Valence state fossils in Proterozoic stromatolites by L‐edge X‐ray absorption spectroscopy
Bibliographic record
Abstract
Paleoproterozioc (ca. 1.9 Ga) stromatolites from the Biwabik Iron‐Formation (Minnesota, United States) contain fossil forms which in the literature have been attributed to ancient microbes including iron‐oxidising bacteria and cyanobacteria. To search for valence state fossils, we measured Fe 3+ /ΣFe (ΣFe = Fe 2+ + Fe 3+ ) transverse to the laminae of a Biwabik, probably biogenic stromatolite and compared the results with a scan across the enclosing, likely abiotic sedimentary rock (or interstromatolite fill). To obtain Fe 3+ /ΣFe and information about the site symmetry and crystal field strength, we used L 3 (2p 3/2 → 3d) soft X‐ray absorption spectroscopy and interpreted the measurements using our calculated spectra. We found that Fe 3+ /ΣFe is approximately constant at 0.3 within the stromatolite, increases steeply to around 0.6 in the region of the stromatolite‐fill boundary, and then reduces significantly in the interstromatolite fill. The crystal field strength for octahedral Fe 3+ increased and became more irregular as the scan moved from the stromatolite into the fill. Our results tentatively suggest that a combination of ancient biological and later diagenetic processes can produce steep gradients in Fe 3+ /ΣFe and alterations in the crystal field strength across a probable abiotic‐biotic boundary. To describe the L 3 absorption spectra in the stromatolite‐fill region, it was necessary to include a significant Fe 3+ tetrahedral component, which suggests the presence of magnetite. At other positions, fits using only octahedral Fe 2+ and Fe 3+ theoretical curves satisfactorily reproduce the spectra. The search for a possible new type of spatial biosignature in the valence state record for ancient stromatolites might help differentiate between the biogenic stromatolites and abiogenic stromatolite‐like structures and could even be relevant to the search for fossil evidence of life in extraterrestrial rocks.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 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".