Constraints on Early Paleozoic Deep‐Ocean Oxygen Concentrations From the Iron Geochemistry of the Bay of Islands Ophiolite
Bibliographic record
Abstract
Abstract The deep ocean is generally considered to have changed from anoxic in the Precambrian to oxygenated by the Late Paleozoic (∼420–400 Ma) due to changes in atmospheric oxygen concentrations. When the transition occurred, that is, in the Early Paleozoic or not until the Late Paleozoic, is less well constrained. To address this, we measured Fe3+/ΣFe of volcanic rocks, sheeted dykes, gabbros, and ultramafic rocks from the Early Paleozoic (∼485 Ma) Bay of Islands (BOI) ophiolite as a proxy for hydrothermal alteration in the presence or absence of O2derived from deep marine fluids. Combining this data with previously published data from the BOI indicates that volcanic rocks are oxidized relative to intrusive crustal rocks (0.35 ± 0.02 vs. 0.19 ± 0.01, 1 standard error), which we interpret to indicate that the volcanic section was altered by marine‐derived fluids that contained some dissolved O2. We compare our results directly to the Macquarie Island and Troodos ophiolites, drilled oceanic crust, previously compiled data for ophiolitic volcanic rocks, and newly compiled data for ophiolitic intrusive rocks. These comparisons show that the BOI volcanic (but not intrusive) rocks are oxidized relative to Precambrian equivalents, but are less oxidized relative to Late Paleozoic to modern equivalents. We interpret these results to indicate that the Early Paleozoic ocean contained dissolved O2, but at concentrations ∼2.4× lower than for the Late Paleozoic to today.
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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 | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".