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January 2010

Volcanoes implicated in ancient ocean oxygen depletion

The onset of a widespread absence of oxygen in the world's deep oceans 94.5 million years ago may have been triggered by sulphur emitted from volcanoes, according to a report published online this week in Nature Geoscience.

Using marine sediments deposited during this event — known as Ocean Anoxic Event 2 — Matthew Hurtgen and colleagues found that unusually low background sulphate levels increased dramatically at the start of the event. Sulphate is a key ingredient for the recycling of organic matter, allowing the bodies of sinking plankton to decompose. The team suggests that once more sulphate was available, it triggered a chain reaction: the nutrients released by the decomposing organisms allowed more and more organisms to grow at the surface. As their bodies sank, they too were decomposed — a process that rapidly consumes oxygen. The chain only stopped once widespread oxygen depletion allowed the sulphur mineral pyrite to develop, consuming the excess sulphate.

The sulphate was most likely supplied by the heightened volcanic activity that is reported to have occurred at this time.

Volcanic triggering of a biogeochemical cascade during Oceanic Anoxic Event 2

Derek D. Adams, Matthew T. Hurtgen and Bradley B. Sageman

Published online: 31 January 2010 | doi 10.1038/ngeo746

Jupiter's moons diverged through bombardment

Differences in the energy delivered by impacts to Jupiter's large moons Callisto and Ganymede during the period of late heavy bombardment, about 3,800 to 4,100 million years ago, can explain their divergent characteristics, according to a study published online this week in Nature Geoscience. The findings explain why Ganymede has a large core of rock and metal, whereas the separation of ice and rock within Callisto is incomplete, even though the moons are very similar in size and composition.

Amy Barr and Robin Canup developed a model of melting and core formation in the presence of planetary impacts. They found that, if sufficient energy is released during a series of impacts, the process of ice—rock separation and core formation can become self-sustaining and will drive itself to completion. Ganymede experienced such self-sustaining separation, they suggest, whereas Callisto did not, because Jupiter's gravity field directed more impact energy towards Ganymede.

Origin of the Ganymede—Callisto dichotomy by impacts during the late heavy bombardment

Amy C. Barr and Robin M. Canup

Published online: 24 January 2010 | doi 10.1038/ngeo746

Long-term contamination in Alaska

Oil spilt from the tanker Exxon Valdez in 1989 can still be found on the Alaskan coast because of the two-layered structure of local beaches, according to a study online this week in Nature Geoscience. As oil exploitation and shipping in the Arctic region becomes more feasible owing to global warming, effective environmental protection and clean-up of spilt oil will become increasingly important.

Michel Boufadel and Hailong Li investigated the groundwater dynamics of a beach on Eleanor Island, Alaska, which was contaminated by the Exxon Valdez oil spill, using field measurements, tracers and numerical simulations. They found that the upper layer of the beach acted as a reservoir for the spilt oil, protecting it from weathering and loss of fluidity. From this reservoir, the oil entered the lower layer whenever the water level fell below the interface between the two layers. Because of the low oxygen content in the lower layer, the oil is not degraded and can persist in the long term.

Long-term persistence of oil from the Exxon Valdez spill in two-layer beaches

Hailong Li and Michel C. Boufadel

Published online: 17 January 2010 | doi 10.1038/ngeo749

Indonesian earthquake risk

The September 2009 earthquake in Indonesia did not significantly relax the threat of a high-magnitude tsunami-generating earthquake in the region, reports a study online this week in Nature Geoscience. Stress on this segment of the fault has been accumulating for about 200 years, suggesting the need for urgent action.

John McCloskey and colleagues investigated the stress released during the September 2009 earthquake of magnitude 7.6 near the Indonesian city of Padang, which is located on Sumatra near Siberut. They found that the quake did not rupture the section of the Sunda megathrust along the coast of Sumatra that had remained unbroken by a series of earthquakes after the 2004 quake and tsunami. The danger of strong shaking and the potential for tsunami generation as forecast earlier therefore is therefore still immanent.

The September 2009 Padang earthquake

John McCloskey, Dietrich Lange, Frederik Tilmann, Suleyman S. Nalbant, Andrew F. Bell, Danny Hilman Natawidjaja and Andreas Rietbrock

Published online: 17 January 2010 | doi 10.1038/ngeo753

India pulled towards Asia

The ongoing movement of India towards Central Asia could be driven by a downward pull from the Indian continental plate, according to a study published online this week in Nature Geoscience. This work provides a different interpretation of the forces that governed the collision of India and Asia between about 55 and 30 million years ago and subsequent convergence.

Fabio Capitanio and colleagues used a numerical model to explore the balance of forces during the collision of the Asian and Indian continental plates, probably the most spectacular convergence of continental plates on Earth. They find that once its upper crust was scraped off in the collision zone, the Indian plate was denser than the underlying mantle, and would have continued to drag India towards Asia. The authors therefore suggest that the collision of the two continental plates that generated the Himalayas did not stop plate movement because the bulk of the Indian plate was sufficiently dense to sink into the underlying mantle.

In an accompanying News and Views article, Dietmar Müller says "Capitanio and colleagues have provided a fresh view to the long-standing problem of understanding the sequence of events before and after the collision of India and Asia".

India—Asia convergence driven by the subduction of the Greater Indian continent

F. A. Capitanio, G. Morra, S. Goes, R. F.Weinberg and L. Moresi

Published online: 10 January 2009 | doi 10.1038/ngeo725

Heat flow on Enceladus

The large jets of water vapour that are observed only in the southern polar region of Saturn's moon Enceladus can be explained as a manifestation of a waning event of catastrophic renewal of the moon's crust. Research, published online this week in Nature Geoscience, suggests that such episodes of strong convection could also have played a part in shaping the surfaces of other icy satellites in the Solar System.

Craig O'Neill and Francis Nimmo simulated convection of Enceladus's ice mantle. They find that occasional episodes during which parts of Enceladus's ice mantle are recycled into the moon's interior could explain the present activity and heat loss in the body's South Polar Region, as well as the heavily deformed surface observed on Enceladus. They estimate that catastrophic convection events occur every 100 to 1,000 million years and last about 10 million years.

The authors conclude that we are currently observing Enceladus in one of its rare phases of resurfacing that make up only about 1–10% of the time.

The role of episodic overturn in generating the surface geology and heat flow on Enceladus

Craig O'Neill and Francis Nimmo

Published online: 10 January 2010 | doi 10.1038/ngeo731


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