ALBERT

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: Biogeochemistry: Riverine carbon dioxide release Nature Geoscience 4, 821 (2011). doi:10.1038/ngeo1333 Author: John Melack Inland waters are increasingly recognized as important to the global carbon cycle. Detailed measurements in the United States suggest that significant amounts of carbon dioxide are released from streams and rivers, particularly the smaller ones.

Description: Geodynamics: Christmas recycling Nature Geoscience 4, 823 (2011). doi:10.1038/ngeo1337 Author: Sally A. Gibson The mechanisms for forming the abundant volcanic islands on ocean floors are debated. The geochemical signature of volcanic rocks from the northeast Indian Ocean suggests that seamounts there formed from melting recycled ancient continental rocks.

Description: Atmospheric science: Aerosols, clouds and climate Nature Geoscience 4, 826 (2011). doi:10.1038/ngeo1340 Author: Seoung-Soo Lee The influence of aerosols on climate is one of the greatest uncertainties in projections of future climate. A long-term observational study suggests that aerosols increase the frequency of rainfall events in convective clouds.

Description: Protracted storage of biospheric carbon in the Ganges–Brahmaputra basin Nature Geoscience 4, 843 (2011). doi:10.1038/ngeo1293 Authors: Valier Galy & Timothy Eglinton The amount of carbon stored in continental reservoirs such as soils, sediments and the biosphere greatly exceeds the amount of carbon in the atmosphere. As such, small variations in the residence time of organic carbon in these reservoirs can produce large variations in the atmospheric inventory of carbon dioxide. One such reservoir is the Ganges–Brahmaputra system draining the Himalayas, which represents one of the largest sources of terrestrial biospheric carbon to the ocean. Here, we examine the radiocarbon content of river sediments collected from the Ganges–Brahmaputra drainage basin to determine the residence time of organic carbon in this reservoir. We show that the average age of biospheric organic carbon in the drainage basin ranges from 0.5 to 17 thousand years. The radiocarbon age of plant-derived fatty acids—a proxy for labile terrestrial vegetation—ranges from just 0.05 to 1.3 thousand years. We propose that the bulk ages can be explained by the existence of a refractory, slowly cycling component of the organic carbon pool that is mixed with a younger labile pool. We estimate that this refractory component has an average age of over 15,000 years, and represents up to 20% of total biospheric carbon exported by the Ganges–Brahmaputra system. We suggest that global warming might destabilize this ancient pool of carbon, if warming stimulates microbial decomposition of organic carbon reserves.

Description: Significant efflux of carbon dioxide from streams and rivers in the United States Nature Geoscience 4, 839 (2011). doi:10.1038/ngeo1294 Authors: David Butman & Peter A. Raymond The evasion of carbon dioxide from inland waters was only recently included in assessments of the global carbon budget. Present estimates of carbon dioxide release from global freshwater systems, including lakes and wetlands, range from 0.7 to 3.3 Pg C yr−1 (refs , , , , ). However, these estimates are based on incomplete spatial coverage of carbon dioxide evasion, and an inadequate understanding of the factors controlling the efflux of carbon dioxide across large drainage networks. Here, we estimate the amount of carbon degassed from streams and rivers in the United States using measurements of temperature, alkalinity and pH, together with high-resolution data on the morphology and surface area of these waterways. We show that streams and rivers in the US are supersaturated with carbon dioxide when compared with the atmosphere, emitting 97±32 Tg carbon each year. We further show that regionally, carbon dioxide evasion from streams and rivers is positively correlated with annual precipitation, which we attribute to climatic regulation of stream surface area, and the flushing of carbon dioxide from soils. Scaling our analysis from the US to temperate rivers between 25° N and 50° N, we estimate a release of around 0.5 Pg carbon to the atmosphere each year.

Description: A role for chance in marine recovery from the end-Cretaceous extinction Nature Geoscience 4, 856 (2011). doi:10.1038/ngeo1302 Authors: Pincelli M. Hull, Richard D. Norris, Timothy J. Bralower & Jonathan D. Schueth Two contrasting ecological models have been proposed for recovery from mass extinctions. The first posits that evolutionary recoveries are structured by trophic interactions alone, resulting in a predictable recovery of species richness and abundance earlier in lower trophic levels. The second, the contingent model, holds that both chance and ecology are key to the structure of recoveries, thus precluding inherent predictability. Documented recovery patterns from the Cretaceous–Palaeogene mass extinction could support either model, as most previous studies have lacked the high-resolution records needed to discriminate between the scenarios. Here we use high-resolution marine sediment records to reconstruct pelagic community structure during the Palaeogene recovery in three sites in the South Atlantic and North Pacific Ocean. We document heterogeneity in the timing of recovery between sites from the alternative community structures characteristic in early pelagic ecosystems. We show that the evolution of species richness and abundance is decoupled between two well-represented groups of phytoplankton and zooplankton, as well as between taxa within a single trophic level. Our results favour the contingent recovery model. We suggest that ecological and environmental mechanisms may account for any similarities in community structure among sites and for the eventual transition from early recovery to late recovery communities, whereas chance may explain intersite differences in the timing and recovery path.

Description: Forty years of plume research Nature Geoscience 4, 813 (2011). doi:10.1038/ngeo1348 Mantle dynamics drive the formation of ocean islands and seamounts in the interior of oceanic plates. Yet the mechanisms for generating these volcanic edifices differ from chain to chain, and their material can be generated at a variety of depths.

Description: Data exchange facilitated Nature Geoscience 4, 814 (2011). doi:10.1038/ngeo1335 Authors: Xiaogang Ma, Kristine Asch, John L. Laxton, Stephen M. Richard, Carlos G. Asato, Emmanuel John M. Carranza, Freek D. van der Meer, Chonglong Wu, Guillaume Duclaux & Koji Wakita

Description: Fragmented slab Nature Geoscience 4, 819 (2011). doi:10.1038/ngeo1343 Author: Amy Whitchurch

Description: Ice and algae Nature Geoscience 4, 819 (2011). doi:10.1038/ngeo1344 Author: Anna Armstrong

Description: Precession control Nature Geoscience 4, 819 (2011). doi:10.1038/ngeo1346 Author: Alicia Newton

Description: Magnetic asymmetry Nature Geoscience 4, 819 (2011). doi:10.1038/ngeo1347 Author: Amy Whitchurch

Description: Planetary science: Chaos and water Nature Geoscience 4, 822 (2011). doi:10.1038/ngeo1341 Author: Heike Langenberg

Description: Early earth: Snowballs limited by weathering Nature Geoscience 4, 824 (2011). doi:10.1038/ngeo1330 Author: A. Joshua West A series of extreme cooling episodes, starting 750 million years ago, could have repeatedly turned the planet into an ice-covered snowball. Carbon cycle modelling suggests that the timing of the glaciations can be explained by chemical weathering rates.

Description: Isotopic evaluation of ocean circulation in the Late Cretaceous North American seaway Nature Geoscience 4, 852 (2011). doi:10.1038/ngeo1312 Authors: Alan B. Coulson, Matthew J. Kohn & Reese E. Barrick During the mid- and Late Cretaceous period, North America was split by the north–south oriented Western Interior Seaway. Its role in creating and maintaining Late Cretaceous global greenhouse conditions remains unclear. Different palaeoceanographic reconstructions portray diverse circulation patterns. The southward extent of relatively cool, low-salinity, low-δ18O surface waters critically distinguishes among these models, but past studies of invertebrates could not independently assess water temperature and isotopic compositions. Here we present oxygen isotopes in biophosphate from coeval marine turtle and fish fossils from western Kansas, representing the east central seaway, and from the Mississippi embayment, representing the marginal Tethys Ocean. Our analyses yield precise seawater isotopic values and geographic temperature differences during the main transition from the Coniacian to the early Campanian age (87–82 Myr), and indicate that the seaway oxygen isotope value and salinity were 2‰ and 3‰ lower, respectively, than in the marginal Tethys Ocean. We infer that the influence of northern freshwater probably reached as far south as Kansas. Our revised values imply relatively large temperature differences between the Mississippi embayment and central seaway, explain the documented regional latitudinal palaeobiogeographic zonation and support models with relatively little inflow of surface waters from the Tethys Ocean to the Western Interior Seaway.

Description: A late Archaean radiating dyke swarm as possible clue to the origin of the Bushveld Complex Nature Geoscience 4, 865 (2011). doi:10.1038/ngeo1308 Authors: Johan R. Olsson, Ulf Söderlund, Michael A. Hamilton, Martin B. Klausen & George R. Helffrich The Bushveld Complex in South Africa represents the world’s largest intrusion of magnesium- and iron-rich magmas. The Bushveld magmas were emplaced beneath the Transvaal basin ∼2.06 billion years ago, but their origin remains elusive. The magmas may have formed in response to an upwelling mantle plume, ancient subduction or melting triggered by a meteorite impact. Here we use U–Pb dating of baddeleyite crystals to date a series of mafic magmatic dykes located east of the Transvaal basin. We find that these dykes formed between 2.70 and 2.66 billion years ago, roughly 600 million years before the Bushveld magmas were emplaced. Collectively, the geometry of the dykes forms a radiating swarm converging towards a focal point in the eastern part of the Bushveld Complex. Such radiating swarms typically record the impact of a mantle plume head that injected large volumes of magma into the crust and at the base of the lithosphere. We propose that subsequent cooling and metamorphism of these mantle-plume-derived rocks caused them to increase in density and sink, triggering subsidence of the Transvaal basin. The dense rocks may later have sunk away into the mantle, with the delamination causing the inflow of hot mantle that initiated production of the voluminous Bushveld magmas about 600 million years after the mantle plume impact.

Description: Abundant Early Palaeogene marine gas hydrates despite warm deep-ocean temperatures Nature Geoscience 4, 848 (2011). doi:10.1038/ngeo1301 Authors: Guangsheng Gu, Gerald R. Dickens, Gaurav Bhatnagar, Frederick S. Colwell, George J. Hirasaki & Walter G. Chapman Abrupt periods of global warming between 57 and 50 million years ago—known as the Early Palaeogene hyperthermal events—were associated with the repeated injection of massive amounts of carbon into the atmosphere. The release of methane from the sea floor following the dissociation of gas hydrates is often invoked as a source. However, seafloor temperatures before the events were at least 4–7 °C higher than today, which would have limited the area of sea floor suitable for hosting gas hydrates. Palaeogene gas hydrate reservoirs may therefore not have been sufficient to provide a significant fraction of the carbon released. Here we use numerical simulations of gas hydrate accumulation at Palaeogene seafloor temperatures to show that near-present-day values of gas hydrates could have been hosted in the Palaeogene. Our simulations show that warmer temperatures during the Palaeogene would have enhanced the amount of organic carbon reaching the sea floor as well as the rate of methanogenesis. We find that under plausible temperature and pressure conditions, the abundance of gas hydrates would be similar or higher in the Palaeogene than at present. We conclude that methane hydrates could have been an important source of carbon during the Palaeogene hyperthermal events.

Description: Mantle plumes persevere Nature Geoscience 4, 816 (2011). doi:10.1038/ngeo1334 Author: Anthony A. P. Koppers The ocean floor is littered with hundreds of thousands of mostly extinct volcanoes. The origin of at least some of these seamounts seems to rest with mantle plumes.

Description: Variability matters Nature Geoscience 4, 819 (2011). doi:10.1038/ngeo1345 Author: Anna Armstrong

Description: Pliocene glaciation Nature Geoscience 4, 734 (2011). doi:10.1038/ngeo1315 Author: Alicia Newton

Description: Influence of African dust on ocean–atmosphere variability in the tropical Atlantic Nature Geoscience 4, 762 (2011). doi:10.1038/ngeo1276 Authors: Amato T. Evan, Gregory R. Foltz, Dongxiao Zhang & Daniel J. Vimont The dominant source of coupled ocean–atmosphere variability in the tropical Atlantic is the so-called Atlantic Meridional Mode. This mode of variability is characterized by an interhemispheric gradient in sea surface temperatures and by oscillations in the strength of surface winds that cross the Equator, thereby reinforcing sea surface temperature anomalies. The Atlantic Meridional Mode is thermodynamically damped and must receive external forcing to persist as observed. However, it is not known which external forcing factors have excited the Atlantic Meridional Mode in the historical record. Here we present simulations with an ocean general circulation model that is forced by a record of surface radiation from anomalous dust concentrations in the atmosphere, reconstructed from a coral proxy and satellite retrievals. We show that the Atlantic Meridional Mode is excited by variability in African dust outbreaks on interannual to decadal timescales. Our analysis indicates that sea surface temperature anomalies resulting from the aerosol direct effect persist in time through the positive ocean–atmosphere feedback that defines the Atlantic Meridional Mode. We conclude that on interannual to decadal timescales, the state of the tropical Atlantic ocean is directly tied to dust emissions over West Africa, which in turn are linked to land-use change.

Description: Climate science: Roofs and roads Nature Geoscience 4, 736 (2011). doi:10.1038/ngeo1311 Author: Anna Armstrong

Description: Changes in North Atlantic circulation at the end of the Cretaceous greenhouse interval Nature Geoscience 4, 779 (2011). doi:10.1038/ngeo1284 Authors: K. G. MacLeod, C. Isaza Londoño, E. E. Martin, Á. Jiménez Berrocoso & C. Basak The mechanics of ocean circulation during the Late Cretaceous greenhouse interval remain contested, with the role of North Atlantic Deep Water in ocean circulation particularly debated: the relative warming of the North Atlantic during the termination of the greenhouse interval has been attributed to heat piracy from North Atlantic Deep Water formation, but the sources of Cretaceous deep water have been difficult to resolve. Nd isotopes as captured by seafloor sediments and expressed as ɛNd(t) reflect the region in which the water mass was formed. Here we present ɛNd(t) measurements from Cretaceous- to Palaeogene-aged sediments from four cores in the tropical North Atlantic. Before 69 Myr ago, we find extremely low ɛNd(t) values of about −16, consistent with the presence of a warm, saline deep water mass formed in the low latitudes. By 62 Myr ago, ɛNd(t) values had risen to −11, similar to values reported from the northern North Atlantic over the past 65 million years, but lower than most contemporaneous values in the South Atlantic and Pacific oceans. We therefore suggest that the ɛNd(t) shift reflects the increasing influence of a northern-sourced water mass at this site, indicating the onset or intensification of deep- or intermediate-water formation in the North Atlantic 69 Myr ago. Our findings support the heat piracy model and imply that circulation patterns during the greenhouse interval were different from those of the subsequent relatively temperate interval.

Description: The age of the megacity Nature Geoscience 4, 733 (2011). doi:10.1038/ngeo1299 Author: Luís M. A. Bettencourt

Description: Australia askew Nature Geoscience 4, 734 (2011). doi:10.1038/ngeo1319 Author: Amy Whitchurch

Description: Second wave Nature Geoscience 4, 734 (2011). doi:10.1038/ngeo1316 Author: Anna Armstrong

Description: Oceanography: Ocean eddies and plankton blooms Nature Geoscience 4, 739 (2011). doi:10.1038/ngeo1307 Author: Richard G. Williams Phytoplankton form the base of the marine food web, but their growth in nutrient-depleted surface waters has remained a puzzle. Two complementary studies suggest that ocean eddies help to control phytoplankton growth and distribution in unexpected ways.

Description: Solar forcing of winter climate variability in the Northern Hemisphere Nature Geoscience 4, 753 (2011). doi:10.1038/ngeo1282 Authors: Sarah Ineson, Adam A. Scaife, Jeff R. Knight, James C. Manners, Nick J. Dunstone, Lesley J. Gray & Joanna D. Haigh An influence of solar irradiance variations on Earth’s surface climate has been repeatedly suggested, based on correlations between solar variability and meteorological variables. Specifically, weaker westerly winds have been observed in winters with a less active sun, for example at the minimum phase of the 11-year sunspot cycle. With some possible exceptions, it has proved difficult for climate models to consistently reproduce this signal. Spectral Irradiance Monitor satellite measurements indicate that variations in solar ultraviolet irradiance may be larger than previously thought. Here we drive an ocean–atmosphere climate model with ultraviolet irradiance variations based on these observations. We find that the model responds to the solar minimum with patterns in surface pressure and temperature that resemble the negative phase of the North Atlantic or Arctic Oscillation, of similar magnitude to observations. In our model, the anomalies descend through the depth of the extratropical winter atmosphere. If the updated measurements of solar ultraviolet irradiance are correct, low solar activity, as observed during recent years, drives cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada, with little direct change in globally averaged temperature. Given the quasiregularity of the 11-year solar cycle, our findings may help improve decadal climate predictions for highly populated extratropical regions.

Description: Equatorial winds on Saturn and the stratospheric oscillation Nature Geoscience 4, 750 (2011). doi:10.1038/ngeo1292 Authors: Liming Li, Xun Jiang, Andrew P. Ingersoll, Anthony D. Del Genio, Carolyn C. Porco, Robert A. West, Ashwin R. Vasavada, Shawn P. Ewald, Barney J. Conrath, Peter J. Gierasch, Amy A. Simon-Miller, Conor A. Nixon, Richard K. Achterberg, Glenn S. Orton, Leigh N. Fletcher & Kevin H. Baines The zonal jets on the giant planets have been thought to be stable in time. A decline in the velocity of Saturn’s equatorial jet has been identified, on the basis of a comparison of cloud-tracking data across two decades, but the differences in cloud speeds have since been suggested to stem from changes in cloud altitude in combination with vertical wind shear, rather than from temporal changes in wind strength at a given height. Here, we combine observations of cloud tracks and of atmospheric temperatures taken by two instruments on the Cassini spacecraft to reveal a significant temporal variation in the strength of the high-altitude equatorial jet on Saturn. Specifically, we find that wind speeds at atmospheric pressure levels of 60 mbar, corresponding to Saturn’s tropopause, increased by about 20 m s−1 between 2004 and 2008, whereas the wind speed has been essentially constant over time in the southern equatorial troposphere. The observations further reveal that the equatorial jet intensified by about 60 m s−1 between 2005 and 2008 in the stratosphere, that is, at pressure levels of 1–5 mbar. Because the wind acceleration is weaker near the tropopause than higher up, in the stratosphere, we conclude that the semi-annual equatorial oscillation of Saturn’s middle atmosphere is also damped as it propagates downwards.

Description: A model for orbital pacing of methane hydrate destabilization during the Palaeogene Nature Geoscience 4, 775 (2011). doi:10.1038/ngeo1266 Authors: Daniel J. Lunt, Andy Ridgwell, Appy Sluijs, James Zachos, Stephen Hunter & Alan Haywood A series of transient global warming events occurred during the late Palaeocene and early Eocene, about 59 to 50 million years ago. The events, although variable in magnitude, were apparently paced by orbital cycles and linked to massive perturbations of the global carbon cycle. However, a causal link between orbital changes in insolation and the carbon cycle has yet to be established for this time period. Here we present a series of coupled climate model simulations that demonstrate that orbitally induced changes in ocean circulation and intermediate water temperature can trigger the destabilization of methane hydrates. We then use a simple threshold model to show that progressive global warming over millions of years, in combination with the increasing tendency of the ocean to remain in a more stagnant state, can explain the decreasing magnitude and increasing frequency of hyperthermal events throughout the early Eocene. Our work shows that nonlinear interactions between climate and the carbon cycle can modulate the effect of orbital variations, in this case producing transient global warming events with varying timing and magnitude.

Description: Planetary science: Martian water aloft Nature Geoscience 4, 738 (2011). doi:10.1038/ngeo1310 Author: Alicia Newton

Description: Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand Nature Geoscience 4, 793 (2011). doi:10.1038/ngeo1283 Authors: K. A. Radloff, Y. Zheng, H. A. Michael, M. Stute, B. C. Bostick, I. Mihajlov, M. Bounds, M. R. Huq, I. Choudhury, M. W. Rahman, P. Schlosser, K. M. Ahmed & A. van Geen

Description: The weather for war Nature Geoscience 4, 815 (2011). doi:10.1038/ngeo1336 Author: Axel Bojanowski

Description: Geodynamics: Asian continental growth Nature Geoscience 4, 827 (2011). doi:10.1038/ngeo1339 Authors: Karel Schulmann & Scott Paterson The ancient Central Asian Orogenic Belt formed during a period of extensive continental crust formation. Comparison with modern continent-building systems suggests that the processes that operate today were already active at that time.

Description: Geochemical zoning of volcanic chains associated with Pacific hotspots Nature Geoscience 4, 874 (2011). doi:10.1038/ngeo1263 Authors: Shichun Huang, Paul S. Hall & Matthew G. Jackson Recent Hawaiian volcanism is manifest as two geographically and geochemically distinct groups of volcanoes, the Loa trend in the south and the Kea trend in the north. The differences between the Loa and Kea lavas are attributed to spatial variations in the geochemical structure of the underlying Hawaiian mantle plume. In turn, the Hawaiian plume structure is thought to reflect heterogeneities in its mantle source. Here we compile geochemical data from the Hawaiian and two other volcanic ocean island chains—the Samoan and Marquesas—that formed above mantle plumes upwelling beneath the Pacific plate. We find that the volcanoes at both Samoa and the Marquesas show geographic and geochemical trends similar to those observed at Hawaii. Specifically, two subparallel arrays of volcanoes exist at both locations. In each case, the southern trend of volcanoes has higher radiogenic lead isotope ratios, 208Pb*/206Pb*, and lower neodymium isotope ratios, ɛNd, than those of the corresponding northern trend. We suggest that geochemical zoning may be a common feature of mantle plumes beneath the Pacific plate. Furthermore, we find that the pattern repeats between island chains, with the highest 208Pb*/206Pb* and the lowest ɛNdfound at Samoa in the south and the lowest 208Pb*/206Pb* and the highest ɛNd observed at Hawaii in the north. We infer that isotopically enriched material is preferentially distributed in the lower mantle of the Southern Hemisphere, within the Pacific low seismic velocity zone.

Description: A deep mantle origin for the primitive signature of ocean island basalt Nature Geoscience 4, 879 (2011). doi:10.1038/ngeo1295 Authors: Frédéric Deschamps, Edouard Kaminski & Paul J. Tackley Seismological observations have identified large-scale compositional heterogeneities in the Earth’s deep mantle. These heterogeneities may represent reservoirs of primitive material that differentiated early in Earth’s history. The volcanic rocks that make up ocean islands are thought to be sourced, in part, from these deep reservoirs, with the primitive material transported to the surface via mantle plumes. Geochemical signatures within the erupted ocean island basalts further support the idea that the regions of heterogeneity are composed of primitive, undegassed mantle material. Here we perform numerical experiments of thermo-chemical convection to simulate the entrainment of primitive material by plumes generated at the top of primitive reservoirs in the deep mantle. We vary the chemical density contrast between the primitive, undegassed and regular, degassed mantle materials. We find that the simulations that reproduce the observed geometry of the heterogeneous regions also explain the geochemical signatures measured in ocean island basalts. In these simulations, the entrainment of primitive material into the mantle plume does not exceed 9%. We conclude that the presence of primitive reservoirs in the deep mantle is dynamically feasible and satisfies both seismological and geochemical constraints.

Description: Timing of Neoproterozoic glaciations linked to transport-limited global weathering Nature Geoscience 4, 861 (2011). doi:10.1038/ngeo1305 Authors: Benjamin Mills, Andrew J. Watson, Colin Goldblatt, Richard Boyle & Timothy M. Lenton The Earth underwent several snowball glaciations between 1,000 and 542 million years ago. The termination of these glaciations is thought to have been triggered by the accumulation of volcanic CO2 in the atmosphere over millions of years. Subsequent high temperatures and loss of continental ice would increase silicate weathering and in turn draw down atmospheric CO2 (ref. ). Estimates of the post-snowball weathering rate indicate that equilibrium between CO2 input and removal would be restored within several million years , potentially triggering a new glaciation. However the transition between deglaciation and the onset a new glaciation was on the order of 107 years. Over long timescales, the availability of fresh rock can become a limiting factor for silicate weathering rates. Here we show that when this transport-determined limitation is incorporated into the COPSE biogeochemical model, the stabilization time is substantially longer, 〉107 years. When we include a simple ice-albedo feedback, the model produces greenhouse–icehouse oscillations on this timescale that are compatible with observations. Our simulations also indicate positive carbon isotope excursions and an increased flux of oxygen to the atmosphere during interglacials, both of which are consistent with the geological record. We conclude that the long gaps between snowball glaciations can be explained by limitations on silicate weathering rates.

Description: Long-term impacts of aerosols on the vertical development of clouds and precipitation Nature Geoscience 4, 888 (2011). doi:10.1038/ngeo1313 Authors: Zhanqing Li, Feng Niu, Jiwen Fan, Yangang Liu, Daniel Rosenfeld & Yanni Ding

Description: Role of the deep mantle in generating the compositional asymmetry of the Hawaiian mantle plume Nature Geoscience 4, 831 (2011). doi:10.1038/ngeo1328 Authors: Dominique Weis, Michael O. Garcia, J. Michael Rhodes, Mark Jellinek & James S. Scoates

Description: Drought-induced carbon loss in peatlands Nature Geoscience 4, 895 (2011). doi:10.1038/ngeo1323 Authors: Nathalie Fenner & Chris Freeman

Description: Origin of Indian Ocean Seamount Province by shallow recycling of continental lithosphere Nature Geoscience 4, 883 (2011). doi:10.1038/ngeo1331 Authors: K. Hoernle, F. Hauff, R. Werner, P. van den Bogaard, A. D. Gibbons, S. Conrad & R. D. Müller The origin of the Christmas Island Seamount Province in the northeast Indian Ocean is enigmatic. The seamounts do not form the narrow, linear and continuous trail of volcanoes that would be expected if they had formed above a mantle plume. Volcanism above a fracture in the lithosphere is also unlikely, because the fractures trend orthogonally with respect to the east–west trend of the Christmas Island chain. Here we combine 40Ar/39Ar age, Sr, Nd, Hf and high-precision Pb isotope analyses of volcanic rocks from the province with plate tectonic reconstructions. We find that the seamounts are 47–136 million years old, decrease in age from east to west and are consistently 0–25 million years younger than the underlying oceanic crust, consistent with formation near a mid-ocean ridge. The seamounts also exhibit an enriched geochemical signal, indicating that recycled continental lithosphere was present in their source. Plate tectonic reconstructions show that the seamount province formed at the position where West Burma began separating from Australia and India, forming a new mid-ocean ridge. We propose that the seamounts formed through shallow recycling of delaminated continental lithosphere entrained in mantle that was passively upwelling beneath the mid-ocean ridge. We conclude that shallow recycling of continental lithosphere at mid-ocean ridges could be an important mechanism for the formation of seamount provinces in young ocean basins.

Description: Tibetan plate overriding the Asian plate in central and northern Tibet Nature Geoscience 4, 870 (2011). doi:10.1038/ngeo1309 Authors: Wenjin Zhao, Prakash Kumar, James Mechie, Rainer Kind, Rolf Meissner, Zhenhan Wu, Danian Shi, Heping Su, Guangqi Xue, Marianne Karplus & Frederik Tilmann The southern boundary between India and the Tibetan Plateau represents a classical case of continental subduction, where the Indian continental lithosphere is subducted northwards beneath the Tibetan Plateau. At the northern boundary, southward subduction of Asian lithosphere beneath the Tibetan Plateau has also been proposed, but imaging has been hampered by inadequate data quality. Here we analyse the plate tectonic structure of the northern boundary between Tibet and Asia using the S receiver function technique. Our passive source seismic data build on, and extend further northwards, the existing geophysical data from the International Deep Profiling of Tibet and the Himalaya project. We detect, beneath central and northern Tibet, a relatively thin, but separate, Tibetan lithosphere overriding the flat, southward subducting Asian lithosphere. We suggest that this overriding Tibetan lithosphere helps to accommodate the convergence between India and Asia in central and northern Tibet. We conclude that the Tibetan–Himalayan system is composed of three major parts: the Indian, Asian and Tibetan lithospheres. In the south, the Indian lithosphere underthrusts Tibet. In central and northern Tibet a separate, thin Tibetan lithosphere exists, which is underthrust by the Asian lithosphere from the north.

Description: Acidification of subsurface coastal waters enhanced by eutrophication Nature Geoscience 4, 766 (2011). doi:10.1038/ngeo1297 Authors: Wei-Jun Cai, Xinping Hu, Wei-Jen Huang, Michael C. Murrell, John C. Lehrter, Steven E. Lohrenz, Wen-Chen Chou, Weidong Zhai, James T. Hollibaugh, Yongchen Wang, Pingsan Zhao, Xianghui Guo, Kjell Gundersen, Minhan Dai & Gwo-Ching Gong Human inputs of nutrients to coastal waters can lead to the excessive production of algae, a process known as eutrophication. Microbial consumption of this organic matter lowers oxygen levels in the water. In addition, the carbon dioxide produced during microbial respiration increases acidity. The dissolution of atmospheric carbon dioxide in ocean waters also raises acidity, a process known as ocean acidification. Here, we assess the combined impact of eutrophication and ocean acidification on acidity in the coastal ocean, using data collected in the northern Gulf of Mexico and the East China Sea—two regions heavily influenced by nutrient–laden rivers. We show that eutrophication in these waters is associated with the development of hypoxia and the acidification of subsurface waters, as expected. Model simulations, using data collected from the northern Gulf of Mexico, however, suggest that the drop in pH since pre-industrial times is greater than that expected from eutrophication and ocean acidification alone. We attribute the additional drop in pH—of 0.05 units—to a reduction in the ability of these carbon dioxide-rich waters to buffer changes in pH. We suggest that eutrophication could increase the susceptibility of coastal waters to ocean acidification.

Description: Textured core Nature Geoscience 4, 734 (2011). doi:10.1038/ngeo1317 Author: Amy Whitchurch

Description: Atmospheric science: Solar cycle and climate predictions Nature Geoscience 4, 735 (2011). doi:10.1038/ngeo1298 Author: Katja Matthes The impact of solar activity on climate has been debated heatedly. Simulations with a climate model using new observations of solar variability suggest a substantial influence of the Sun on the winter climate in the Northern Hemisphere.

Description: Active submarine eruption of boninite in the northeastern Lau Basin Nature Geoscience 4, 799 (2011). doi:10.1038/ngeo1275 Authors: Joseph A. Resing, Kenneth H. Rubin, Robert W. Embley, John E. Lupton, Edward T. Baker, Robert P. Dziak, Tamara Baumberger, Marvin D. Lilley, Julie A. Huber, Timothy M. Shank, David A. Butterfield, David A. Clague, Nicole S. Keller, Susan G. Merle, Nathaniel J. Buck, Peter J. Michael, Adam Soule, David W. Caress, Sharon L. Walker, Richard Davis, James P. Cowen, Anna-Louise Reysenbach & Hans Thomas

Description: Nordic carbon in flux Nature Geoscience 4, 734 (2011). doi:10.1038/ngeo1318 Author: Anna Armstrong

Description: High abundances of noble gas and chlorine delivered to the mantle by serpentinite subduction Nature Geoscience 4, 807 (2011). doi:10.1038/ngeo1270 Authors: Mark A. Kendrick, Marco Scambelluri, Masahiko Honda & David Phillips

Description: Increased capture of magma in the crust promoted by ice-cap retreat in Iceland Nature Geoscience 4, 783 (2011). doi:10.1038/ngeo1269 Authors: Andrew Hooper, Benedikt Ófeigsson, Freysteinn Sigmundsson, Björn Lund, Páll Einarsson, Halldór Geirsson & Erik Sturkell Climate warming at the end of the last glaciation caused ice caps on Icelandic volcanoes to retreat. Removal of surface ice load is thought to have decreased pressures in the underlying mantle, triggering decompression melting, enhanced magma generation and increased volcanic activity. Present-day climate change could have the same effect, although there may be a time lag of hundreds of years between magma generation and eruption. However, in addition to increased magma generation, pressure changes associated with ice retreat should also alter the capacity for storing magma within the crust. Here we use a numerical model to evaluate the effect of the current decrease in ice load on magma storage in the crust at the Kverkfjöll volcanic system, located partially beneath Iceland’s largest ice cap. We compare the model results with radar and global positioning system measurements of surface displacement and changes in crustal stress between 2007 and 2008, during the intrusion of a deep dyke at Upptyppingar. We find that although the main component of stress recorded during dyke intrusion relates to plate extension, another component of stress is consistent with the stress field caused by the retreating ice cap. We conclude that the retreating ice cap led to enhanced capture of magma within the crust. We suggest that ice-cap retreat can promote magma storage, rather than eruption, at least in the short term.

Description: City lights and urban air Nature Geoscience 4, 730 (2011). doi:10.1038/ngeo1300 Authors: H. Stark, S. S. Brown, K. W. Wong, J. Stutz, C. D. Elvidge, I. B. Pollack, T. B. Ryerson, W. P. Dube, N. L. Wagner & D. D. Parrish

Description: Urban lead Nature Geoscience 4, 729 (2011). doi:10.1038/ngeo1322 The world is undergoing a phenomenally fast wave of urban growth. Research that can help tackle some of the ensuing problems is likely to originate in cities themselves.

Description: Iceland's impact Nature Geoscience 4, 732 (2011). doi:10.1038/ngeo1304 Author: Axel Bojanowski

Description: Submarine volcanism: Deeply explosive Nature Geoscience 4, 737 (2011). doi:10.1038/ngeo1306 Author: Richard Arculus Volcanic activity is much more common beneath the oceans than on land, yet has been observed only rarely. Direct measurements of an eruption in the southwest Pacific Ocean reveal unexpectedly explosive activity at great depths.

Description: Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change Nature Geoscience 4, 741 (2011). doi:10.1038/ngeo1296 Authors: David W. J. Thompson, Susan Solomon, Paul J. Kushner, Matthew H. England, Kevin M. Grise & David J. Karoly