Volume 4 Issue 5, May 2011

Research into the biological threat of reduced ocean pH has yielded many insights over the past decade. Further progress requires a better understanding of how the interplay between ocean acidification and other anthropogenic stresses impacts marine biota.

Mitigating greenhouse gas emissions from agriculture is important and achievable. However, cutting emissions to meet the UK's legal targets for 2050 will bring technical and political challenges, and may affect food production.

Why broad fields of volcanism are found in the interior of tectonic plates is hard to explain. Spatial correlations between sheared mantle flow and volcanism suggest that differential motion between surface plates and the mantle generates upwelling and melt.

Mountain-forming systems on Earth occur at present either at the edge of continental plates or in their centre. Isotopic signatures from orogenic rocks worldwide indicate that these two distinct systems have existed for at least 550 million years.

Empirical data on mangrove carbon pools and fluxes are scarce. A field survey in the Indo-Pacific region suggests that the sediments below these remarkable trees hold exceptionally high quantities of carbon.

Estimates of sea level during the mid-Pliocene warm period three million years ago vary by 35 m. Model simulations of glacial isostatic adjustment reconcile these values and indicate little to no melting of the East Antarctic ice sheet during this time.

Carbonate rocks of Middle Ediacaran age record the largest excursion in carbon isotopic compositions in Earth history. A review of the data offers two intriguing explanations: an extraordinary perturbation of the carbon cycle, or post-depositional alteration that is global, rather than local.

The areal extent of mangrove forests has declined by 30–50% over the past half century. An analysis of mangrove forests across the Indo-Pacific suggests that mangrove deforestation generates losses of 0.02–0.12 Pg C yr⁻¹, equivalent to up to 10% of carbon emissions from global deforestation.

Monomethylmercury is a neurotoxin that accumulates in marine organisms. Incubation experiments suggest that methylation of inorganic mercury accounts for around half of the monomethylmercury present in polar marine waters.

Melting glaciers in the Patagonian Icefields are thought to have contributed 10% of the sea-level rise attributable to mountain glaciers over the past 50 years. A reconstruction of Patagonian glacier decline since ad 1650 suggests that melt rates increased markedly in the twentieth century.

The subduction of topographic features has been linked both to earthquake generation and the segmentation of faults. Seismic imaging reveals a seamount subducted to 40-km depth below Sumatra that is associated with an aseismic zone, suggesting that at this location, the seamount reduces coupling of the slab and overriding plate.

Earthquakes trigger damaging aftershocks in nearby regions, but whether they also trigger large earthquakes over significant distances has been unclear. Analysis of seismic data since 1979 suggests that earthquakes triggered more large shocks only over distances of about 1,000 km, and did not increase the seismic hazard risk on a global scale.

Volcanism observed far from plate boundaries, in the interior of oceanic and continental plates, may result from flow in the underlying mantle. Comparison between a numerical model of mantle flow and the spatial distribution of intraplate volcanism indicates that rapid shear motion in the mantle may drive melting that causes intraplate eruptions.

Submarine melting has been suggested as a trigger for the widespread acceleration of tidewater glaciers in Greenland. An analysis of oceanographic data from the fjord off Helheim Glacier, Greenland, suggests the presence of light Arctic and dense Atlantic waters in the fjord and that the melting circulation is more complex than thought.

Estimates for sea level three million years ago, a period with similar atmospheric CO₂ levels to today, vary from 10 to 40 m above present. Glacial isostatic adjustment modelling suggests that variations in the height of palaeoshorelines result from the residual adjustment of continental flexure following recent glaciations.

Two types of mountain-forming systems exist on Earth — external orogens form during oceanic–continental plate collisions and internal orogens form where continental plates collide. Hafnium isotope data from orogens worldwide show that each has produced a distinctive signature over the past 550 Myr, reflecting the contrasting subduction geometry in each setting.

Individual rock units are predicted by tectonic models to undergo numerous, complex cycles of subduction. Analysis of high-pressure rocks exposed in the Sesia zone, Italian Western Alps, suggest that slices of the crust underwent two distinct episodes of subduction to mantle depths in fewer than 20 million years.

Daniel Donato and colleagues exercised their acrobatic skills while measuring mangrove carbon stocks.