Impact of silica on the identity of minerals formed in Archean oceans during Fe cycling by cyanobacteria and iron(III)-reducing bacteria
Bibliographic record
Abstract
Banded Iron Formations (BIF) are iron- and silica-rich marine sediments deposited between 3.8 and 1.85 Ga. With the evolution of cyanobacteria, iron cycling via abiotic Fe(II) oxidation with O 2 produced by cyanobacteria and Fe(III) reduction by Fe(III)-reducing microorganisms is hypothesized to be responsible for BIF mineral deposition and transformation both in the water column and in the resulting sedimentary deposits. However, the impact of silica on iron mineralogy during microbially influenced iron cycling has not been determined experimentally under relevant ancient conditions. Hence, we set up batch incubation experiments with different concentrations of silica (0 to 2.2 mM) in the presence of varying concentrations of Fe(II) (0.5 to 5 mM), a marine cyanobacterium ( Synechococcus PCC7002) and a marine dissimilatory Fe(III)-reducer ( Shewanella colwelliana ). We found that the fluctuation between microbial production of O 2 (cyanobacteria) and the activity of Fe(III)-reducing bacteria led to alternating Fe(II) oxidation and Fe(III) reduction and the precipitation of various Fe(II)- and Fe(III)-bearing minerals. Using a combination of X-ray diffraction and 57 Fe Moessbauer spectroscopy we identified poorly crystalline Fe(III)-bearing minerals (e.g., ferrihydrite), and the well crystalline ones (e.g., goethite, and lepidocrocite) in the absence of silica. Fe minerals precipitated in the presence of silica showed more pronounced reflections in µXRD, indicating higher crystallinity, while 57 Fe Moessbauer spectroscopy suggested the formation of Fe(II) silicate minerals and Fe(III) (oxyhydr)oxide minerals associated with silica. Furthermore, the presence of silica led to higher oxidation and reduction rates but more incomplete Fe(II) oxidation, while the reduction extent was higher in the presence of silica. In summary, our experiments showed that the presence of silica clearly affects Fe cycling and Fe mineral (trans)formation by cyanobacteria and Fe(III)-reducing bacteria, with relevance for the deposition of Precambrian Banded Iron Formations.
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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 | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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".