Geochemistry of C, O, and Sr isotopes in the Neoproterozoic carbonates from the southwestern Patom paleobasin, southern Middle Siberia
Bibliographic record
Abstract
Scatter of δ13C (from −11.6 to 8.4‰) and sequence of carbon isotope events in the Vendian carbonates from the southwestern Patom paleobasin (Vitim, Bol’shaya Chuya, and Chaya rivers) are similar to those previously established in the reference northeastern sections. It was found that cap dolomites crowning the glaciogenic diamictites of the Dzhemkukan Formation are characterized by moderately low δ13C (from −7.4 to −5.1‰), while the limestones, marls, and calcareous aleurolites of the Zhuya Group (Nikol’skoe and Chencha formations) correlable with the Shuram-Wonoka event (580–650 Ma) have extremely low δ13C values (up to −11.9‰). Intervals with negative δ13C are separated by positive excursion of δ13C (most part of the Barakun, Valyukhta, and Uluntui formations) with extreme up to 8.4‰. One more positive excursion (δ13C up to 6.1‰) was identified in the dolomitic unit sandwiched between the limestones of the Chencha Formation and the sandstones of the Zherba Formation on the Vitim and Bol’shaya Chuya rivers; in the northeastern sections of the Patom Complex, the positive excursion corresponds to the hiatus, but occurs at similar level (∼550 Ma) in many sections of Oman, China, Namibia, Australia, and other regions. Ultrahigh δ13C (from 11.7 to 15.2‰) values were found in ∼150-m-thick succession of “coaly” carbonates in the Lokatyk Block (Bol’shaya Chuya River), which were presumably erroneously ascribed to the Barakun Formation. These carbonates likely marking the Neoproterozoic anoxic peak are not known among the Vendian (Ediakaran) successions, but have previously been described in the sediments with an inferred age of 720–650 Ma in the Bambui Group (eastern Brazil) and Keele Formation of the Neoproterozoic Windermere Group (northwestern Canada). Scatter of δ18O in the studied carbonates (from 6.3 to 28.5‰) was mainly caused by postsedimentary transformations. General correlation between δ18O and δ13C is absent. Both positive and negative correlations can be observed within limited intervals, indicating that the behavior of O and C isotope systems was not controlled by a common mechanism. The lowest 87Sr/86Sr = 0.7084 in the main chemostratigraphic marker (Chencha Formation) in the extreme west (Chaya River) does not differ much from the 87Sr/86Sr value in the Chencha Formation in the northeastern sections, which confirms the hydrological homogeneity of the Patom paleobasin. The lowest 87Sr/86Sr (0.70815) in the carbonates of the Uluntui Formation of the Chaya section, however, is much lower than in the carbonates from the stratotype area (0.70842) of the Uluntui Formation (Kuznetsov et al., 2013). This casts some doubts on the correlation of these stratigraphic units.
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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 | 0.000 | 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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 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".