Letters in 2009

The cause of high electrical conductivity in the middle crust beneath the Pacific Northwest region of the US is not clear. New electrical-resistivity data reveal a connection between this regional conductor and a localized conductor beneath a prominent volcano in the region, suggesting that the anomalous conductivity is due to the presence of partial melts.

Sea ice is a critical component of the climate and oceanographic system in the North Atlantic Ocean. A biomarker record reveals millennial-scale and glacial–interglacial fluctuations in sea-ice coverage in the northernmost Atlantic Ocean over the past 30,000 years.

Sprite discharges above thunderclouds, at altitudes of 40–90 km, are usually created by a strong positive cloud-to-ground lightning flash. A numerical discharge model of the process suggests that sprite streamers are generated through the collapse of a downward-propagating screening-ionization wave in the lower ionosphere.

Magnesium silicate perovskite, the dominant mineral in the lower mantle, is thought to transform into a post-perovskite phase in the mantle’s lowermost region. Laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post-perovskite, which could explain the anomalous physical properties of the lowermost mantle.

Both core formation and the late addition of extraterrestrial material have been invoked to explain the abundances and relative proportions of iron-loving elements in the Earth’s mantle. High-temperature experiments suggest that the concentration of gold is consistent with core formation, but the amounts of osmium and iridium require later inputs of extraterrestrial material.

Changes in the sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation. Shipboard measurements of turbulence kinetic-energy dissipation rate indicate that seasonal surface cooling in the central equatorial Pacific may be largely caused by mixing induced by tropical instability waves.

Banded iron formations are plentiful in the rocks representing early Earth, but the mechanisms by which they formed remain controversial. Geochemical modelling indicates that the hydrothermal leaching of low-aluminium ocean crust and subsequent chemical reactions in iron- and silica-rich hydrothermal fluids could have triggered the alternating deposition of iron and silica-dominated sediments.

Geophysical data reveal that at subduction zones oceanic plates could be pervasively hydrated for several kilometres below the crust–mantle boundary. Numerical experiments suggest that such deep hydration is facilitated by negative pressure gradients that lead to the downward pumping of water along bending-related normal faults.

Numerical simulations that assume realistic core-fluid viscosities have been unsuccessful in fully reproducing the unique characteristics of the Earth’s geomagnetic field. An evaluation of boundary conditions suggests that the prescription of a uniform heat flux at the core’s surface could generate a more Earth-like magnetic field.

Rocks near the San Andreas fault are pervasively crushed at distances of up to 400 m from its core. Laboratory experiments and calculations suggest that the rocks were pulverized at high strain rates (>150 s⁻¹) associated with a supershear rupture—a rupture propagating at a velocity equal to greater than that of seismic shear waves.

The dynamic friction along faults controls earthquake ruptures in the crust, but many previous studies have quantified this value only for constant slip rates. Experiments accounting for the more realistic condition of changing slip rates suggest that faults undergo a sequence of strengthening, weakening and healing during acceleration and deceleration of slip.

Phosphonates, compounds with a carbon–phosphorus bond, are a key component of the marine-dissolved organic phosphorus pool. Nuclear magnetic resonance spectroscopy measurements suggest that the cyanobacteria Trichodesmium is a significant source of phosphonates in nutrient-poor regions of the ocean.

1.1-billion-year-old volcanic rocks in North America are thought to record asymmetric geomagnetic reversals, indicating non-axial dipolar behaviour of the magnetic field. High-resolution data from Ontario suggest that the reversals were instead symmetric, and that the apparent reversal asymmetry is an aliasing effect of the low resolution of earlier samples combined with the rapid motion of North America.

Now that stratospheric ozone depletion has been controlled by the Montreal Protocol, interest has turned to the effects of climate change on the ozone layer. An atmospheric chemistry model suggests that climate change will increase the stratosphere-to-troposphere ozone flux by 23% globally between 1965 and 2095, altering the amount of ultraviolet radiation reaching Earth’s surface.

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.

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.