<sup>210</sup>Pb–<sup>226</sup>Ra–<sup>230</sup>Th systematics in very low sedimentation rate sediments from the Mendeleev Ridge (Arctic Ocean)This article is one of a series of papers published in this Special Issue on the theme<i> Polar Climate Stability Network</i>.GEOTOP Publication 2008-0031.
Bibliographic record
Abstract
U-series isotope behaviour in subsurface sediment of the Arctic Ocean is investigated based on high resolution measurements of natural radionuclides (210Pb, 226Ra, 230Th) and a few analyses of anthropogenic 137Cs in cores collected during the 2005 Healy-Oden Trans-Arctic Expedition (HOTRAX). Cores from the Mendeleev Ridge, representing distinct bathymetric settings, are analyzed in more detail as a means to assess the dating potential of such radionuclides at sites characterized by very low sedimentation rates (∼3 mm ka–1). The sediment consists of variable proportions of fine-grained carbonates, clays, and ice-rafted debris and shows excesses in 210Pb (210Pbxs) over parent 226Ra content, down to ∼1 cm below core top. This 210Pbxs distribution is due to shallow mixing by benthic organisms and (or) diffusion from the sediment–water interface, as also indicated by 137Cs activities. From ∼1 to 7 cm downcore, 210Pb activities closely follow 226Ra activities. Below 7 cm downcore, 226Ra activities are controlled by variable excesses in parent 230Th (230Thxs) resulting from its scavenging in the overlying water column. 226Ra diffusion is observed towards the water column occuring from the upper ∼7 cm of sediment below the seafloor (with a flux of ∼0.043 disintegrations per minute (dpm) cm–2 a–1) and deeper in the sediment below 230Thxs peaks but with lesser fluxes. Both cores show identical 210Pb profiles despite their 1 km bathymetric difference. This suggests negligible 230Th and 210Pb scavenging below water depths of ∼1.6 km, i.e., the bathymetry of the shallower core. In such settings where sedimentation rates are very low and vertical particle rain is the major sediment source, estimates of the actual 210Pbxs require precise knowledge of the 226Ra-supported fraction, which is controlled by 230Thxs, Ra diffusion, and thus sedimentation rates and porosity.
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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".