Letters in 2013

Relatively little is known about the dynamics of the Antarctic Circumpolar Current during the last glacial period. Estimates of current speeds over the past 20,000 years based on sediment grain size suggest that average flow speeds during the last glacial were comparable to modern speeds, but not in the areas with overlying winter sea ice.

Surface melt water from the Greenland ice sheet can become trapped in firn, delaying its journey to the sea. Radar and ice-core observations provide direct evidence of a perennial aquifer in the firn layer in southern Greenland that represents a potentially significant contribution to the Greenland mass budget.

Climate variations over the past 1,000 years correspond to solar fluctuations, but the magnitude of the solar variability is unclear. An analysis of numerical simulations and climate reconstructions suggests that the amplitude of solar forcing was small over this interval, with the main climate forcing derived from volcanic eruptions and greenhouse gas concentrations.

Part of the subduction zone plate interface beneath Costa Rica was previously locked, which allowed strain to accumulate. Analyses using GPS and geomorphic data show that almost the entire locked region ruptured during a megathrust quake in 2012, implying that plate-interface mapping towards the end of the earthquake cycle can aid seismic hazard assessments.

The characteristics of magmas typically associated with porphyry copper deposits are thought to be imparted in the mantle. Statistical assessment of over 40,000 geochemical analyses of magmatic rocks formed in subduction zones worldwide, however, shows that the characteristics of these magmatic rocks are systematically controlled by the thickness of the arc crust.

The Archaean rocks of Isua, West Greenland, contain graphite, but its origins are debated. Geochemical and microscopic analyses suggest that the graphite was formed from biologically formed carbon that was deposited at least 3.7 billion years ago.

In many planetary atmospheres, including that of Earth, the base of the stratosphere—the tropopause—occurs at an atmospheric pressure of 0.1 bar. A physically based model demonstrates that the pressure-dependence of transparency to infrared radiation leads to a common tropopause pressure that is probably applicable to many planetary bodies with thick atmospheres.

At mid-ocean ridges, the movements between rift segments are usually accommodated by transform faults that are oriented perpendicular to the rift axis. Analysis of seismic data from rift segments exposed in Iceland shows that such movements can also occur through the rotation of several small faults and crustal blocks that slip like books tilting on a shelf.

Great earthquakes in the Himalaya are thought to occur mostly along the range front. Field mapping and radiocarbon dating reveal earthquake rupture on a previously unrecognized fault in the interior of the western Himalaya, which forms part of a key structural component of the mountain range, implying that seismic risk evaluations should be revised.

On Jupiter’s icy moon Europa, enigmatic chaos terrain—where the icy crust is cut by a jumble of ridges and cracks—occurs most commonly at lower latitudes. Simulations of convection in the ocean underlying Europa’s icy crust suggest that ocean dynamics can control an enhanced flow of heat to Europa’s equatorial surface, and hence geological activity.

The volume of Archaean crust preserved at Earth’s surface today is low. Thermodynamic calculations and geodynamic modelling show that the thick, primary crust that would have formed on a much hotter Archaean Earth was denser than the underlying mantle, and would have therefore been recycled back into the mantle as drips.

The Canterbury earthquake sequence that struck New Zealand in 2010 and 2011 was characterized by an extended series of aftershocks. Analysis of seismic data show that a broad region of previously strong crustal rocks was weakened during the mainshock, and variations in crustal strength may have contributed to the protracted seismic activity.

Several volcanoes exist in Marie Byrd Land, a highland range in West Antarctica, but none were thought to be currently active. Analysis of seismic data from the region shows that magma could be moving beneath the West Antarctic Ice Sheet, with potential implications for ice flow.

Felsic rocks have not been identified on Mars, a planet that lacks plate tectonics to drive the magmatic processes that lead to evolved silica-rich melts. Spectral observations by the Mars Reconnaissance Orbiter indicate that felsic lithologies occur at multiple localities on Mars and suggest prolonged magmatic activity on ancient Mars.

The first known phosphorus-rich deposits formed 2 billion years ago, but their origins are unclear. Geochemical and palaeontological analyses of 2-billion-year-old deposits from northwest Russia suggest that the presence of sulphur-oxidizing bacteria and a sharp oxic–anoxic transition in the sediments allowed for phosphorus accumulation in this setting.

The formation of the silicate mineral anorthosite is thought to require magmatic processes that are not expected on Mars because of its predominately mafic terrains. Localized spectral detections by the Mars Reconnaissance Orbiter are consistent with anorthosite, suggestive of ancient intrusive igneous processes similar to those active on Earth.

The pressures and temperatures experienced by material flung from craters following impact events are expected to preclude survival of organics. The preservation of biomarkers in impact glass from the Darwin crater in Tasmania suggests that organic matter can survive in the distal products of meteorite impact.

The flow of ductile rocks in the deep crust and uppermost mantle is thought to add stress to faults in the shallow crust, potentially bringing the faults closer to rupture. Measurements of fault offsets in the Italian Apennines show that earthquake recurrence is largely controlled by viscous flow of deeper rocks in localized zones.

The boundary between Earth’s rigid lithosphere and ductile asthenosphere is marked by a seismic discontinuity. Laboratory experiments on basaltic magmas show that melts should pond at pressures that correspond to the lithosphere–asthenosphere boundary. Thus, magma ponding could explain the observed seismic discontinuity.

Modern grasslands are dominated by grasses that use the C₄ photosynthetic pathway, and were established about 8 million years ago. A sediment record suggests that in southwestern Africa, the expansion of grasslands was associated with increasing aridity and fire activity, both of which favour grasses that use the C₄ pathway.