Research articles

The Slumgullion landslide in the western United States has been moving almost continuously for over 100 years. Observations and numerical modelling show that air pressure changes from atmospheric tides trigger daily movement.

Oceanic nitrogen concentrations are controlled by the balance between nitrogen fixation and denitrification. Examination of nutrient concentrations in the North and South Atlantic Ocean suggests that nitrogen fixation is controlled by the supply of dissolved iron.

Many mountain ranges have asymmetric topography and drainage patterns. Laboratory experiments show that tectonic uplift combined with a precipitation gradient will cause the drainage divide to migrate towards the drier side of the mountain range, thereby triggering the splitting of drainage basins on the dry side of the range.

The Eocene Thermal Maximum 2 occurred 53.5 million years ago in response to elevated atmospheric carbon dioxide levels. Geochemical and microfossil analyses of Arctic sediments show that the during this event the surface of the Arctic Ocean warmed and freshened, and the coldest month mean temperatures did not fall below 8 ^∘C.

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.

The devastating Wenchuan earthquake in 2008 struck along a fault zone that showed low rates of deformation. Analysis of GPS and InSAR data suggests that, as structural barriers failed during a single earthquake, the rupture cascaded across multiple fault segments, which may explain the high magnitude of the event.

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.

The initial production of oxygen in early Earth’s oceans altered the redox chemistry and cycling of nitrogen. A record of nitrogen isotopes from preserved organic matter indicates nitrogen cycling in the presence of free oxygen 2.67 billion years ago, about 200 million years before the first geochemical evidence for atmospheric free oxygen.

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.