Letters in 2012

Oceanic crust is formed at mid-ocean ridges, but there is little consensus on where crystallization of melt actually occurs within the crust or mantle. Geochemical analyses of melt inclusions from two Pacific Ocean mid-ocean ridges indicate that 25% of the melt crystallizes below the melt lens to form the lower oceanic crust.

Subduction zone models often assume that the shallowest part of the plate interface slips aseismically. Images of the subduction trench next to the Tohoku-oki epicentre, captured using seismic reflection data 11 days after the 2011 earthquake, reveal deformation structures in sediments next to the trench, indicating that fault slip did reach the sea floor.

Subducting slabs can influence mantle flow, but the importance of neighbouring continental cratons is little understood. Geodynamical modelling, constrained by seismic data that identify regions of mantle flow beneath the Caribbean–South American Plate margin, shows that the deep-rooted South American craton acts to deflect and enhance mantle flow into a narrow channel.

Subglacial seismicity reveals information about glacier behaviour. Analysis of repeated seismic events beneath an Antarctic outlet glacier is consistent with sliding of debris-laden ice over a bedrock asperity with an event frequency that is modulated by the ocean tides.

Glacier snowlines in both the European Alps and the Southern Alps of New Zealand have retreated over the past century. An analysis of glacier fluctuations in New Zealand over the past 11,000 years suggests that this synchronous behaviour is unique to the past few hundred years.

Before it was destroyed by slash and burn practices, Brazil’s Atlantic Forest was one of the largest tropical forest biomes on Earth. Measurements from a river draining the region suggest that significant quantities of black carbon generated by the burning continue to be exported from the former forest.

Glacial landscapes exhibit both high- and low-relief land surfaces. A comparison of fjord erosion and offshore deposition suggests that glacier erosion created both the dramatic fjords and high-elevation low-relief surfaces in western Scandanavia.

High topography in eastern Tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin. Thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the Indo-Asian collision, implying that crustal flow alone could not have created the high topography.

The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming. Analyses of observational and reanalysis data suggest that the strength of the western North American carbon sink declined by 30–298 Tg carbon per year during the drought at the turn of the century.

In marine and freshwater ecosystems, anaerobic ammonium oxidation is coupled to nitrite reduction, and accounts for a significant fraction of ecosystem nitrogen loss. Laboratory incubations suggest that ammonium oxidation coupled to iron reduction contributes to nitrogen loss in anaerobic slurries of tropical forest soils.

The Bonnet Carré Spillway diverts floodwaters from the Mississippi River to Lake Pontchartrain, and was opened for 42 days during the 2011 flood. According to measurements of the newly deposited sediments, at least 31–46% of the river’s sand load was diverted into the spillway at this time, suggesting that such diversions can help mitigate coastal wetland loss.

The mantles of the terrestrial planets contain elemental abundances that suggest accretion continued at a late stage, after core formation. Geochemical data of meteorites from differentiated asteroids are consistent with such a late accretion event, suggesting that the phenomenon occurred throughout the Solar System and was related to planet formation.

The mantle and continental crust contain excessive amounts of radiogenic lead, implying that a complementary reservoir of unradiogenic lead should exist somewhere on Earth. Isotopic analyses of mantle rocks exposed on the Atlantic Ocean floor reveal that sulphide inclusions can have extremely unradiogenic lead compositions, suggesting that the reservoir could exist within the mantle itself.

Volcanic eruptions can inject hazardous ash clouds into the atmosphere. Numerical simulations and experiments on volcanic rock samples show that clasts initially formed deep in the volcanic conduit break-up during collisions in the conduit, thus generating fine-grained clouds of ash.

Biologically available nitrogen limits phytoplankton growth over much of the ocean. Data-constrained model simulations suggest that bioavailable nitrogen losses match gains in the global ocean, indicative of a balanced budget.

The exchange of water between subtropical North Atlantic Deep Water and the Antarctic Circumpolar Current is important, but poorly constrained. A subsurface acoustic image taken in the confluence region shows a prominent swirling structure, 500 m high and 10 km wide, that could be either a thermohaline intrusion or a localized and intermittent overturning event.

The axis of the geomagnetic field is offset eastwards from Earth’s centre by more than 500 km. Simulations of Earth’s geomagnetic field using a numerical dynamo model show that lopsided growth of the inner core, with faster solidification occurring in one hemisphere, could cause the offset.

The structure of the European crust and upper mantle is precisely known only in limited regions. A new tomographic model for the entire European upper mantle identifies northeastward subduction of the Adria plate beneath the Dinarides Mountains, volcanism related to the upwelling Eifel hotspot and mantle delamination beneath Scandinavia.

Star dunes are common in sand seas, but the mechanisms driving their formation are unclear. Numerical modelling indicates that the morphology of the dunes is controlled by the frequency of changes in the wind regime.

Global warmth 20–15 million years ago allowed vegetation to grow on formerly ice-covered areas of Antarctica. Leaf wax and pollen data show that this growth was supported by increased hydrologic activity over the Antarctic coast, derived from a local moisture source.