Volume 4 Issue 12, December 2011

The ocean floor is littered with hundreds of thousands of mostly extinct volcanoes. The origin of at least some of these seamounts seems to rest with mantle plumes.

Inland waters are increasingly recognized as important to the global carbon cycle. Detailed measurements in the United States suggest that significant amounts of carbon dioxide are released from streams and rivers, particularly the smaller ones.

The mechanisms for forming the abundant volcanic islands on ocean floors are debated. The geochemical signature of volcanic rocks from the northeast Indian Ocean suggests that seamounts there formed from melting recycled ancient continental rocks.

A series of extreme cooling episodes, starting 750 million years ago, could have repeatedly turned the planet into an ice-covered snowball. Carbon cycle modelling suggests that the timing of the glaciations can be explained by chemical weathering rates.

The influence of aerosols on climate is one of the greatest uncertainties in projections of future climate. A long-term observational study suggests that aerosols increase the frequency of rainfall events in convective clouds.

The ancient Central Asian Orogenic Belt formed during a period of extensive continental crust formation. Comparison with modern continent-building systems suggests that the processes that operate today were already active at that time.

Volcanoes formed above the Hawaiian mantle plume exhibit a striking contrast in the geochemical characteristics of the lavas erupted at the northern Kea compared with the southern Loa volcanoes. Isotopic data show that these trends have persisted for more than 5 million years and may mirror compositional heterogeneities in the deep mantle.

Current estimates of carbon dioxide evasion from inland waters are based on incomplete spatial coverage. Streams and rivers in the United States release 97 Tg of carbon to the atmosphere each year, according to an analysis of chemical and morphological data.

The Ganges–Brahmaputra drainage basin represents one of the largest sources of terrestrial biospheric carbon to the ocean. Radiocarbon analyses suggest that 20% of the carbon exported from this system has an average age of more than 15,000 years.

Gas hydrates have been suggested as a carbon source for Palaeogene hyperthermal events, but warm seafloor temperatures are thought to have limited their accumulation. Numerical simulations suggest that enhanced organic carbon sedimentation and methanogenesis could have compensated for the smaller area of hydrate stability.

The role of the North American seaway in creating and maintaining Late Cretaceous global greenhouse conditions has been unclear. An isotopic analysis of marine turtle and fish fossils from western Kansas and the Mississippi embayment reveals that the inflow of Tethyan surface waters to the seaway was limited.

Two competing models have been suggested to explain the recovery of ecosystems from mass extinctions. An analysis of the recovery of marine pelagic communities from the Cretaceous–Palaeogene mass extinction supports a model of contingent recovery, rather than one based on trophic structure.

The Neoproterozoic Snowball Earth glaciations were separated by tens of millions of years, although models suggest glacial inception should occur within millions. Numerical modelling suggests that the delay could be explained by inherent limits on silicate weathering rates controlled by the availability of fresh rock.

The origin of the Bushveld Complex, South Africa, is elusive. Identification and dating of a radial swarm of volcanic dykes suggests that a mantle plume affected the region, but was active 600 million years before the Bushveld Complex formed, implying that the Bushveld magmas could be an indirect consequence of the plume.

At the southern boundary between India and Tibet, the Indian tectonic plate subducts northwards beneath the Tibetan Plateau. Seismic imaging shows that at the northern boundary of Tibet, the Asian plate is also subducting southwards beneath the plateau, and the Tibetan lithosphere is separate.

Lavas erupted from volcanoes in the north of the Hawaiian volcanic island chain have a different geochemical signature from those in the south. Analysis of the geochemistry of lavas erupted in the Samoan and Marquesas volcanic chains reveals similar trends, implying that the lowermost mantle beneath the southern Pacific is isotopically enriched.

Seismic data have identified large-scale compositional heterogeneities in Earth’s deep mantle, but their origin is debated. Numerical modelling demonstrates that seismological and geochemical constraints on the character of these heterogeneities can be satisfied if they are composed of primitive material formed early in Earth’s history.

The origin of the Christmas Island Seamount Province in the northeast Indian Ocean is enigmatic. Radiometric dating and isotopic analysis of volcanic rocks from the seamounts imply that they were derived from the recycling of continental lithosphere and brought to the surface at a mid-ocean ridge.

Atmospheric aerosols can both suppress and foster the development of clouds and precipitation, depending on meteorological conditions. Ten years of observations, together with model simulations, suggest that aerosols stimulate the vertical development of warm-base mixed-phase clouds.

Peatlands store vast amounts of organic carbon, owing to anoxic conditions, which prevent decay. Laboratory and field experiments suggest that drought-induced increases in oxygen stimulate microbial growth and the breakdown of peatland carbon.

Volcanic eruptions in the middle of tectonic plates, far from any volcanically-active plate boundaries, have created many of the thousands of ocean islands and seamounts that cover Earth's ocean floors. The geochemistry of their lavas may provide a window into processes occurring deep inside our planet. In this issue we collect research articles and opinion pieces that explore some possible mechanisms for the creation of ocean islands and seamounts, and highlight their connections to the deep and upper mantle.