Mass-dependent and mass-independent sulfur isotope fractionation in Precambrian sediments as a key to early atmospheric and oceanic evolution

Abstract

The redox state of Earth's early atmosphere and oceans continues to be controversial, the majority view is that the early Earth was relatively reduced, becoming oxygenated through the activities of photosynthetic organisms, with a postulated transition to an O2 atmosphere around 2.3 Ga. However, other researchers propose that the Earth has always been relatively oxic. There is conflicting geochemical evidence supporting both hypotheses. The recent identification of an anomalous mass-independent sulfur isotope effect in some Precambrian minerals has provided a new geochemical tool for investigating the chemistry of the early atmosphere.This study demonstrates a new, rapid Continuous-Flow Isotope-Ratio-Mass-Spectrometer analytical technique utilising 48SO, 49SO, 50SO isotopomers as proxies for 32S, 33S, 34S for the determination of sulfur isotope mass-independent fractionation in sulfide and sulfate samples. With this technique we have performed 246 analyses on 90 sulfide and sulfate samples from six localities including the Isua Greenstone Belt in south west Greenland, the East Pilbara Granite Greenstone Terrane in Western Australia, the Barberton Greenstone Belt in southern Africa, the Hamersley Basin in Western Australia, the Griqualand-West Basin in South Africa and the Pechenga Greenstone Belt in north west Russia spanning a time interval between 3.7 and 1.9 Ga. This study has revealed the largest positive △33S yet published (10.86‰) from a shale-hosted pyrite of the Mt. McRae Shale in the Hamersley Basin, and confirmed the youngest MIF age for widespread mass-independent fractionation (MIF) of 2.450 Ga from pyrites of the Gamohaan Formation in the Griqualand-West Basin. Our results are broadly consistent with previously published results although frequently extend the previously known range, and include stratigraphic sequences not analysed previously. We have also modeled the response of o33S, o34S and △A33S during Raleigh distillation in a three-isotope system.This study combines our results with previously published results and makes interpretations within the context of stratigraphy, chronology, sulfur species, lithology and locality, concluding that mesophilic sulfate reducing microbes emerged around 2.6 Ga and were initially restricted to shallow-water environments before widespread colonisation of deeper water environments around 2.5 Ga. It supports the proposition that negative MIF sulfides are the result of sulfate reducing microbes by relating negative MIF to 34S-depleted o34S values.Our findings suggest there were three redox stages in Earth's early history, the Early Archean may have been more oxic than is generally believed, while the Late Archean was relatively reduced until a dramatic transition to a significantly more oxic Paleoproterozoic. We propose that the occurrence of negative versus positive MIF is correlated with lithological facies prior to 2.5 Ga and we have identified three reservoirs depicted in the sulfur MIF data including an oceanic particulate sulfur reservoir with △33S of around l‰ a soluble-sulfate oceanic reservoir with △33S of around -l‰ and a particulate sulfur atmospheric reservoir with △33S >1‰. We postulate that large magnitude MIF around 2.5 Ga is the result of MIF (photochemical) processes operating under Raleigh distillation conditions in the atmosphere. Our chronological interpretation leads us to place the transition event 100 Ma earlier than previously accepted at 2.4 Ga.