Research articles

The Southern Ocean makes a substantial contribution to the oceanic carbon sink. Observationally based estimates of carbon subduction suggest that carbon sequestration depends on physical properties, such as mixed layer depth, ocean currents, wind and eddies, that are potentially sensitive to climate variability and change.

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.

The great distance travelled by long-runout landslides, observed previously on the Earth and Mars, requires a mechanism of friction reduction. Identification and analysis of long-runout landslides on Saturn’s moon Iapetus suggests that the Iapetian landslides are enabled by flash heating of the icy sliding surface.

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.

The Axial Seamount submarine volcano exhibits an inflation–deflation cycle comparable to similar volcanoes on land. Measurements of ocean bottom pressure document the entire inflation–deflation cycle between eruptions at Axial Seamount in 1998 and 2011, and imply that the timing of submarine eruptions could be more predictable than that of their subaerial counterparts.

For volcanoes at submarine rift zones, a direct link between seismicity, seafloor deformation and magma intrusion has not been demonstrated. Recordings from ocean-bottom hydrophones and bottom-pressure recorders map an increasing rate of seismicity at Axial Seamount, northeast Pacific, over several years before its eruption in April 2011.

At frequently active submarine volcanoes, it is difficult to distinguish between new and pre-existing lava flows. A combination of high-resolution bathymetric surveys taken before and after an eruption at Axial Seamount in 2011 allows detailed mapping of the 2011 lava flows, and highlights the tendency of new flows to mimic older ones.

Many terrestrial surfaces are covered by photoautotrophic communities, which are capable of synthesizing their own food from inorganic substances using sunlight. According to an analysis of previously published data, these communities account for nearly half of the biological nitrogen fixation on land.

Both marine- and land-terminating glaciers in southeast Greenland have experienced dramatic recent retreat. An 80-year record of historical aerial photographs and satellite imagery shows that many land-terminating glaciers in this region retreated more rapidly in the 1930s than today, whereas marine-terminating glaciers have retreated faster in the 2000s.

The steep topography of mountain landscapes arises from interactions between tectonic rock uplift, valley incision and landslide erosion on hillslopes. An analysis of more than 15,000 landslides in the eastern Himalaya, mapped from satellite images, shows that steep uplands primarily respond to uplift and river incision by increases in landslide erosion rates rather than by steepened hillslope angles.