Oxygenation of the Earth's surface is increasingly thought to have occurred in two steps. The first step, which occurred approximately 2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period ( approximately 800-542 Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates, and may have led to oxygenation of the deep ocean. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (approximately 635 to approximately 548 Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period, our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon and sulphur cycles that occurred during this significant period of Earth's history.

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The martian surface is a natural laboratory for testing our understanding of the physics of aeolian (wind-related) processes in an environment different from that of Earth. Martian surface markings and atmospheric opacity are time-variable, indicating that fine particles at the surface are mobilized regularly by wind'"'. Regolith (unconsolidated surface material) at the Mars Exploration Rover Opportunity's landing site has been affected greatly by wind, which has created and reoriented bedforms, sorted grains, and eroded bedrock. Aeolian features here preserve a unique record of changing wind direction and wind strength. Here we present an in situ examination of a martian bright wind streak, which provides evidence consistent with a previously proposed formational model'''^ for such features. We also show that a widely used criterion for distinguishing between aeolian saltation-and suspension-dominated grain behaviour is different on Mars, and that estimated wind friction speeds between 2 and 3ms~', most recently from the northwest, are associated with recent global dust storms, providing ground truth for climate model predictions.Pre-landing orbiter observations of Opportunity's landing site showed bright and dark streaks tapering away from craters on the Meridiani plains. From observations of similar features distributed across many locations on Mars, streak orientations indicate formative wind directions'""*. High-resolution images within the 117 km X 18 km landing ellipse obtained over several years show bright streak directions that indicate winds from the northwest and southeast'. What has not been recognized previously, however, is that this apparent bimodality of wind direction has a significant time dependence. Images obtained before the major 2001 dust storm are more likely to show bright streaks oriented in the opposite direction from images obtained after the storm waned. Rare overlapping image pairs provide two examples of an individual streak changing orientation after the intervening 2001 dust storm (Fig.