Ferruginous oceans during OAE1a and the collapse of the seawater sulphate reservoir
Bibliographic record
Abstract
Seawater sulfate is one of the largest oxidant pools at Earth’s surface today and its concentration in the oceans is generally assumed to have varied between 5 and 28 mM since the early Phanerozoic Eon. Intermittent and potentially global Oceanic Anoxic Events (OAEs) are accompanied by changes in seawater sulfate concentrations and signal perturbations in the Earth system associated with major climatic anomalies and biological crises. Ferruginous (Fe-rich) ocean conditions developed transiently during multiple OAEs, implying strong variability in seawater chemistry and global biogeochemical cycles. The precise evolution of seawater sulfate concentrations during OAEs, however, is uncertain and thus models that aim to mechanistically link oceanic anoxia to broad-scale disruptions in the Earth system remain equivocal. Here, we use analyses of Fe-speciation and redox sensitive trace metals in slope sediments deposited in the Tethys and Pacific oceans to constrain seawater sulfate concentrations and underlying dynamics in marine chemistry during OAE1a, ~120 Ma. We find that large parts of the global oceans were anoxic and ferruginous for more than 1 million years. Calculations show that the development of ferruginous conditions requires that seawater sulfate concentrations drop below 300 µM and possibly below 100 µM, which is an order of magnitude lower than previous minimum estimates. Such a collapse of the seawater sulfate pool over a time scale of only one-hundred thousand years is a key and previously unrecognized feature of Phanerozoic Earth surface redox budgets. Critically, this unprecedented sensitivity has potential to dramatically alter global biogeochemical cycles, marine biology, and climate on remarkably short timescales.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| 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".