Rare earth elements as indicators of post-Marinoan (∼635 Ma) paleoceanographic changes from the Amazon Craton
Résumé
The extensive deglaciation linked to snowball Earth climate elapsed in terminal Neoproterozoic caused dramatic changes in the ocean's chemistry, exceptionally recorded in the cap carbonate sequence. One of the most spectacular examples is the post-Marinoan Puga cap carbonate (~635-622 Ma), exposed in the Amazon Craton associated with expressive sea level changes related to the glacial-isostatic adjustment (GIA) resulting in continuous mixed-waters, coastal upwelling succeeded by long-term transgression under greenhouse climax. To better understand the complex dynamics during this period, we conducted a comprehensive multiproxy study encompassing sedimentological, rare earth elements, and stable isotopic analysis. The Rare Earth Element + Yttrium (REY) patterns in the Puga cap carbonate reflect the marine conditions aftermath of the aftermath of the snowball Earth. Specifically, the Y/Ho ratios in the cap dolostones consistently fall below modern seawater values interpreted as seawater composition dilution due to the meltwater influx from the continents during deglaciation. However, superchondritic Y/Ho ratios (up to 70) and positive Eu anomalies (Eu/Eu* up to 3.6) are observed in the basal portion of the cap dolostone. These results suggest the upwelling of hypersaline seawater with some contribution from contemporaneous hydrothermal fluids in shallow coastal water. These findings are interpreted as evidence of ocean destratification related to upwelling, ocean circulation, and glacioisostatic adjustment. The cap dolostone occurs with minimal siliciclastic contributions, consistent with a siliciclastic starvation model. It could be attributed to the persistent shallow-water conditions in the coastal zone, significantly influenced by glacial isostatic adjustment (GIA). This process led to continuous uplift, resulting in a shallow shelf isolated from direct oceanic processes. Barrier-inland facies were not observed in the studied succession, suggesting that irregular diamicton morphologies may have created protected areas against waves. This scenario maintained a consistently shallow water column and followed a significant landward shoreline migration depositional pattern. These dramatic changes have been assumed to have resulted from the rapid recovery of primary productivity during deglaciation, with the proliferation of microbial communities and induced dolomite precipitation. The δ 13 C isotopic signature is consistent with these interpretations and with the values found for the Ediacaran in other cap carbonates. Mg concentration was constant during restricted paleoenvironmental conditions on dolomitic platforms. The rapid rise in sea level led to the dispersion of Mg ions, leading to the definitive interruption of dolomicrite precipitation, abruptly replaced by seas supersaturated in CaCO3 with massive aragonite precipitation coinciding with the increase in detrital components under the influence of a long-term transgression in the initial Ediacaran.
| Origine | Fichiers produits par l'(les) auteur(s) |
|---|---|
| licence |
