Nature Geoscience最新文献

Earth’s atmosphere underwent permanent oxidation during the Great Oxidation Event approximately 2.45–2.22 billion years ago (Ga) due to excess oxygen (O₂) generated by marine cyanobacteria. However, understanding the timing and tempo of seawater oxygenation before the Great Oxidation Event has been hindered by the absence of sensitive tracers. Nitrogen (N) isotopes can be an indicator of marine oxygenation. Here we present an ~200 Myr nitrogen isotope oscillation recorded by Neoarchaean and Palaeoproterozoic banded iron formations from the Hamersley Basin, Western Australia, that were deposited in relatively deep marine shelf environments. Paired with the Jeerinah Formation shale record, our data from the Marra Mamba Iron Formation suggest that oxic conditions expanded to banded iron formation depositional environments from ~2.63 to 2.60 Ga. Subsequently, a positive δ¹⁵N excursion occurred in the ~2.48 Ga Dale Gorge Member, marking a decline in seawater O₂ and enhanced denitrification. This O₂ deficit was followed by a second phase of increasing O₂ levels as indicated by a gradual return to moderately positive δ¹⁵N values in the ~2.46 Ga Joffre Member and 2.45 Ga Weeli Wolli Iron Formation. These variations underscore a nonlinear history of marine oxygenation and reveal a previously unrecognized oscillation in seawater O₂ levels preceding the Great Oxidation Event.

The subduction of pelagic sediments and altered oceanic crust modulates the global cycle of volatile elements. Sulfate and carbonate fluids released when one plate descends beneath another modify the redox state of the mantle, and generate the return of water and reactive gases to the atmosphere and hydrosphere via arc volcanism, affecting planetary habitability over geologic timescales. However, the timing of the onset of subduction remains uncertain, hindering our understanding of how deep geochemical cycles operated on the early Earth. Here we measure sulfur and neodymium isotope data on Eoarchaean mantle-derived rocks of the Innuksuac Complex in northern Québec, Canada, with petrological characteristics of arc magmas. These rocks exhibit anomalous sulfur isotopic compositions originally produced by photochemical reactions in the atmosphere more than 3.8 Gyr ago. Combined sulfur and neodymium isotope data suggest that these signatures were transferred to the Innuksuac mantle through devolatilization and partial melting of terrigenous sediments derived from a Hadean (4.3–4.4 Gyr ago) continental source, providing a record of an early continental margin subduction environment. This result pushes back direct evidence of a subduction-driven volatile cycle to the onset of the terrestrial rock record, approximately 1 Gyr earlier than previously inferred from diamond inclusions.

A growing body of evidence suggests that molecular oxygen (O₂) accumulated in some shallow marine environments beneath the effectively anoxic Archaean atmosphere 4.0 to 2.5 billion years (Ga) ago. Yet, the temporal and spatial distribution of these oxygen oases is not well known. Here we use thallium (Tl) isotope ratios, which are sensitive to manganese oxide burial, to place constraints on the timing and tempo of marine oxygen oases between about 2.65 Ga and 2.50 Ga. Lower-than-crustal authigenic ²⁰⁵Tl/²⁰³Tl ratios are common in shales from the approximately 2.65 Ga Jeerinah Formation (Western Australia) and the 2.50 Ga Klein Naute Formation (South Africa). Particularly low ²⁰⁵Tl/²⁰³Tl ratios are found at 2.50 Ga, coincident with a pronounced ‘whiff’ of O₂. These data can be explained by widespread seafloor manganese oxide burial, a scenario that requires persistent O₂ penetration into marine sediments beneath regionally extensive marine oxygen oases. By contrast, ²⁰⁵Tl/²⁰³Tl ratios from the 2.60–2.52 Ga Nauga Formation (South Africa) do not deviate from crustal values, suggesting an intervening period of muted seafloor Mn oxide burial. Our data suggest that O₂ accumulated over greater spatial extents and to greater depths than previously thought at about 2.65 Ga and that marine oxygenation was spatially and temporally dynamic well before the Great Oxidation Event began at about 2.4 Ga.

Earth’s first rise in atmospheric oxygen between about 2.43 billion and 2.1 billion years ago fundamentally transformed the atmosphere and oceans, setting the foundation for the evolution of complex life. However, geochemical evidence reveals intermittent oceanic oxygen oases before the rise of atmospheric oxygen, although the mechanisms that drove the production and accumulation of oxygen remain poorly constrained. Here we present redox-sensitive trace metal and iron speciation data, and phosphorus phase partitioning results, for a 2.93-billion-year-old drill core from the Red Lake area, Canada, to reconstruct oceanic phosphorus cycling and links to oxygen production in the dominantly anoxic, iron-rich Archaean ocean. Our data document one of the earliest known intervals of surface water oxygen accumulation, predating the first accumulation of atmospheric oxygen by about 500 Ma. These intervals were preceded by ferruginous intervals and intervals of enhanced sulfide availability, which led to pulsed increases in oceanic phosphorus bioavailability via anoxic recycling from sediments. Enhanced phosphorus bioavailability would have helped stimulate photosynthetic primary productivity and organic carbon burial, probably exerting a major control on the episodic development of oxygen oases in the late Archaean ocean. This, in turn, led to a critical transitional phase in the development of an oxygenated surface environment.

Periodic sediment patterns have been observed on Earth in riverbeds and sand and snow deserts, but also in other planetary environments. One of the most ubiquitous patterns, familiar wind or ‘impact’ ripples, adorns sand beaches and arid regions on Earth. The observation of aeolian impact ripples on Mars the same size as their terrestrial counterparts despite a thinner atmosphere raises questions about their formation. Here we show in a numerical simulation that the emergent wavelength of impact ripples is controlled by the mechanics of grain–bed impacts and not the characteristic trajectories of grains above the bed. We find that the distribution of grain trajectories in transport is essentially scale-free, invoking the proximity of a critical point and precluding a transport-related length scale that selects ripple wavelengths. By contrast, when a grain strikes the bed, the process leading to grain ejections introduces a collective granular length scale that determines the scale of the ripples. We propose a theoretical model that predicts a relatively constant ripple size for most planetary conditions. In addition, our model predicts that for high-density atmospheres, such as on Venus, or for sufficiently large sand grains on Earth, impact ripples propagate upwind. Although wind-tunnel and field experiments are needed to confirm the existence of such ‘antiripples’, we suggest that our quantitative model of wind-blown sediment transport may be used to deduce geological and environmental conditions on other planets from the sizes and propagation speeds of impact ripples.