Letters in 2008

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.

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.

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.

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.

Coral records from a range of sites extend the index of the Indian Ocean Dipole back to 1846. Indian Ocean Dipole events increased in strength and frequency in the twentieth century, coincident with the development of direct feedbacks with the Asian Monsoon.

The North Atlantic Oscillation controls winter climate variability in eastern North America and Europe. Coral-derived records of sea surface temperature in Bermuda suggest that multidecadal variability of the North Atlantic Oscillation has increased in the past few decades relative to the early nineteenth century.

Sustained nitrogen deposition has had a detrimental effect on ecosystems in Europe and North America. Now a grassland in Slovakia is showing symptoms of extreme soil acidification not previously observed in association with nitrogen deposition.

Understanding heat exchange in the Indian Ocean requires knowledge of the magnitudes and locations of both meridional deep-water transport and mixing. Observations from a fracture zone in the Southwest Indian Ridge quantify the flow through this narrow region to 20–30% of the total meridional overturning circulation in the Indian Ocean, and provide an example of elevated turbulence in a deep sheared flow.