Latitudinal variations in <i>δ</i> <sup>30</sup> Si and <i>δ</i> <sup>15</sup> N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years
Bibliographic record
Abstract
The stable sedimentary nitrogen isotope compositions of bulk organic matter ( δ 15 N bulk ) and the silicon isotope composition of diatoms ( δ 30 Si BSi ) both mainly reflect the degree of past nutrient utilization by primary producers. However, in ocean areas where anoxic and suboxic conditions prevail, the δ 15 N bulk signal ultimately recorded within the sediments is also influenced by water column denitrification, causing an increase in the subsurface δ 15 N signature of dissolved nitrate (δ15NO3-) upwelled to the surface. Such conditions are found in the oxygen minimum zone off the coast of Peru, where, at present, an increase in subsurface δ15NO3- from north to south along the shelf is observed due to ongoing denitrification within the poleward-flowing subsurface waters, while the δ 30 Si signature of silicic acid ( δ 30 Si(OH) 4 ) at the same time remains unchanged. Here, we present three new δ 30 Si BSi records between 11 and 15 ∘ S and compare these to previously published δ 30 Si BSi and δ 15 N bulk records from Peru covering the past 600 years. We present a new approach to calculate past subsurface δ15NO3- signatures based on the direct comparison of δ 30 Si BSi and δ 15 N bulk signatures at a latitudinal resolution for different time periods. Our results show that, during the Current Warm Period (CWP, since 1800 CE) and prior short-term arid events, source water δ15NO3- compositions have been close to modern values, increasing southward from 7 to 10 ‰ (between 11 and 15 ∘ S). In contrast, during the Little Ice Age (LIA) we calculate low δ15NO3- values between 6 ‰ and 7.5 ‰. Furthermore, the direct δ 30 Si BSi versus δ 15 N bulk comparison also enables us to relate the short-term variability in both isotope compositions to changes in the ratio of nutrients (NO3-:Si(OH)4) taken up by different dominating phytoplankton groups (diatoms and non-siliceous phytoplankton) under the variable climatic conditions of the past 600 years. Accordingly, we estimate a shift from a 1:1 (or 1:2) ratio during the CWP and a 2:1 (up to 15:1) ratio during the LIA, associated with a shift from overall high nutrient utilization to NO3--dominated (and thus non-siliceous phytoplankton) utilization.
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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".