ALBERT

Description: Nature Geoscience 6, 828 (2013). doi:10.1038/ngeo1931 Authors: Shang-Ping Xie, Bo Lu & Baoqiang Xiang Spatial variations in ocean warming have been linked to regional changes in tropical cyclones, precipitation and monsoons. But development of reliable regional climate projections for climate change mitigation and adaptation remains challenging. The presence of anthropogenic aerosols, which are highly variable in space and time, is thought to induce spatial patterns of climate response that are distinct from those of well-mixed greenhouse gases. Using CMIP5 climate simulations that consider aerosols and greenhouse gases separately, we show that regional responses to changes in greenhouse gases and aerosols are similar over the ocean, as reflected in similar spatial patterns of ocean temperature and precipitation. This similarity suggests that the climate response to radiative changes is relatively insensitive to the spatial distribution of these changes. Although anthropogenic aerosols are largely confined to the Northern Hemisphere, simulations that include aerosol forcing predict decreases in temperature and westerly wind speed that reach the pristine Southern Hemisphere oceans. Over land, the climate response to aerosol forcing is more localized, but larger scale spatial patterns are also evident. We suggest that the climate responses induced by greenhouse gases and aerosols share key ocean–atmosphere feedbacks, leading to a qualitative resemblance in spatial distribution.

Description: Nature Geoscience 6, 856 (2013). doi:10.1038/ngeo1928 Authors: Chloé Michaut, Yanick Ricard, David Bercovici & R. Steve J. Sparks Eruptions at active silicic volcanoes are often cyclical. For example, at the Soufrière Hills volcano in Montserrat, Mount Pinatubo in the Philippines, and Sakurajima in Japan, episodes of intense activity alternate with repose intervals over periods between several hours and a day. Abrupt changes in eruption rates have been explained with the motion of a plug of magma that alternatively sticks or slides along the wall of the volcanic conduit. However, it is unclear how the static friction that prevents the plug from sliding is periodically overcome. Here we use two-phase flow equations to model a gas-rich, viscous magma ascending through a volcanic conduit. Our analyses indicate that magma compaction yields ascending waves comprised of low- and high-porosity bands. However, magma ascent to lower pressures also causes gas expansion. We find that the competition between magma compaction and gas expansion naturally selects pressurized gas waves with specific periods. At the surface, these waves can induce cyclical eruptive behaviour with periods between 1 and 100 hours, which compares well to the observations from Soufrière Hills, Mount Pinatubo and Sakurajima. We find that the period is insensitive to volcano structure, but increases weakly with magma viscosity. We conclude that observations of a shift to a longer eruption cycle imply an increase in magma viscosity and thereby enhanced volcanic hazard.

Description: Nature Geoscience 6, 866 (2013). doi:10.1038/ngeo1933 Authors: Suzanne M. Carbotte, Milena Marjanović, Helene Carton, John C. Mutter, Juan Pablo Canales, Mladen R. Nedimović, Shuoshuo Han & Michael R. Perfit The global mid-ocean ridge is segmented in its seafloor morphology and magmatic systems, but the origin of and relationships between this tectonic and magmatic segmentation are poorly understood. At fast-spreading ridges, tectonic segmentation is observed on a fine scale, but it is unclear whether this partitioning also occurs in the magmatic system. Fine-scale tectonic segmentation could have a deep origin, arising from the distribution of upwelling mantle melt, or a shallow origin, linked to offset intruding dikes from long, more continuous crustal reservoirs. Here we use seismic reflection data from the fast-spreading East Pacific Rise, between 8° 20′ N and 10° 10′ N, which includes a unique area where two documented volcanic eruptions have occurred, to image the crustal magma bodies in high resolution. We find that the magma reservoirs form 5- to 15-km-long segments that coincide with the fine-scale tectonic segmentation at the seafloor and that three lens segments fed the recent eruptions. Transitions in composition, volume and morphology of erupted lavas coincide with disruptions in the lens that define magmatic segments. We conclude that eruptions at the East Pacific Rise are associated with the vertical ascent of magma from lenses that are mostly physically isolated, leading to the eruption of distinct lavas at the surface that coincide with fine-scale tectonic segmentation.

Description: Nature Geoscience 6, 871 (2013). doi:10.1038/ngeo1911 Authors: Judith A. Coggon, Ambre Luguet, Geoffrey M. Nowell & Peter W. U. Appel Partial melting of the Earth’s mantle is a key process in the generation of crustal material and the formation of continents. Crustal samples record the generation of crust up to 4.4 billion years (Gyr) ago, yet the complementary record in the mantle extends to only 3.3 Gyr ago, with sparse evidence for differentiation occurring 3.9–4.1 Gyr ago. Here we use the Pt–Os isotope chronometer to show that a Hadean record of mantle depletion is preserved in Earth’s oldest known ultramafic rocks, the Ujaragssuit Nunât intrusion of southwest Greenland. We identify two distinct age populations at approximately 4.1 and 2.9 Gyr. We suggest that the younger age population records a regional metamorphic event and the older one records mantle depletion. We also identify individual sample ages of up to 4.36 Gyr old, thus extending the record of large mantle-melting events into the Hadean. Furthermore, the preservation of Hadean model ages in Os-rich mantle-derived rocks supports the theory that re-enrichment of Os in the mantle during the Late Heavy Bombardment—after expected partitioning into the Earth’s core—occurred at least 0.2 Gyr earlier than previously thought. This also implies that the Earth could have been habitable by 4.1 Gyr ago.

Description: Nature Geoscience 6, 875 (2013). doi:10.1038/ngeo1903 Authors: Jun Wu & Peter R. Buseck A significant fraction of Earth’s carbon resides in the mantle, but the mode of carbon storage presents a long-standing problem. The mantle contains fluids rich in carbon dioxide and methane, carbonate-bearing melts, carbonate minerals, graphite, diamond and carbides, as well as dissolved carbon atoms in metals. However, it is uncertain whether these can sufficiently account for the total amount of carbon thought to be stored in the mantle and the volume of carbon degassed from the mantle at volcanoes. Moreover, such carbon hosts should significantly affect the physical and chemical behaviour of the mantle, including its melting temperature, electrical conductivity and oxidation state. Here we use in situ transmission electron microscopy to measure the storage of carbon within common mantle mineral analogues—nickel-doped lanthanum chromate perovskite and titanium dioxide—in laboratory experiments at high pressure and temperature. We detect elevated carbon concentrations at defect sites in the nanocrystals, maintained at high pressures within annealed carbon nanocages. Specifically, our experiments show that small stacking faults within the mantle analogue materials are effective carbon sinks at mantle conditions, potentially providing an efficient mechanism for carbon storage in the mantle. Furthermore, this carbon can be readily released under lower pressure conditions, and may therefore help to explain carbon release in volcanic eruptions.

Description: Nature Geoscience 6, 885 (2013). doi:10.1038/ngeo1922 Authors: Lars Möller, Todd Sowers, Michael Bock, Renato Spahni, Melanie Behrens, Jochen Schmitt, Heinrich Miller & Hubertus Fischer

Description: Nature Geoscience 6, 847 (2013). doi:10.1038/ngeo1905 Authors: Marcello Campione & Gian Carlo Capitani Earthquakes generated in subduction zones are caused by unstable movements along faults. This fault-slip instability is determined by frictional forces that depend on the temperature, pressure, morphology and deformation state of the fault rocks. Fault friction may also be influenced by preferred mineral orientations. Over-thrusting of rocks at the interface between a subducting slab and the overlying mantle wedge generates shear deformation that causes minerals to align, and this preferred mineral orientation affects the propagation of shear seismic waves. Here we use laboratory experiments to simulate fault slip in antigorite, the most abundant hydrous mineral phase within Earth’s upper mantle. Using atomic force microscopy, we show that antigorite single crystals possess strong frictional anisotropy on their basal slip surface and that preferred mineral alignment extends this property to a regional scale. Depending on the alignment, fault movements can occur along a high-friction direction, creating stick-slip behaviour that generates earthquakes. In contrast, if movements occur along a low-friction direction, the mantle wedge will deform aseismically. Our results imply that mantle rocks in subduction-zone thrust faults can exhibit two opposite frictional behaviours, seismic and aseismic.

Description: Nature Geoscience 6, 801 (2013). doi:10.1038/ngeo1983 The latest report on global warming brings yet another rise in confidence that human actions are altering the Earth's climate. But in contrast to its 2007 predecessor, it is unlikely to cause a stir.

Description: Nature Geoscience 6, 802 (2013). doi:10.1038/ngeo1971 Authors: Thomas Shea, Julia Hammer & Emily First

Description: Nature Geoscience 6, 803 (2013). doi:10.1038/ngeo1970 Authors: Melissa D. Rotella, Colin J. N. Wilson, Simon J. Barker & Ian C. Wright

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1981 Author: Amy Whitchurch

Description: Nature Geoscience 6, 807 (2013). doi:10.1038/ngeo1960 Author: Julie Brigham-Grette During the Last Glacial Maximum, ice sheets in Eurasia terminated at the edge of the Laptev Sea. Seismic data now suggest that a separate ice sheet was repeatedly centred further east, in the East Siberian Sea, during previous glacial periods.

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1979 Author: Tamara Goldin

Description: Nature Geoscience 6, 811 (2013). doi:10.1038/ngeo1951 Author: W. Roger Buck Intrusions of magma into the crust help accommodate the divergence between tectonic plates. A magnetotelluric survey of the crust and mantle beneath Afar, Ethiopia, has identified enough magma to accommodate plate separation there for about 50,000 years.

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1978 Author: Anna Armstrong

Description: Nature Geoscience 6, 813 (2013). doi:10.1038/ngeo1955 Authors: Stefanie Kirschke, Philippe Bousquet, Philippe Ciais, Marielle Saunois, Josep G. Canadell, Edward J. Dlugokencky, Peter Bergamaschi, Daniel Bergmann, Donald R. Blake, Lori Bruhwiler, Philip Cameron-Smith, Simona Castaldi, Frédéric Chevallier, Liang Feng, Annemarie Fraser, Martin Heimann, Elke L. Hodson, Sander Houweling, Béatrice Josse, Paul J. Fraser, Paul B. Krummel, Jean-François Lamarque, Ray L. Langenfelds, Corinne Le Quéré, Vaishali Naik, Simon O'Doherty, Paul I. Palmer, Isabelle Pison, David Plummer, Benjamin Poulter, Ronald G. Prinn, Matt Rigby, Bruno Ringeval, Monia Santini, Martina Schmidt, Drew T. Shindell, Isobel J. Simpson, Renato Spahni, L. Paul Steele, Sarah A. Strode, Kengo Sudo, Sophie Szopa, Guido R. van der Werf, Apostolos Voulgarakis, Michiel van Weele, Ray F. Weiss, Jason E. Williams & Guang Zeng

Description: Nature Geoscience 6, 805 (2013). doi:10.1038/ngeo1980 Author: Alicia Newton

Description: Nature Geoscience 6, 809 (2013). doi:10.1038/ngeo1953 Author: Klaus Hasselmann

Description: Nature Geoscience 6, 804 (2013). doi:10.1038/ngeo1965 Author: Emily Lakdawalla

Description: Nature Geoscience 6, 833 (2013). doi:10.1038/ngeo1887 Authors: J. N. Bassis & S. Jacobs Iceberg calving has been implicated in the retreat and acceleration of glaciers and ice shelves along the margins of the Greenland and Antarctic ice sheets. Accurate projections of sea-level rise therefore require an understanding of how and why calving occurs. Unfortunately, calving is a complex process and previous models of the phenomenon have not reproduced the diverse patterns of iceberg calving observed in nature. Here we present a numerical model that simulates the disparate calving regimes observed, including the detachment of large tabular bergs from floating ice tongues, the disintegration of ice shelves and the capsizing of smaller bergs from grounded glaciers that terminate in deep water. Our model treats glacier ice as a granular material made of interacting boulders of ice that are bonded together. Simulations suggest that different calving regimes are controlled by glacier geometry, which controls the stress state within the glacier. We also find that calving is a two-stage process that requires both ice fracture and transport of detached icebergs away from the calving front. We suggest that, as a result, rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.

Description: Nature Geoscience 6, 824 (2013). doi:10.1038/ngeo1923 Authors: C. T. Adcock, E. M. Hausrath & P. M. Forster If the chemistry essential to life was present in water-containing environments on Mars, the processes that led to life on Earth may have also occurred on the red planet. Phosphate is one of the chemical nutrients thought to be essential for life and is also considered critical to reactions that may have led to life on Earth. However, low prebiotic availability of phosphate may have been a complicating factor in terrestrial abiogenesis, suggesting that a similar hurdle may have confronted the development of life on Mars. Phosphate available for biological reactions can be introduced into aqueous environments through dissolution of primary phosphate minerals during water–rock interactions, but little is known about the dissolution of the dominant phosphate minerals found in martian meteorites and presumably on Mars. Here we present dissolution rates, phosphate release rates and solubilities of phosphate minerals found in martian rocks as determined from laboratory measurements. Our experimental findings predict phosphate release rates during water–rock interactions on Mars that are as much as 45 times higher than on Earth and phosphate concentrations of early wet martian environments more than twice those of Earth. We suggest that available phosphate may have mitigated one of the hurdles to abiogenesis on Mars.

Description: Nature Geoscience 6, 837 (2013). doi:10.1038/ngeo1924 Authors: María Fernanda Sánchez Goñi, Edouard Bard, Amaelle Landais, Linda Rossignol & Francesco d’Errico A period of continental ice growth between about 80,000 and 70,000 years ago was controlled by a decrease in summer insolation, and was among the four largest ice expansions of the past 250,000 years. The moisture source for this ice sheet expansion, known as the Marine Isotope Stage (MIS) 5a/4 transition, has been proposed to be the warm subpolar and northern subtropical Atlantic Ocean. However, the mechanism by which glaciers kept growing through three suborbital cooling events within this period, which were associated with iceberg discharge in the North Atlantic and cooling over Greenland, is unclear. Here we reconstruct parallel records of sea surface and air temperatures from marine microfossil and pollen data, respectively, from two sediment cores collected within the northern subtropical gyre. The thermal gradient between the cold air and warmer sea increased throughout the MIS5a/4 transition, and was marked by three intervals of even more pronounced thermal gradients associated with the C20, C19 and C18’ cold events. We argue that the warm ocean surface along the western European margin provided a source of moisture that was transported, through northward-tracking storms, to feed ice sheets in colder Greenland, northern Europe and the Arctic.

Description: Nature Geoscience 6, 842 (2013). doi:10.1038/ngeo1904 Authors: Frank Niessen, Jong Kuk Hong, Anne Hegewald, Jens Matthiessen, Rüdiger Stein, Hyoungjun Kim, Sookwan Kim, Laura Jensen, Wilfried Jokat, Seung-Il Nam & Sung-Ho Kang During the Pleistocene glaciations, Arctic ice sheets on western Eurasia, Greenland and North America terminated at their continental margins. In contrast, the exposed continental shelves in the Beringian region of Siberia are thought to have been covered by a tundra landscape. Evidence of grounded ice on seafloor ridges and plateaux off the coast of the Beringian margin, at depths of up to 1,000 m, have generally been attributed to ice shelves or giant icebergs that spread oceanwards during glacial maxima. Here we identify marine glaciogenic landforms visible in seismic profiles and detailed bathymetric maps along the East Siberian continental margin. We interpret these features, which occur in present water depths of up to 1,200 m, as traces from grounding events of ice sheets and ice shelves. We conclude that the Siberian Shelf edge and parts of the Arctic Ocean were covered by ice sheets of about 1 km in thickness during several Pleistocene glaciations before the most recent glacial period, which must have had a significant influence on albedo and oceanic and atmospheric circulation.

Description: Nature Geoscience 6, 861 (2013). doi:10.1038/ngeo1925 Authors: M. Desissa, N. E. Johnson, K. A. Whaler, S. Hautot, S. Fisseha & G. J. K. Dawes Shallow magma reservoirs exist in the crust beneath volcanoes and mid-ocean ridges, yet there are no reports of extensive magma bodies within the uppermost mantle. Indeed the buoyancy of magma should cause it to intrude into the crust, preventing it from ponding in the mantle below. The Dabbahu magmatic segment in Afar, Ethiopia, marks the late stages of continental rifting. This segment has been active since 2005 and has experienced repeated magma intrusions. Here we use magnetotelluric data to image magma bodies beneath it. We identify a 30-km-wide region of very high electrical conductivity that reaches down to about 35 km depth. We interpret this region as a large volume of magma of at least 500 km3 that extends well into the mantle and contains about 13% melt fraction. The magma volume is orders of magnitude larger than that intruded during a typical rifting episode, implying that the magma reservoir persists for several tens of thousands of years. This is in marked contrast to the situation beneath mid-ocean ridges, where melt supply is thought to be episodic. Large magma reservoirs within the mantle may therefore be responsible for the localization of strain that accompanies the final stages of continental break-up.

Description: Nature Geoscience 6, 852 (2013). doi:10.1038/ngeo1927 Authors: Pascal Audet & Susan Y. Schwartz Subduction zones can exhibit variable seismic behaviour, ranging from great earthquakes to slow slip. This variability may be linked to fault frictional properties, and the rheology and structure of the upper plate. The subduction zone beneath the Nicoya Peninsula, Costa Rica, is characterized by strong variations in fault-slip behaviour and a lateral change in the origin of the subducting plate. In the northwest, the plate interface is locked, and experiences large, infrequent earthquakes, and the subducting plate is formed at the East Pacific Rise. In contrast, in the southeast, slow-slip events occur frequently and the subducting plate is formed at the Cocos–Nazca spreading centre. Here we use seismic receiver-function data to analyse the structure of the subduction zone beneath the Nicoya Peninsula. We find extremely high P–S seismic-velocity ratios within the entire subducting oceanic crust that we interpret as high pore-fluid pressure. Velocity ratios in the overriding continental crust, however, change from lower values in the northwest to higher ones in the southeast, indicating a disparity in fluid accumulation. We infer that this disparity is caused by a higher supply of fluid from the subducting slab in the southeast, owing to the permeability structure of oceanic crust formed at the Cocos–Nazca spreading centre. We suggest that the spatial gradient in fluid content influences upper-plate strength and controls the segmentation of seismogenic behaviour in this subduction zone.

Description: Nature Geoscience 6, 806 (2013). doi:10.1038/ngeo1929 Author: Matthew Pasek Phosphorus is an important element for biogeochemical development. According to a set of experiments, martian phosphate minerals dissolve more quickly than terrestrial ones, possibly providing nutrients in aqueous environments for early martian life.

Description: Nature Geoscience 6, 810 (2013). doi:10.1038/ngeo1967 Author: Daniel Cossa The neurotoxin methylmercury can accumulate in marine food webs, contaminating seafood. An analysis of the isotopic composition of fish in the North Pacific suggests that much of the mercury that enters the marine food web originates from low-oxygen subsurface waters.

Description: Nature Geoscience 6, 879 (2013). doi:10.1038/ngeo1918 Authors: Joel D. Blum, Brian N. Popp, Jeffrey C. Drazen, C. Anela Choy & Marcus W. Johnson

Description: Nature Geoscience 7, 860 (2014). doi:10.1038/ngeo2301 Author: Jay J. Ague Aqueous subduction-zone fluids contain CO2 and methane. New calculations indicate that these fluids also host a wide array of organic carbon species, in concentrations sufficient to influence the deep carbon cycle.

Description: Nature Geoscience 7, 925 (2014). doi:10.1038/ngeo2278 Authors: Devin McPhillips, Paul R. Bierman & Dylan H. Rood

Description: Nature Geoscience 7, 920 (2014). doi:10.1038/ngeo2284 Authors: Peter B. Reich, Sarah E. Hobbie & Tali D. Lee

Description: Nature Geoscience 7, 931 (2014). doi:10.1038/ngeo2296 Author: Colin Goldblatt, Tyler D. Robinson, Kevin J. Zahnle & David Crisp

Description: Tweets on Earth Nature Geoscience 4, 209 (2011). doi:10.1038/ngeo1137 Nature Geoscience has joined Twitter. We share our take on exciting developments in the Earth and planetary sciences as they happen.

Description: Closed ranks in oceanography Nature Geoscience 4, 211 (2011). doi:10.1038/ngeo1113 Authors: LuAnne Thompson, Renellys C. Perez & Amelia E. Shevenell

Description: Not the earthquake's fault Nature Geoscience 4, 210 (2011). doi:10.1038/ngeo1116 Authors: Brian G. McAdoo & Lisabeth Paravisini-Gebert

Description: Coal's true cost Nature Geoscience 4, 209 (2011). doi:10.1038/ngeo1136 The deaths of birds have become a rallying point against the proliferation of wind farms. Yet the loss of human life in mines is rarely linked with coal as an energy source.

Description: Our choice from the recent literature Nature Geoscience 4, 215 (2011). doi:10.1038/ngeo1131

Description: Science-based insurance Nature Geoscience 4, 213 (2011). doi:10.1038/ngeo1117 Authors: Molly E. Brown, Daniel E. Osgood & Miguel A. Carriquiry Crops are at risk in a changing climate. Farmers in the developing world will be able to insure against harvest failure if robust insurance packages, based on a geophysical index rather than individual loss, become widely available.

Description: Earth's core: A new twist on inner-core spin Nature Geoscience 4, 216 (2011). doi:10.1038/ngeo1091 Author: Mathieu Dumberry An observed hemispheric structure in the Earth's inner core has been hard to reconcile with evidence that it rotates faster than the mantle. Detection of a shift of the hemisphere boundary that occurred over geological timescales removes the contradiction.

Description: Structural geology: Natural fault lubricants Nature Geoscience 4, 217 (2011). doi:10.1038/ngeo1126 Author: Ben van der Pluijm Motion along faults can occur in sudden earthquakes or through steady, aseismic creep. Rock samples retrieved by drilling deep into a creeping section of the San Andreas Fault show that clay minerals in fault rock promote creep behaviour.

Description: Transformation of tectonic and climatic signals from source to sedimentary archive Nature Geoscience 4, 231 (2011). doi:10.1038/ngeo1087 Authors: John J. Armitage, Robert A. Duller, Alex C. Whittaker & Philip A. Allen The Earth’s sedimentary successions are an archive of past tectonic and climate events. The physical characteristics of the sediment record are controlled by three main factors: the sediment supply from the eroding source region, the grain size distribution of that sediment supply, and the area available for sediment accumulation in the downstream regions. The interplay of these factors can make the interpretation of a climatic or tectonic signal complex, particularly as these processes are nonlinear. Here we assess the evolution of a tectonically active landscape undergoing erosional and depositional processes, using a numerical model that incorporates variations in grain size and the volumetric sediment budget. Our simulations indicate that changes in precipitation and tectonic uplift both generate characteristic patterns of grain size and stratigraphic geometry. An increase in catchment precipitation results in the deposition of a laterally extensive sheet of coarse gravel. The responses to a changing tectonic regime are more diverse: a large increase in uplift rate results first in the deposition of sediments of larger grain size at proximal sites, followed by a reduction in grain size at distal locations. We conclude that the stratigraphic record is strongly controlled by the grain size of sediments released from catchments undergoing tectonic or climatic change.

Description: Early Solar System: Gathering dust Nature Geoscience 4, 219 (2011). doi:10.1038/ngeo1112 Author: Jeff Cuzzi Chondritic meteorites are remnants of the ancient Solar System. Analysis of the dust rims often found on their constituent particles shows that the rims were swept up while the particles wafted about and collided in a weakly turbulent protoplanetary nebula.

Description: Oceanography: Chemical twins, separated Nature Geoscience 4, 220 (2011). doi:10.1038/ngeo1125 Author: Martin Frank How the chemical composition of sea water changes on its journey through the world's oceans is poorly understood. Systematic measurements of dissolved trace metals across the Pacific Ocean suggest that these metals may help track sources and mixing of water masses.

Description: Weakening of the equatorial Atlantic cold tongue over the past six decades Nature Geoscience 4, 222 (2011). doi:10.1038/ngeo1078 Authors: Hiroki Tokinaga & Shang-Ping Xie Seasonal and interannual variations of the equatorial cold tongue are defining features of the tropical Atlantic Ocean, with significant climatic and biogeochemical effects. However, its long-term changes are poorly understood owing to biases in observations and climate models. Here we use a suite of bias-corrected observations, and find that cold-tongue variability has weakened during the past six decades. We find that sea surface temperature has increased across the basin, with a local enhancement over the eastern equatorial Atlantic. This warming pattern of the sea surface is most pronounced during boreal summer, reducing the annual cycle through a positive ocean–atmosphere feedback. Specifically, the eastward-intensified warming leads to enhanced atmospheric convection in the equatorial eastern Atlantic region, as well as to less vigorous trade winds. These in turn deepen the thermocline in the east, and reinforce the sea surface warming pattern. The flattened thermocline and reduced thermocline feedback weaken interannual variability of equatorial sea surface temperatures and Guinea coast precipitation associated with the Atlantic Niño. We suggest that the observed changes could be associated with cooling by anthropogenic aerosols, an effect that is stronger in the Northern than in the Southern Hemisphere. If the aerosol emissions decrease in the next decades, the tropical Atlantic may experience yet another shift as the greenhouse gas forcing increases.

Description: Strong elemental fractionation of Zr–Hf and Nb–Ta across the Pacific Ocean Nature Geoscience 4, 227 (2011). doi:10.1038/ngeo1114 Authors: M. Lutfi Firdaus, Tomoharu Minami, Kazuhiro Norisuye & Yoshiki Sohrin Understanding the circulation of water masses in the world’s oceans is critical to our knowledge of the Earth’s climate system. Trace elements and their isotopes have been explored as tracers for the movement of water masses . One type of candidate elements are the high-field-strength elements zirconium (Zr), hafnium (Hf), niobium (Nb) and tantalum (Ta). Here we measure the distributions of dissolved Zr, Hf, Nb and Ta along two meridional sections in the Pacific Ocean that extend from 65° to 10° S and from 10° to 50° N. We find that all four elements tend to be depleted in surface water. In the deep oceans, their concentrations rise along our transects from the Southern Ocean to the North Pacific Ocean, and show strong correlations with the concentration of silicate. These results indicate that terrigenous sources are important to the budget of Zr, Hf, Nb and Ta in sea water, compared with hydrothermal input. Unexpectedly, the weight ratios for Zr/Hf fall between 45 and 350 and those for Nb/Ta between 14 and 85 in Pacific sea water, higher than the ratios observed in fresh water, in the silicate Earth or in chondritic meteorites. We conclude that the fractionation of Zr/Hf and Nb/Ta ratios will be useful for tracing water masses in the ocean.

Description: Earliest rock fabric formed in the Solar System preserved in a chondrule rim Nature Geoscience 4, 244 (2011). doi:10.1038/ngeo1120 Authors: Philip A. Bland, Lauren E. Howard, David J. Prior, John Wheeler, Robert M. Hough & Kathryn A. Dyl Rock fabrics—the preferred orientation of grains—provide a window into the history of rock formation, deformation and compaction. Chondritic meteorites are among the oldest materials in the Solar System and their fabrics should record a range of processes occurring in the nebula and in asteroids. However, owing to abundant fine-grained material, chondrites have largely resisted traditional in situ fabric analysis. Here we use high-resolution electron backscatter diffraction to map the orientation of submicrometre grains in the Allende CV carbonaceous chondrite. We look at the fine-grained rims surrounding the chondrules—spherical grains cooled from molten droplets before accretion in the meteorite—as well as the matrix material between the chondrules. Although the matrix exhibits a bulk uniaxial fabric indicative of a compressive event in the parent asteroid, we find that the chondrule rims preserve a spherically symmetric fabric centred on the chondrule. We define a method to quantitatively relate fabric intensity to net compression, and reconstruct an initial rim porosity of 70–80%. Our calculations provide meteoritic evidence that the first solids formed in the Solar System accreted with high porosity, similar to modelling and laboratory estimates. We conclude that the chondrule rim textures formed in a nebula setting and may therefore represent the first rock fabric in the Solar System.

Description: The protracted development of the continent–ocean transition in Afar Nature Geoscience 4, 248 (2011). doi:10.1038/ngeo1095 Authors: Ian D. Bastow & Derek Keir Continental breakup and the transition to seafloor spreading is characterized by extensional faulting, thinning of the lithosphere and, at magmatic margins, voluminous intrusive and extrusive magmatism. It is difficult to discriminate between different mechanisms of extension and magmatism at ancient continental margins because the continent–ocean transition is buried beneath thick layers of volcanic and sedimentary rocks and the tectonic activity that characterized breakup has ceased. Instead, the timing of these mechanisms is inferred from theoretical models or from the geological record preserved at the fully developed, ancient rifted margins. Ongoing rifting in Ethiopia offers a unique opportunity to address these problems because it exposes subaerially the transition between continental rifting towards the south and seafloor spreading further northward. Here we synthesize constraints on the spatial and temporal evolution of magmatism and extension in Ethiopia. We show that although intrusion of magma maintains crustal thickness during the early stages of the continent–ocean transition, subsidence of the margin below sea level, and eruption of voluminous basalt flows, is initiated by late-stage thinning of the heavily intruded, weakened plate just before the onset of seafloor spreading. We thus conclude that faulting, stretching and magma intrusion are each important, but at different times during breakup.

Description: Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas Nature Geoscience 4, 260 (2011). doi:10.1038/ngeo1104 Authors: Christoph Helo, Marc-Antoine Longpré, Nobumichi Shimizu, David A. Clague & John Stix The abundance of volatile compounds, and particularly CO2, in the upper oceanic mantle affects the style of volcanic eruptions. At mid-ocean ridges, eruptions are generally dominated by the gentle effusion of basaltic lavas with a low volatile content. However explosive volcanism has been documented at some ocean spreading centres, indicative of abundant volatile compounds. Estimates of the initial CO2 concentration of primary magmas can be used to constrain the CO2 content of the upper oceanic mantle, but these estimates vary greatly. Here we present ion microprobe measurements of the CO2 content of basaltic melt trapped in plagioclase crystals. The crystals are derived from volcanic ash deposits erupted explosively at Axial Seamount, Juan de Fuca Ridge, in the northeast Pacific Ocean. We report unusually high CO2 concentrations of up to 9,160 ppm, which indicate that the upper oceanic mantle is more enriched in carbon than previously thought. We furthermore suggest that CO2 fluxes along mid-ocean ridges vary significantly. Our results demonstrate that elevated fluxes of CO2 from the upper oceanic mantle can drive explosive eruptions at mid-ocean ridges.

Description: Fan-delta uplift and mountain subsidence during the Haiti 2010 earthquake Nature Geoscience 4, 255 (2011). doi:10.1038/ngeo1115 Authors: Manabu Hashimoto, Yo Fukushima & Yukitoshi Fukahata The relative motion between the Caribbean and North American plates is accommodated by several active faults around Hispaniola Island. The Enriquillo–Plantain Garden fault in southern Haiti is one of these structures. Strain equivalent to a magnitude 7.2 earthquake is estimated to have accumulated along this fault since its last significant activity. The Haiti earthquake of 12 January 2010 was initially reported to have occurred along this fault, but more recent studies proposed slips on previously unrecognized, neighbouring faults. Here we use interferometric synthetic aperture radar data to show that surface deformation caused by the earthquake does not correspond to the present topography. Alluvial fan deltas were uplifted on the north side of the Enriquillo–Plantain Garden fault, whereas mountains located on the south side of the fault subsided, implying that faults other than the Enriquillo–Plantain Garden fault were responsible for the deformation. To determine fault structure, we fit the satellite surface deformation data to a fault model. We show that slip occurred on a fault dipping northward at 42°, with large thrust components. The maximum displacement on the fault was about 4 m at 10–20 km depth, offshore from the Tiburon peninsula. We confirm that the earthquake ruptured a blind thrust fault and show that the fault could not be identified from large-scale present-day topography.

Description: Pleistocene water intrusions from the Mediterranean and Caspian seas into the Black Sea Nature Geoscience 4, 236 (2011). doi:10.1038/ngeo1106 Authors: S. Badertscher, D. Fleitmann, H. Cheng, R. L. Edwards, O. M. Göktürk, A. Zumbühl, M. Leuenberger & O. Tüysüz The hydrological balance of the Black Sea is governed by riverine input and by the exchange with the Mediterranean Sea through the shallow Bosporus Strait. These sources have distinctly different oxygen isotope (δ18O) signatures. Therefore, the δ18O of Black Sea water directly reflects the presence or absence of a connection with the Mediterranean Sea, as well as hydrological changes in the vast watersheds of the Black and Caspian seas. However, the timing of late to middle Pleistocene water intrusions to the Black Sea is poorly constrained in sedimentary sequences. Here we present a stacked speleothem δ18O record from Sofular Cave in northern Turkey that tracks the isotopic signature of Black Sea surface water, and thus allows a reconstruction of the precise timing of hydrological shifts of the Black Sea. Our record, which extends discontinuously over the last 670,000 years, suggests that the connection between the Black Sea and Mediterranean Sea has been open for a significant period at least twelve times since 670,000 yr ago, more often than previously suggested. Distinct minima in the Sofular δ18O record indicate at least seven intervals when isotopically depleted freshwater from the Caspian Sea entered the Black Sea. Our data provide precisely dated evidence for a highly dynamic hydrological history of the Black Sea.

Description: Haematite pseudomicrofossils present in the 3.5-billion-year-old Apex Chert Nature Geoscience 4, 240 (2011). doi:10.1038/ngeo1084 Authors: Craig P. Marshall, Julienne R. Emry & Alison Olcott Marshall Microstructures in the ∼3.5 Gyr Apex Chert Formation were initially described as the oldest bacterial fossils on Earth over 20 years ago. However, the identification of the structures (which resemble cyanobacteria) as biological in origin remains controversial. Here we determine the petrology and geochemistry of similar structures from the original Apex Chert locality using thin sections and Raman spectroscopy. Based on the microscopic examination of thin sections, we identify features similar to those previously identified as microfossils as a series of quartz and haematite-filled fractures. Raman spectroscopy of the fractures shows that carbonaceous material is not, as previously reported, associated with the structures, but is instead disseminated in the surrounding quartz matrix. We suggest that although the microstuctures analysed are not microfossils, the presence of carbonaceous material in the surrounding matrix is consistent with the existence of microbial life at this time, and with evidence of early Archaean life found at other sites. Furthermore, we caution against identifying microstructures as biological in origin without a full morphological and geochemical assessment.

Description: Weakness of the San Andreas Fault revealed by samples from the active fault zone Nature Geoscience 4, 251 (2011). doi:10.1038/ngeo1089 Authors: B. M. Carpenter, C. Marone & D. M. Saffer Understanding the strength and slip behaviour of tectonic faults is a central problem in earthquake physics and seismic-hazard assessment. Many major faults, including the San Andreas Fault, are weak compared with the surrounding rock, but the cause of this weakness is debated. Previous measurements of the frictional strength of San Andreas Fault rocks are too high to explain the observed weakness. However, these measurements relied on samples taken at a distance from the active fault or from weathered surface samples. Recent drilling into the San Andreas Fault has provided material from the actively slipping fault at seismogenic depths. Here we present systematic measurements of the frictional properties and composition of the San Andreas Fault at 2.7 km depth, including the wall rock and active fault. We find that the fault is weak relative to the surrounding rock and that the fault rock exhibits stable sliding friction behaviour. The fault zone contains the weak mineral smectite and exhibits no frictional healing—bonds in the material do not heal after rupture. Taken together, the low inherent strength and lack of healing of the fault-zone material could explain why the San Andreas Fault slips by aseismic creep and small earthquakes in central California, rather than by large, destructive earthquakes.

Description: Core surprise Nature Geoscience 4, 268 (2011). doi:10.1038/ngeo1128 Brett Carpenter and colleagues got more than they bargained for when cleaning the mud off a metre-long piece of core from the San Andreas Fault.

Description: Reconciling the hemispherical structure of Earth’s inner core with its super-rotation Nature Geoscience 4, 264 (2011). doi:10.1038/ngeo1083 Authors: Lauren Waszek, Jessica Irving & Arwen Deuss Earth’s solid inner core grows through solidification of material from the fluid outer core onto its surface at rates of about 1 mm per year, freezing in core properties over time and generating an age–depth relation for the inner core. A hemispherical structure of the inner core is well-documented: an isotropic eastern hemisphere with fast seismic velocities contrasts with a slower, anisotropic western hemisphere. Independently, the inner core is reported to super-rotate at rates of up to 1° per year. Considering the slow growth, steady rotation rates of this magnitude would erase ’frozen-in’ regional variation and cannot coexist with hemispherical structure. Here, we exploit the age–depth relation, using the largest available PKIKP–PKiKP seismic travel time data set, to confirm hemispherical structure in the uppermost inner core, and to constrain the locations of the hemisphere boundaries. We find consistent eastward displacement of these boundaries with depth, from which we infer extremely slow steady inner core super-rotation of 0.1°–1° per million years. Our estimate of long-term super-rotation reconciles inner core rotation with hemispherical structure, two properties previously thought incompatible. It is in excellent agreement with geodynamo simulations, while not excluding the possibility that the much larger rotation rates inferred earlier correspond to fluctuations in inner core rotation on shorter timescales.

Description: Find faults and funds Nature Geoscience 4, 345 (2011). doi:10.1038/ngeo1180 Seismic risk is poorly known in many places on Earth. To save lives it is necessary — but by no means sufficient — to map the faults that pose a threat more accurately.

Description: Not just family matters Nature Geoscience 4, 346 (2011). doi:10.1038/ngeo1163 Author: Helena L. Filipsson

Description: Uncharted seismic risk Nature Geoscience 4, 348 (2011). doi:10.1038/ngeo1168 Authors: Philip England & James Jackson The potential for earthquakes along the plate boundaries has been mapped with reasonable success. Our attention should now focus on the threat posed by unanticipated quakes located in the continental interiors.

Description: Ocean science: If Gaia could talk Nature Geoscience 4, 351 (2011). doi:10.1038/ngeo1175 Author: Maurice Levasseur Sulphur aerosols contribute to the regulation of the Earth's climate. Biogenic sulphur emissions from the Southern Ocean have been underestimated, and could be further enhanced owing to climate warming, according to observations and model simulations.

Description: Our choice from the recent literature Nature Geoscience 4, 350 (2011). doi:10.1038/ngeo1178

Description: Atmospheric science: Asia under a high-level brown cloud Nature Geoscience 4, 352 (2011). doi:10.1038/ngeo1166 Author: Mark G. Lawrence Gaseous pollutants such as ozone and carbon monoxide from Asia are lifted to altitudes of more than 10 km during the summer monsoon season. Satellite observations show that aerosol particles, too, can rise high and spread across thousands of kilometres.

Description: Biogenicity of Apex Chert microstructures Nature Geoscience 4, 346 (2011). doi:10.1038/ngeo1172 Authors: J. William Schopf & Anatoliy B. Kudryavtsev

Description: Geography: Strabo's Greece Nature Geoscience 4, 355 (2011). doi:10.1038/ngeo1177 Author: Alicia Newton

Description: Planetary science: Titan's nitrogenesis Nature Geoscience 4, 356 (2011). doi:10.1038/ngeo1162 Author: Catherine Neish Observations from the Cassini–Huygens mission have produced potentially contradictory constraints on the origin of Titan's atmosphere. Experiments and a simple model demonstrate that a new mechanism for late formation is plausible.

Description: Tectonics: Unexpected Andean earthquakes Nature Geoscience 4, 357 (2011). doi:10.1038/ngeo1167 Author: Nadine McQuarrie Great earthquakes along the western, subduction zone boundary of the Andes Mountains in South America are expected. Measurements of surface motion along the eastern boundary highlight the potential for equally large earthquakes in the east.

Description: Replacement and late formation of atmospheric N2 on undifferentiated Titan by impacts Nature Geoscience 4, 359 (2011). doi:10.1038/ngeo1147 Authors: Yasuhito Sekine, Hidenori Genda, Seiji Sugita, Toshihiko Kadono & Takafumi Matsui Saturn’s moon Titan has attracted much attention because of its massive nitrogen atmosphere, but the origin of this atmosphere is largely unknown. Massive secondary atmospheres on planets and satellites usually form only after a substantial differentiation of the body’s interior and chemical reactions during accretion, yet Titan’s interior has been found to be incompletely differentiated. Here we propose that Titan’s nitrogen atmosphere formed after accretion, by the conversion from ammonia that was already present on Titan during the period of late heavy bombardment about four billion years ago. Our laser-gun experiments show that ammonia ice converts to N2 very efficiently during impacts. Numerical calculations based on our experimental results indicate that Titan would acquire sufficient N2 to sustain the current atmosphere and that most of the atmosphere present before the late heavy bombardment would have been replaced by impact-induced N2. Our scenario is capable of generating a N2-rich atmosphere with little primordial Ar on undifferentiated Titan. If this mechanism generated Titan’s atmosphere, its N2 was derived from a source in the solar nebula different from that for Earth, and the origins of N2 on Titan and Triton may be fundamentally different from the origin of N2 on Pluto.

Description: Hydrothermal vents as a kinetically stable source of iron-sulphide-bearing nanoparticles to the ocean Nature Geoscience 4, 367 (2011). doi:10.1038/ngeo1148 Authors: Mustafa Yücel, Amy Gartman, Clara S. Chan & George W. Luther Hydrothermal vents emit sulphur and metals to the ocean. Particular attention has been paid to hydrothermal fluxes of iron, a limiting micronutrient of marine primary production. Vent-derived iron was previously thought to rapidly oxidize and precipitate around vents. However, organic matter can bind to and stabilize dissolved and particulate iron in hydrothermal plumes, facilitating its dispersion into the open ocean. Here, we report measurements of the chemical speciation of sulphide and iron in high-temperature fluids emanating from vents in the East Pacific Rise and the Eastern Lau Spreading Center. We show that pyrite nanoparticles—composed of iron and sulphur—account for up to 10% of the filterable iron (less than 200 nm in size) in these fluids. We suggest that these particles form before the discharge of the vent fluid. We estimate that pyrite nanoparticles sink more slowly than larger plume particles, and are more resistant to oxidation than dissolved Fe(II) and FeS. We suggest that the discharge of iron in the form of pyrite nanoparticles increases the probability that vent-derived iron will be transported over long distances in the deep ocean.

Description: Reply to 'Not just family matters' Nature Geoscience 4, 346 (2011). doi:10.1038/ngeo1165 Authors: LuAnne Thompson, Renellys C. Perez & Amelia E. Shevenell

Description: Possible evolution of mobile animals in association with microbial mats Nature Geoscience 4, 372 (2011). doi:10.1038/ngeo1142 Authors: Murray Gingras, James W. Hagadorn, Adolf Seilacher, Stefan V. Lalonde, Ernesto Pecoits, Daniel Petrash & Kurt O. Konhauser Complex animals first evolved during the Ediacaran period, between 635 and 542 million years ago, when the oceans were just becoming fully oxygenated. In situ fossils of the mobile forms of these animals are associated with microbial sedimentary structures, and the animal’s trace fossils generally were formed parallel to the surface of the seabed, at or below the sediment–water interface. This evidence suggests the earliest mobile animals inhabited settings with high microbial populations, and may have mined microbially bound sediments for food resources. Here we report the association of mobile animals—insect larvae, oligochaetes and burrowing shore crabs—with microbial mats in a modern hypersaline lagoon in Venezuela. The lagoon is characterized by low concentrations of dissolved O2 and pervasive biomats dominated by oxygen-producing cyanobacteria, both analogous to conditions during the Ediacaran. We find that, during the day, O2 levels in the biomats are four times higher than in the overlying water column. We therefore conclude that the animals harvest both food and O2 from the biomats. In doing so, the animals produce horizontal burrows similar to those found in Ediacaran-aged rocks. We suggest that early mobile animals may have evolved in similar environments during the Ediacaran, effectively exploiting oases rich in O2 that formed within low oxygen settings.

Description: Young poorly crystalline graphite in the 〉3.8-Gyr-old Nuvvuagittuq banded iron formation Nature Geoscience 4, 376 (2011). doi:10.1038/ngeo1155 Authors: D. Papineau, B. T. De Gregorio, G. D. Cody, J. O’Neil, A. Steele, R. M. Stroud & M. L. Fogel Carbonaceous material present in ancient rocks can be used as an indicator of life during the time the rocks were formed. In particular, evidence for the existence of life more than 3,800 million years ago might come from mineral associations between apatite and graphite in rocks from southern West Greenland. However, this interpretation is partly based on the assumption that the graphite was formed at the same time as the host rocks, an assumption that has been difficult to prove. Here we investigate the origins of poorly crystalline graphite associated with apatite in metamorphosed banded iron formations from northern Canada that are 3,750 to 4,280 million years old. We measured average δ13Cgraphite values of −22.8±1.9‰ (1σ), similar to values from West Greenland sedimentary rocks of comparable age, and that point to a biological source for this carbon. Our microscopic and spectroscopic analyses suggest, however, that the graphite experienced much lower temperatures than the host rocks during metamorphism. We conclude that the poorly crystalline graphite in these rocks was deposited by fluids after peak metamorphism of the banded iron formations. We suggest that the occurrence of carbonaceous material with low δ13C values in Eoarchaean rocks cannot be used to indicate the presence of a microbial biosphere on the earliest Earth unless the syngeneity of the carbonaceous material in the host rock can be confirmed.

Description: Orogenic-wedge deformation and potential for great earthquakes in the central Andean backarc Nature Geoscience 4, 380 (2011). doi:10.1038/ngeo1143 Authors: Benjamin A. Brooks, Michael Bevis, Kelin Whipple, J Ramon Arrowsmith, James Foster, Tomas Zapata, Eric Kendrick, Estella Minaya, Arturo Echalar, Mauro Blanco, Pablo Euillades, Mario Sandoval & Robert J. Smalley Subduction of the Nazca plate beneath South America has driven the growth of the Andes Mountains. Subduction has routinely generated earthquakes larger than magnitude 8.0 along the western margin of the mountain belt, but the potential size of less frequent earthquakes in the eastern, backarc margin is unknown. Continued support of the high Andean Plateau at the centre of the Andes can be explained only if deformation of the backarc margin is ongoing. Here we present GPS data that record surface motions in the Subandean ranges that are part of the backarc margin. We find that the velocity of surface movement decreases sharply from west to east across the Subandean ranges. We suggest that a subhorizontal fault underlying the ranges slips freely at depth in the west, but is locked for up to 100 km in shallower sections further east. Analysis of fault scarps formed where the subhorizontal fault intersects the surface indicates that the fault has generated repeated large earthquakes. We suggest that rupture of the entire locked section of the fault could generate an earthquake of magnitude 8.7–8.9. We attribute the large seismic potential to the unusual width of the Subandean ranges, and suggest that deformation of the Subandean ranges, at a rate unmatched by erosion, causes the mountain range to widen.

Description: Seasonal and spatial variations of Southern Ocean diapycnal mixing from Argo profiling floats Nature Geoscience 4, 363 (2011). doi:10.1038/ngeo1156 Authors: Lixin Wu, Zhao Jing, Steve Riser & Martin Visbeck The Southern Ocean is thought to be one of the most energetic regions in the world’s oceans. As a result, it is a location of vigorous diapycnal mixing of heat, salt and biogeochemical properties. At the same time, the Southern Ocean is poorly sampled, not least because of its harsh climate and remote location. Yet the spatial and temporal variation of diapycnal diffusivity in this region plays an important part in the large-scale ocean circulation and climate. Here we use high-resolution hydrographic profiles from Argo floats in combination with the Iridium communications system to investigate diapycnal mixing in the Southern Ocean. We find that the spatial distribution of turbulent diapycnal mixing in the Southern Ocean at depths between 300 and 1,800 m is controlled by the topography, by means of its interaction with the Antarctic Circumpolar Current. The seasonal variation of this mixing can largely be attributed to the seasonal cycle of surface wind stress and is more pronounced in the upper ocean over flat topography. We suggest that additional high-resolution profiles from Argo floats will serve to advance our understanding of mixing processes in the global ocean interior.

Description: Triggered creep as a possible mechanism for delayed dynamic triggering of tremor and earthquakes Nature Geoscience 4, 384 (2011). doi:10.1038/ngeo1141 Authors: David R. Shelly, Zhigang Peng, David P. Hill & Chastity Aiken The passage of radiating seismic waves generates transient stresses in the Earth’s crust that can trigger slip on faults far away from the original earthquake source. The triggered fault slip is detectable in the form of earthquakes and seismic tremor. However, the significance of these triggered events remains controversial, in part because they often occur with some delay, long after the triggering stress has passed. Here we scrutinize the location and timing of tremor on the San Andreas fault between 2001 and 2010 in relation to distant earthquakes. We observe tremor on the San Andreas fault that is initiated by passing seismic waves, yet migrates along the fault at a much slower velocity than the radiating seismic waves. We suggest that the migrating tremor records triggered slow slip of the San Andreas fault as a propagating creep event. We find that the triggered tremor and fault creep can be initiated by distant earthquakes as small as magnitude 5.4 and can persist for several days after the seismic waves have passed. Our observations of prolonged tremor activity provide a clear example of the delayed dynamic triggering of seismic events. Fault creep has been shown to trigger earthquakes, and we therefore suggest that the dynamic triggering of prolonged fault creep could provide a mechanism for the delayed triggering of earthquakes.

Description: Winter warming in West Antarctica caused by central tropical Pacific warming Nature Geoscience 4, 398 (2011). doi:10.1038/ngeo1129 Authors: Qinghua Ding, Eric J. Steig, David S. Battisti & Marcel Küttel

Description: Saltwater surprise Nature Geoscience 4, 412 (2011). doi:10.1038/ngeo1171 Murray Gingras and colleagues examined lagoon goo for signs of life from the comfort of a moonlit rooftop in the Caribbean.

Description: Palaeontology: Breathing room for early animals Nature Geoscience 4, 354 (2011). doi:10.1038/ngeo1170 Author: Jake Bailey Animals originated in a world with marine oxygen levels only a fraction of those found in today's oceans. Observations of microbial habitats in present-day lagoons suggest that early animals could have found refuge in oxygen-producing mats.

Description: Characteristic slip for five great earthquakes along the Fuyun fault in China Nature Geoscience 4, 389 (2011). doi:10.1038/ngeo1158 Authors: Y. Klinger, M. Etchebes, P. Tapponnier & C. Narteau The seismic hazard associated with an individual fault can be assessed from the distributions of slip and recurrence times of earthquakes. However, seismic cycle models that aim to predict rupture lengths and fault displacements of successive earthquakes on one fault remain poorly validated. It is therefore unknown whether individual fault segments rupture independently, producing earthquakes with a diverse range of magnitudes and recurrence times, or slip by characteristic amounts, with characteristic magnitudes. Here we use high-resolution satellite data to document the horizontal offsets of stream channels and terraces created by strike-slip motion on the Fuyun fault, Xinjiang, China, during five historical earthquakes. We find that the  Ms 7.9 11 August 1931 earthquake produced a surface rupture with a length of 160 km, dispersed over three different fault segments. The 290 measured stream channel and terrace offsets record an average slip of 6.3 m. We use the degree of preservation of geomorphological markers to assign relative ages to individual fault offsets and identify at least four distinct older earthquakes. We find that these older earthquakes also produced fault offsets with a similar distribution to the 1931 earthquake. As the slip distributions during five successive earthquakes were so similar, we conclude that ruptures on the Fuyun fault obey a characteristic slip model.

Description: Rapid tremor reversals in Cascadia generated by a weakened plate interface Nature Geoscience 4, 404 (2011). doi:10.1038/ngeo1157 Authors: Heidi Houston, Brent G. Delbridge, Aaron G. Wech & Kenneth C. Creager

Description: Recycled gabbro signature in hotspot magmas unveiled by plume–ridge interactions Nature Geoscience 4, 393 (2011). doi:10.1038/ngeo1121 Authors: N. A. Stroncik & C. W. Devey Lavas erupted within plate interiors above upwelling mantle plumes have chemical signatures that are distinct from mid-ocean ridge lavas. When a plume interacts with a mid-ocean ridge, the compositions of both their lavas changes, but there is no consensus as to how this interaction occurs. For the past 15 Myr, the Pacific–Antarctic mid-ocean ridge has been approaching the Foundation hotspot and erupted lavas have formed seamounts. Here we analyse the noble gas isotope and trace element signature of lava samples collected from the seamounts. We find that both intraplate and on-axis lavas have noble gas isotope signatures consistent with the contribution from a primitive plume source. In contrast, near-axis lavas show no primitive noble gas isotope signatures, but are enriched in strontium and lead, indicative of subducted former oceanic lower crust melting within the plume source. We propose that, in a near-ridge setting, primitive, plume-sourced magmas formed deep in the plume are preferentially channelled to and erupted at the ridge-axis. The remaining residue continues to rise and melt, forming the near-axis seamounts. With the deep melts removed, the geochemical signature of subduction contained within the residue becomes apparent. Lavas with strontium and lead enrichments are found worldwide where plumes meet mid-ocean ridges, suggesting that subducted lower crust is an important but previously unrecognised plume component.

Description: Nature Geoscience 6, 153 (2013). doi:10.1038/ngeo1768 The upper atmosphere is not at the focus of public attention. Yet it is becoming clear that human-induced changes reach high above Earth's surface, with potential repercussions down below.

Description: Nature Geoscience 6, 154 (2013). doi:10.1038/ngeo1742 Authors: Carina Hoorn, Volker Mosbrugger, Andreas Mulch & Alexandre Antonelli

Description: Problematic plate reconstruction Nature Geoscience 5, 676 (2012). doi:10.1038/ngeo1596 Authors: Brian E. Tucholke & Jean-Claude Sibuet

Description: Tributaries through time Nature Geoscience 5, 681 (2012). doi:10.1038/ngeo1598 Author: Anna Armstrong

Description: Tsunami: Onshore shake-up Nature Geoscience 5, 682 (2012). doi:10.1038/ngeo1602 Author: Amy Whitchurch

Description: Planetary science: Mars bedevilled Nature Geoscience 5, 682 (2012). doi:10.1038/ngeo1603 Author: Tamara Goldin

Description: Biogeochemistry: Riverine carbon unravelled Nature Geoscience 5, 684 (2012). doi:10.1038/ngeo1599 Author: Anna Armstrong

Description: An update on Earth's energy balance in light of the latest global observations Nature Geoscience 5, 691 (2012). doi:10.1038/ngeo1580 Authors: Graeme L. Stephens, Juilin Li, Martin Wild, Carol Anne Clayson, Norman Loeb, Seiji Kato, Tristan L'Ecuyer, Paul W. Stackhouse, Matthew Lebsock & Timothy Andrews

Description: The state of the seas Nature Geoscience 5, 680 (2012). doi:10.1038/ngeo1592 Author: Mark Schrope

Description: An expanded role for river networks Nature Geoscience 5, 678 (2012). doi:10.1038/ngeo1593 Authors: Jonathan P. Benstead & David S. Leigh Estimates of stream and river area have relied on observations at coarse resolution. Consideration of the smallest and most dynamic streams could reveal a greater role for river networks in global biogeochemical cycling than previously thought.

Description: Biogeochemistry: Arctic nitrogen fix Nature Geoscience 5, 682 (2012). doi:10.1038/ngeo1605 Author: Anna Armstrong

Description: Climate change: Tropical extremes Nature Geoscience 5, 689 (2012). doi:10.1038/ngeo1587 Author: Geert Lenderink Climate model projections of future precipitation extremes in the tropics are highly uncertain. Observations of year-to-year variations in extremes of present-day climate help to narrow down these projections to a rise in extreme rainfall by 6–14% per °C of warming.

Description: Planetary science: Uninhabitable martian clays? Nature Geoscience 5, 683 (2012). doi:10.1038/ngeo1560 Author: Brian Hynek Clay minerals on Mars have been interpreted as an indication for a warm, wet early climate. A new hypothesis proposes that the minerals instead formed during brief periods of magmatic degassing, diminishing the prospects for signs of life in these settings.

Description: Significant contribution to climate warming from the permafrost carbon feedback Nature Geoscience 5, 719 (2012). doi:10.1038/ngeo1573 Authors: Andrew H. MacDougall, Christopher A. Avis & Andrew J. Weaver Permafrost soils contain an estimated 1,700 Pg of carbon, almost twice the present atmospheric carbon pool. As permafrost soils thaw owing to climate warming, respiration of organic matter within these soils will transfer carbon to the atmosphere, potentially leading to a positive feedback. Models in which the carbon cycle is uncoupled from the atmosphere, together with one-dimensional models, suggest that permafrost soils could release 7–138 Pg carbon by 2100 (refs , ). Here, we use a coupled global climate model to quantify the magnitude of the warming generated by the feedback between permafrost carbon release and climate. According to our simulations, permafrost soils will release between 68 and 508 Pg carbon by 2100. We show that the additional surface warming generated by the feedback between permafrost carbon and climate is independent of the pathway of anthropogenic emissions followed in the twenty-first century. We estimate that this feedback could result in an additional warming of 0.13–1.69 °C by 2300. We further show that the upper bound for the strength of the feedback is reached under the less intensive emissions pathways. We suggest that permafrost carbon release could lead to significant warming, even under less intensive emissions trajectories.

Description: Mantle flow deflected by interactions between subducted slabs and cratonic keels Nature Geoscience 5, 726 (2012). doi:10.1038/ngeo1553 Authors: Meghan S. Miller & Thorsten W. Becker Oceanic lithosphere is rapidly recycled into the mantle through subduction, an important part of the dynamic evolution of the Earth. Cratonic continental lithosphere, however, can exist for billions of years, moving coherently with the tectonic plates. At the Caribbean–South American Plate margin, a complex subduction system and continental transform fault is adjacent to the South American cratonic keel. Parallel to the transform fault plate boundary, an anomalous region of seismic anisotropy—created when minerals become aligned during mantle flow—is observed. This region of anisotropy has been attributed to stirring of the mantle by subducting slabs. Here we use seismological measurements and global geodynamic models adapted to this unique region to investigate how mantle flow, induced by subduction beneath the Antilles volcanic arc, is influenced by the stiff, deep continental craton. We find that three components—a stiff cratonic keel, a weak asthenospheric layer beneath the oceans and an accurate representation of the subducted slabs globally—are required in the models to match the unusual observed seismic anisotropy in the southeast Caribbean region. We conclude that mantle flow near the plate boundary is deflected and enhanced by the keel of the South American craton, rather than by slab stirring.

Description: Episodic tremor and slow slip potentially linked to permeability contrasts at the Moho Nature Geoscience 5, 731 (2012). doi:10.1038/ngeo1559 Authors: Ikuo Katayama, Tatsuya Terada, Keishi Okazaki & Wataru Tanikawa Slow earthquakes occur at the plate interface in subduction zones. These low-frequency tremors and slow-slip events are often located at about 30 km depth, near the boundary between the crust and mantle (Moho) on the overriding plate. Slow earthquakes occur on fault patches with extremely low frictional strength. This weakness is generally assumed to result from increased pore-fluid pressures and may be linked to the release of fluids from the descending plate. Here we propose that a contrast in permeability across the Moho results in the accumulation of water and the build-up of pore-fluid pressure at the corner of the mantle wedge that overlies the subducting plate. We use laboratory measurements of permeability to show that gabbroic rock layers in the crust are two orders of magnitude less permeable than serpentinite layers in the underlying hydrated mantle rocks. Inserting our experimental data into a numerical model that simulates pore pressure evolution across the Moho, we show that the pore-fluid pressure at this boundary can be as high as lithostatic pressure. We suggest that water released from the descending plate is trapped at the corner of the mantle wedge owing to this permeability barrier, and then causes the localized slow earthquakes that are triggered by fault instabilities.

Description: Elevation-dependent influence of snow accumulation on forest greening Nature Geoscience 5, 705 (2012). doi:10.1038/ngeo1571 Authors: Ernesto Trujillo, Noah P. Molotch, Michael L. Goulden, Anne E. Kelly & Roger C. Bales Rising temperatures and declining water availability have influenced the ecological function of mountain forests over the past half-century. For instance, warming in spring and summer and shifts towards earlier snowmelt are associated with an increase in wildfire activity and tree mortality in mountain forests in the western United States. Temperature increases are expected to continue during the twenty-first century in mountain ecosystems across the globe, with uncertain consequences. Here, we examine the influence of interannual variations in snowpack accumulation on forest greenness in the Sierra Nevada Mountains, California, between 1982 and 2006. Using observational records of snow accumulation and satellite data on vegetation greenness we show that vegetation greenness increases with snow accumulation. Indeed, we show that variations in maximum snow accumulation explain over 50% of the interannual variability in peak forest greenness across the Sierra Nevada region. The extent to which snow accumulation can explain variations in greenness varies with elevation, reaching a maximum in the water-limited mid-elevations, between 2,000 and 2,600 m. In situ measurements of carbon uptake and snow accumulation along an elevational transect in the region confirm the elevation dependence of this relationship. We suggest that mid-elevation mountain forest ecosystems could prove particularly sensitive to future increases in temperature and concurrent changes in snow accumulation and melt.

Description: Nature Geoscience 6, 93 (2013). doi:10.1038/ngeo1710 Authors: B. J. Tkalcec, G. J. Golabek & F. E. Brenker Diogenite meteorites are thought to represent mantle rocks that formed as cumulates in magma chambers on 4 Vesta or a similar differentiated asteroid. Northwest Africa 5480 is a harzburgitic diogenite, composed mainly of heterogeneously distributed olivine and orthopyroxene. Here we present a microstructural analysis of olivine grains from Northwest Africa 5480, using electron backscatter diffraction techniques to quantify any preferred orientation of crystallographic lattice. We find that the preferred orientation in the olivine-dominated zones can be explained neither by cumulate formation nor by impact reprocessing near the asteroid’s surface. Rather, they represent high-temperature solid-state plastic deformation by the pencil-glide slip system. The detected type of preferred orientation is well known from dry ultramafic rocks on Earth, where it is typically formed by mantle shear at temperatures between 1,273 and 1,523 K. Numerical modelling indicates that our observations can be explained by large-scale downwelling inside the asteroid’s mantle, within the first 50 million years after formation of calcium–aluminium-rich inclusions. The discovery of solid-state plastic deformation in an asteroidal ultramafic rock represents compelling evidence of dynamic planet-like processes in asteroids. We conclude that long-lasting enhanced mass exchange occurred in the dynamic interior of a differentiated asteroid such as Vesta, and enabled accelerated chemical, structural and thermal equilibration.

Description: Nature Geoscience 6, 125 (2013). doi:10.1038/ngeo1679 Authors: Clint Scott, Noah J. Planavsky, Chris L. Dupont, Brian Kendall, Benjamin C. Gill, Leslie J. Robbins, Kathryn F. Husband, Gail L. Arnold, Boswell A. Wing, Simon W. Poulton, Andrey Bekker, Ariel D. Anbar, Kurt O. Konhauser & Timothy W. Lyons The redox state of the oceans strongly influences the concentration of dissolved trace metals in sea water. Changes in the redox state of the oceans are thought to have limited the availability of some trace metals in the past, particularly during the Proterozoic eon, 2,500 to 542 million years ago. Of these trace metals, zinc (Zn) is of particular importance to eukaryotic organisms, because it is essential for a wide range of basic cellular functions. It has been suggested that during the Proterozoic, marine environments were broadly euxinic—that is, anoxic and sulphidic—which would have resulted in low Zn availability. Low Zn bioavailability could therefore be responsible for an observed delay in eukaryote diversification. Here we present a compilation of Zn abundance data from black shales deposited under euxinic conditions from the Precambrian time to the present. We show that these values track first-order trends in seawater Zn availability. Contrary to previous estimates, we find that Zn concentrations during the Proterozoic were similar to modern concentrations, supporting recent studies that call for limited euxinia at this time. Instead, we propose that predominantly anoxic and iron-rich deep oceans, combined with large hydrothermal fluxes of Zn, maintained high levels of dissolved Zn throughout the oceans. We thus suggest that the protracted diversification of eukaryotic Zn-binding proteins was not a result of Znbiolimitation.

Description: Nature Geoscience 6, 79 (2013). doi:10.1038/ngeo1716 Authors: P. J. Armitage, D. R. Faulkner & R. H. Worden

Description: Nature Geoscience 6, 81 (2013). doi:10.1038/ngeo1711 Author: Raymond T. Pierrehumbert The dawn of exoplanet discovery has unearthed a rich tapestry of planets different from anything encountered in the Solar System. Geoscientists can and should be in the vanguard of investigating what is out there in the Universe.

Description: Nature Geoscience 6, 77 (2013). doi:10.1038/ngeo1729 Extrasolar planet research is booming. We welcome submissions with links to the geosciences.