Volume 7 Issue 4, April 2014

The tropical belt has become wider over the past decades, but climate models fall short of capturing the full rate of the expansion. The latest analysis of climate simulations suggests that a long-term swing of the Pacific Decadal Oscillation is the main missing cause from the coupled climate models.

As Mercury's interior cools and its massive iron core freezes, its surface feels the squeeze. A comprehensive global census of compressional deformation features indicates that Mercury has shrunk by at least 5 km in radius over the past 4 billion years.

Oxygen-producing photosynthesis must have evolved before the pervasive oxidation of the atmosphere around 2.4 billion years ago, but how long before is unclear. Geochemical analyses of ancient sedimentary rocks now suggest that cyanobacteria generated oxygen at least 3 billion years ago.

The climate regimes of monsoon regions and deserts are connected. Satellite data and numerical experiments reveal that an increase in dust aerosol loading over the Arabian Sea and West Asia can lead to enhanced summer monsoon rainfall over central India on timescales of days to weeks.

The oxygenation of the Earth's deep oceans is often thought to have triggered the evolution of simple animals. A review article proposes that instead, the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea.

Ozone-depleting substances emitted through human activities cause large-scale damage to the stratospheric ozone layer, and influence global climate. An analysis of unpolluted air sampled from Tasmania and firn snow reveals the emergence of four new ozone-depleting substances in the atmosphere since the 1960s.

The tropical belt has expanded by several degrees latitude over the past 30 years, following an earlier period of contraction. Climate simulations indicate that tropical belt width is controlled by multidecadal sea surface temperature variability associated with the Pacific Decadal Oscillation and anthropogenic aerosols.

Fluctuations in North Atlantic climate and hydrography over the past 1,000 years are seemingly linked to changes in solar irradiance. Reconstructions of marine conditions compared with an analysis of climate model output indicate that low solar irradiance is associated with the development of a high-pressure system in the eastern basin that affects the dynamics of the subpolar gyre.

Following the Chicxulub impact, many foraminifera in near-surface waters perished, but bottom-dwelling species survived. Impact experiments suggest that sulphate in Chicxulubs target rocks was released as predominantly sulphur trioxide, which would have been converted to sulphuric acid in the atmosphere and swept down swiftly by larger particles, acidifying the ocean surface.

The evolution of oxygenic photosynthesis should have occurred some time before the oxidation of Earth’s atmosphere 2.5 billion years ago. The molybdenum isotopic signature of shallow marine rocks that formed at least 2.95 billion years ago is consistent with deposition in waters that were receiving oxygen from photosynthesis at least half a billion years before the oxidation of the atmosphere.

Regions of intense continental deformation, termed continental slivers, have been identified in Chile, Bolivia and Ecuador. Analyses of GPS data now identify another large sliver in Peru, the Inca Sliver, that is moving away from a neighbouring sliver in Ecuador—implying that moving continental slivers control the deformation of almost the entire Andean mountain range.

Large subduction-zone earthquakes are thought to occur where the down-going and overriding tectonic plates are strongly locked. Analysis of geodetic and seismic data collected in the decade before the 2010 Chile earthquake shows that variations in pore-fluid pressure correlate with the degree of plate-interface locking, and may therefore control earthquake rupture.

As continents are stretched apart, deep rift valleys form and volcanoes can erupt both inside and outside of the valley. Numerical modelling suggests that gravitational unloading, caused by thinning of the stretched crust, can deflect rising magma towards the edges of the rift valley, causing off-rift eruptions.

Observations of compressional structures on Mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet's interior. Mapping of folds and faults across Mercury's surface using MESSENGER spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history.

The Indian summer monsoon is influenced by numerous factors, including aerosol-induced changes to clouds, surface and atmospheric heating, and atmospheric circulation. An analysis of satellite data and global climate model simulations suggests that dust aerosol levels over the Arabian Sea, West Asia and the Arabian Peninsula are positively correlated with the intensity of the Indian summer monsoon.

Low levels of iron limit primary productivity across much of the Southern Ocean. Measurements of dissolved iron levels combined with hydrographic data suggest that much of the iron in the surface waters of the Southern Ocean is supplied by deep mixing during winter.

Today, life on Earth depends on the availability of free oxygen, whether in the atmosphere, oceans, or aquatic systems. However, oxygen concentrations were low and variable for most of the first four billion years of Earth’s history. In this web focus we bring together a collection of research and review articles and opinion pieces tracing the origins of oxygenic photosynthesis and the factors that allowed oxygen to accumulate in the oceans and atmosphere.