Fate of the preglacial regolith beneath the Laurentide Ice Sheet
Bibliographic record
Abstract
Subglacial erosion and transport of deformable sediment influence the size, stability, and sensitvity to climate of large ice sheets. These processes may cause or sustain ice-sheet instabilities [1], and may have dictated the periodicity of the Cenozoic ice ages, in particular the enigmatic mid-Pleistocene transition from small and frequent to large and infrequent glaciations [2,3]. Subglacial erosion, however, is difficult to study. Where active at present, it is hard to observe, and, like other erosional processes, it continuously removes the evidence of its previous actions. Here we use the cosmic-ray-produced radionuclide 10 Be, which is abundant in deeply weathered soils but absent in fresh bedrock, to investigate the sources of subglacial sediment eroded from the Canadian Shield by the Laurentide Ice Sheet (LIS) and deposited as till in the north-central U.S. Some tills have extraordinarily high 10 Be concentrations, as high as those in deeply weathered regolith in unglaciated areas that has accumulated 10 Be over millions of years. In general, the lowermost tills have high 10 Be concentrations, tills directly overlying them have 10 Be concentrations that are lower by an order of magnitude, and Wisconsinan tills have the lowest 10 Be concentrations. There are exceptions to this general picture in two boreholes, where intermediate-age tills have higher 10 Be concentrations than the oldest tills. These results, combined with a box model for 10 Be and sediment transport during a series of glaciations that we use to interpret the results, show the following: First, some tills cannot be the result of deep subglacial erosion of unweathered rock, but must have originated by mobilization of a relatively thin layer of deeply weathered surface sediment. This source can only have been the preglacial regolith and saprolite, the legacy of millions of years of Tertiary weathering, that was present on the Canadian Shield before the onset of Northern Hemisphere glaciation. Second, the preglacial regolith formed a source for early tills, but was rapidly removed from the core area of the ice sheet by the first few Plio-Pleistocene ice sheet advances. Third, renewed export of 10 Be-rich till sometime after 1 Ma appears to reflect an increase in the area exposed to subglacial erosion. It was likely a consequence, and not a cause, of a change in ice sheet dynamics at the mid-Pleistocene transition.
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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 | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".