Letters

Following the spring-time polar sunrise, ozone concentrations in the lower troposphere episodically decline to near-zero levels. Measurements on Alaskan snow and sea ice suggest that photochemical reactions in surface snow serve as a major source of reactive bromine to the overlying atmosphere, contributing to episodic ozone depletion.

In the northwestern United States, two neighbouring volcanic chains—the Snake River Plain and High Lava Plains—decrease in age in opposing directions. Laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two.

Multi-decadal variability in coral growth rates has been documented throughout the Caribbean over the past 150–200 years. Analyses of observational and model data suggest that anthropogenic aerosols were a key driver of variations in coral growth in the western Caribbean in the second half of the twentieth century.

In the Southern Ocean, the biological cycling of dissolved CO₂ is thought to be influenced by the delivery of iron by dust particles. Reconstructions of nutrient utilization from the South Atlantic Ocean show millennial-scale links between dust flux and the efficiency of the biological pump.

Lavas erupted from individual volcanic centres often have one of two distinct compositions. High pressure and temperature experiments on lava samples collected from St Vincent Volcano in the Caribbean, combined with thermal modelling, show that this compositional bimodality is generated by volcanic systems with low heat and water content.

In the Antarctic interior, assessments of surface mass balance may overestimate accumulation because high winds remove some of the annual snowfall. Geophysical observations reveal localized zones of persistent wind scour (where little or no snow accumulates) that are predicted to occur across approximately 5% of the Antarctic surface.

Subducting slabs of oceanic lithosphere often stagnate before reaching the lower mantle. Laboratory experiments under high pressures and temperatures show that pyroxene, a common mineral in such slabs, is transformed to its dense, high-pressure counterpart, majorite garnet, at a very slow rate, temporarily keeping the slabs buoyant compared to the surrounding mantle.

In sharp contrast to events in the Arctic region, sea ice surrounding Antarctica has expanded slightly in the past few years. A combination of observations and climate model simulations suggests that cooling of the surface ocean by meltwater from the Antarctic ice shelves has contributed significantly to this sea ice expansion.

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.

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.

The continents are thought to insulate and heat the underlying mantle. Geochemical analyses of lava samples formed at a mid-ocean ridge in the Atlantic Ocean immediately after continental breakup show that the mantle was up to 150 °C hotter than today and took about 70 million years to cool.

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.

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.

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.

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.

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 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 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.

The fast flow of glaciers in Greenland during the summer season has been attributed to seasonal increases in subglacial melt water. Tracking the flow of subglacial water using geochemical tracers reveals the establishment of an increasingly efficient drainage network as the melt season progresses.