Rheological properties of continental lower crust (CLC) are essential for understanding many geodynamical processes in the Earth. Here we performed a series of deformation experiments on synthetic felsic granulite (65% plagioclase + 15% quartz + 9% clinopyroxene + 6% orthopyroxene + 5% amphibole) using a 5 GPa modified Griggs-type deformation apparatus at 827-927 {degree sign}C and 1 GPa. All experiments reached a steady-state creep at strain rates ranging from 3.0×10-6 s-1 to 1.0×10-4 s-1, yielding a stress exponent of 4.2 {plus minus} 0.1, a pre-exponential factor of 10-4.4{plus minus}0.2 MPa-4.2 s-1, and an activation energy of 260 {plus minus} 30 kJ/mol. Microstructural observations show that plagioclase and pyroxene in deformed samples develop noticeable intracrystalline plasticity and shape preferred orientation (SPO). Electron backscatter diffraction (EBSD) measurements demonstrate more significant crystallographic preferred orientations (CPOs) of pyroxene and plagioclase in deformed granulites compared to those in hot-pressed ones. By contrast, quartz develops nearly random fabrics probably due to the simultaneous activation of multiple slip systems during deformation. These features indicate that dislocation creep dominates the deformation of these minerals and felsic granulite, consistent with the obtained stress exponent. Our data imply that a CLC consisting of felsic granulite is weaker rheologically compared to the quartz-dominated upper crust and the olivine-dominated uppermost mantle, which supports the ‘jelly sandwich’ model for the strength of continental lithosphere. Additionally, extrapolations of flow laws of our felsic granulite and formerly-reported mafic granulite strongly favor the hypothesis of delamination-induced decratonization of the North China Craton.

Mixed pixels are the rule, not the exception, in decameter terrestrial imaging. By definition, the reflectance spectrum of a mixed pixel is a function of more than one generative process. Physically-based surface biology or geology retrievals must therefore isolate the component of interest from a myriad of unrelated processes, heterogenously distributed across hundreds of square meters. Foliar traits, for example, must be isolated from canopy structure and substrate composition which can dominate overall variance of spatially integrated reflectance. We propose a new approach to isolate low-variance spectral signatures. The reflectance of each pixel is modeled assuming linear geographic mixing due to a small library of generic endmembers. The difference between the modeled and observed spectra is deemed the Mixture Residual (MR). The MR, a residual reflectance spectrum that is presumed to carry the subtler and variable signals of interest, is then leveraged as a source of signal. We illustrate the approach using three datasets: synthetic composites computed from field reflectance spectra, NEON AOP airborne image compilations, and DESIS satellite data. The MR discriminates between land cover versus plant trait signals and accentuates subtle absorption features. Mean band-to-band correlations within the visible, NIR, and SWIR wavebands decrease from 0.97, 0.94, and 0.97 to 0.95, 0.04 and 0.31. The number of dimensions required to explain 99% of image variance increases from 4 to 13. We focus on vegetation as an illustrative example, but note that the concept can be extended to other applications and used as an input to other algorithms.

Earth's millennial climate cycle has been studied by scientists for decades, but its cause has been lacking. It warms the Earth for several hundred years and melts some of the ice sheets in Greenland and Antarctica. My analysis discovered that it is caused by the astronomical conjunction of the planets Jupiter, Saturn and Neptune. This paper describes the observations that led to its discovery and explains how its 1,470-year spin-orbit resonance cycle warms the Earth with geothermal heat. The current cycle started warming the Earth in the year 1791 and will probably reach peak temperature in year 2060 at the next Jupiter, Saturn and Neptune conjunction. Then it will cause cooling oscillations for the rest of the cycle.

The retrieval of the asymmetry parameter from nephelometer measurements can be challenging due to the inability to detect the whole angular range. Here, we present a new method for retrieving the asymmetry parameter of ice crystals with relatively large size parameters (>50) from polar nephelometer measurements. We propose to fit the angular scattering measurement with a series of Legendre polynomials and the best fitted coefficients give the asymmetry parameter. The accuracy of the retrieval is analyzed by accessing the smoothness of the phase function, which is closely linked to the complexity of ice particle. It is found that the uncertainty of retrieval could be smaller than 0.01, provided the measured intensity profile is smooth enough. As an application, we report an case study on Arctic cirrus, which shows a mean value for the asymmetry parameter of 0.72.

The origin of widespread volcanism far from plate boundaries and mantle plumes remains a fundamental unsolved question. An example of this puzzle is the Anatolian region, where abundant intraplate volcanism has occurred since 10 Ma, but a nearby underlying plume structure in the deep mantle is lacking. We employed a combination of seismic and geochemical data to link intraplate volcanism in Anatolia to a trail of magmatic centers leading back to East Africa and its mantle plume, consistent with northward asthenospheric transport of over ~2500 km distance. Joint modeling of seismic imaging and petrological data indicates that the east Anatolian mantle potential temperature is higher than the ambient mantle (~1420C). Based on multiple seismic tomography models, the Anatolian upper mantle is likely connected to East Africa by an asthenospheric channel with low seismic velocities. Along the channel, isotopic signatures among volcanoes are consistent with a common mantle source, and petrological data demonstrate similar elevated mantle temperatures, consistent with little cooling in the channel during the long-distance transport. Horizontal asthenospheric pressure gradients originating from mantle plume upwelling beneath East Africa provide a mechanism for high lateral transport rates that match the relatively constant mantle potential temperatures along the channel. Rapid long-distance asthenospheric flow helps explain the widespread occurrence of global intraplate magmatism in regions far from deeply-rooted mantle plumes throughout Earth history.

The commonly applied pedotransfer functions (PTFs), which predict soil hydraulic properties (SHPs) from easily measured soil properties such as texture information, often account only for capillary forces. Recent advances in soil hydraulic modeling suggest that, to improve the prediction of SHPs under dry conditions, the impact of adsorption forces has to be taken into account. However, the lack of observations in particularly dry conditions, due to the difficult and time-consuming measurement, hinders the development of PTFs that predict SHPs from saturation to oven dryness. In this paper, we first present a simple method for predicting complete SHPs with limited measurements that cover only a relatively high matric potential range. With this method, we extended a public dataset to cover dry conditions, and then applied it to develop PTFs that can predict SHPs from saturation to oven dryness. This was achieved by applying the complete soil hydraulic model proposed by Wang et al. (2021), which accounts for both capillary and adsorptions forces and overcomes the unrealistic decrease near saturation for fine-textured soils. The impact of vapor diffusion was also considered. We further applied this method in extending an existing capillary-based PTF to dry conditions. The results showed that: 1) the proposed method performs very well in describing SHPs over the entire moisture range; 2) the PTFs developed with the extended observations and the complete model show a superior prediction performance, especially for the hydraulic conductivity; and 3) the extended capillary-based PTF improves the performance in describing SHPs under dry conditions.

Mantle transition zone (MTZ) is the layer between two seismic upper mantle discontinuities, which are generally believed to be caused by the transformation of high-pressure polymorphs of olivine occurred around depth of 410 km (d410) and 660 km (d660). The fine structure around the discontinuities, especially the undulation of them can be used to infer the variation of temperature and/or composition in the deep mantle. Seismic images have revealed a subhorizontally deflected slab beneath Northeast China, and thus the seismic mapping of the interaction between the slab and the deep mantle is important in understanding the mantle circulation as well as the upper mantle composition. Recently, a temporary NECsaids Array has been installed, and it consists mainly of two nearly orthogonal dense profiles with station spacing of 15–20 km, which provides us an unprecedented opportunity to obtain a high-resolution 3D topography of the MTZ. Considering the dipping and stagnant structure of the slab, we used a novel Ps scatter kernel migration technique to obtain the detailed image of the d410 and d660, which has been proved to be efficient and better for more complex structures. Our results show that the boundaries of the subducting Northwest Pacific slab can be clearly identified in the image, with a dip angle of about 30° in the MTZ. The depth of the d660 depressed 20 - 40 km around the stagnant part of the subducting slab, which could be explained with the lower temperature caused by the cold Pacific slab. Moreover, the d660 is uplifted about 5 - 15 km beneath northwest of Changbaishan and Longgang Volcano, suggesting that the upwelling feeding the volcanoes might originate from the MTZ. Apart from the prominent depression of the d410 observed in the volcanic area, we also observed a wide-distributed depression of the d410 with value ~10 km over most studied areas, seeming inconsistent with the thermal anomaly associated with the subduction environment. We argue that subtle variations of water and/or chemical composition contribute to the observed depth variation of the d410, and the apparent depressed “410” can be related to the wet/dry wadsleyite boundary, which might be caused by the high water content at the top of the MTZ.

Our understanding of the past behavior of the geomagnetic field arises from magnetic signals stored in geological materials, e.g. (volcanic) rocks. Bulk rock samples, however, often contain magnetic grains that differ in chemistry, size and shape; some of them record the Earth’s magnetic field well, others are unreliable. The presence of a small amount of adverse behaved magnetic grains in a sample may already obscure important information on the past state of the geomagnetic field. Recently it was shown that it is possible to determine magnetizations of individual grains in a sample by combining X-ray computed tomography and magnetic surface scanning measurements. Here we establish this new Micromagnetic Tomography (MMT) technique and make it suitable for use with different magnetic scanning techniques, and for both synthetic and natural samples. We acquired reliable magnetic directions by selecting subsets of grains in a synthetic sample, and we obtained rock-magnetic information of individual grains in a volcanic sample. This illustrates that MMT opens up entirely new venues of paleomagnetic and rock-magnetic research. MMT’s unique ability to determine the magnetization of individual grains in a nondestructive way allows for a systematic analysis of how geological materials record and retain information on the past state of the Earth’s magnetic field. Moreover, by interpreting only the contributions of known magnetically well-behaved grains in a sample MMT has the potential to unlock paleomagnetic information from even the most complex, crucial, or valuable recorders that current methods are unable to recover.

South China, India and their derivative terranes/blocks preserve a larger amount of similar magmatic and sedimentary records related to the tectonic transition from Rodinia to Gondwana. They provide crucial insights into not only the paleogeographic correlation between them but also the geodynamic mechanism for such a transition. Our new results together with published big data from these terranes/blocks point out that South China kept a linkage with India at least from the late Tonian (ca. 830 Ma) to Early Cambrian and formed the South China-India Duo located at the western margin of Rodinia. The identical magmatism and sedimentation reflect that double late Neoproterozoic rift systems in the South China-India Duo could have developed owing to the rollback of subducting oceanic slab beneath them, including an intracontinental rift (the Nanhua-Aravalli-Delhi rift) separating the Yangtze-Marwar from Cathaysia-Bundelkhand terranes and a contemporaneous intra-arc rift along the northern and western margins of the Yangtze Terrane, through the Marwar Terrane of western India, and then into the Seychelles and Madagascar terranes. Such an intra-arc rift is also the most feasible explanation for the common development of coeval arc-like and extension-related magmatic rocks and extensional sedimentary sequences on the western margin of the South China-India Duo and in Seychelles and Madagascar, and even other subduction zones.

Earth’s magnetotail is filled with solar wind and ionospheric electrons, whose initial energies are significantly lower than the typical energies (temperatures) of plasmasheet electrons. One of the most common mechanisms responsible for heating of solar wind and ionospheric electrons in Earth’s magnetotail is adiabatic heating caused by earthward convection of these electrons from the deep tail (i.e., from the region of a weak magnetic field) towards the region of stronger magnetic fields closer to Earth. This heating is moderated by electron losses into the ionosphere due to local wave scattering. In this study, we compare electron spectra from simultaneous observations of The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at different radial distances with spectra obtained from a simple model that includes adiabatic heating and losses. Our comparison shows that the model heating significantly overestimates the increase in energetic (>1 keV) electron fluxes, indicating that losses are essential for accurate modelling of the observed spectra. The required electron losses are similar to or even greater than the losses in the strong diffusion limit (when the loss cone is full). The latter can be interpreted as loss cone widening by field-aligned electron acceleration.

The GEOSS Water Strategy–From Observations to Decisions (Lawford et al., 2014) identifies several key water variables as Essential Water Variables (EWVs). This was based on broad meta-surveys (Unninayar et al., 2010) of water-related observational needs for GEO Societal Benefit Aeas (SBAs) that included energy and agriculture among others. This paper summarizes currently defined EWVs required by key research and applications sectors involved in decision support within the Water-Energy-Food (WEF) Nexus. There is a notable overlap between the EWVs and a preliminary assessment of Essential WEF Variables. We also highlight EWVs that are relevant to the indicator monitoring objectives of the UN Sustainable Development Goals (SDGs), and GEOGLOWS priority thematic communities—especially those that overlap with the WEF Nexus. Besides primary EWVs such as precipitation, soil moisture, evapotranspiration and water levels/storage, supplemental EWVs are identified that support the integrated multi-sectoral information needed by WEF decision support applications. Examples of supplemental EWVs include surface meteorology and winds, solar radiation, land use/land cover and vegetation. Both Remote Sensing (RS) platforms and In-Situ observing networks are required to address the broad range of space/time resolutions, accuracies, and data latencies that end-users need. The AGU-2021 is invited to comment on, endorse and/or recommend additional EWVs that should be considered for adoption by GEOGLOWS and the GEO (Group on Earth Observations).

We use seismic ambient noise recorded by ocean bottom nodes (OBNs) in the Gorgon gas field, Western Australia to compute time-lapse seafloor models. The extracted hourly cross-correlation (CC) functions of 0.1 – 1 Hz contain mainly Scholte waves with very high signal to noise ratio. The conventional time-lapse analysis suggests relative velocity variations (dv/v) up to 1% assuming a spatially homogeneous dv/v, with a likely 24-hour cycling pattern. With high-resolution baseline models from full waveform inversion of Scholte waves, we propose a double-difference waveform inversion (DD-WI) method using travel time differences for localizing the time-lapse dv/v in the heterogeneous subsurface in depth. The time-lapse velocity models show velocity increase/decrease patterns in agreement with that from conventional analysis, with more notable changes at the shallower depths. We demonstrate the feasibility of using ambient noise for quantitative monitoring of subsurface property changes in the horizontal and depth domain at an hourly basis.

Venus is the planet in the Solar System most similar to Earth in terms of size and (probably) bulk composition. Until the mid-20th century, scientists thought that Venus was a verdant world—inspiring science-fictional stories of heroes battling megafauna in sprawling jungles. At the start of the Space Age, people learned that Venus actually has a hellish surface, baked by the greenhouse effect under a thick, CO2-rich atmosphere. In popular culture, Venus was demoted from a jungly playground to (at best) a metaphor for the redemptive potential of extreme adversity. However, whether Venus was much different in the past than it is today remains unknown. In this review, we show how now-popular models for the evolution of Venus mirror how the scientific understanding of modern Venus has changed over time. Billions of years ago, Venus could have had a clement surface with water oceans. Venus perhaps then underwent at least one dramatic transition in atmospheric, surface, and interior conditions before present day. This review kicks off a topical collection about all aspects of Venus’s evolution and how understanding Venus can teach us about other planets, including exoplanets. Here we provide the general background and motivation required to delve into the other manuscripts in this collection. Finally, we discuss how our ignorance about the evolution of Venus motivated the prioritization of new spacecraft missions that will essentially rediscover Earth’s nearest planetary neighbor—beginning a new age of Venus exploration.

We present a study of the Schumann Resonance (SR) regular variations (March 2013-February 2017) using the ground-based magnetometers from the Sierra Nevada station, Spain (37◦02’N, 3◦19’W). The study is based on the fitting parameters obtained by the Lorentzian fit, calculated for each 10-min interval record, namely, peak amplitudes, central frequencies, width of the resonances and the power spectrum integral for the first 3 SR modes. We consider three time-scales in the study: seasonal, monthly and daily variations. The processed data collected by the Sierra Nevada station are also made public with this work. The general characteristics of the long-term evolution of the Schumann resonance are confirmed, but discrepancies appear that require further study comparing recent measurements from different stations. Signatures of the influences of the El Niño phenomenon and the solar cycle to SR have been found.

Strains occur at shallow depths in response to pressure changes during well tests in an underlying aquifer, and recent developments in instrumentation have made it feasible to measure essentially the full strain tensor. Simulations using poroelastic analyses indicate that shallow normal strains are approximately proportional to the logarithm of time when a well is injecting into or pumping from a uniform aquifer or reservoir. The drawdown is also a function of log time, as shown by the classic Cooper-Jacob type-curve analysis. The time when the semi-log straight line intercepts the zero-strain axis is similar to the time determined from the Cooper-Jacob pressure analysis, and it can be used to estimate hydraulic diffusivity, suggesting that horizontal strain data can be used directly to estimate aquifer properties. This approach is applied to data measured with shallow (30 m) borehole strainmeters during an injection test at a 530-m-deep sandstone aquifer/reservoir in Oklahoma. The results show intercept times for the shallow normal strain data are essentially the same as for deep pressure data from an equivalent radial distance. The slopes of the semi-log plots of the pressure and the strain increase at the same time, suggesting that they both respond to a lateral aquifer boundary. These results confirm the type-curve approach for interpreting strain data. Significantly, though, strain was measured at shallow depths while the pressure data was measured at 530 m depth. This suggests that strain data from shallow depths could be an effective way to improve the characterization of an underlying aquifer.

A geomorphological key paradigm predicts that intact forests are erosional idle, however comprise an efficient weathering machine sustaining high soil production rates. Only during times of disturbance, e.g., by earthquakes, those forests are observed to jump up to high-erosion-state, then being capable of releasing some of Earth’s highest sediment yields involving massive pulses of organic carbon. Coastal temperate rainforests, in particular, do not only store unparalleled carbon stocks building up a globally important carbon sink, but are also home to high (endemic) biodiversity. Here we document extraordinarily high catchment-averaged denudation rates, across multiple disturbance cycles, under the dense vegetation of the Patagonian rainforests. There, 10 Be-derived denudation rates of >0.8 m kyr^-1 exceed any known value from the entire Chilean Andes orogen, a highly variable >3.000 km long natural laboratory involving steep climatic and topographic gradients. We argue that such high denudation rates are consistent with a first-order control of the rainforest itself. High biomass loads exert a soil surcharge that promotes landsliding already along a relatively low critical slope angle. In contrast, denudation rates from more arid, and less forested sectors of the Chilean Andes though going along with steeper critical slope angles remain below half of our new rates derived from the Patagonian rainforests. Taken together, our study provides indication that denudation, to a higher degree than hitherto agreed on, operates as a continuous process involving soil production, vegetation, physical erosion and ecohydrological processes. Such a holistic denudational continuum, finally, is different from prevailing views that vegetation generally stabilizes hillslopes, thus promoting steep slope gradients, however, limiting landsliding activity.

Drought risk assessment can identify high-risk areas and bridge the gap between impacts and adaptation. However, very few dynamic drought risk assessments and projections have been performed worldwide at high spatial resolution (e.g., 0.5{degree sign} × 0.5{degree sign}) under different greenhouse gas emission scenarios. Here, future global drought risk is projected combing three components (i.e., hazard, exposure, and vulnerability) during 2021-2100 under combined scenarios of Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs): SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. This study first investigates dynamic drought risks and exposed population and GDP across the six continents (Antarctica is not examined due to data availability). The results show that high-risk regions mainly concentrate in southeastern China, India, Western Europe, eastern United States, and western and eastern Africa. Drought risk will further strengthen in the future under four scenarios, with the highest under SSP5-8.5 and the lowest under SSP3-7.0. Populations exposed to high drought risk for Asia and Africa are much more than other continents. Among four SSP-RCPs, populations exposed to high risk are the largest under SSP3-7.0 for Africa, Asia, and South America, while under SSP5-8.5 for Australia, Europe, and North America. GDP exposed to high drought risk is the largest for Asia among the six continents and the largest under SSP5-8.5 among the SSP-RCPs. The most significant increases in population and GDP under high drought risk both occur in Africa. This study provides a scientific basis for effective adaptation measures to enhance drought resilience in potential high-risk areas.

A document by Hao Chen. Click on the document to view its contents.