Articles in 2013

Volcanic eruptions are often preceded by long-period seismic events that were thought to be generated by the resonance of cracks filled with magmatic fluid. Analysis and modelling of long-period seismicity at volcanoes in Italy, Costa Rica and Peru shows that it could instead be caused by slow rupture along faults in the upper volcanic edifice.

Dark streaks that appear on the surface of Mars during warm seasons have been observed at the mid-latitudes and tentatively attributed to the flow of briny water. Imagery from the Mars Reconnaissance Orbiter over multiple Mars years suggests that these seasonally active features are also present in equatorial regions, where liquid surface water is not expected.

Ancient valleys suggest a warm early Mars where liquid water flowed, but a greenhouse effect strong enough to offset a dim early Sun has been difficult to explain. Climate simulations suggest that sufficient concentrations of the greenhouse gases CO₂ and H₂ — outgassed during volcanic eruptions — could have warmed Mars above water’s freezing point.

Vast quantities of carbon are stored in shallow Arctic reservoirs, such as subsea and terrestrial permafrost. Observations in the Laptev Sea suggest that bubbles deliver significant quantities of the methane stored in subsea permafrost to the overlying water column.

The causal connection between human activities and the evolution of climate warming over the past century is not fully understood. A state-of-the-art statistical analysis of time series of temperature and radiative forcing reveals that reductions in ozone-depleting substances and methane have contributed to the slow-down in warming since the late 1990s.

The predictability of heat waves in the mid-latitudes has been limited to the 10-day range of weather forecasts. An integration of a climate model that spans 12,000 years reveals a pattern in atmospheric planetary waves that tends to precede heat waves in the US, extending potential predictability to 20 days.

Glacial Termination II was marked by a rise in atmospheric CO₂ concentrations and global temperature. An analysis of air bubbles from an Antarctic ice core suggests that during the first phase of deglaciation, Antarctic temperature and atmospheric CO₂ concentrations increased together, whereas CO₂ lagged behind temperature rise during the second phase.

Some mantle plumes are enriched in ³He, but the source of this primordial isotope is unclear. The partitioning behaviour of helium between silicate and iron melts—as determined by experiments—suggests that sufficient helium may have been incorporated into the core when the Earth differentiated to explain the anomalous leakage at the Earth’s surface.

Comets harbour the organic precursors of amino acids. High-velocity impact experiments into icy targets suggest that impacts involving icy planetary bodies could be a viable pathway to synthesize the complex organic compounds needed for life.

The structure of oceanic plateaux is unclear, as they are remote and submerged beneath the seas. Seismic images of the Tamu Massif, part of the Shatsky Rise oceanic plateau in the northwestern Pacific Ocean, show that it is a single immense volcano, potentially the largest on Earth.

Mercury enters marine food webs in the form of microbially generated monomethylmercury. An analysis of the mercury isotopic composition of nine species of North Pacific fish suggests that microbial production of monomethylmercury below the surface mixed layer contributes significantly to the mercury contamination of marine food webs.

Atmospheric methane concentrations varied substantially over the last glacial cycle. An analysis of the δ¹³C of the methane suggests that the relative strength of various methane sources and sinks varied independently of overall changes in methane concentrations.

Sea level during the last interglacial period reached a peak of between 5 and 9 m above the present-day level. A detailed reconstruction of sea level and isostatic rebound from Western Australia indicates a prolonged period of sea-level stability at 3–4 m above present, followed by an abrupt sea-level rise of 5–6 m.

As a moist atmosphere warms, it will reach a limit after which it is unable to radiate incoming solar radiation back to space, and a runaway greenhouse will occur. Calculations suggest that this limit is lower than previously thought and, for a water-saturated atmosphere, a runaway greenhouse can occur under present-day solar radiation.

Ocean Anoxic Event 2 was marked by rapid global warming and loss of O₂ from the ocean. Lithium isotope data suggest that the warming was accompanied by enhanced silicate weathering, which stimulated marine productivity and helped stabilize atmospheric CO₂ levels.

Marine cyanobacteria supply much of the nitrogen that supports open ocean food webs and biogeochemical cycles. An experimental study suggests that the relationship between nitrogen fixation and carbon dioxide concentration varies significantly between cyanobacterial strains.

The marine nitrogen cycle was altered during the transition from glacial to interglacial conditions. An analysis of δ¹⁵N records throughout the world’s oceans suggests that rates of denitrification in the water column accelerated during the last deglaciation.

Hydrogen is commonly produced during the high-temperature hydration of mafic and ultramafic rocks. Laboratory experiments suggest that water–rock reactions also generate hydrogen at lower temperatures, potentially fuelling microbial life in ultramafic aquifers in oceanic and terrestrial crust.

The Indo-Pacific warm pool is the largest source of heat and moisture vapour to the atmosphere. Proxy reconstructions and model simulations suggest that during the Last Glacial Maximum, the exposure of the Sunda Shelf of Southeast Asia weakened deep convection over the warm pool.

Earth’s inner core rotates at a different rate than the mantle, and discrepancies exist between rotation rates derived from geophysical observations and geodynamical simulations. An inverse analysis of seismic data from repeating earthquakes over the past 50 years suggests that the rotation rate of the inner core fluctuates on decadal timescales.