Research articles

Nearly 40% of the Earth’s ice-free surface area is cropland or pasture. Measurements of dissolved organic matter along a gradient of agricultural land use suggest that agricultural watersheds contain less complex, more microbially derived dissolved organic matter than natural wetlands.

Seismic anisotropy in Earth’s oceanic lithosphere and in the mantle wedge above subduction zones is associated with crystallographic preferred orientations of olivine. Experiments at high pressure and temperature suggest that a pressure of ∼3 GPa can induce the same changes in the crystal structure of olivine as high water activity at lower pressures.

The effect of volcanism on low-latitude climate has been difficult to quantify. A compilation of tropical and subtropical annually resolved climate reconstructions shows a correlation between low sea surface temperatures and low-latitude volcanic activity over the past four centuries.

Climate variability in the tropical Pacific Ocean influences climate across much of the planet. A diatom-based record of sea surface temperatures from El Junco lake, Galapagos, reveals that the most recent half-century is the warmest 50-year period in the past 1,200 years.

The Earth constantly loses matter, mostly in the form of H⁺ and O⁺ ions. Analysis of measurements from the Cluster spacecraft reveals that the total loss of cold ions, below 1 eV in thermal energy, is larger than the previously observed, more energetic, outflow.

The break-up of an asteroid led to the rapid delivery of meteorites to Earth around 470 million years ago. The widespread occurrence of sedimentary megabreccias deposited at the same time suggests this high influx of meteorites could have triggered mass wasting — downslope movement of rock and sediment — at continental margins, by inducing the destabilization of sedimentary slopes.

The rising abundance of greenhouse gases in the atmosphere has been associated with a change in the stratospheric circulation and a decrease in the mean age of stratospheric air. Balloon-borne measurements of trace gases over 30 years suggest that, in contrast to model predictions, there have been no significant trends (at a 90% confidence limit) in the age of stratospheric air.

As a single-plate planet, Mars cannot support large-scale motion of the lithosphere, according to the standard theory of stagnant-lid convection, but such motion is required to explain the position of the Tharsis Rise. Modelling work suggests that differential rotation of the lithosphere can resolve this problem.

Observational studies show a clear response in European climate to El Niño/Southern Oscillation in late winter. Simulations with an atmospheric general circulation model identify a long-distance pathway connecting climate variability in the Pacific region and Europe via the stratosphere, the upper layer of the atmosphere.

Impacts of extraterrestrial objects were frequent on the early Earth. Shock experiments suggest that the impact of the most common type of meteorites into sea water could have synthesized the organic molecules that served as the building blocks of life.

Biological nitrogen fixation limits plant growth and carbon exchange at local to global scales. Long-term nutrient manipulation experiments in forests and short-term manipulation experiments in microcosms suggest that the micronutrient molybdenum, a component of the nitrogen-fixing enzyme nitrogenase, limits nitrogen fixation by asymbiotic bacteria in tropical soils in Panama.

Pacific salmon deliver substantial quantities of nutrients to freshwater streams when they spawn. Experiments with a recirculation flume support the idea that bacterially mediated aggregation of salmon organic matter, as well as inorganic particulate matter, is responsible for nutrient delivery to these stream beds.

Deep convection in the subpolar North Atlantic, an important component of the global ocean circulation, has been absent in recent years. Profiling float data from the Argo programme document the return of deep mixing to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008.

Peridotites from the Horoman massif have the least radiogenic lead isotope ratios reported from any mantle material, and unlike any inferred from the compositions of mid-ocean ridge basalts. These data hint at the existence of ancient mantle domains that are not sampled by mid-ocean ridge basalts.

The formation and circulation of Antarctic Intermediate Water has varied over glacial–interglacial timescales. A neodymium record from the Atlantic Ocean basin suggests that changes in circulation may have been driven by changes both in Antarctic Intermediate Water formation in the Southern Ocean and in the strength of North Atlantic meridional overturning.

The response of ocean circulation in the Southern Ocean to changes in wind stress and surface buoyancy fluxes is under debate. An analysis of Argo data and historical measurements suggests that transport in the Antarctic Circumpolar Current and the meridional overturning circulation in the Southern Ocean are insensitive to decadal changes in wind stress.

Future climate warming is predicted to accelerate the decomposition of labile soil organic matter, but to have little impact on the degradation of biochemically resistant organic compounds such as leaf cuticles and lignin. However, 14 months of soil warming in a temperate mixed forest resulted in a build-up of leaf-cuticle-derived carbon and an increased decomposition of lignin in soils.

During the early stages in the formation of divergent margins, the lithosphere experiences large changes in temperature that can determine its strength and influence magma generation. Heat-flow data from the Eastern Gulf of Aden indicate a thermal anomaly that has persisted after continental break-up. This anomaly may have been caused by small-scale convection that occurred during and after rifting.

Large ice streams and outlet glaciers drain Greenland and Antarctica. An observed acceleration of ice velocity in one of these outlet glaciers, Byrd Glacier, East Antarctica coincides with a large water discharge from two subglacial lakes, allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice.

Global warming is likely to increase soil organic carbon decomposition, and thus CO₂ release to the atmosphere, creating a positive feedback cycle. Inclusion of realistic estimates of soil black carbon in a climate model results in a decrease in soil CO₂ emission in Australia by up to 24.4% following a 3 ^∘C warming over 100 years, suggesting that black carbon reduces the strength of this feedback.