ALBERT

Description: Nature Geoscience 8, 655 (2015). doi:10.1038/ngeo2531 During several intervals in Earth's history, ice sheets expanded to cover the globe. These glaciations may be intricately linked to the evolution of life.

Description: Nature Geoscience 8, 657 (2015). doi:10.1038/ngeo2527 Authors: B. L. Cooper, K. Thaisen, B. C. Chang, T. S. Lee & D. S. McKay

Description: Nature Geoscience 8, 668 (2015). doi:10.1038/ngeo2504 Author: Philip Allen Evidence for a Neoproterozoic Snowball Earth in the sedimentary record has been controversial. A weathered horizon preserved in sedimentary rocks from Svalbard may provide a rare signature of prolonged global glaciation.

Description: Nature Geoscience 8, 663 (2015). doi:10.1038/ngeo2509 Author: Jeff Peakall Braided channels are rare on ocean floors, but abundant on land. Experiments and theory suggest that deeper flows and rapid overbank deposition restrict braiding in underwater rivers relative to their terrestrial counterparts.

Description: Nature Geoscience 8, 659 (2015). doi:10.1038/ngeo2523 Authors: Georg Feulner, Christian Hallmann & Hendrik Kienert The Earth underwent two snowball glaciation events between 720 and 635 million years ago. The preceding expansion of eukaryotic algae and a consequent rise in emissions of organic cloud condensation nuclei may have contributed to the dramatic cooling.

Description: Nature Geoscience 8, 686 (2015). doi:10.1038/ngeo2508 Authors: Ryuki Hyodo & Keiji Ohtsuki Saturn’s F ring is a narrow ring of icy particles, located 3,400 km beyond the outer edge of the main ring system. Enigmatically, the F ring is accompanied on either side by two small satellites, Prometheus and Pandora, which are called shepherd satellites. The inner regular satellites of giant planets are thought to form by the accretion of particles from an ancient massive ring and subsequent outward migration. However, the origin of a system consisting of a narrow ring and shepherd satellites remains poorly understood. Here we present N-body numerical simulations to show that a collision of two of the small satellites that are thought to accumulate near the main ring’s outer edge can produce a system similar to the F ring and its shepherd satellites. We find that if the two rubble-pile satellites have denser cores, such an impact results in only partial disruption of the satellites and the formation of a narrow ring of particles between two remnant satellites. Our simulations suggest that the seemingly unusual F ring system is a natural outcome at the final stage of the formation process of the ring–satellite system of giant planets.

Description: Nature Geoscience 8, 708 (2015). doi:10.1038/ngeo2518 Authors: Jean-Philippe Avouac, Lingsen Meng, Shengji Wei, Teng Wang & Jean-Paul Ampuero Large earthquakes are thought to release strain on previously locked faults. However, the details of how earthquakes are initiated, grow and terminate in relation to pre-seismically locked and creeping patches is unclear. The 2015 Mw 7.8 Gorkha, Nepal earthquake occurred close to Kathmandu in a region where the prior pattern of fault locking is well documented. Here we analyse this event using seismological records measured at teleseismic distances and Synthetic Aperture Radar imagery. We show that the earthquake originated northwest of Kathmandu within a cluster of background seismicity that fringes the bottom of the locked portion of the Main Himalayan Thrust fault (MHT). The rupture propagated eastwards for about 140 km, unzipping the lower edge of the locked portion of the fault. High-frequency seismic waves radiated continuously as the slip pulse propagated at about 2.8 km s−1 along this zone of presumably high and heterogeneous pre-seismic stress at the seismic–aseismic transition. Eastward unzipping of the fault resumed during the Mw 7.3 aftershock on 12 May. The transfer of stress to neighbouring regions during the Gorkha earthquake should facilitate future rupture of the areas of the MHT adjacent and updip of the Gorkha earthquake rupture.

Description: Nature Geoscience 8, 664 (2015). doi:10.1038/ngeo2497 Author: Ruth M. Doherty Tropospheric ozone is generated from precursor pollutants, but can be blown far afield. Satellite observations show rising ozone levels over China — and almost stable levels over western North America despite stricter regulations.

Description: Nature Geoscience 8, 417 (2015). doi:10.1038/ngeo2460 Complex ecological and evolutionary controls of forest dynamics make projecting the future difficult.

Description: Nature Geoscience 8, 419 (2015). doi:10.1038/ngeo2444 Authors: Chris Brierley, Natalie Burls, Christina Ravelo & Alexey Fedorov

Description: Nature Geoscience 8, 420 (2015). doi:10.1038/ngeo2445 Authors: Charlotte L. O'Brien, Gavin L. Foster, James W. B. Rae & Richard D. Pancost

Description: Nature Geoscience 8, 423 (2015). doi:10.1038/ngeo2442 Author: Jérôme Vialard Global surface warming has slowed since the start of the twenty-first century, while Pacific heat uptake was enhanced. Analyses of ocean heat content suggest that the warm water was transferred to the Indian Ocean, through the Indonesian straits.

Description: Nature Geoscience 8, 428 (2015). doi:10.1038/ngeo2431 Author: Magali Billen The Indian Plate moved north unusually quickly during the late Cretaceous. Numerical simulations suggest that this rapid migration was caused by the pull of two coupled, narrowing subduction zones.

Description: Nature Geoscience 8, 429 (2015). doi:10.1038/ngeo2446 Author: Morgan F. Schaller An ancient carbon release resulted in widespread dissolution of carbonates at the sea floor. Numerical simulations suggest that the pattern of dissolution can be explained by a top-down invasion of corrosive bottom waters from the North Atlantic.

Description: Nature Geoscience 8, 424 (2015). doi:10.1038/ngeo2450 Author: Jane K. Willenbring It is intuitive, but evidence that high levels of precipitation increase erosion rates has been elusive. The ages of exposed porphyry copper deposits reveal that rocks emplaced at depth travel to the surface faster where precipitation rates are high.

Description: Nature Geoscience 8, 426 (2015). doi:10.1038/ngeo2457 Author: Tamara Goldin

Description: Nature Geoscience 8, 421 (2015). doi:10.1038/ngeo2455 Author: Richard Guthrie Natural landscapes are shaped by frequent moderate-sized events, except for the rare catastrophe. Human modifications to the Earth's surface are, compared with natural processes, increasingly catastrophic.

Description: Nature Geoscience 8, 426 (2015). doi:10.1038/ngeo2433 Author: Richard Keil Fjords account for less than 0.1% of the surface of Earth's oceans. A global assessment finds that organic carbon is buried in fjords five times faster than other marine systems, accounting for 11% of global marine organic carbon burial.

Description: Nature Geoscience 8, 417 (2015). doi:10.1038/ngeo2461 Nature Geoscience introduces 3,000-word Methods sections that are integrated with the online paper.

Description: Nature Geoscience 8, 450 (2015). doi:10.1038/ngeo2421 Authors: Richard W. Smith, Thomas S. Bianchi, Mead Allison, Candida Savage & Valier Galy The deposition and long-term burial of organic carbon in marine sediments has played a key role in controlling atmospheric O2 and CO2 concentrations over the past 500 million years. Marine carbon burial represents the dominant natural mechanism of long-term organic carbon sequestration. Fjords—deep, glacially carved estuaries at high latitudes—have been hypothesized to be hotspots of organic carbon burial, because they receive high rates of organic material fluxes from the watershed. Here we compile organic carbon concentrations from 573 fjord surface sediment samples and 124 sediment cores from nearly all fjord systems globally. We use sediment organic carbon content and sediment delivery rates to calculate rates of organic carbon burial in fjord systems across the globe. We estimate that about 18 Mt of organic carbon are buried in fjord sediments each year, equivalent to 11% of annual marine carbon burial globally. Per unit area, fjord organic carbon burial rates are one hundred times as large as the global ocean average, and fjord sediments contain twice as much organic carbon as biogenous sediments underlying the upwelling regions of the ocean. We conclude that fjords may play an important role in climate regulation on glacial–interglacial timescales.

Description: Nature Geoscience 8, 454 (2015). doi:10.1038/ngeo2440 Authors: Anne M. Kellerman, Dolly N. Kothawala, Thorsten Dittmar & Lars J. Tranvik Whether intrinsic molecular properties or extrinsic factors such as environmental conditions control the decomposition of natural organic matter across soil, marine and freshwater systems has been subject to debate. Comprehensive evaluations of the controls that molecular structure exerts on organic matter’s persistence in the environment have been precluded by organic matter’s extreme complexity. Here we examine dissolved organic matter from 109 Swedish lakes using ultrahigh-resolution mass spectrometry and optical spectroscopy to investigate the constraints on its persistence in the environment. We find that degradation processes preferentially remove oxidized, aromatic compounds, whereas reduced, aliphatic and N-containing compounds are either resistant to degradation or tightly cycled and thus persist in aquatic systems. The patterns we observe for individual molecules are consistent with our measurements of emergent bulk characteristics of organic matter at wide geographic and temporal scales, as reflected by optical properties. We conclude that intrinsic molecular properties are an important control of overall organic matter reactivity.

Description: Nature Geoscience 8, 462 (2015). doi:10.1038/ngeo2429 Authors: Brian J. Yanites & Stephen E. Kesler The processes that build and shape mountain landscapes expose important mineral resources. Mountain landscapes are widely thought to result from the interaction between tectonic uplift and exhumation by erosion. Both climate and tectonics affect rates of exhumation, but estimates of their relative importance vary. Porphyry copper deposits are emplaced at a depth of about 2 km in convergent tectonic settings; their exposure at the surface therefore can be used to track landscape exhumation. Here we analyse the distribution, ages and spatial density of exposed Cenozoic porphyry copper deposits using a global data set to quantify exhumation. We find that the deposits exhibit young ages and are sparsely distributed—both consistent with rapid exhumation—in regions with high precipitation, and deposits are older and more abundant in dry regions. This suggests that climate is driving erosion and mineral exposure in deposit-bearing mountain landscapes. Our findings show that the emplacement ages of porphyry copper deposits provide a means to estimate long-term exhumation rates in active orogens, and we conclude that climate-driven exhumation influences the age and abundance of exposed porphyry copper deposits around the world.

Description: Nature Geoscience 8, 466 (2015). doi:10.1038/ngeo2434 Authors: Romain Guilbaud, Simon W. Poulton, Nicholas J. Butterfield, Maoyan Zhu & Graham A. Shields-Zhou Eukaryotic life expanded during the Proterozoic eon, 2.5 to 0.542 billion years ago, against a background of fluctuating ocean chemistry. After about 1.8 billion years ago, the global ocean is thought to have been characterized by oxygenated surface waters, with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron-containing (ferruginous) deeper waters. The spatial extent of sulphidic waters probably varied through time, but this surface-to-deep redox structure is suggested to have persisted until the first Neoproterozoic glaciation about 717 million years ago. Here we report an analysis of ocean redox conditions throughout the Proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops. We find a global transition from sulphidic to ferruginous mid-depth waters in the earliest Neoproterozoic, coincident with the amalgamation of the supercontinent Rodinia at low latitudes. We suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate, driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on Rodinia. We propose that this transition essentially detoxified ocean margin settings, allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago.

Description: Nature Geoscience 8, 471 (2015). doi:10.1038/ngeo2427 Authors: Robert Herrendörfer, Ylona van Dinther, Taras Gerya & Luis Angel Dalguer A supercycle describes a long-term cluster of differently-sized megathrust earthquakes, leading up to the final complete failure of a subduction zone segment. The precise controls on supercycles are unclear, although structural and frictional heterogeneities are proposed. We recognize that supercycles are suggested to occur in those regions where the estimated downdip width of the seismogenic zone is larger than average. Here we investigate the link between supercycles and the seismogenic zone downdip width using a two-dimensional numerical model. In our simulations, the first megathrust earthquakes in a supercycle generally rupture only the outermost parts of the seismogenic zone. These partial ruptures are stopped owing to a large excess of strength over stress, and transfer stresses towards the centre of the seismogenic zone. In addition to the continued tectonic loading, they thereby gradually reduce the strength excess so that the largest megathrust events finally rupture the entire seismogenic zone and release most of the accumulated stress. A greater width increases the average strength excess and thus favours supercycles over ordinary cycles of only similarly sized complete ruptures. Our results imply that larger than thus far observed earthquakes could conclude a supercycle where seismogenic zone widths are larger than average.

Description: Nature Geoscience 8, 475 (2015). doi:10.1038/ngeo2418 Authors: Oliver Jagoutz, Leigh Royden, Adam F. Holt & Thorsten W. Becker Before its collision with Eurasia, the Indian Plate moved rapidly, at rates exceeding 140 mm yr−1 for a period of 20 million years. This motion is 50 to 100% faster than the maximum sustained rate of convergence of the main tectonic plates today. The cause of such high rates of convergence is unclear and not reproduced by numerical models. Here we show that existing geological data support the existence of two, almost parallel, northward dipping subduction zones between the Indian and Eurasian plates, during the Early Cretaceous period. We use a quantitative model to show that the combined pull of two subducting slabs can generate anomalously rapid convergence between India and Eurasia. Furthermore, in our simulations a reduction in length of the southern subduction system, from about 10,000 to 3,000 km between 90 and 80 million years ago, reduced the viscous pressure between the subducting slabs and created a threefold increase in plate convergence rate between 80 and 65 million years ago. Rapid convergence ended 50 million years ago, when the Indian Plate collided with the southern subduction system. Collision of India with Eurasia and the northern subduction system had little effect on plate convergence rates before 40 million years ago. We conclude that the number and geometry of subduction systems has a strong influence on plate migration rates.

Description: Nature Geoscience 8, 479 (2015). doi:10.1038/ngeo2437 Authors: J. M. Whittaker, J. C. Afonso, S. Masterton, R. D. Müller, P. Wessel, S. E. Williams & M. Seton Plate tectonic motions are commonly considered to be driven by slab pull at subduction zones and ridge push at mid-ocean ridges, with motion punctuated by plumes of hot material rising from the lower mantle. Within this model, the geometry and location of mid-ocean ridges are considered to be independent of lower-mantle dynamics, such as deeply sourced plumes that produce voluminous lava eruptions—termed large igneous provinces. Here we use a global plate model to reconstruct the locations of large igneous provinces relative to plumes and mid-ocean ridges at the time they formed. We find that large igneous provinces repeatedly formed at specific locations where mid-ocean ridges and plumes interact. We calculate how much mantle material was converted to oceanic lithosphere at the mid-ocean ridges and find that slowly migrating ridge systems that have been stabilized by upwelling plumes have extracted large volumes of material from the same part of the upper mantle over periods up to 180 million years. The geochemical signatures of mid-ocean ridge basalts and seismic tomographic data show that upper-mantle temperatures are elevated at significant distances from ridge–plume interactions, indicating a far-field, indirect influence of plume–ridge interactions on the upper-mantle structure. We conclude that strong feedbacks exist between the dynamics of slowly migrating ridges and deeply sourced plumes.

Description: Nature Geoscience 8, 445 (2015). doi:10.1038/ngeo2438 Authors: Sang-Ki Lee, Wonsun Park, Molly O. Baringer, Arnold L. Gordon, Bruce Huber & Yanyun Liu Global mean surface warming has stalled since the end of the twentieth century, but the net radiation imbalance at the top of the atmosphere continues to suggest an increasingly warming planet. This apparent contradiction has been reconciled by an anomalous heat flux into the ocean, induced by a shift towards a La Niña-like state with cold sea surface temperatures in the eastern tropical Pacific over the past decade or so. A significant portion of the heat missing from the atmosphere is therefore expected to be stored in the Pacific Ocean. However, in situ hydrographic records indicate that Pacific Ocean heat content has been decreasing. Here, we analyse observations along with simulations from a global ocean–sea ice model to track the pathway of heat. We find that the enhanced heat uptake by the Pacific Ocean has been compensated by an increased heat transport from the Pacific Ocean to the Indian Ocean, carried by the Indonesian throughflow. As a result, Indian Ocean heat content has increased abruptly, which accounts for more than 70% of the global ocean heat gain in the upper 700 m during the past decade. We conclude that the Indian Ocean has become increasingly important in modulating global climate variability.

Description: Nature Geoscience 8, 441 (2015). doi:10.1038/ngeo2413 Authors: William R. Wieder, Cory C. Cleveland, W. Kolby Smith & Katherine Todd-Brown The size of the terrestrial sink remains uncertain. This uncertainty presents a challenge for projecting future climate–carbon cycle feedbacks. Terrestrial carbon storage is dependent on the availability of nitrogen for plant growth, and nitrogen limitation is increasingly included in global models. Widespread phosphorus limitation in terrestrial ecosystems may also strongly regulate the global carbon cycle, but explicit considerations of phosphorus limitation in global models are uncommon. Here we use global state-of-the-art coupled carbon–climate model projections of terrestrial net primary productivity and carbon storage from 1860–2100; estimates of annual new nutrient inputs from deposition, nitrogen fixation, and weathering; and estimates of carbon allocation and stoichiometry to evaluate how simulated CO2 fertilization effects could be constrained by nutrient availability. We find that the nutrients required for the projected increases in net primary productivity greatly exceed estimated nutrient supply rates, suggesting that projected productivity increases may be unrealistically high. Accounting for nitrogen and nitrogen–phosphorus limitation lowers projected end-of-century estimates of net primary productivity by 19% and 25%, respectively, and turns the land surface into a net source of CO2 by 2100. We conclude that potential effects of nutrient limitation must be considered in estimates of the terrestrial carbon sink strength through the twenty-first century.

Description: Nature Geoscience 8, 458 (2015). doi:10.1038/ngeo2430 Authors: Kaitlin Alexander, Katrin J. Meissner & Timothy J. Bralower The Palaeocene–Eocene Thermal Maximum, approximately 55 million years ago, was a period of rapid warming linked to a massive release of carbon to the ocean–atmosphere system. This warming event was also marked by widespread dissolution of carbonates at the sea floor. The acidification of deep waters was generally more extensive and severe in the Atlantic and Caribbean, with more modest changes in the Southern and Pacific oceans. Here we use the UVic ESCM global climate model to show that corrosive deep water spreading from the North Atlantic can explain the spatial variations in carbonate dissolution during the Palaeocene–Eocene Thermal Maximum. In our simulations, highly corrosive waters accumulate in the deep North Atlantic at the onset of the event. Several thousand years after an imposed atmospheric carbon release, warming of the deep ocean destabilizes the North Atlantic water column and triggers deep-water formation. This deep convection causes the corrosive bottom water to spill over an equatorial sill into the South Atlantic. The bottom water then spreads through the Southern and Pacific oceans, progressively gaining alkalinity. We conclude that the pattern of sediment dissolution simulated along the path taken by the corrosive water is consistent with most dissolution estimates from the sediment record.

Description: Nature Geoscience 8, 490 (2015). doi:10.1038/ngeo2447 Authors: M. Crespo-Medina, C. D. Meile, K. S. Hunter, A-R. Diercks, V. L. Asper, V. J. Orphan, P. L. Tavormina, L. M. Nigro, J. J. Battles, J. P. Chanton, A. M. Shiller, D-J. Joung, R. M. W. Amon, A. Bracco, J. P. Montoya, T. A. Villareal, A. M. Wood & S. B. Joye

Description: Nature Geoscience 8, 484 (2015). doi:10.1038/ngeo2436 Authors: H. W. Green II, F. Shi, K. Bozhilov, G. Xia & Z. Reches

Description: Nature Geoscience 8, 431 (2015). doi:10.1038/ngeo2454 Authors: Daniel J. Conley & Joanna C. Carey The Earth's long-term silica cycle is intimately linked to weathering rates and biogenic uptake. Changes in weathering rates and the retention of silica on land have altered silica availability in the oceans for hundreds of millions of years.

Description: Nature Geoscience 8, 433 (2015). doi:10.1038/ngeo2424 Authors: Joseph Kidston, Adam A. Scaife, Steven C. Hardiman, Daniel M. Mitchell, Neal Butchart, Mark P. Baldwin & Lesley J. Gray

Description: Nature Geoscience 9, 561 (2016). doi:10.1038/ngeo2787 Forests are important for the global carbon cycle, and for mitigating greenhouse gas emissions. However, the role forests play in carbon sequestration should not eclipse everything else we value them for.

Description: Nature Geoscience 9, 564 (2016). doi:10.1038/ngeo2777 Authors: Ye Qi, Nicholas Stern, Tong Wu, Jiaqi Lu & Fergus Green Slowing GDP growth, a structural shift away from heavy industry, and more proactive policies on air pollution and clean energy have caused China's coal use to peak. It seems that economic growth has decoupled from growth in coal consumption.

Description: Nature Geoscience 9, 562 (2016). doi:10.1038/ngeo2780 Author: Reiner Grundmann

Description: Nature Geoscience 9, 569 (2016). doi:10.1038/ngeo2766 Authors: Nathaniel L. Bindoff & William R. Hobbs Conversion of Antarctic circumpolar upwelling waters to less dense water has mainly been attributed to surface heat fluxes. An analysis of water-mass transformation shows that the dominant process is the formation of sea ice near Antarctica and its melt offshore.

Description: Nature Geoscience 9, 567 (2016). doi:10.1038/ngeo2758 Author: Pablo Ortega Summer temperatures in Europe varied markedly over the past millennium. Climate models and palaeoclimate records indicate that changes in cloud cover related to storm tracks contributed to the variations — and may continue to do so in the future.

Description: Nature Geoscience 9, 572 (2016). doi:10.1038/ngeo2765 Author: Anitra E. Ingalls TEX86-based records of sea surface temperature from the Early Eocene suggest polar warmth that is not seen in climate models. A reassessment of the TEX86 proxy adjusts these temperatures, lending confidence to simulations of greenhouse climates.

Description: Nature Geoscience 9, 568 (2016). doi:10.1038/ngeo2755 Author: Erik Asphaug The two small satellites of Mars are thought to have accreted from a debris disk formed in a giant impact. Simulations suggest the moons were shepherded into formation by the dynamical influence of one or more short-lived massive inner moons.

Description: Nature Geoscience 9, 584 (2016). doi:10.1038/ngeo2761 Authors: Filippo Giorgi, Csaba Torma, Erika Coppola, Nikolina Ban, Christoph Schär & Samuel Somot Global climate projections consistently indicate a future decrease in summer precipitation over the European Alps. However, topography can substantially modulate precipitation change signals. For example, the shadowing effect by topographic barriers can modify winter precipitation change patterns, and orographic convection might also play an important role. Here we analyse summer precipitation over the Alpine region in an ensemble of twenty-first-century projections with high-resolution (∼12 km) regional climate models driven by recent global climate model simulations. A broad-scale summer precipitation reduction is projected by both model ensembles. However, the regional models simulate an increase in precipitation over the high Alpine elevations that is not present in the global simulations. This is associated with increased convective rainfall due to enhanced potential instability by high-elevation surface heating and moistening. The robustness of this signal, which is found also for precipitation extremes, is supported by the consistency across models and future time slices, the identification of an underlying mechanism (enhanced convection), results from a convection-resolving simulation, the statistical significance of the signal and the consistency with some observed trends. Our results challenge the picture of a ubiquitous decrease of summer precipitation over the Alps found in coarse-scale projections.

Description: Nature Geoscience 9, 596 (2016). doi:10.1038/ngeo2749 Authors: Ryan P. Abernathey, Ivana Cerovecki, Paul R. Holland, Emily Newsom, Matt Mazloff & Lynne D. Talley Ocean overturning circulation requires a continuous thermodynamic transformation of the buoyancy of seawater. The steeply sloping isopycnals of the Southern Ocean provide a pathway for Circumpolar Deep Water to upwell from mid depth without strong diapycnal mixing, where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch. While brine rejection from sea ice is thought to contribute to the lower branch, the role of sea ice in the upper branch is less well understood, partly due to a paucity of observations of sea-ice thickness and transport. Here we quantify the sea-ice freshwater flux using the Southern Ocean State Estimate, a state-of-the-art data assimilation that incorporates millions of ocean and ice observations. We then use the water-mass transformation framework to compare the relative roles of atmospheric, sea-ice, and glacial freshwater fluxes, heat fluxes, and upper-ocean mixing in transforming buoyancy within the upper branch. We find that sea ice is a dominant term, with differential brine rejection and ice melt transforming upwelled Circumpolar Deep Water at a rate of ∼22 × 106 m3 s−1. These results imply a prominent role for Antarctic sea ice in the upper branch and suggest that residual overturning and wind-driven sea-ice transport are tightly coupled.

Description: Nature Geoscience 9, 590 (2016). doi:10.1038/ngeo2751 Authors: Gerald A. Meehl, Julie M. Arblaster, Cecilia M. Bitz, Christine T. Y. Chung & Haiyan Teng Antarctic sea-ice extent has been slowly increasing in the satellite record that began in 1979. Since the late 1990s, the increase has accelerated, but the average of all climate models shows a decline. Meanwhile, the Interdecadal Pacific Oscillation, an internally generated mode of climate variability, transitioned from positive to negative, with an average cooling of tropical Pacific sea surface temperatures, a slowdown of the global warming trend and a deepening of the Amundsen Sea Low near Antarctica that has contributed to regional circulation changes in the Ross Sea region and expansion of sea ice. Here we show that the negative phase of the Interdecadal Pacific Oscillation in global coupled climate models is characterized by anomalies similar to the observed sea-level pressure and near-surface 850 hPa wind changes near Antarctica since 2000 that are conducive to expanding Antarctic sea-ice extent, particularly in the Ross Sea region in all seasons, involving a deepening of the Amundsen Sea Low. These atmospheric circulation changes are shown to be mainly driven by precipitation and convective heating anomalies related to the Interdecadal Pacific Oscillation in the equatorial eastern Pacific, with additional contributions from convective heating anomalies in the South Pacific convergence zone and tropical Atlantic regions.

Description: Nature Geoscience 9, 611 (2016). doi:10.1038/ngeo2741 Authors: Samuel Howell, Bridget Smith-Konter, Neil Frazer, Xiaopeng Tong & David Sandwell The San Andreas Fault System, one of the best-studied transform plate boundaries on Earth, is well known for its complex network of locked faults that slowly deform the crust in response to large-scale plate motions. Horizontal interseismic motions of the fault system are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Here we show that when carefully treated for spatial consistency, global positioning system-derived vertical velocities expose a small-amplitude (±2 mm yr−1), but spatially considerable (200 km), coherent pattern of uplift and subsidence straddling the fault system in southern California. We employ the statistical method of model selection to isolate this vertical velocity field from non-tectonic signals that induce velocity variations in both magnitude and direction across small distances (less than tens of kilometres; ref. ), and find remarkable agreement with the sense of vertical motions predicted by physical earthquake cycle models spanning the past few centuries. We suggest that these motions reveal the subtle, but identifiable, tectonic fingerprint of far-field flexure due to more than 300 years of fault locking and creeping depth variability. Understanding this critical component of interseismic deformation at a complex strike–slip plate boundary will better constrain regional mechanics and crustal rheology, improving the quantification of seismic hazards in southern California and beyond.

Description: Nature Geoscience 9, 581 (2016). doi:10.1038/ngeo2742 Authors: Pascal Rosenblatt, Sébastien Charnoz, Kevin M. Dunseath, Mariko Terao-Dunseath, Antony Trinh, Ryuki Hyodo, Hidenori Genda & Stéven Toupin Phobos and Deimos, the two small satellites of Mars, are thought either to be asteroids captured by the planet or to have formed in a disc of debris surrounding Mars following a giant impact. Both scenarios, however, have been unable to account for the current Mars system. Here we use numerical simulations to suggest that Phobos and Deimos accreted from the outer portion of a debris disc formed after a giant impact on Mars. In our simulations, larger moons form from material in the denser inner disc and migrate outwards due to gravitational interactions with the disc. The resulting orbital resonances spread outwards and gather dispersed outer disc debris, facilitating accretion into two satellites of sizes similar to Phobos and Deimos. The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. The proposed scenario can explain why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor.

Description: Nature Geoscience 9, 575 (2016). doi:10.1038/ngeo2757 Authors: Donald E. Penman, Sandra Kirtland Turner, Philip F. Sexton, Richard D. Norris, Alexander J. Dickson, Slah Boulila, Andy Ridgwell, Richard E. Zeebe, James C. Zachos, Adele Cameron, Thomas Westerhold & Ursula Röhl

Description: Nature Geoscience 9, 624 (2016). doi:10.1038/ngeo2739 Authors: Pradeep K. Aggarwal, Ulrike Romatschke, Luis Araguas-Araguas, Dagnachew Belachew, Frederick J. Longstaffe, Peter Berg, Courtney Schumacher & Aaron Funk

Description: Nature Geoscience 9, 602 (2016). doi:10.1038/ngeo2767 Authors: Jasper A. Wassenburg, Stephan Dietrich, Jan Fietzke, Jens Fohlmeister, Klaus Peter Jochum, Denis Scholz, Detlev K. Richter, Abdellah Sabaoui, Christoph Spötl, Gerrit Lohmann, Meinrat O. Andreae & Adrian Immenhauser The North Atlantic Oscillation is the dominant atmospheric pressure mode in the North Atlantic region and affects winter temperature and precipitation in the Mediterranean, northwest Europe, Greenland, and Asia. The index that describes the sea-level pressure difference between Iceland and the Azores is correlated with a dipole precipitation pattern over northwest Europe and northwest Africa. How the North Atlantic Oscillation will develop as the Greenland ice sheet melts is unclear. A potential past analogue is the early Holocene, during which melting ice sheets around the North Atlantic freshened surface waters, affecting the strength of the meridional overturning circulation. Here we present a Holocene rainfall record from northwest Africa based on speleothem δ18O and compare it against a speleothem-based rainfall record from Europe. The two records are positively correlated during the early Holocene, followed by a shift to an anti-correlation, similar to the modern record, during the mid-Holocene. On the basis of our simulations with an Earth system model, we suggest the shift to the anti-correlation reflects a large-scale atmospheric and oceanic reorganization in response to the demise of the Laurentide ice sheet and a strong reduction of meltwater flux to the North Atlantic, pointing to a potential sensitivity of the North Atlantic Oscillation to the melting of ice sheets.

Description: Nature Geoscience 9, 571 (2016). doi:10.1038/ngeo2746 Author: Mladen R. Nedimović At mid-ocean ridges, the directions in which plates spread and the underlying mantle flows were thought to broadly align. A synthesis of results from ridges that spread at a variety of rates reveals that instead there may be a systematic skew.

Description: Nature Geoscience 9, 630 (2016). doi:10.1038/ngeo2752 Authors: Mary H. Gagen, Eduardo Zorita, Danny McCarroll, Matthias Zahn, Giles H. F. Young & Iain Robertson

Description: Nature Geoscience 9, 606 (2016). doi:10.1038/ngeo2763 Authors: Sze Ling Ho & Thomas Laepple The early Eocene (49–55 million years ago) is a time interval characterized by elevated surface temperatures and atmospheric CO2 (refs ,), and a flatter-than-present latitudinal surface temperature gradient. The multi-proxy-derived flat temperature gradient has been a challenge to reproduce in model simulations, especially the subtropical warmth at the high-latitude surface oceans, inferred from the archaeal lipid-based palaeothermometry, ngeo2763-m5jpg20K4522. Here we revisit the ngeo2763-m5jpg20K4522 interpretation by analysing a global collection of multi-proxy temperature estimates from sediment cores spanning millennia to millions of years. Comparing the variability between proxy types, we demonstrate that the present ngeo2763-m5jpg20K4522 interpretation overestimates the magnitude of past climate changes on all timescales. We attribute this to an inappropriate calibration, which reflects subsurface ocean but is calibrated to the sea surface, where the latitudinal temperature gradient is steeper. Recalibrating the proxy to the temperatures of subsurface ocean, where the signal is probably formed, yields colder ngeo2763-m5jpg20K4522-temperatures and latitudinal gradient consistent with standard climate model simulations of the Eocene climate, invalidating the apparent, extremely warm polar sea surface temperatures. We conclude that there is a need to reinterpret ngeo2763-m5jpg20K4522-inferred marine temperature records in the literature, especially for reconstructions of past warm climates that rely heavily on this proxy as reflecting subsurface ocean.

Description: Nature Geoscience 9, 643 (2016). doi:10.1038/ngeo2768 Author: Willem W. Verstraeten, Jessica L. Neu, Jason E. Williams, Kevin W. Bowman, John R. Worden & K. Folkert Boersma

Description: Nature Geoscience 9, 615 (2016). doi:10.1038/ngeo2760 Authors: Michael S. Steckler, Dhiman Ranjan Mondal, Syed Humayun Akhter, Leonardo Seeber, Lujia Feng, Jonathan Gale, Emma M. Hill & Michael Howe The Indo-Burman mountain ranges mark the boundary between the Indian and Eurasian plates, north of the Sumatra–Andaman subduction zone. Whether subduction still occurs along this subaerial section of the plate boundary, with 46 mm yr−1 of highly oblique motion, is contentious. About 21 mm yr−1 of shear motion is taken up along the Sagaing Fault, on the eastern margin of the deformation zone. It has been suggested that the remainder of the relative motion is taken up largely or entirely by horizontal strike-slip faulting and that subduction has stopped. Here we present GPS measurements of plate motions in Bangladesh, combined with measurements from Myanmar and northeast India, taking advantage of a more than 300 km subaerial accretionary prism spanning the Indo-Burman Ranges to the Ganges–Brahmaputra Delta. They reveal 13–17 mm yr−1 of plate convergence on an active, shallowly dipping and locked megathrust fault. Most of the strike-slip motion occurs on a few steep faults, consistent with patterns of strain partitioning in subduction zones. Our results strongly suggest that subduction in this region is active, despite the highly oblique plate motion and thick sediments. We suggest that the presence of a locked megathrust plate boundary represents an underappreciated hazard in one of the most densely populated regions of the world.

Description: Nature Geoscience 9, 619 (2016). doi:10.1038/ngeo2759 Authors: Marcia Maia, Susanna Sichel, Anne Briais, Daniele Brunelli, Marco Ligi, Nicolas Ferreira, Thomas Campos, Bérengère Mougel, Isa Brehme, Christophe Hémond, Akihisa Motoki, Denise Moura, Carla Scalabrin, Ivo Pessanha, Eliane Alves, Arthur Ayres & Pedro Oliveira Mantle exhumation at slow-spreading ridges is favoured by extensional tectonics through low-angle detachment faults, and, along transforms, by transtension due to changes in ridge/transform geometry. Less common, exhumation by compressive stresses has been proposed for the large-offset transforms of the equatorial Atlantic. Here we show, using high-resolution bathymetry, seismic and gravity data, that the northern transform fault of the St Paul system has been controlled by compressive deformation since ∼10 million years ago. The long-lived transpression resulted from ridge overlap due to the propagation of the northern Mid-Atlantic Ridge segment into the transform domain, which induced the migration and segmentation of the transform fault creating restraining stepovers. An anticlockwise change in plate motion at ∼11 million years ago initially favoured extension in the left-stepping transform, triggering the formation of a transverse ridge, later uplifted through transpression, forming the St Peter and St Paul islets. Enhanced melt supply at the ridge axis due to the nearby Sierra Leone thermo chemical anomaly is responsible for the robust response of the northern Mid-Atlantic Ridge segment to the kinematic change. The long-lived process at the origin of the compressive stresses is directly linked to the nature of the underlying mantle and not to a change in the far-field stress regime.

Description: Nature Geoscience 9, 636 (2016). doi:10.1038/ngeo2745 Authors: Brandon P. VanderBeek, Douglas R. Toomey, Emilie E. E. Hooft & William S. D. Wilcock

Description: Nature Geoscience 9, 395 (2016). doi:10.1038/ngeo2686 Authors: Brian D. Collins & Greg M. Stock

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 8, 383 (2015). doi:10.1038/ngeo2422 Authors: Thomas K. Bauska, Fortunat Joos, Alan C. Mix, Raphael Roth, Jinho Ahn & Edward J. Brook The stability of terrestrial carbon reservoirs is thought to be closely linked to variations in climate, but the magnitude of carbon–climate feedbacks has proved difficult to constrain for both modern and millennial timescales. Reconstructions of atmospheric CO2 concentrations for the past thousand years have shown fluctuations on multidecadal to centennial timescales, but the causes of these fluctuations are unclear. Here we report high-resolution carbon isotope measurements of CO2 trapped within the ice of the West Antarctic Ice Sheet Divide ice core for the past 1,000 years. We use a deconvolution approach to show that changes in terrestrial organic carbon stores best explain the observed multidecadal variations in the δ13C of CO2 and in CO2 concentrations from 755 to 1850 CE. If significant long-term carbon emissions came from pre-industrial anthropogenic land-use changes over this interval, the emissions must have been offset by a natural terrestrial sink for 13C-depleted carbon, such as peatlands. We find that on multidecadal timescales, carbon cycle changes seem to vary with reconstructed regional climate changes. We conclude that climate variability could be an important control of fluctuations in land carbon storage on these timescales.

Description: Nature Geoscience 8, 398 (2015). doi:10.1038/ngeo2405 Authors: Cheng Li & Andrew P. Ingersoll

Description: Nature Geoscience 8, 409 (2015). doi:10.1038/ngeo2419 Author: Heidi Houston

Description: Nature Geoscience 8, 339 (2015). doi:10.1038/ngeo2399 Authors: Dake Chen, Tao Lian, Congbin Fu, Mark A. Cane, Youmin Tang, Raghu Murtugudde, Xunshu Song, Qiaoyan Wu & Lei Zhou

Description: Nature Geoscience 8, 352 (2015). doi:10.1038/ngeo2397 Authors: Megan Bruck Syal, Peter H. Schultz & Miriam A. Riner Mercury’s surface is darker than that of the Moon. Iron-bearing minerals and submicroscopic metallic iron produced by space weathering are the primary known darkening materials on airless bodies. Yet Mercury’s iron abundance at the surface is lower than the Moon’s; another material is therefore likely to be responsible for Mercury’s dark surface. Enhanced darkening by submicroscopic metallic iron particles under intense space weathering at Mercury’s surface is insufficient to reconcile the planet’s low reflectance with its low iron abundance. Here we show that the delivery of cometary carbon by micrometeorites provides a mechanism to darken Mercury’s surface without violating observational constraints on iron content. We calculate the micrometeorite flux at Mercury and numerically simulate the fraction of carbonaceous material retained by the planet following micrometeorite impacts. We estimate that 50 times as many carbon-rich micrometeorites per unit surface area are delivered to Mercury, compared with the Moon, resulting in approximately 3–6 wt% carbon at Mercury’s surface (in graphite, amorphous, or nanodiamond form). Spectroscopic analysis of products of hypervelocity impact experiments demonstrates that the incorporation of carbon effectively darkens and weakens spectral features, consistent with remote observations of Mercury. Carbon delivery by micrometeorites provides an explanation for Mercury’s globally low reflectance and may contribute to the darkening of planetary surfaces elsewhere.

Description: Nature Geoscience 8, 346 (2015). doi:10.1038/ngeo2414 Authors: Thorsten Mauritsen & Bjorn Stevens

Description: Nature Geoscience 8, 357 (2015). doi:10.1038/ngeo2412 Authors: F. Javier Martín-Torres, María-Paz Zorzano, Patricia Valentín-Serrano, Ari-Matti Harri, Maria Genzer, Osku Kemppinen, Edgard G. Rivera-Valentin, Insoo Jun, James Wray, Morten Bo Madsen, Walter Goetz, Alfred S. McEwen, Craig Hardgrove, Nilton Renno, Vincent F. Chevrier, Michael Mischna, Rafael Navarro-González, Jesús Martínez-Frías, Pamela Conrad, Tim McConnochie, Charles Cockell, Gilles Berger, Ashwin R. Vasavada, Dawn Sumner & David Vaniman Water is a requirement for life as we know it. Indirect evidence of transient liquid water has been observed from orbiter on equatorial Mars, in contrast with expectations from large-scale climate models. The presence of perchlorate salts, which have been detected at Gale crater on equatorial Mars by the Curiosity rover, lowers the freezing temperature of water. Moreover, perchlorates can form stable hydrated compounds and liquid solutions by absorbing atmospheric water vapour through deliquescence. Here we analyse relative humidity, air temperature and ground temperature data from the Curiosity rover at Gale crater and find that the observations support the formation of night-time transient liquid brines in the uppermost 5 cm of the subsurface that then evaporate after sunrise. We also find that changes in the hydration state of salts within the uppermost 15 cm of the subsurface, as measured by Curiosity, are consistent with an active exchange of water at the atmosphere–soil interface. However, the water activity and temperature are probably too low to support terrestrial organisms. Perchlorates are widespread on the surface of Mars and we expect that liquid brines are abundant beyond equatorial regions where atmospheric humidity is higher and temperatures are lower.

Description: Nature Geoscience 8, 362 (2015). doi:10.1038/ngeo2406 Authors: Benjamin Charnay, Erika Barth, Scot Rafkin, Clément Narteau, Sébastien Lebonnois, Sébastien Rodriguez, Sylvain Courrech du Pont & Antoine Lucas The equatorial regions of Saturn’s moon Titan are covered by linear dunes that propagate eastwards. Global climate models (GCMs), however, predict westward mean surface winds at low latitudes on Titan, similar to the trade winds on Earth. This apparent contradiction has been attributed to Saturn’s gravitational tides, large-scale topography and wind statistics, but none of these hypotheses fully explains the global eastward propagation of dunes in Titan’s equatorial band. However, above altitudes of about 5 km, Titan’s atmosphere is in eastward super-rotation, suggesting that this momentum may be delivered to the surface. Here we assess the influence of equatorial tropical methane storms—which develop at high altitudes during the equinox—on Titan’s dune orientation, using mesoscale simulations of convective methane clouds with a GCM wind profile that includes super-rotation. We find that these storms produce fast eastward gust fronts above the surface that exceed the normal westward surface winds. These episodic gusts generated by tropical storms are expected to dominate aeolian transport, leading to eastward propagation of dunes. We therefore suggest a coupling between super-rotation, tropical methane storms and dune formation on Titan. This framework, applied to GCM predictions and analogies to some terrestrial dune fields, explains the linear shape, eastward propagation and poleward divergence of Titan’s dunes, and implies an equatorial origin of dune sand.

Description: Nature Geoscience 8, 367 (2015). doi:10.1038/ngeo2400 Authors: William R. L. Anderegg, Alan Flint, Cho-ying Huang, Lorraine Flint, Joseph A. Berry, Frank W. Davis, John S. Sperry & Christopher B. Field The projected responses of forest ecosystems to warming and drying associated with twenty-first-century climate change vary widely from resiliency to widespread tree mortality. Current vegetation models lack the ability to account for mortality of overstorey trees during extreme drought owing to uncertainties in mechanisms and thresholds causing mortality. Here we assess the causes of tree mortality, using field measurements of branch hydraulic conductivity during ongoing mortality in Populus tremuloides in the southwestern United States and a detailed plant hydraulics model. We identify a lethal plant water stress threshold that corresponds with a loss of vascular transport capacity from air entry into the xylem. We then use this hydraulic-based threshold to simulate forest dieback during historical drought, and compare predictions against three independent mortality data sets. The hydraulic threshold predicted with 75% accuracy regional patterns of tree mortality as found in field plots and mortality maps derived from Landsat imagery. In a high-emissions scenario, climate models project that drought stress will exceed the observed mortality threshold in the southwestern United States by the 2050s. Our approach provides a powerful and tractable way of incorporating tree mortality into vegetation models to resolve uncertainty over the fate of forest ecosystems in a changing climate.

Description: Nature Geoscience 8, 335 (2015). doi:10.1038/ngeo2432 Author: Jed O. Kaplan Analyses of ice-core carbon isotopes show that variations in atmospheric CO2 levels during the past millennium are controlled by changes in land reservoirs. But whether climate variations or human activity were mainly responsible is uncertain.

Description: Nature Geoscience 8, 337 (2015). doi:10.1038/ngeo2435 Author: Amy Whitchurch

Description: Nature Geoscience 8, 388 (2015). doi:10.1038/ngeo2401 Authors: A. Koptev, E. Calais, E. Burov, S. Leroy & T. Gerya Although many continental rifts and passive margins are magmatic, some are not. This observation prompted endmember views of the mechanisms driving continental rifting, where magma-rich or active rifts would be caused by deep mantle plumes, whereas magma-poor or passive rifts would result from the stretching of the lithosphere under far-field plate forces. The Central East African Rift provides a unique setting to investigate the mechanisms of continental rifting because it juxtaposes a magma-rich (eastern) branch and magma-poor (western) branch on either side of the 250-km-thick Tanzanian craton. Here we investigate this contrasted behavior using a high-resolution rheologically consistent three-dimensional thermo-mechanical numerical model. The model reproduces the rise of a mantle plume beneath a craton experiencing tensional far-field stress. In our numerical experiments the plume is deflected by the cratonic keel and preferentially channelled along one of its sides. This leads to the coeval development of magma-rich and magma-poor rifts along opposite craton sides, fed by melt from a single mantle source. Our numerical experiments show strong similarities to the observed evolution of the Central East African Rift, reconcile the passive and active rift models, and demonstrate the possibility of developing both magmatic and amagmatic rifts in identical geotectonic environments.

Description: Nature Geoscience 8, 393 (2015). doi:10.1038/ngeo2416 Authors: John M. O’Connor, Kaj Hoernle, R. Dietmar Müller, Jason P. Morgan, Nathaniel P. Butterworth, Folkmar Hauff, David T. Sandwell, Wilfried Jokat, Jan R. Wijbrans & Peter Stoffers Ocean islands, seamounts and volcanic ridges are thought to form above mantle plumes. Yet, this mechanism cannot explain many volcanic features on the Pacific Ocean floor and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions. A distinctive bend in the Hawaiian–Emperor volcanic chain has been linked to changes in the direction of motion of the Pacific Plate, movement of the Hawaiian plume, or a combination of both. However, these links are uncertain because there is no independent record that precisely dates tectonic events that affected the Pacific Plate. Here we analyse the geochemical characteristics of lava samples collected from the Musicians Ridges, lines of volcanic seamounts formed close to the Hawaiian–Emperor bend. We find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid-ocean ridge basalts. We infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting, due to deformation of the Pacific Plate. 40Ar/39Ar dating reveals that the Musicians Ridges formed during two time windows that bracket the time of formation of the Hawaiian–Emperor bend, 53–52 and 48–47 million years ago. We conclude that the Hawaiian–Emperor bend was formed by plate–mantle reorganization, potentially triggered by a series of subduction events at the Pacific Plate margins.

Description: Nature Geoscience 8, 404 (2015). doi:10.1038/ngeo2417 Authors: K. J. Walowski, P. J. Wallace, E. H. Hauri, I. Wada & M. A. Clynne