Research articles

Forest fires release significant amounts of carbon dioxide into the atmosphere, but also convert a fraction of the burning vegetation to charred black carbon. Examination of 845 soil samples in Scandinavian forests shows that the charcoal content of boreal soils is highly variable, and more susceptible to degradation than has been thought.

Atmospheric concentrations of nitrous oxide, a greenhouse gas, have increased since 1860. A regression model indicates that conversion of 2% of manure nitrogen and 2.5% of fertilizer nitrogen could explain the pattern of increasing nitrous oxide concentrations between 1860 and 2005, including a rise in the rate of increase around 1960.

The depth at which particulate organic carbon sinking from the surface ocean is converted back to carbon dioxide is known as the remineralization depth. A three-dimensional global ocean biogeochemistry model suggests that a modest change in remineralization depth can have a substantial impact on atmospheric carbon dioxide concentrations.

The oxidation of ammonia to nitrate, known as nitrification, is a key process in the nitrogen cycle. Real-time polymerase chain-reaction measurements show that nitrification is driven by bacteria rather than archaea in nitrogen-rich grassland soils in New Zealand.

Earthquakes often occur in areas that lack an array of seismometers, resulting in a scarcity of local measurements from some regions of great geological interest. In such regions, some earthquakes themselves may be turned into virtual seismometers that are capable of measuring strain caused by passing waves from other earthquakes.

It has been debated whether rivers or glaciers are more effective agents of erosion. A global compilation of erosion rates reveals that both are capable of generating rates of erosion that match or exceed the highest rates of rock uplift.

Gigantic jets emerge from the top of thunderstorms and extend all the way to the ionosphere at altitudes of 90 km. Simultaneous video images and magnetic field measurements of a gigantic jet demonstrate an electric charge transfer between the thunderstorm and the ionosphere that is comparable to that observed in cloud-to-ground lightning.

The formation of dunes is controlled by the direction of the prevailing winds and the characteristics of the sediments. Linear dunes in the Qaidam Basin, China, are shown to form from cohesive sediments, a model that could be applicable to dunes on Titan.

The sedimentary wedge in the Nankai Trough off the coast of Japan is cut by a landward-dipping thrust fault, which is an important plate boundary structure in this region. Geological and seismic reflection data indicate that the fault initiated ∼1.95 million years ago and underwent uplift and reactivation ∼1.55 million years ago.

The earliest fossils of cyanobacteria have been controversially reported from the 3,465-million-year-old Apex chert in Western Australia. Mineralogical analyses of the rock near the fossil site indicate that the site experienced repeated alterations at low and medium temperatures, and is therefore unlikely to have preserved any early forms of life.

The Australian–Indonesian summer monsoon affects rainfall variability across the Indo–Pacific region. Reconstructions of monsoon strength from stalagmites show that precipitation increased from 11,000 to 7,000 years ago, as rising global sea level caused the flooding of the Indonesian continental shelf.

The micronutrient molybdenum is necessary for nitrogen fixation, but is very rare and highly soluble in soils. X-ray spectroscopy analysis of forest soil samples indicates that most of the molybdenum in the litter layer binds to organic matter.

The Salton Sea is an evolving pull-apart basin located between the San Andreas and Imperial faults in Southern California. Seismic and geological data reveal a rapidly subsiding southern sub-basin that is bounded by a hinge zone to the north, and northwest-dipping normal faults to the south.

Sea level fluctuated substantially over the past 22,000 years. A simple model based on these fluctuations estimates between 7 and 86 cm of sea-level rise by the end of the twenty-first century—in agreement with climate model projections.

It has been proposed that hydrocarbons could be produced abiogenically under the high pressure, high temperature conditions characteristic of the upper mantle. In situ Raman spectroscopy indicates that methane forms saturated hydrocarbons, containing two to four carbons, when exposed to upper-mantle conditions.

Most discharge from large ice sheets takes place through fast-flowing ice streams. A combination of radar and seismic data reveal megascale glacial lineations at the bed of a West Antarctic ice stream that undergo significant change by erosion and deposition on decadal timescales.

Following the crystallization of a magma ocean, the martian mantle probably underwent an overturning event, but its initiation, timing and geochemical consequences are poorly constrained. Isotopic data for martian meteorites and numerical simulations provide strong evidence for early overturning in the martian mantle.

Mineral dust can be transported long distances in the lower atmosphere. Satellite measurements and model simulations show that dust generated during a storm in the Taklimakan Desert, China, in 2007 was transported more than once around the globe.

Phosphorus is frequently the limiting nutrient in marine and terrestrial ecosystems. Analysis of lightning-derived glassy compounds from North America, Africa and Australia suggests that cloud-to-ground lightning increases the bioavailability of this nutrient.

About 55 million years ago global surface temperatures increased by 5–9 ^∘C within a few thousand years, following a pulse of carbon released to the atmosphere. Analysis of existing data with a carbon cycle model indicates that this carbon pulse was too small to cause the full amount of warming at accepted values for climate sensitivity.