Letters in 2010

Magma transports metals to the Earth’s surface to form ore deposits, but only sulphide-undersaturated magmas were thought to be capable of generating large amounts of ore. Laboratory experiments indicate that large volumes of gold ore can also be generated by sulphide-saturated magma, if the redox conditions of the magma are suitable.

Biogenic aerosol particles of terrestrial origin, including bacteria and pollen, trigger ice formation in the atmosphere. Laboratory experiments reveal that biogenic particles of marine origin also initiate ice formation under typical tropospheric conditions.

The amount of fluid delivered to subduction zones by the oceanic crust and penetrating sea water is not matched by that leaving through volcanic emissions or transfer to the deep mantle. Electromagnetic images of the Costa Rican subduction zone reveal an extra reservoir in the crust that may account for some of the missing fluid.

The tropical African rainbelt is an important component of atmospheric circulation and the global hydrological cycle. Reconstructions of vegetation in tropical Africa over the past 23,000 years suggest that the rainbelt expanded and contracted in response to changes in high-latitude climate conditions.

Modelling studies have postulated a possible impact of soil-moisture deficit and drought on hot extremes. An analysis of observational indices from central and southeastern Europe confirms that summer hot extremes are linked to soil-moisture deficits in southeastern Europe but does not detect a similar effect in central Europe.

Deformation in the crust and mantle — measured using seismic anisotropy — is poorly constrained in the western United States because of inconsistencies in the existing data. A three-dimensional model that reconciles these discrepancies reveals that seismic anisotropy in the crust correlates with different geological provinces, but is unrelated to anisotropy in the underlying mantle.

During the last deglaciation, climate changes over Greenland and Antarctica on millennial timescales were asynchronous. A temperature record from the Talos Dome in Antarctica confirms this asynchrony and shows clear regional differences in deglacial warming between the Indo-Pacific and Atlantic sectors of Antarctica.

Climate change has increased the area affected by forest fires in boreal North America. An analysis of the depth of burning in forests and peatlands in Alaska indicates that ground-layer combustion has accelerated regional carbon losses.

Marine sediments contain large quantities of carbon, primarily in the form of gas hydrate. Isotopic analyses suggest that carbon derived from fossil methane accounts for up to 28% of the dissolved organic carbon in sea water overlying hydrate-bearing seeps in the northeastern Pacific Ocean.

In the polar atmosphere, non-reactive gaseous elemental mercury is converted to a highly reactive form of mercury by halogens such as bromine. Measurements over the Dead Sea suggest that bromine also triggers reactive mercury formation over the mid-latitude ocean.

Hydrothermal fluids circulate through the upper portion of the oceanic crust. Isotopic analyses suggest that chemosynthetic microbial communities in the crust synthesize dissolved organic carbon in hydrothermal ridge-flank fluids.

Climate change could potentially destabilize marine ice sheets such as the West Antarctic ice sheet. A suite of predictions of sea-level change following grounding-line migration suggests that the gravitational effects of melting on local sea levels can exert a stabilizing influence on marine ice sheets on a reverse slope.

The Palaeocene–Eocene Thermal Maximum warm event about 56 million years ago was caused by the release of large amounts of carbon to the ocean and atmosphere. Estimates of the rate of recovery from the event suggest that about 2,000 Pg of the carbon released was sequestered as organic carbon.

Northern South America experienced significant changes in drainage patterns during the opening of the South Atlantic. Numerical modelling of the influence of mantle processes on the South American continent indicates that mantle convection was partly responsible for the formation of the Amazon River, the largest river on Earth.

Continental rifting creates narrow ocean basins, where coastal ocean upwelling and enhanced silicate weathering remove carbon dioxide from the atmosphere. Evidence from seismic data, sonar backscatter and seafloor images, and geochemical water analyses suggest that in young sedimented rifts, active magmatism occurs in a broader region than appreciated and releases carbon from the sediments.

Soils comprise the largest terrestrial carbon store on the planet. Soil respiration measurements suggest that the more biogeochemically recalcitrant the soil organic matter, the greater the temperature sensitivity of soil respiration.

Deep convection in the tropics is observed generally above a threshold for sea surface temperatures of about 26–28 °C. An analysis of satellite observations of tropical rainfall shows that the threshold has varied in the past 30 years in parallel with tropical mean sea surface temperatures.

Archaea are prevalent in the deep sea, and comprise a major fraction of the biomass in marine sediments. ¹³C-labelling experiments on the sea floor suggest that benthic archaea use sedimentary organic compounds to construct their membranes.

Skilful predictions of hurricane frequency have been limited to lead times of one season, and evidence for external forcing has been indirect. Simulations with nine variants of one global climate model show an influence of external forcing on hurricane frequency, and predictability on multi-year timescales.

Extant or relict martian volcanic hydrothermal systems have been sought in the pursuit of evidence for habitable environments. Detection from orbit of hydrated silica deposits on the flanks of a volcanic cone in the martian Syrtis Major caldera complex suggests the possible preservation of a recent habitable microenvironment.