Letters in 2012

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.

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.

During subduction, the seawater-altered lithosphere becomes dehydrated and expels fluids. Isotopic analysis of an exhumed oceanic slab in the Tianshan Mountain Range shows that although subduction can continue for many millions of years, fluids are expelled in short-lived channels over periods of just a few hundred years.

Changes in terrestrial water storage are likely to affect sea level, but comprehensive and reliable data are scarce. Simulations of global terrestrial water stocks and flows, with an integrated model that specifically accounts for human activities, indicate that groundwater depletion and reservoir storage have together led to a sea-level rise of about 0.66 mm yr⁻¹ between 1961 and 2003, about 36% of the observed rise.

The bed of the West Antarctic ice sheet is, in places, more than 1.5 km below sea level. Radio-echo sounding data from the Weddell Sea sector of Antarctica reveal a large subglacial basin immediately upstream of the ice sheet’s grounding line, with a steep reverse gradient and a smooth floor.

Changes in solar emissions can be amplified in atmospheric circulation patterns and lead to climate changes. Proxy data from lake sediments and long-term climate models support such a top-down mechanism of Late Holocene cooling induced by a concurrent grand solar minimum.

At subduction zones, slip along the shallowest parts of the plate boundary is generally thought to be aseismic. Observations of very-low-frequency earthquakes occurring at shallow levels on the plate boundary of the Nankai Trough subduction zone imply that slow but seismic slip can occur there, and could potentially generate tsunamigenic earthquakes.

The role of permanent versus transient crustal stress changes in triggering earthquakes is debated. Analysis of the stress imparted by the Joshua Tree and nearby Landers earthquakes in California in 1992 implies that a permanent drop in stress can halt aftershocks, so such static stress changes should be incorporated into seismic-hazard assessments.

The origin of the dichotomy between the lunar nearside and farside is unclear. Analysis of spectral reflectance data from the Kaguya lunar orbiter indicates a systematic difference in the degree of differentiation in the oldest lunar crustal terrains, linking the lunar dichotomy to crystallization of the magma ocean.

The size of the caldera formed when the surface collapses after a large volcanic eruption is thought to reflect the size of the evacuated magma chamber. Numerical modelling shows that magma stored in different parts of the chamber can be mobile or locked, so caldera size may only correspond to the volume of evacuated mobile magma.

Continental breakup and volcanism in Afar, Africa, has been linked to mantle plume activity. Seismic imaging of the mantle beneath Afar, however, identifies an increase in seismic velocities at shallow depths that is consistent with decompression melting and magmatism in the absence of strong plume activity today.

Uncertainty in the future atmospheric burden of methane—a potent greenhouse gas—represents an important challenge to the development of realistic climate projections. Airborne observations of methane suggest that the remote Arctic Ocean could prove to be a potentially important methane source.

Mid-ocean ridge transform faults experience more foreshocks than continental faults, yet the mainshock rarely ruptures the entire fault. Analysis of seismic data from the Gofar transform fault at the East Pacific Rise indicates that the foreshock region has different material properties from the mainshock region, and acts as a barrier to rupture propagation.

The transition between the weak lithosphere of the Tibetan plateau and the surrounding rigid crustal blocks has a key role in the ongoing collision between India and Asia. A reanalysis of existing magnetotelluric data suggests that crustal melt penetrates north from the Tibetan plateau beyond the Kunlun Fault, and weakens the crust beneath the Kunlun Shan.

The mass balance of Hindu-Kush–Karakoram–Himalaya glaciers has been debated, partly because of a severe lack of observations from the region. An analysis of the regional mass balance of Karakoram glaciers by comparison of digital elevation models from 1999 to 2008 reveals a small glacier mass gain in the area.

The Chilean subduction zone, where the oceanic Nazca plate subducts beneath the continental South American plates, is a very active convergent margin. Wide-angle seismic refraction and reflection data, combined with records of aftershocks following the 2007 Tocopilla earthquake, document an abrupt change in the dip of the subducting plate from less than 10° to about 22°.

In contrast to those in other regions, the North Atlantic storm track responds to anthropogenic greenhouse-gas forcing by strengthening and extending farther east. A regression analysis of an ensemble of coupled climate model simulations shows that this response is, to a large part, shaped by the interaction between ocean and atmosphere.