Lack of synsedimentary chemical alteration in polar carbonates (Ross Sea, Antarctica): Resolution of a conundrum
Bibliographic record
Abstract
ABSTRACT Although rare in space and time, skeletal carbonates deposited on polar shelves hold great potential for improving understanding of the oceanography of the high latitudes. Low temperatures, low carbonate saturation states, and strong seasonality govern not only the nature of carbonate communities, but also how their deposits translate into the rock record. To understand the effects of early seafloor processes on preservation, we investigated late Quaternary carbonates recovered in piston cores from the Ross Sea, Antarctica. Sediments are unconsolidated skeletal gravels and sands that mantle areas of the outer shelf swept by strong bottom currents. Deposits are dominated locally by either stylasterine hydrocorals, barnacles, or bryozoans, which comprise assemblages with strong similarities to modern benthic communities. Radiocarbon ages indicate that carbonate factories were most prolific during the lead-up to the Last Glacial Maximum (Tartanian), when sediment input was minimized, and have been mostly dormant since. Results show that synsedimentary alteration is not substantially different in the temperate and polar realms with the significant exception of chemical diagenesis. As is common in temperate deposits, skeletal grains undergo disarticulation, fracturing, abrasion, and intense bioerosion. By contrast, cementation is absent and rare aragonite grains are preserved, indicating that taphonomic loss is not as prevalent as in temperate deposits. Primary skeletal microstructures and stable-isotope compositions are preserved, indicating that chemical alteration of grains is negligible. The preservation of aragonite in polar settings is herein attributed to low rates of organic-matter burial and very low temperatures, which strongly limit microbial activity. These factors allow interstitial waters to remain weakly supersaturated with respect to aragonite. Comparison with Permian analogs indicates that lithification is delayed until deposits reach burial depths at which chemical compaction proceeds. The ultimate end product is limestone with prominent compaction features and a tightly packed fabric. Calcitic skeletal material can retain primary geochemical compositions through the lithification process, although growth of burial cement in intraparticle porosity complicates selective sampling of unaltered material. In providing a cold-water end member for the spectrum of synsedimentary diagenetic processes, results highlight specific differences that should be accounted for when interpreting the deposits of polar, cold-water carbonate systems.
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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 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".