Post-Marinoan paleoredox and paleoproductivity record in Puga cap carbonate: Implication for coastal life colonization at the Amazon Craton marginal Sea

Abstract

The geochemical signatures in cap carbonate successions are critical records of paleoceanographic conditions following Snowball Earth events. These deposits offer insight into the shifts in redox conditions and the re-establishment of biogeochemical cycles during postglacial periods, providing a window into the evolving marine environments and potential drivers of early oxygenation. To track redox changes during this transition, we present improved high-resolution analyses of redox proxies across Puga cap carbonate (∼ 635 Ma) on the Southern Amazon Craton, Brazil, allowing for the identification of temporal redox transitions during the post-Marinoan transgression. The depletion of trace elements, particularly redox-sensitive elements (RSEs), such as Mo, U, and V in microbialites formed in basal cap dolostone, was deposited under oxic conditions. Following the initial melting of Marinoan glaciers, microbial mats flourished and grew in a semi-restricted shallow marine environment in the coastal paleoenvironment along the Amazon cratonic margin, where nutrient-rich surface waters fueled primary productivity. In contrast, the increase in RSEs in upper wave-dominated dolostone facies indicates predominantly dysoxic conditions in continuous sea level rise, resulting in the drowning of these early microbial environments and the precipitation of cap limestones. The sequential oxic-dysoxic redox marks the transition from shallow sea to deepening CaCO₃-oversaturated platform conditions. These results demonstrate an unequivocal synchronous relationship between the initial oxygenation of the Amazon margin and the local microbial mat flourishment shortly after the Marinoan glaciation.