Coupled δ56Fe and δ18O analysis of the Frere iron formation, Western Australia, and Paleoproterozoic ocean circulation
Bibliographic record
Abstract
• The 1.89 Ga Frere granular Fe formation was deposited on a redox stratified shelf. • Trend in whole-rock and magnetite δ 56 Fe data tracks changes in sea level. • The δ 56 Fe of iron formations are a robust tracer for Paleoproterozoic ocean circulation. • (Ce/Ce*) PAAS holds a primary seawater signature in spite of terrestrial input. • Primary δ 18 O values of minerals were overprinted by diagenesis. The Paleoproterozoic (ca.1.89 Ga) Frere Formation is a ca. 300-m-thick sedimentary succession of five stacked, Fe- and silicate-rich shallowing upwards cycles or parasequences that were deposited during marine transgression. Deposition in an oxygen-stratified water column is supported by the occurrence of magnetite-rich mudstones (anoxic and distal) beneath shallower, hematite- and chert-rich grainstones (suboxic and proximal). Minor (LREE/HREE) PAAS depletion and positive Ce anomalies in grainstones support this interpretation. Sequence stratigraphic relationships suggest that the Frere Formation accumulated on an unrimmed continental shelf associated with coastal upwelling of Fe. Large variations in oxygen isotope ratios ( δ 18 O) of magnetite (–0.6 ‰ ≤ δ 18 O ≤ +2.8 ‰, n = 12) and quartz (+8.8 ‰ ≤ δ 18 O ≤ +20.1 ‰, n = 11) from lithofacies composing the Frere Formation indicate that pristine δ 18 O of minerals were overprinted by diagenesis. Highly variable δ 18 O values calculated at 200 °C in equilibrium with quartz (–3.0 to + 8.3 ‰) and magnetite (+8.0 to + 12.7 ‰) suggest that fluids with heterogeneous δ 18 O evolved through oxygen isotope exchange between minerals and porewater in accumulating sediment during diagenesis. The Fe isotope ratios ( δ 56 Fe) of bulk rock (–0.50 ‰ ≤ δ 56 Fe ≤ +1.55 ‰, n = 20) and magnetite separates (–0.09 ‰ ≤ δ 56 Fe ≤ +2.06 ‰, n = 11) from deeper anoxic and shallow suboxic paleoenvironments define a decreasing trend through stratigraphically stacked parasequences. The high δ 56 Fe values of magnetite (+0.7 ‰ to + 2.06 ‰) characterizing the base of the Frere Formation is interpreted to reflect equilibrium isotopic fractionation between dissolved Fe 2+ aq and freshly precipitated Fe-(oxyhydr)oxide (Fe(OH) 3 ) resulting from sluggish recharge of Fe 2+ aq from deeper shelf environments. These high δ 56 Fe values of magnetite from shallow paleoenvironments may also reflect microbial dissimilatory Fe reduction at the seafloor. As sea level rose, continued deepening and encroachment of a well-mixed water mass increased the oxidation of Fe 2+ aq , which ultimately increased the influence of kinetic isotopic fractionation between Fe 2+ aq and Fe(OH) 3 . Such a process produced δ 56 Fe compositions of magnetite that gradually decrease through the stratigraphic thickness of the Frere Formation (+2‰ to −0.1 ‰). Thus, Fe isotopic data framed in a sequence stratigraphic context provides greater insight into water column processes on Fe-rich, oxygen stratified Paleoproterozoic and Mesoproterozoic shelves, as well as geologically younger restricted basins with a redox gradient.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".