N‐loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment
Bibliographic record
Abstract
Abstract Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N‐loss as well as the ocean's N isotope budget. They also represent “natural tracer experiments” with intensified biogeochemical signals that can be exploited to understand the large‐scale processes that control N‐loss and associated isotope effects ( ε ; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO 3 − ), nitrite (NO 2 − ), and biogenic N 2 associated with an anticyclonic mode‐water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO 3 − was nearly exhausted, we measured the highest δ 15 N values for both NO 3 − and NO 2 − (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N 2 ‐N concentrations were also the highest near the eddy's center (up to ~40 µmol L −1 ). δ 15 N‐N 2 also varied with biogenic N 2 production, following kinetic isotopic fractionation during NO 2 − reduction to N 2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N‐loss. We found apparent variable ε for NO 3 − reduction (up to ~30‰ in the presence of NO 2 − ). However, the overall ε for N‐loss was calculated to be only ~13–14‰ (as compared to canonical values of ~20–30‰) assuming a closed system and only slightly higher assuming an open system (16–19‰). Our results were similar whether calculated from the disappearance of DIN (NO 3 − + NO 2 − ) or from the appearance of N 2 and changes in isotopic composition. Further, we calculated the separate ε values for NO 3 − reduction to NO 2 − and NO 2 − reduction to N 2 of ~16–21‰ and ~12‰, respectively, when the effect of NO 2 − oxidation could be removed. These results, together with the relationship between N and O of NO 3 − isotopes and the difference in δ 15 N between NO 3 − and NO 2 − , confirm a role for NO 2 − oxidation in increasing the apparent ε associated with NO 3 − reduction. The lower ε for N‐loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N‐loss.
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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 | 0.000 | 0.000 |
| 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 | 0.000 |
| 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".