sediment depth profile and in a simple food chain. He went on to correlate sequentially extracted metal concentrations with Hg ratios in the sediment profile and identified zones of anthropogenically derived mercury. In this comment, we question the main conclusion of the paper, which is that this investigation demonstrates a systematic isotope fractionation of Hg in a sediment profile or in a food chain. The paper is lacking simple statistical evaluation of the data and neglects basic rules of stable isotope analysis. If the data were valid, it would demonstrate that more recent, presumably anthropogenic sources of mercury to lakes have a different isotopic composition than historically deposited Hg. This would be a great tool to elucidate natural mercury cycling and very useful to regulators developing policies on the controls of mercury emissions. In addition, if there were isotope discrimination in food chains, this would enable us to better understand mercury bioaccumulation in food webs, using similar strategies that have been applied in the study of other elements (e.g., C, N, and S, Peterson and Fry 1987). We do agree, however, with his conclusion that more precise and sensitive mass spectrometric instrumentation should be used in any future work. Only high-precision instrumentation, such as a multicollector ICP/MS, will provide the ability to determine potential variations in isotope ratios in the environment; these will almost certainly be smaller than the variations reported in this paper. It is well known in the field of precise and accurate isotope ratio measurements by ICP/MS that the instrument used in this study (quadrupole ICP/MS) is incapable of measuring isotope ratios with the precision necessary to observe the level of discrimination reported in the paper (Becker and Dietze 2000; Heumann et al. 1998). With respect to the data on isotope fractionation in sediments, the author based most of his conclusion on one selected pair of Hg isotopes (199Hg/201Hg) that apparently showed a trend with depth in the sediment (the heavier Hg isotope is enriched in deeper segments, see fig. 1 in Jackson 2001). The conclusion was that more recently deposited mercury had a different isotopic ratio than mercury deposited in historic times. However, the author failed to include the uncertainty of his measurements in the interpretation. As the standard deviations of the reported Hg ratio determinations are given in Jackson’s table 5, we were able to reconstruct part of his fig. 1 to included error bars (Fig. 1, this paper). We show means ± 2 standard deviations (SD) to illustrate that there are no significant differences between ratios measured in surface sediments and in deeper sections of the core. We further believe that mean ratios have to be compared rather than median values because isotope ratios in different samples can only be compared considering the external reproducibility of replicate measurements. A more serious and fundamental criticism of the paper by Jackson is that the author seems to have selectively chosen to examine a single pair of Hg isotopes (199Hg/201Hg). The 199Hg/201Hg ratio is in fact the only pair of Hg isotopes that apparently shows such a trend and even then, only if a statistical evaluation is neglected. None of the other 15 possible pairs of Hg isotopes confirms the postulated trend. Indeed, some pairs of isotopes even suggest the opposite trend, that is, the lighter isotope is enriched with depth. If fractionation processes
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
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 | 0,010 | 0,043 |
| Méta-épidémiologie (sens strict) | 0,002 | 0,001 |
| Méta-épidémiologie (sens large) | 0,002 | 0,002 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,004 | 0,005 |
| Communication savante | 0,003 | 0,005 |
| Science ouverte | 0,006 | 0,003 |
| Intégrité de la recherche | 0,030 | 0,026 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,011 | 0,011 |
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 ».