Ediacaran iron formations and carbonates of Uruguay: palaeoceanographic, palaeoclimatic and palaeobiologic implications.

摘要

The Ediacaran in Uruguay preserves a unique record of deposits generated during the assembly of the palaeocontinent Gondwana and concurrent with major changes in the atmosphere and oceans, and the rise of animal life.;The association of negative delta13C excursions in Neoproterozoic carbonates and large-scale glaciations has become a tempting explanation for the short-term perturbation of the global carbon cycle. Not surprisingly, negative delta13C shifts in Ediacaran-aged carbonates from Uruguay have been interpreted as recording post-Gaskiers glacial events. New high-resolution delta13C-chemostratigraphy of carbonates shows negative fractionations in deep facies with a progressive rise towards shallow-water settings, and suggests a deposition across a stratified ocean. Furthermore, 87Sr/86Sr chemostratigraphy coupled with radiometric data allowed a more precise chronostratigraphy, which supports an age of ∼600-575 Ma for the unit, and suggests a deposition concurrent with the Gaskiers glaciation. Notwithstanding whether associated delta 13C variations in shallow water facies were produced by glacially-related conditions or by the dynamic of the basin itself remains unresolved. Although these conclusions are particularly valid for these deposits, they carry important implications for the understanding of other negative delta13C excursions recorded in the Precambrian.;Finally, bilaterian burrows occur in Gaskiers age glaciomarine rocks in Uruguay implying that these are the oldest definite animal tracks yet reported. Crucially, our new discovery unites the palaeontological and molecular data pertaining to the origin of bilaterians, and brings the origin of animals firmly into the interval of the Neoproterozoic glaciations. It also implies that ancestral bilaterians likely evolved first in relatively shallow seas, and only colonized the deep-sea floor once sufficient bottom water oxygenation had taken place.;Recent studies have suggested that the deep oceans remained anoxic and highly ferruginous throughout the Ediacaran and possibly into the Cambrian. Unfortunately, acceptance of this idea has been hindered by the virtual absence of iron formations (IF). Detailed studies of Ediacaran IF in Uruguay confirm that ferruginous conditions dominated the pre-Gaskiers (∼580 Ma), and interestingly, they also extended well into the upper Ediacaran before complete ocean ventilation occurred. Significantly, a simple two-layer stratified system that argues for an oxygenated surface layer overlying a suboxic zone is proposed.