Volume 6 Issue 4, April 2013

The sea floor around mid-ocean ridges is often carpeted by hummocky lava flows. Images from the Southwest Indian Ridge sea floor, however, show a smooth texture created by exhumation and widespread exposure of altered mantle rocks.

Gold is often deposited in Earth's crust by fluids that percolate through rock fractures. Earthquakes cause rock fractures to expand rapidly and could cause the fluids to evaporate, triggering almost instantaneous gold deposition.

Iron limits plankton productivity in large regions of the global ocean. Analyses of meltwater from the Greenland ice sheet suggests that subglacial weathering delivers significant quantities of biologically available iron to the North Atlantic Ocean.

Scarce food supplies could hinder biological activity in the ocean's depths. However, measurements at Mariana Trench point to an unexpectedly active microbial community in the deepest seafloor setting on the planet.

A whirling vortex has been observed in the atmosphere at the south pole of Venus. Cloud motions tracked by the Venus Express spacecraft suggest that the south polar vortex is long-lived, erratic and baroclinic in character.

Aerosols scatter and absorb incoming solar radiation, with consequences for the energy balance of the atmosphere. An analysis of satellite data suggests that the regional redistribution of aerosols over the past decade had little net effect on the global radiative forcing of the atmosphere.

The water vapour content of the atmosphere has increased as a result of global warming, strengthening the hydrological cycle. An analysis of observational data suggests that wet seasons have become wetter, and dry seasons drier, in recent decades.

Assessing potential future carbon loss from tropical forests is important for evaluating the efficacy of programmes for reducing emissions from deforestation and degradation (REDD). An exploration of results from 22 climate models in conjunction with a land surface scheme suggests that in the Americas, Africa and Asia, the resilience of tropical forests to climate change is higher than expected, although uncertainties are large.

The micronutrient iron is thought to limit primary production in large regions of the global ocean. Meltwater measurements suggest that the Greenland ice sheet serves as a significant source of potentially bioavailable iron to the surrounding coastal ocean

The elemental composition of marine organic matter is used to infer a variety of oceanic ecosystem processes. A compilation of observational data suggests that elemental ratios differ substantially from the Redfield ratio, but exhibit a clear latitudinal trend.

Microbes regulate the decomposition of organic matter in marine sediments. Measurements at the deepest oceanic site on Earth reveal high rates of microbial activity, potentially fuelled by the deposition of organic matter.

The 100,000-year problem refers to an apparent mismatch between the strength of solar forcing associated with the 100,000-year cycle of eccentricity in the Earth’s orbit and the amplitude of glacial–interglacial cycles. Numerical analyses suggest that recent glacial–interglacial cycles can instead be explained by a phase locking between internal climate oscillations and the 413,000-year eccentricity cycle.

Fluids flowing through cavities in Earth’s crust can deposit gold. Thermo-mechanical modelling of a fluid-filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously.

Foreshocks precede some—but not all—earthquakes. Analysis of all earthquakes larger than magnitude 6.5 that occurred between 1999 and 2011 shows that earthquakes at plate boundaries are often preceded by increasing foreshock activity in the days leading up to the quake, whereas earthquakes in plate interiors often are not.

Lunar samples suggest that the inner Solar System was bombarded by asteroids about 4 Gyr ago. Radiometric ages of meteorites suggest an unusual number of high-velocity asteroids at this time, consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits.

Nitrite, a central intermediate in the marine nitrogen cycle, accumulates at the base of the sunlit surface ocean. Isotopic measurements suggest that ammonia oxidation is the primary source of nitrite in the primary nitrite maximum in the Arabian Sea.

The sea floor at the easternmost Southwest Indian mid-ocean ridge is smooth, unlike that at other mid-ocean ridges. Sonar imaging and analysis of rock samples show that the sea floor here is composed almost entirely of sea-water-altered mantle rocks that have been brought to the surface by large faults on both sides of the ridge axis over the past 11 million years.