Letters in 2011

Blooms of the nitrogen-fixing cyanobacterium Trichodesmium, which fuel primary production in tropical and subtropical waters, require large quantities of iron. Laboratory incubations suggest that Trichodesmium accelerates the dissolution of iron oxides and dust, increasing the rate of iron uptake.

Variations in the volume, geochemistry and location of volcanism in Hawaii cannot be explained as the simple consequence of an underlying mantle plume. A numerical model of the Hawaiian plume suggests that small-scale convection in the mantle erodes the base of the overlying tectonic plate and may help generate the anomalous volcanism.

Sediments accumulated in a subduction trench are usually unconsolidated and impede the updip propagation of fault rupture during an earthquake. Seismic images of the southern Sumatra–Andaman trench reveal blocks of consolidated sediment that may have enabled fault rupture in 2004 to propagate up fault dip, thus further seaward, increasing the tsunami magnitude.

Afforestation, the conversion of croplands or marginal lands into forests, is considered one of the key climate-change mitigation strategies available to governments. Model simulations suggest that the temperature benefits of realistic afforestation efforts are marginal.

Evapotranspiration of soil moisture can affect rainfall and the development of convective storms. Satellite observations of cloud and land-surface temperatures over the Sahel suggest that convective storms are more likely to form over strong mesoscale gradients in soil moisture.

V-shaped ridges of thickened oceanic crust above the Iceland plume are thought to record variations in the convection of the mantle below. Geochemical analyses of basalt samples taken from the ridges suggest the thickened crust formed as the result of blobs of hot mantle rising up in the underlying plume.

Wetlands take up and store carbon, and release methane, through the decomposition of organic matter. Model simulations suggest that the areal extent of wetlands declines when permafrost thaws.

Feedbacks between soil moisture and precipitation are among the most important land–atmosphere interactions. An analysis of evaporative fluxes over North America suggests that the probability of afternoon rainfall in Mexico and the eastern United States is increased during periods of enhanced evaporation.

Photolysis of nitrous acid generates hydroxyl radicals — a key atmospheric oxidant. Measurements at a forest in Michigan suggest that sunlight-induced breakdown of nitric acid on the canopy surface serves as a significant source of nitrous acid to the overlying atmosphere.

Mixing in the Southern Ocean plays an important part in large-scale ocean circulation and in climate. An analysis of high-resolution hydrographic profiles from the Argo float programme reveals that the seasonal cycle of mixing in the Southern Ocean is controlled by seasonal variations in the wind stress, in particular over flat topography.

The seismic hazard associated with individual faults can be assessed from the distribution of slip and the recurrence time of earthquakes. Analysis of the offsets of stream channels and terraces caused during five historical earthquakes on the Fuyun fault, China, reveals characteristic fault slip of about six metres for each event.

Carbonaceous minerals in 3.8-billion-year-old rocks from West Greenland have been used as evidence for early life on Earth. Geochemical analyses of similar carbon minerals in 3.75- to 4.2-billion-year-old rocks from Canada show that they can be derived from subsequent alteration more than a billion years after the initial formation.

The evolution of marine complex animals about 635 million years ago took place in relatively low-oxygen waters. An analysis of a low-oxygen, hypersaline lagoon suggests these early animals may have obtained both oxygen and food from widespread microbial mats.

Shallow earthquakes lead to the uplift of mountain ranges, but also trigger landslides that remove mass. An analysis of the 2008 Wenshuan earthquake suggests that more material was removed from the orogen by widespread landslides than was added by coseismic uplift.

The origin of Titan’s massive nitrogen atmosphere is largely unknown. Laser-gun experiments and numerical calculations suggest that the nitrogen could have been generated by conversion from ammonia during the period of Late Heavy Bombardment.

Hydrothermal vents emit metals such as iron to the ocean. A chemical analysis of hydrothermal fluids suggests that up to 10% of the iron is present as stable, iron-bearing nanoparticles.

Radiating seismic waves can generate tremor on faults that are far away from the original earthquake source. Analysis of seismic wave data from the San Andreas fault reveals episodes of tremor, triggered by far-off earthquakes, which record a prolonged creeping of the fault that could potentially trigger another earthquake.

The western margin of the Andes Mountains often experiences large earthquakes, but the potential size of earthquakes along the eastern margin is unknown. Analysis of GPS data shows that a substantial section of the eastern margin is locked and could rupture in an earthquake with a magnitude of up to 8.9.

Monomethylmercury is a neurotoxin that accumulates in marine organisms. Incubation experiments suggest that methylation of inorganic mercury accounts for around half of the monomethylmercury present in polar marine waters.

The geochemistry of lavas erupted at locations where mantle plumes interact with mid-ocean ridges reflects the mixing between the two sources. Analysis of lavas erupted above the Foundation hotspot, near the Pacific–Antarctic Ridge, reveal a geochemical signature indicative of both a primitive mantle plume source and a recycled oceanic lithosphere source.