Glacial‐interglacial variability in the eastern tropical North Pacific oxygen minimum zone recorded by redox‐sensitive trace metals
Bibliographic record
Abstract
Changes in the intensity of the oxygen minimum zone (OMZ) in the eastern tropical North Pacific over the past 140 kyr are recorded as enrichments and depletions of redox‐sensitive metals in sediments of two piston cores, one within and one below the modern OMZ, from the continental margin off Mazatlán, Mexico (22°41′N, 106°28′W). Concentrations of Al (7.7 ± 0.6%), Ti (0.37 ± 0.03%), Fe (3.1 ± 0.25%), Mn (320 ± 31 ppm), and Ba (560 ± 82 ppm) in core NH15P (within the OMZ at 420 m water depth) were relatively constant over the last 110 kyr. In contrast, concentrations of Cd (4.8 ± 2 ppm), Cu (29.8 ± 9.1 ppm), U (9.1 ± 3.7 ppm), Mo (12.1 ± 5.0 ppm), V (138.2 ± 51.9 ppm), and Re (45.8 ± 25.5 ppb) were all at least 30% higher in interglacial stages compared to glacial stages. Concentrations of Al (7.4 ± 0.3%), Ti (0.35 ± 0.03%), Fe (3.5 ± 0.4%), and Mn (385 ± 77 ppm) in core NH22P (below the OMZ at 2025 m water depth) were comparable to those in core NH15P, while concentrations of Ba (1662 ± 292 ppm) were about a factor of three higher. In contrast, concentrations of Cd (0.9 ± 0.3 ppm), U (6.6 ± 0.9 ppm), Mo (3.2 ± 1.2 ppm), V (81.8 ± 10.1 ppm), and Re (25.4 ± 12 ppb) were lower in sediments of comparable age than the concentrations of these same metals in the OMZ core, and the differences in their concentrations over glacial‐interglacial cycles were less pronounced than those in shallower core. Comparison of the nonlithogenic fraction of metals in the sediments with their estimated contribution from plankton suggests that organic matter is probably the major source of Cu, Ba, and perhaps Cd to the sediment, whereas the indirect effects of organic carbon and low bottom water oxygen concentrations on sediment redox state appear to be more important controls on the distributions U, Re, Mo, and V. Changes in the depth at which Re and Mo precipitated in the sediments and in the Re/Mo ratio suggest that the redox state of the surface sediment and overlying water at both core depths varied over time. Re and Mo removal depths were shallower and Re/Mo ratios were lower at the OMZ site than in deeper water, suggesting that a more reducing environment prevailed over time at the shallower site. Although the redox state of the waters and sediment at both sites varied over glacial‐interglacial cycles, the OMZ likely was never anoxic in the last 140 kyr. This variability in redox state could be attributed to changes in regional export productivity, changes in ocean circulation, or a combination of both processes. A paleoproductivity reconstruction from biogenic Ba data suggests that glacial productivity was considerable lower than it was during interglacial stages. Model results suggest that the oxygen penetration depth, an indicator of sediment redox state, changed less than 1 cm as a result of the change in productivity. Changes in oxygen penetration depth estimated from Re and U removal depths are significantly larger, suggesting that changes in ventilation are a more important control on sediment redox state. Overall, trace metal results confirm the tight coupling between ocean circulation, marine productivity, and global change.
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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 | 0.001 |
| 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) | 0.001 | 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".