Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Sulfur mass-independent fractionation(S-MIF)preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry.Small ranges in Δ^(33)S that expanded into larger fractionations leading up to the Great Oxygenation Event(GOE;2.45–2.2 Ga)are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons.These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons,thus obscuring local for global sulfur cycling dynamics.By expanding the Δ^(33)S record to include the relatively underrepresented São Francisco Craton in Brazil,we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE.In an early Neoarchean sequence(2763–2730 Ma)from the Rio das Velhas Greenstone Belt,we propose that low δ^(13)Corg(<-30‰)and dampened Δ^(33)S(-0.4‰to-0.7‰)in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation.The overlying black shale(TOC up to 7.8%)with higher δ^(13)Corg(-33.4‰to-19.2‰)and expanded Δ^(33)S(2.3‰±0.8‰),recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur.The sequence culminates in a metasandstone,where concomitant changes to more uniform δCorg(-30‰to--25‰),potentially associated with the RuBisCO I enzyme,and near-zero Δ^(33)S(-0.04‰to 0.38‰)is mainly interpreted as evidence for local oxygen production.When placed in the context of other sequences worldwide,the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes.Our data suggest that prokaryotic sulfur,iron,and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.

Unraveling one billion years of geological evolution of the southeastern Amazonia Craton from detrital zircon analyses

Despite representing one of the largest cratons on Earth,the early geological evolution of the Amazonia Craton remains poorly known due to relatively poor exposure and because younger metamorphic and tectonic events have obscured initial information.In this study,we investigated the sedimentary archives of the Carajás Basin to unravel the early geological evolution of the southeastern Amazonia Craton.The Carajás Basin contains sedimentary rocks that were deposited throughout a long period spanning more than one billion years from the Mesoarchean to the Paleoproterozoic.The oldest archives preserved in this basin consist of a few ca.3.6 Ga detrital zircon grains showing that the geological roots of the Amazonia Craton were already formed by the Eoarchean.During the Paleoarchean or the early Mesoarchean(<3.1 Ga),the Carajás Basin was large and rigid enough to sustain the formation and preservation of the Rio Novo Group greenstone belt.Later,during the Neoarchean,at ca.2.7 Ga,the southeastern Amazonia Craton witnessed the emplacement of the Parauapebas Large Igneous Province(LIP)that probably covered a large part of the craton and was associated with the deposition of some of the world largest iron formations.The emplacement of this LIP immediately preceded a period of continental extension that formed a rift infilled first by iron formations followed by terrigenous sediments.This major change of sedimentary regime might have been controlled by the regional tectonic evolution of the Amazonia Craton and its emergence above sea-level.During the Paleoproterozoic,at ca.2.1 Ga,the Rio Fresco Group,consisting of terrigenous sediments from the interior of the Amazonia Craton,was deposited in the Carajás Basin.At that time,the Amazonian lithosphere could have either underwent thermal subsidence forming a large intracratonic basin or could have been deformed by long wavelength flexures that induced the formation of basins and swells throughout the craton under the influence of the growing Transamazonian mountain belt.