The Sun was substantially less bright in the distant past, yet Earth's surface temperature remained above freezing. Higher concentrations of the greenhouse gases CO 2 and CH 4 were likely responsible for keeping the early climate warm. CH 4 concentrations of 1000 ppm or higher are predicted for the Late Archean/Paleoproterozoic atmosphere prior to the rise of O 2 . Photolysis of this CH 4 may have created an optically thin organic haze during much of this time. The rise of O 2 at 2.3 Ga eliminated most of the methane and probably triggered the Paleoproterozoic glaciations. CH 4 concentrations could have remained elevated throughout much of the Proterozoic, however, as a consequence of low concentrations of dissolved O 2 and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis.

Methane and climate during the Precambrian Era

Mountain belts modulate denudation flux and hydrologic processes and are thus fundamental to nutrient cycling on Earth's surface. We used europium anomalies in detrital zircons to track mountain-building processes over Earth's history. We show that the average thickness of active continental crust varied on billion-year time scales, with the thickest crust formed in the Archean and Phanerozoic. By contrast, the Proterozoic witnessed continuously decreasing crustal thickness, leaving the continents devoid of high mountains until the end of the eon. We link this gradually diminished orogenesis to the long-lived Nuna-Rodinia supercontinent, which altered the mantle thermal structure and weakened the continental lithosphere. This prolonged orogenic quiescence may have resulted in a persistent famine in the oceans and stalled life's evolution in Earth's middle age.

Orogenic quiescence in Earth’s middle age

The long history of life on Earth has unfolded as a cause-and-effect relationship with the evolving amount of oxygen (O2) in the oceans and atmosphere. Oxygen deficiency characterized our planet's ...

Oxygenation, Life, and the Planetary System during Earth's Middle History: An Overview.

Abstract Mineral–microbe interactions play important roles in environmental change, biogeochemical cycling of elements and formation of ore deposits. Minerals provide both beneficial (physical and chemical protection, nutrients, and energy) and detrimental (toxic substances and oxidative pressure) effects to microbes, resulting in mineral-specific microbial colonization. Microbes impact dissolution, transformation and precipitation of minerals through their activity, resulting in either genetically controlled or metabolism-induced biomineralization. Through these interactions, minerals and microbes co-evolve through Earth history. Mineral–microbe interactions typically occur at microscopic scale but the effect is often manifested at global scale. Despite advances achieved through decades of research, major questions remain. Four areas are identified for future research: integrating mineral and microbial ecology, establishing mineral biosignatures, linking laboratory mechanistic investigation to field observation, and manipulating mineral–microbe interactions for the benefit of humankind.

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A critical review of mineral–microbe interaction and co-evolution: mechanisms and applications

A coupled carbon-silicon cycle model over Earth history: Reverse weathering as a possible explanation of a warm mid-Proterozoic climate

Oceanic sulfate concentrations are widely thought to have reached millimolar levels during the Proterozoic Eon, 2.5 to 0.54 billion years ago. Yet the magnitude of the increase in seawater sulfate concentrations over the course of the Eon remains largely unquantified. A rise in seawater sulfate concentrations has been inferred from the increased range of marine sulfide δ34S values following the Great Oxidation Event and was induced by two processes: enhanced oxidative weathering of sulfides on land, and the onset of marine sulfur redox cycling. Here we use mass balance and diagenetic reaction-transport models to reconstruct the sulfate concentrations in Proterozoic seawater. We find that sulfate concentrations remained below 400 µM, and were possibly as low as 100 µM, throughout much of the Proterozoic. At these low sulfate concentrations, relatively large sulfate–pyrite sulfur isotope differences cannot be explained by sulfate reduction alone and are only possible through oxidative sediment sulfur cycling. This requires oxygen concentrations of at least 10 µM in shallow Proterozoic seawater, which translates to 1–10% of present atmospheric oxygen concentrations. At these oxygen and sulfate concentrations, the oceans would have been a substantial source of methane to the atmosphere (60–140 Tmol yr−1). This methane would have accumulated to high concentrations (more than 25 ppmv) and supported greenhouse warming during much of the Proterozoic Eon, with notable exceptions during the Palaeoproterozoic and Neoproterozoic eras. In the Proterozoic, sulfate concentrations in the oceans were low and atmospheric methane levels high, according to mass balance and diagenetic models that investigate the oxidation state of the Proterozoic oceans.

Olivier Hancisse

Papers

Proterozoic seawater sulfate scarcity and the evolution of ocean–atmosphere chemistry