Letters in 2011

Formic acid contributes significantly to acid rain in remote environments. Satellite measurements combined with model simulations indicate that annual formic acid production is two to three times larger than current estimates suggest.

Mercury contamination affects many aquatic ecosystems. Measurements of mercury concentrations in air of stratospheric origin suggest that mercury is oxidized in the stratosphere and subsequently lost to the troposphere, most likely following attachment to stratospheric particles.

Oceanic lithosphere contains a record of plate-spreading rates, but the oldest oceanic plates have been subducted into the mantle. Measurements of seismic wave velocities in the subducted part of the Cocos Plate beneath central Mexico reveal an anisotropy that was created when the plate formed, preserving an archive of ancient plate-spreading rates on Earth.

The planet Mercury rotates three times about its spin axis for every two orbits around the Sun. Numerical modelling suggests that this unusual pattern could result from initial retrograde rotation that was captured into a stable synchronous orbit, and subsequent disturbance by a large impact.

During the early twenty-first century, the Greenland Ice Sheet experienced the largest ice mass loss on instrumental record. An analysis of sand deposition in Sermilik Fjord, off Helheim Glacier in east Greenland, suggests that despite strong variability over the past 120 years, similarly high rates of iceberg calving have only occurred once before, in the 1930s.

The Athabasca Basin, Canada, is home to some of the world’s largest uranium deposits. Analysis of preserved ore-forming fluids and experimental measurement of uranium solubility in analogous solutions show that the giant deposits could have formed relatively rapidly from extremely uranium-rich brines under acidic conditions.

The formal detection of climate warming and its attribution to human influence has so far relied on the differences between natural and anthropogenic warming patterns. An alternative and entirely independent attribution method that relies on the principle of conservation of energy instead, confirms greenhouse gas warming by 0.85 °C since the mid-twentieth century, half of which was offset by aerosol cooling.

East Asian summer monsoon precipitation varied on millennial timescales during the last glacial period. Sediment records and climate modelling suggest that the winter monsoon was also affected by millenial scale variability, and that the abrupt changes were driven by changes in the strength of the Atlantic meridional overturning circulation.

The origin of the Christmas Island Seamount Province in the northeast Indian Ocean is enigmatic. Radiometric dating and isotopic analysis of volcanic rocks from the seamounts imply that they were derived from the recycling of continental lithosphere and brought to the surface at a mid-ocean ridge.

The timing of onset of modern-style plate tectonics on Earth is debated. Analysis of rocks in the West African metamorphic province, which is more than 2 Gyr old, reveals that some minerals formed under conditions similar to those in modern-day subduction zones, suggesting that subduction occurred on the Palaeoproterozoic Earth.

Subduction modifies the cycling of Earth’s volatile elements. Geochemical analyses of fragments of mantle rocks collected above the Batan Island subduction zone, Philippines, suggest that wet sediment melts are released from the subducted slab, followed later by release of aqueous fluids, yet a significant amount of water is retained in the wedge.

The role of the North American seaway in creating and maintaining Late Cretaceous global greenhouse conditions has been unclear. An isotopic analysis of marine turtle and fish fossils from western Kansas and the Mississippi embayment reveals that the inflow of Tethyan surface waters to the seaway was limited.

The electric discharge of a thundercloud in the troposphere is often accompanied by upper-atmospheric electric discharges such as sprites or halos. Numerical simulations of the electric response of the mesosphere to lightning suggest that the process of electron associative detachment is fundamental to upper-atmospheric electrodynamics.

Massive-turbidite deposits are common in deep-water environments. Numerical simulations suggest that when turbulence is extinguished as turbidity currents reach areas of minimal slope, sediment reworking ceases, which allows the deposition of massive units.

Gas hydrates have been suggested as a carbon source for Palaeogene hyperthermal events, but warm seafloor temperatures are thought to have limited their accumulation. Numerical simulations suggest that enhanced organic carbon sedimentation and methanogenesis could have compensated for the smaller area of hydrate stability.

The Neoproterozoic Snowball Earth glaciations were separated by tens of millions of years, although models suggest glacial inception should occur within millions. Numerical modelling suggests that the delay could be explained by inherent limits on silicate weathering rates controlled by the availability of fresh rock.

The origin of the Bushveld Complex, South Africa, is elusive. Identification and dating of a radial swarm of volcanic dykes suggests that a mantle plume affected the region, but was active 600 million years before the Bushveld Complex formed, implying that the Bushveld magmas could be an indirect consequence of the plume.

At the southern boundary between India and Tibet, the Indian tectonic plate subducts northwards beneath the Tibetan Plateau. Seismic imaging shows that at the northern boundary of Tibet, the Asian plate is also subducting southwards beneath the plateau, and the Tibetan lithosphere is separate.

Two competing models have been suggested to explain the recovery of ecosystems from mass extinctions. An analysis of the recovery of marine pelagic communities from the Cretaceous–Palaeogene mass extinction supports a model of contingent recovery, rather than one based on trophic structure.

Seismic data have identified large-scale compositional heterogeneities in Earth’s deep mantle, but their origin is debated. Numerical modelling demonstrates that seismological and geochemical constraints on the character of these heterogeneities can be satisfied if they are composed of primitive material formed early in Earth’s history.