Abstract
Iron formations (IFs) are well-studied chemical archives of Precambrian seawater,thus offering evidence of past oceanic redox conditions, and providing insights onhow the iron (Fe) cycle was operating throughout key periods of Earth’s history. This work focuses on applying elemental and isotopic geochemical measurements in IFs and carbonates deposited in Neoarchean Carajás (~2.74 Ga), before the Great Oxidation Event (GOE). There is still a large debate regarding possible oxygenation events prior to the GOE, the role of microbial activity in reducing environments, nutrient availability in paleoseawater, and post-depositional effects imprinted in the sedimentary record. These topics will be addressed after a general introduction (Chapter 1), an overview of geological setting from which our samples came from (Chapter 2), and a description of the specific methods utilized to reach our main conclusions (Chapter 3).In an attempt to reconstruct paleoenvironmental conditions in Neoarchean Carajás,petrographic work combined with major and trace element geochemistry, Fe isotope composition of bulk-rock, and C isotope measurements in organic matter have provided support for anoxic conditions (i.e. lack of negative Ce anomalies, positive and homogeneous δ56Fe values). Organic carbon isotopes also favored the role of bacteria in oxidizing dissolved Fe(II) through anoxygenic photosynthesis (i.e. δ13Corg similar to autotrophic organisms). We have also demonstrated that Fe had a prominent hydrothermal source shown by significantly positive Eu anomalies (Chapter 4). Additionally, by measuring phosphorus (P) and Fe content in IFs we have shown that these chemical sediments are indeed suitable archives of ancient seawater as shown by their analogous behavior with modern seawater. Hence, we estimated dissolved phosphorous contents in paleoseawater locally and globally, based on measured, experimental, and literature data. Our data suggests P-limited conditions prevailed throughout most the Archean Eon (Chapter 5). Finally, after measuring Fe isotopes in carbonate leachates and magnetite (see Methods section, Chapter 3), C isotopes in carbonates, and applying Raman spectroscopy in organic-rich carbonate samples, we demonstrated that microbial dissimilatory iron reduction (DIR) was likely responsible for depleted 13C isotopic signature measured in carbonates. However, the large Fe isotope fractionation expected for DIR as recorded by experimental studies was not observed. In fact, a constant fractionation was recorded in a basinal scale between carbonates and oxides, possibly arguing for a reequilibration of the Fe-isotopic system at relatively lowtemperature conditions (Chapter 6). These results hopefully contributed to the effort of reconstructing the key aspects of the Fe cycle in Neoarchean Carajás, and importantly showed the potential of these samples as significant paleoenvironmental archives.