Origin and migration of dissolved organic carbon fractions in a clay‐rich aquitard: <sup>14</sup>C and δ<sup>13</sup>C evidence
Bibliographic record
Abstract
The origin and long‐term transport of dissolved organic carbon (DOC) and its high molecular weight (HMW) and low molecular weight (LMW) fractions were investigated in a clay‐rich till aquitard system. DOC concentrations decreased systematically with depth from 168 mg L−1 in the surficial fractured zone (1.2 m) to minimum values of 12–16 mg L−1 from 15 to 43 m depth in the nonfractured aquitard. The HMW and LMW DOC fractions and their corresponding δ13C values also decreased systematically with depth. Numerical transport simulations of the downward migration of DOC from the soil zone into the aquitard closely approximated field‐measured values and show transport of DOC was controlled by diffusive mixing. The depth trends for HMW and LMW DOC fractions and δ13C support hypothesized transport via diffusion, as well as earlier findings that microbial respiration in this nonfractured aquitard was negligible over the time period studied. The 14C content of the HMW DOC fraction decreased systematically with depth from 87 percent modern carbon (pmC) at 1.2 m to 13–15 pmC from 15 to 43 m depth. A simple model accounting for diffusive mixing and radioactive decay described the 14C profile in the aquitard. Radiocarbon measurements, transport modeling, and δ13C data suggest DOC and its fractions were derived from two distinct end‐members: soil organic carbon formed since the Holocene (∼10 ka B.P.) and DOC in connate pore water (∼15 ka B.P.).
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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 | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 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 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".