Nature Geoscience 1, 719 (2008). doi:10.1038/ngeo354

Climate warming is not the only consequence of rising levels of atmospheric greenhouse gases. The only way to counter all effects, including those on rainfall and ocean acidity, is to remove carbon from the climate system.

Nature Geoscience 1, 722 (2008). doi:10.1038/ngeo348

Author: Philip W. Boyd

Geo-engineering proposals for mitigating climate change continue to proliferate without being tested. It is time to select and assess the most promising ideas according to efficacy, cost, all aspects of risk and, importantly, their rate of mitigation.

Nature Geoscience 1, 732 (2008). doi:10.1038/ngeo344

Author: Simon H. Brocklehurst

The interactions between climate and tectonics in active mountain ranges are complex and important. Field and geophysical data from the St Elias Range of Alaska show that glacial erosion can influence the dynamics of the lithosphere in such settings.

Nature Geoscience 1, 727 (2008). doi:10.1038/ngeo349

Author: Timothy I. Eglinton

Riverine transport of terrestrial organic carbon to the oceans exerts an important long-term control on atmospheric carbon dioxide levels. Tropical cyclones participate in this process by delivering recently fixed carbon to the sea.

Nature Geoscience 1, 730 (2008). doi:10.1038/ngeo341

Author: Meghan S. Miller

Two chains of seamounts on the Pacific plate subduct beneath central Japan. In the process, a fragment of the Pacific slab has become wedged in the subduction zone and may be the source of recurring deep-thrust earthquakes beneath Tokyo.

Nature Geoscience 1, 728 (2008). doi:10.1038/ngeo346

Authors: Andrew Monaghan & David Bromwich

Natural climate variability and limited observational records have made identifying human-influenced climate change at the poles difficult. But a human signature is now emerging in rising Arctic and Antarctic temperatures.

Nature Geoscience 1, 767 (2008). doi:10.1038/ngeo339

Authors: William D. Bowman, Cory C. Cleveland, Ĺuboš Halada, Juraj Hreško & Jill S. Baron

Anthropogenic nitrogen deposition over the past half century has had a detrimental impact on temperate ecosystems in Europe and North America, resulting in soil acidification and a reduction in plant biodiversity. During the acidification process, soils release base cations, such as calcium and magnesium, neutralizing the increase in acidity. Once these base cations have been depleted, aluminium is released from the soils, often reaching toxic levels. Here, we present results from a nitrogen deposition experiment that suggests that a long legacy of acid deposition in the Western Tatra Mountains of Slovakia has pushed soils to a new threshold of acidification usually associated with acid mine drainage soils. We show that increases in nitrogen deposition in the region result in a depletion of both base cations and soluble aluminium, and an increase in extractable iron concentrations. In conjunction with this, we observe a nitrogen-deposition-induced reduction in the biomass of vascular plants, associated with a decrease in shoot calcium and magnesium concentrations. We suggest that this site, and potentially others in central Europe, have reached a new and potentially more toxic level of soil acidification in which aluminium release is superseded by iron release into soil water.

Nature Geoscience 1, 750 (2008). doi:10.1038/ngeo338

Authors: Nathan P. Gillett, Dáithí A. Stone, Peter A. Stott, Toru Nozawa, Alexey Yu. Karpechko, Gabriele C. Hegerl, Michael F. Wehner & Philip D. Jones

The polar regions have long been expected to warm strongly as a result of anthropogenic climate change, because of the positive feedbacks associated with melting ice and snow. Several studies have noted a rise in Arctic temperatures over recent decades, but have not formally attributed the changes to human influence, owing to sparse observations and large natural variability. Both warming and cooling trends have been observed in Antarctica, which the Intergovernmental Panel on Climate Change Fourth Assessment Report concludes is the only continent where anthropogenic temperature changes have not been detected so far, possibly as a result of insufficient observational coverage. Here we use an up-to-date gridded data set of land surface temperatures and simulations from four coupled climate models to assess the causes of the observed polar temperature changes. We find that the observed changes in Arctic and Antarctic temperatures are not consistent with internal climate variability or natural climate drivers alone, and are directly attributable to human influence. Our results demonstrate that human activities have already caused significant warming in both polar regions, with likely impacts on polar biology, indigenous communities, ice-sheet mass balance and global sea level.

Nature Geoscience 1, 759 (2008). doi:10.1038/ngeo333

Authors: Robert G. Hilton, Albert Galy, Niels Hovius, Meng-Chiang Chen, Ming-Jame Horng & Hongey Chen

The transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle. More than one third of this organic carbon flux comes from sediment-laden rivers that drain the mountains in the western Pacific region. This region is prone to tropical cyclones, but their role in sourcing and transferring vegetation and soil is not well constrained. Here we measure particulate organic carbon load and composition in the LiWu River, Taiwan, during cyclone-triggered floods. We correct for fossil particulate organic carbon using radiocarbon, and find that the concentration of particulate organic carbon from vegetation and soils is positively correlated with water discharge. Floods have been shown to carry large amounts of clastic sediment. Non-fossil particulate organic carbon transported at the same time may be buried offshore under high rates of sediment accumulation. We estimate that on decadal timescales, 77–92% of non-fossil particulate organic carbon eroded from the LiWu catchment is transported during large, cyclone-induced floods. We suggest that tropical cyclones, which affect many forested mountains within the Intertropical Convergence Zone, may provide optimum conditions for the delivery and burial of non-fossil particulate organic carbon in the ocean. This carbon transfer is moderated by the frequency, intensity and duration of tropical cyclones.

Nature Geoscience 1, 763 (2008). doi:10.1038/ngeo331

Authors: Takeshi Ise, Allison L. Dunn, Steven C. Wofsy & Paul R. Moorcroft

Historically, northern peatlands have functioned as a carbon sink, sequestering large amounts of soil organic carbon, mainly due to low decomposition in cold, largely waterlogged soils. The water table, an essential determinant of soil-organic-carbon dynamics, interacts with soil organic carbon. Because of the high water-holding capacity of peat and its low hydraulic conductivity, accumulation of soil organic carbon raises the water table, which lowers decomposition rates of soil organic carbon in a positive feedback loop. This two-way interaction between hydrology and biogeochemistry has been noted, but is not reproduced in process-based simulations. Here we present simulations with a coupled physical–biogeochemical soil model with peat depths that are continuously updated from the dynamic balance of soil organic carbon. Our model reproduces dynamics of shallow and deep peatlands in northern Manitoba, Canada, on both short and longer timescales. We find that the feedback between the water table and peat depth increases the sensitivity of peat decomposition to temperature, and intensifies the loss of soil organic carbon in a changing climate. In our long-term simulation, an experimental warming of 4 ∘C causes a 40% loss of soil organic carbon from the shallow peat and 86% from the deep peat. We conclude that peatlands will quickly respond to the expected warming in this century by losing labile soil organic carbon during dry periods.

Nature Geoscience 1, 755 (2008). doi:10.1038/ngeo340

Authors: J. A. MacKinnon, T. M. S. Johnston & R. Pinkel

The Indian Ocean harbours an important but poorly understood part of the global meridional ocean overturning circulation, which transports heat to high latitudes. Understanding heat exchange in the Indian Ocean requires knowledge of the magnitudes and locations of both meridional deep-water transport and mixing, but in particular the latter is poorly constrained at present. Here we present detailed measurements of transport and mixing in the Atlantis II fracture zone in the Southwest Indian Ridge, one of the main conduits for equatorward-flowing deep water. We observe a northward jet of deep and bottom water extending 1,000 m vertically with a transport rate of 3×106 m3 s−1. Turbulent diffusivity within the jet was up to two orders of magnitude above typical deep ocean levels in our measurements. Our results quantify the flow through this narrow fracture zone to 20 to 30% of the total meridional overturning circulation in the Indian Ocean, and provide an example of elevated turbulence in a deep sheared flow that is not hydraulically controlled, in contrast to many other fracture zones.

Nature Geoscience 1, 745 (2008). doi:10.1038/ngeo332

Authors: D. J. McCleese, J. T. Schofield, F. W. Taylor, W. A. Abdou, O. Aharonson, D. Banfield, S. B. Calcutt, N. G. Heavens, P. G. J. Irwin, D. M. Kass, A. Kleinböhl, W. G. Lawson, C. B. Leovy, S. R. Lewis, D. A. Paige, P. L. Read, M. I. Richardson, N. Teanby & R. W. Zurek

Current understanding of weather, climate and global atmospheric circulation on Mars is incomplete, in particular at altitudes above about 30 km. General circulation models for Mars are similar to those developed for weather and climate forecasting on Earth and require more martian observations to allow testing and model improvements. However, the available measurements of martian atmospheric temperatures, winds, water vapour and airborne dust are generally restricted to the region close to the surface and lack the vertical resolution and global coverage that is necessary to shed light on the dynamics of Mars’ middle atmosphere at altitudes between 30 and 80 km (ref. 7). Here we report high-resolution observations from the Mars Climate Sounder instrument on the Mars Reconnaissance Orbiter. These observations show an intense warming of the middle atmosphere over the south polar region in winter that is at least 10–20 K warmer than predicted by current model simulations. To explain this finding, we suggest that the atmospheric downwelling circulation over the pole, which is part of the equator-to-pole Hadley circulation, may be as much as 50% more vigorous than expected, with consequences for the cycles of water, dust and CO2 that regulate the present-day climate on Mars.

Nature Geoscience 1, 777 (2008). doi:10.1038/ngeo336

Authors: Satish C. Singh, Hélène Carton, Paul Tapponnier, Nugroho D. Hananto, Ajay P. S. Chauhan, Djoko Hartoyo, Martin Bayly, Soelistijani Moeljopranoto, Tim Bunting, Phil Christie, Hasbi Lubis & James Martin

The great Sumatra earthquake of 26 December 2004 was the third largest event to occur in a subduction zone in the past 50 years. The rupture initiated at 30–40 km depth northwest of Simeulue Island and propagated for ∼1,300 km to the northern Andaman Islands. The earthquake was caused by sudden slip along the plate interface between the subducting Indo-Australian plate and the overriding Sunda plate. Although detailed knowledge of the structure of the subduction interface is important to define potential sources of large megathrust earthquakes, available data have not provided such information so far. Here we present a high-quality seismic section of the focal region, from the abyssal plain down to 40 km depth below the fore-arc. The seismic data reveal that the subducting crust and oceanic Moho—the crust–mantle boundary—are broken and displaced by landward-dipping thrust ramps, suggesting that the megathrust now lies in the oceanic mantle. We image active thrust faults at the front of the accretionary wedge, consistent with thrust aftershocks on steeply dipping planes. Our observations imply that very strong coupling leading to brittle failure of mantle rocks accounts for the initiation of such an exceptionally large earthquake.

Nature Geoscience 1, 771 (2008). doi:10.1038/ngeo318

Authors: Shinji Toda, Ross S. Stein, Stephen H. Kirby & Serkan B. Bozkurt

The Philippine Sea and Pacific plate slabs both subduct beneath Tokyo, and so their configuration and seismic potential have been subject to intensive study. Previous work suggests that the Philippine Sea slab extends up to 100 km northwest of Tokyo and subducts to a depth of 90 km beneath the Kanto basin, where it is folded against the underlying Pacific slab. Here we evaluate seismic data in three dimensions and delineate a distinct 25-km-thick and 100-km-wide body beneath the Kanto basin that has hitherto been considered to be part of the Philippine Sea slab. We find that several of its characteristics—such as its high seismic velocity and the presence of a double seismic zone—point instead to an affinity with the Pacific slab, implying that the Philippine Sea slab penetrates to depths of only 35–40 km beneath the Kanto basin. We propose that the body is a fragment of Pacific lithosphere that formed by the collision of two intersecting seamount chains with the Japan Trench 2–3 million years ago. We suggest that such slab fragments may not be uncommon, especially where seamount subduction deforms trenches and near triple junctions.

Nature Geoscience 1, 782 (2008). doi:10.1038/ngeo335

Authors: Iuliana Vasiliev, Christine Franke, Johannes D. Meeldijk, Mark J. Dekkers, Cor G. Langereis & Wout Krijgsman

Magnetotactic bacteria produce chains of magnetite and/or greigite crystals within their cell bodies called magnetosomes that are permanently magnetized. They use these magnets to navigate along geomagnetic field lines to reach their preferred habitat. Greigite magnetosomes have been well documented in modern sedimentary environments, but their identification in the fossil record remains controversial. Here we use transmission electron microscopy, electron diffraction patterns and rockmagnetic analyses to assess the origins of nanometre-scale greigite crystals found in Pliocene claystones from the Carpathian foredeep of Romania. We find that, like modern magnetosomal greigite grains, the crystals are single domain, with few crystallographic defects and an overall shape consistent with an intracellular origin. We suggest these crystals are magnetosomal in origin, which would place them among the oldest greigite magnetofossils identified so far. The crystals also carry a primary magnetic signal, which has remained stable since its acquisition 5.3–2.6 million years ago. We suggest that greigite magnetofossils could therefore provide reliable records of ancient geomagnetic field variations, and that they could also be used as a proxy to assess palaeoenvironmental conditions in low-oxygen sedimentary environments.

Nature Geoscience 1, 793 (2008). doi:10.1038/ngeo334

Authors: Aaron L. Berger, Sean P. S. Gulick, James A. Spotila, Phaedra Upton, John M. Jaeger, James B. Chapman, Lindsay A. Worthington, Terry L. Pavlis, Kenneth D. Ridgway, Bryce A. Willems & Ryan J. McAleer

Nature Geoscience 1, 787 (2008). doi:10.1038/ngeo311

Authors: Peter Huybers & George Denton

Nature Geoscience 1, E18 (2008). doi:10.1038/ngeo345

William Bowman and colleagues braved beverages of pig fat and vodka in their attempt to understand the impact of long-term nitrogen deposition on Slovakian soils.

Nature Geoscience 1, 802 (2008). doi:10.1038/ngeo343

Aaron Berger and colleagues leapt out of helicopters in the snow and fog in their quest to understand the effects of glacial erosion on mountain formation.