Towards an astrochronological framework for the lower Paleoproterozoic Kuruman and Brockman Iron Formations
Bibliographic record
Abstract
Abstract Recent evidence for astronomical-induced cycles in banded iron formations (BIFs) hints at the intriguing possibility of developing astrochronological, i.e. precise time-stratigraphic, frameworks for the earliest Proterozoic as also reconstructed for parts of the Mesozoic and Paleozoic. The Kuruman Iron Formation (IF) (Griqualand West Basin, South Africa) and Dales Gorge Member of the Brockman IF (Hamersley Basin, Western Australia) are of special interest in this regard, given their inferred temporal overlap at ca. 2.47 Ga and similar long-period orbital eccentricity imprint. This suggests that these two BIFs may be correlated on the basis of their large-scale cycle patterns and using additional radio-isotopic age constraints. To examine the possibility of establishing such a framework, we generated and analysed several high-resolution proxy records from both drill-core and outcrop, combined with high-precision U-Pb dating of zircon from interbedded shale horizons. Time-series analysis of these records yields a variety of spectral peaks, of which a prominent ~5 m and ~16 m cycle can be linked to the basic stratigraphic alternations and bundling as observed in the field. New and revised 207Pb/206Pb ages calculated from the U-Pb data of the Dales Gorge Member and Kuruman IF, respectively, indicate a comparable average sedimentation rate of 10 to 12 m/Myr for both units. Based on this depositional rate, we attribute the ~5 m cycle to the long (~405 kyr) orbital eccentricity cycle. More tentatively, we interpret the ~16 m cycle as the very long (presently ~2.4-Myr) eccentricity cycle, having a reduced period of ~1.3 Myr due to chaotic behaviour in the solar system. Other identified cycles (~560 kyr, ~700 kyr and ~1.8 Myr) can be explained in terms of weaker orbital eccentricity components and/or as harmonics and combination tones of these cycles resulting from nonlinear responses. An initial attempt to establish cyclostratigraphic correlations between the Kuruman IF and Dales Gorge Member solely based on their characteristic cycle patterns proved unsuccessful, which may be due to a difference in the recording of the astronomical signal between different depositional environments. Next, we used the zircon ages to first constrain correlations at the scale of the ~16 m cycle, followed by a correlation of the basic ~5 m cycles. The resultant framework remains problematic and debatable at the individual ~405 kyr cycle-level, but provides a starting point for future studies. Particularly, our findings highlight the need for further investigations into how Milankovitch forcing influenced BIF sedimentation and paleoenvironmental conditions at a time when the Earth and solar system behaved fundamentally different from today.
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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 | 0.001 | 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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| 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 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".