Experimental evidence for mass-independent fractionation of even-mass mercury isotopes related to the nuclear volume effect
Bibliographic record
Abstract
Investigating stable mercury (Hg) isotope ratios is a novel approach for tracing biogeochemical transformations of this global pollutant in environmental systems. The Hg isotope system is unique in the sense that it encompasses not only mass-dependent (MDF) but also multiple types of mass-independent fractionation (MIF) in both experimental and natural systems. One of the processes causing MIF for heavy elements such as Hg is the nuclear volume effect (NVE) which occurs during kinetic and equilibrium reactions and can exceed the magnitude of the conventional mass difference effect (MDE). It is well established that the NVE can cause significant MIF of odd-mass Hg isotopes (199Hg, 201Hg) because they have slightly smaller nuclear charge radii than predicted by a linear scaling with mass (based on 198Hg and 202Hg). Small deviations from this linear relationship have also been theoretically predicted for the even-mass isotopes 200Hg and 204Hg, but the effect was so far believed to be negligible. In this study, we investigated Hg isotope fractionation during Hg(II) reduction by Fe(II) by analyzing both reactants and products of laboratory experiments. Reduction of Hg(II) by dissolved Fe(II) led to MDF with enrichment factors (up to -2.4‰) that are larger than for other abiotic reduction pathways. A positive MIF of odd-mass Hg isotopes was observed in all experiments (E199Hg up to 0.34±0.02‰ and E201Hg up to 0.21±0.02‰) with a consistent Δ199Hg/Δ201Hg slope of ≈1.6 indicating that the MIF was likely caused by the NVE. Additionally, we report the first experimental evidence for small MIF of even-mass Hg isotopes related to NVE (E200Hg up to 0.04±0.01‰ and E204Hg up to ‑0.05±0.01‰), which aligns with theoretical predictions based on the non-linearity of nuclear charge radii (Figure 1). Our results provide further insights into Hg isotope fractionation systematics and constraints for the interpretation of Hg isotope signatures. Despite the small magnitude of the documented even-mass MIF caused by the NVE, we suggest that this effect should be considered when interpreting small even-mass MIF signals detected in environmental samples, which are frequently assumed to be solely produced by atmospheric processes.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".