Compositions of Carbonate Rocks and Carbon Isotope Ratios of Graphite in Nulliak Supracrustal Rocks, Labrador: The Oldest Evidence for Life and Seawater Composition
Notice bibliographique
Résumé
The Earth is the only planet where liquid water and life have existed through geologic time. Therefore, it is important to investigate the surface environments of the early Earth to understand its early life. However, little is known about the environments of the early Earth because of a lack of geologic evidence. Recently, > 3.95 Ga supracrustal rocks including carbonate rocks, named Nulliak supracrustal rocks, were found in the Saglek Block, northern Labrador. The seawater composition of the early Earth and vestiges of life from the oldest supracrustal rocks are investigated. The carbonate rocks occur in four areas of the Saglek Block, and are accompanied by meta-chert and meta-pelitic rocks or metamorphosed banded iron formation and meta-basalt. They all are regionally metamorphosed under the amphibolite or the granulite facies conditions (> 500°C) to yield metamorphic minerals such as amphibole and pyroxene. The whole-rock compositions of major, trace, and rare earth elements of the carbonate rocks are assessed. The carbonate rocks are composed of CaO, MgO, FeO, and SiO2 with trace amounts of Al2O3, Zr, and Ba. Carbonate rocks with low Al2O3 (< 0.2 wt.%), Zr (< 2 ppm), and Ba (< 5 ppm) contents are selected to avoid the influences of detrital and volcanic materials on their compositions. Post-Archean Australian Average Shale (PAAS)-normalized Rare Earth Element (REE) patterns of detritus-free samples show flat to slightly light REE-depleted patterns with positive La, Eu, and Y anomalies and without a Ce anomaly. The positive La and Y anomalies indicate that the carbonate rocks originated from chemical sediments precipitated from seawater. The lack of the Ce anomaly indicates that Eoarchean seawater was anoxic. Because the carbonate rocks accompanied by the pelitic rocks have obvious Eu anomaly, even seawater near a continent had the Eu anomaly and was widely dominated by hydrothermal influx. Pelitic rocks, conglomerate, carbonate rocks, and chert nodules within the carbonate rocks in St. John's Harbour South, St. John's Harbour East, Shuldham Island, and Big Island contain graphite, whereas no graphite is found in the sedimentary rocks in Pangertok Inlet. Some of the pelitic rocks and conglomerate have low δ13Corg values with a nadir of −28.2‰, which is comparable to the minimum value of δ13C values of graphite in the Isua supracrustal belt. The δ13Corg values of graphite in the politic rocks show a weak negative correlation with total organic carbon contents. On the other hand, carbon isotope values (δ13Ccarb) of carbonate in the carbonate rocks range from −2.6‰ to −3.8‰. The δ13Corg values positively correlate with the metamorphic grade and negatively with total organic carbon contents. The correlation indicates that the variation in δ13Corg values is possibly due to carbon loss during metamorphism. It is concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, provides the oldest evidence for organisms over 3.95 Ga.
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Comment cette classification a été obtenuedéplier
Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».