AN OXYGENATED DEMISE TO THE SNOWBALL EARTH IN THE BAMBUÍ BASIN, BRAZIL

Feedback loops have long been proposed between metazoan diversification, tectonic and paleogeographic conditions, rearrangement of global biogeochemical cycles, and a putative rise in atmospheric oxygen levels following the harsh conditions of the last Snowball Earth ca. 635 Ma. While there is purported evidence for global oxygenation following the Marinoan glaciation, the local redox conditions at all areas investigated to date (e.g., in South China and NW Canada) suggest anoxic conditions. Here, we present new evidence for oxic deep waters during deglaciation of the Marinoan Snowball, based on iron speciation and redox-sensitive trace metal data from the Bambuí Group, covering the São Francisco craton in eastern Brazil. Above the glacial diamictite, a 2 m-thick mudstone containing dropstones records pervasively oxic conditions. Above the dropstone-bearing unit, black shales interleaved within the early Ediacaran cap dolostone of the Sete Lagoas Formation show iron speciation evidence for fluctuating redox conditions between ferruginous and oxic states, with clear (albeit muted) enrichments of trace metals such as Mo and V. Our results represent the first tangible evidence for oxygenated waters immediately during deglaciation and before cap dolostone deposition, predating current estimates on the timing of Ediacaran oxygen rise. Expansion of the redox-sensitive trace metals oceanic inventory is delayed by at least a couple of million years, coherent with previous literature data. Thus, nutrient limitation, along with co-occurring phenomena such as salinity/temperature/density-driven stratification, might have exerted the main control on metazoan diversification after the advent of oxygenated bottom waters. This work is supported by Instituto Serrapilheira - Brazil through grant number Serra-1912-31510 (Project MOBILE, geolifemobile.com), by WUN (Worldwide Universities Network, UK) through the Research Development Fund 2021 grant, by CNPq-Brazil through grants number 408815/2021-3 and 304509/2021-3, and by FAPEMIG-Brazil through grant number PPM-00618-18.