Chang’E-4, the first soft landing mission on the far side of the Moon, provided high-resolution close-range images up to 2.3 cm/pixel, especially suitable for the study of the degradation in the population of small-sized impact craters around the landing site. This paper presents a preliminary analysis of the population degradation mechanism of the small-sized impact craters. From mosaicked descent camera image, 6316 impact craters in the landing area were extracted, identified, and classified into four types according to their morphology: complete, slightly degraded, moderately degraded, and severely degraded craters. In-situ verification using the lander topographic camera image supported the crater degradation analysis around the landing site. The small-sized impact crater population equilibrium analysis was also carried out. Over 99% of the impact craters in the study area are in degradation, which suggesting the area is severely degraded. Our results suggest that similar to the findings for impact crater populations above the meter scale, the size of meter to centimeter scale impact craters is also strongly correlated with degradation, with smaller impact craters being more likely to degrade. Images from topographic cameras also confirm the severe degradation of small-scale craters. The crater populations in equilibrium of different resolutions areas shows that the cumulative SFD slope is contrary to previous research results, the smaller the size of the impact crater, the more difficult for them to achieve impact equilibrium, which due to secondary craters and the ground resurfacing caused by neighboring craters’ ejecta.

Flow in formations having both low-porosity and low-permeability prevalently occurs in fracture networks. Fracture connectivity and permeability are the main features governing flow in these media, and affect the effectiveness of any subsurface operation (e.g. enhanced oil recovery, enhanced geothermal systems). Moreover, the rheology of the fluid is often non-Newtonian due to their complex microstructure, resulting in a non-linear constitutive law at the continuum scale. Modeling flow of complex fluids in a fracture is challenging, when both medium heterogeneity and a realistic fluid rheology, typically shear-thinning, are considered. Full 3-D simulations are computationally expensive and time-consuming; alternatively, a lubrication-based approach can be adopted to implement an efficient 2-D flow solver able to produce vast statistics in reasonable time. We propose a numerical code that solves the 2-D generalized non-linear Reynolds equation for Ellis fluid flow in a single variable aperture fracture, adopting the finite volume method. The inexact Newton-Krylov method solves the associated non-linear system of equations, while a preconditioned conjugate gradient algorithm is used to solve the linear system at each Newton iteration. A parameter continuation strategy is also introduced to handle strongly non-linear cases. A synthetic fracture aperture field w(x) is generated and the flow problem is solved. The numerical scheme is particularly efficient and robust for most input parameters of practical interest. Numerical parameters of the Inexact Newton-Krylov method have been optimized to simulate flow on large mesh (e.g. 1010x1010), considering a range of fracture closure (σw/‹w›) from 0 to 1, a shear-thinning index (n) of the Ellis rheologic model varying from 0.1 (strongly shear-thinning behavior) to 1 (Newtonian case). Pressure gradients (∇P) typical of subsurface flow, either natural or forced, have been considered to study natural phenomena and industrial applications.

The Gangdese arc magmatism was active in response to the Mesozoic subduction of the Neo-Tethyan oceanic lithosphere and the Cenozoic collision between the Indian and Asian continents. The Paleocene Gangdese batholith in southern Tibet is intruded by several voluminous peraluminous leucogranites, with the origin of these leucogranites remaining controversial. In this study, we investigate a small garnet-bearing leucogranite intrusion in the Nyingchi Complex that represents the exposed lower crust of the Lhasa Terrane and underwent multi-stages of amphibolite- to granulite-facies metamorphism since the Cretaceous. Bulk-rock mineralogy and geochemistry display S-type granite signatures, indicating its anatexic origin. The center of the ~300 m-wide pluton contains abundant garnet in a perthite-quartz matrix. The garnets, from core to rim, display stepwise concentrations of major and rare earth elements, indicating multiple stages of growth. The cores are featured by three groups of oriented needles of rutile, which are regarded as products of exsolution due to rapid decompression or cooling and have been found almost exclusively in extreme temperature and/or pressure conditions. The Ti-in-rutile temperatures range from 900 to 850 oC, consistent with the temperatures derived from the ternary feldspar thermometer. These rutile-rich garnet cores were likely seeded from the lower crustal resitites and incorporated in an upwelling of hot mantle-derived magma. The peritectic garnet mantle continued to grow in the melt at 800-650 oC, as suggested by the Zircon saturation temperature and the Ti-in-zircon thermometer. The diffusion profiles between the Ti-rich core and mantle yield a short ~25 kyr duration of cooling from 870 °C to 700 °C, when the leucogranite magma migrated and emplaced. Zircons from the granite show complex textures; most of the cores and magmatic mantles yield U-Pb ages between 60 and 50 Ma, suggesting a syn-collisional origin of the melt. We speculate that the Paleogene slab roll-back and slab break-off would have resulted in enhanced asthenospheric corner flow and upwells, supplying a long-lived heat source for coeval crustal anatexis and metamorphism in southern Lhasa during the early phase of continental collision.

Understanding the physical and chemical dynamics of caldera magma chambers prior to VEI 7+ eruptions is important for eruption forecasting and societal preparedness. The 1.26 Ma, 400 km3 Tshirege Member of the Bandelier Tuff is the later of two zoned rhyolitic ignimbrites erupted from the Valles caldera in the Jemez Mountains volcanic field, New Mexico, USA. To avoid effects of crystal-glass sorting, lithic contamination, and post-emplacement thermal alteration of glasses and mineral phases, we sampled whole pumices from non-welded tuff, including zones of otherwise welded tuff quenched against cold paleovalley walls. The tuff is broadly systematically chemically zoned from early-erupted high-silica rhyolite enriched in incompatible trace elements to late-erupted low-silica rhyolite. Whole-pumices indicate the upper portions of the Tshirege magma system were unaffected by convective stirring prior to the eruption, while some mixing and overturn is reflected by pumices from deeper in the system (i.e. stratigraphically higher in tuff sheet), likely closer to recharge sources. Analyses of well-preserved minerals, glasses and melt inclusions, and application of mineral thermometers and barometers, show a vertical stratification of temperature (710-840 °C), pressures of ~0.16 GPa, and variable [H2O] (~2.0-3.5 wt. %). Whole-pumice and melt inclusion chemistry supports crystal accumulation and subsequent melting and remobilization of the cumulate pile as a major contributor to the overall compositional zoning. 2-D thermal models indicate that the creation of the temperature gradient over ~10ka from mush development to mobilization would require volumetrically unreasonable influx of recharge magma (flux rate >30 km3/yr), suggesting that recharge heating is likely only affecting the lowest portions of the chamber. As a result, the thermal gradient likely existed prior to mobilization of the mush. Heating alone cannot affect thermal loosening of a mechanically locked mush to an eruptible crystallinity in reasonable time frames, and so input of volatiles such as H2O or CO2 from a second boiling of recharge magmas is required to melt and remobilize the crystal mush pile. The 1.26 Ma eruption was thus triggered by a final recharge event recorded as dacite pumice clasts distributed throughout the tuff.

The Sandynallah valley (Western Ghats, India) features one of the oldest peat accumulations in the world at >50 kyr and has been central to the reconstruction of late Quaternary paleoclimate using paleovegetation changes in the forest-grassland vegetation mosaic that coexist here. It is well-known that short-term disturbances (fire, frost, intense drought) can also cause vegetation switches when multiple stable states exist, but this framework has seldom been considered in paleoecology investigations. Using stable carbon isotope signatures (relative C3-C4 vegetation abundance) on the cellulose fraction from two well-dated peat cores ~170 m apart - Core 1 closer to the hillslope (32000 years old) and Core 2 from the centre of the valley floor (45,000 years old) - we looked at paleovegetation changes and the implications for paleoclimate reconstruction within the alternative stable states framework. Charcoal data from another sediment profile from the same valley was used to correlate with paleofires. We propose that the valley floor is bistable, switching between peat-forming vegetation ‘sedgeland’ and montane stunted evergreen forest ‘shola’, maintained by level of waterlogging. Core 1 shows shola-sedgeland dynamics with vegetation switching at c.22ka from shola (possibly due to fires) to a prolonged unstable state until 13 ka sustained by low waterlogging. Following a hiatus c.13-7 ka, sedgeland dominates, with a shift into shola at 3.75 ka driven by increasing aridity. Core 2 shows a relatively stable signature, enriched in C3-vegetation in the last glacial (45-20 ka) compared to the Holocene. Given temperature is the primary driver of abundance in C3-C4 mixed-grasslands, C4 dominance beginning c.18.5 ka followed by C4 enrichment is indicative of deglacial warming that continues into the Holocene except for a departure at ~10 ka. The record at Core 2 is indicative of changing climate while Core 1 shows disturbance-based vegetation dynamics. The simultaneously distinctive vegetation states in Cores 1 and 2 within the same valley is the first record of alternative stable states in the past in the montane tropics. Our results point to the need to account for short-term disturbances and site attributes before ascribing vegetation changes to changing climate in alternative stable states landscapes.

This paper examines the impact of climate-sea level controls on the vegetation and evolution of the Niger Delta during the Late Quaternary. The extraneous controls on the environment outlined in this context confirm a direct link between vegetation dynamics (pollen data), sediment supply, and the landscape evolution of the Niger Delta between 20 ka and 6.5 ka. Two phases of sedimentation are recognized based on multiple proxies analyzed in three gravity cores obtained from the shallow offshore at ~40 m water depth. Phase I records abundant occurrences of Poaceae, Cyperaceae, and Podocarpus pollen from a dry hinterland, charred grass cuticles, nonmarine alga Pediastrum, high Ti/Zr ratio, and lower sedimentation from 20-11.7 ka. Phase II records an expansion of mangrove vegetation, high Fe/S ratio, and increase in planktonic foraminifera between 11.7 ka and 6.5 ka. This second phase is attributed to sea-level rise and higher sedimentation during the development of delta plain and mangrove vegetation on the gently sloping shelf. These sequential records provide a new clue about the link between the evolutionary stages of the Niger Delta landscape and vegetation dynamics during two distinct time-bound intervals, which potentially delineate the boundary between two Marine Isotope Stages: MIS2 (late glacial period) and MIS1 (interglacial period).

Glacier velocity reflects the dynamics of ice flow, and its change over time serves a key role in predicting the future sea-level rise. Glacier feature tracking (also known as offset tracking or pixel tracking) is one of the most widely-used approaches for mapping glacier velocity using remote sensing data. However, running this workflow relies on multiple empirical parameter choices such as correlation kernel selection, image filter, and template size. As each target glacier area has different data availability, surface feature density, and ice flow width, there is no one-size-fits-all parameter set for glacier feature tracking. Finding an ideal parameter set for a given glacier requires quantitative and objective metrics to determine the quality of resulting velocity maps. The objective of our Glacier feature tracking test (gftt) project is both to devise a set of widely applicable metrics and to build a Python-based tool for calculating them. These metrics can be thus used for comparing the performance of different tracking parameters. We use Kaskawulsh glacier, Canada, as a test case to compare the velocity mapping results using Landsat 8 and Sentinel-2 images, various software packages (including Auto-RIFT, CARST, GIV, and vmap), and a range of input parameters. To begin with, we calculate random error over stable terrain, a metric that has been used for evaluating the uncertainty of the velocity products. We develop two other workflows for exploring new metrics and validating existing metrics, including the test with synthetic pixel offsets and the comparison with GNSS records. These existing and new metrics, calculated through the gftt software, will help determine optimal parameter sets for feature tracking of Kaskawulsh glacier and any other glacier around the world. This work is supported by the NSF Earth Cube Program under awards 1928406, 1928374.

The potential for molecular hydrogen (H-OH- groundwaters bearing up to 4.05 μmol⋅L-1 H2 , 3.81 μmol⋅L-1 methane (CH4) and 946 μmol⋅L-1 sulfate (SO42-) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H2 oxidation coupled to SO42- reduction. In shallower, oxidized Mg2+-HCO3- groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High δ13C and δD of CH4 (up to 23.9 ‰ VPDB and 45 ‰ VSMOW , respectively), indicated microbial CH4 oxidation, particularly in Ca2+-OH- waters with evidence of mixing with Mg2+-HCO3- waters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.

Recent increasing wildfire activities in California have raised widespread public concerns. This raises a question as to whether the increase of of fire weather this year and in recent decades is mainly due to weather pattern change dominated by natural variability or anthropogenic warming. We observed that vapor pressure deficit (VPD), an important fire weather risk index, in the warm season (May to September) of California, has increased significantly since 1979, with a positive trend of 0.70 hPa or 0.50 σ per decade, and this trend is also significant for VPD in large fire days; trend analysis of saturated and actual vapor pressure suggests that direct warming and drying contributes to about 70% and 30%, respectively, to the positive VPD trend. This increase of VPD and warming for such a short period cannot be simply linked to anthropogenic factors as it can also be affected by circulation changes. Our self-organizing maps analysis reveals the close relationship between VPD and circulation. Using a flow analogue approach, we estimated circulation changes contribute to about half (54%) of the observed VPD trend. Since circulation changes represent the upper limit of internal variability of the climate system because both the internal climate variability and anthropogenic forcings can contribute to such a change, our results suggest that anthropogenic warming must explain at least the other half of the observed VPD trend.

Emission of methane (CH and CO2. In situ mole fractions of CH4 and CO2 were measured in the subglacial air at a subglacial river outlet where emissions of CH4 and CO2 had been identified. Water samples were analysed for dissolved CH4 and CO2 concentrations. CH4 and CO2 in gas and water samples were analyzed for their isotopic composition of 13C and 2H. Close correlation between gaseous and dissolved CH4 and CO2, respectively, show degassing of CH4 and CO2 from the subglacial meltwater. The diurnal variability of in situ mole fractions of CH4 and CO2 in subglacial air was related to meltwater runoff. Maximum in situ mole fractions decreased after the peak of the melt season, but estimated net emissions increased because the size of the subglacial river outlet increased. The isotopic signature of CH4 in the subglacial air, estimated with a Keeling plot, indicated that subglacial CH4 likely originated from acetoclastic methanogenesis. Isotopic signatures of gaseous CO2 indicate that both microbial oxidation of CH4 in the subglacial system and remineralization of carbon in subglacial sediments contribute to subglacial CO2.

Knowledge about the early life history of Antarctic toothfish (Dissostichus mawsoni) is still incomplete, particularly on the spatial and temporal extent of spawning and the subsequent transport of eggs and juveniles from the offshore spawning areas to the continental shelf. This study used a high-resolution hydrodynamic model to investigate the impact of ocean circulation and sea-ice drift on the dispersal of eggs and juvenile Antarctic toothfish. The virtual eggs were released on seamounts of the Pacific-Antarctic ridge in the northern Ross Sea and advected using hydrodynamical model data. Particles were seeded annually over a 14-year period (2002 to 2016) and tracked for three years after release. Spawning success was evaluated based on the number of juveniles that reached known coastal recruitment areas, in the eastern Ross and Amundsen Sea, within three years. Observations show that juveniles (50-100 cm size class) are abundant on the shelf and slope of the Ross and Amundsen Seas. Sensitivities to certain juvenile behaviours were explored and showed that spawning success was reduced by around 70% if juveniles drifted with sea-ice during the second winter season as this carried them into the open ocean away from the shelf region. Spawning success increased during the second winter season if juveniles were entrained in the Ross Gyre circulation or if they actively swam towards the shelf. These modelling results suggest that the ecological advantage of sea-ice association in the early life cycle of toothfish diminishes as they grow, promoting a behaviour change during their second winter.

The 1601 Common Era earthquake (Mw ca. 5.9) in “Unterwalden”, Central Switzerland triggered multiple subaqueous mass movements and a subaerial rockfall that generated tsunami waves with devastating run-up heights of up to 4 m and several hundred meters of inundation along the coastal lowland plain of Lake Lucerne. In the shallow Lucerne Bay at the outlet of the perialpine lake, historical chronicles reported a seiche with an initial amplitude of ~1–2 m and a period of 10 min that decreased with time but persisted for several days after the event. The impact and erosion potential of the tsunami wave on the Lucerne Bay is assessed with sediment core analysis and numerical simulation of wave propagation. A 60 cm thick offshore event deposit was recovered and radiocarbon dated along a sediment-core transect. The event deposit has a sharp basal contact with carbonate shell fragments and a normal graded succession of siliciclastic sand to silt with high amount of terrestrial-derived horizontally bedded wooden particles. The simulated tsunami waves have a water-surface displacement of up to 1.5 m and bed shear-stresses that are likely capable of remobilizing large amounts of sediment in the Lucerne Bay area. Our study thus successfully links the sedimentology of event deposits with physical principles of sediment mobilization derived from numerical wave modeling, providing a tool to improve the identification and interpretation of potential tsunami deposits.

In field observations from a sinuous estuary, the drag coefficient C, much greater than expected from bottom friction alone. C are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of CD, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with CD during flood tides. While CD generally increased with water depth, CD decreased for the highest water levels that corresponded with overbank flow. The decrease in CD may be due to inhibition of flow separation with flow over the vegetated marsh. The dependence of CD during ebbs on discharge corresponds with inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple-scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.

D. Jiang submitted a comment entitled « Metamorphic data from global subduction zones do not call for excessive overpressures » to Nature Geoscience. His comment on our paper entitled « Metamorphic record of catastrophic pressure drops in subduction zones » (Nature Geoscience 10, 46-50, 2017) will not be published. However, because his comment remains available on the web via ESSOAR (https://www.essoar.org/doi/abs/10.1002/essoar.10504134.3), I here propose to share my reply.

The importance of air-sea coupling in the simulation and prediction of the Madden-Julian Oscillation (MJO) has been well established. However, it remains unclear how air-sea coupling modulates the convection and related oceanic features on the subdaily scale. Based on a regional cloud-resolving coupled model, we evaluated the impact of the air-sea coupling on the convection during the active phase of the MJO by varying the coupling frequency. The model successfully reproduced the atmospheric and oceanic variations observed by satellite and measurements but with some quantitative biases. According to the sensitivity experiments, we found that stronger convection was mainly caused by the higher sea surface temperatures (SSTs) generated in highly coupled experiments, especially when the coupling frequency was 1 hour or shorter. A lower coupling frequency would generate the phase lags in the diurnal cycle of SST and related turbulent heat fluxes. Our analyses further demonstrated that the phase-lagged diurnal cycle of SST suppressed deep convection through a decrease in daytime moistening in the lower troposphere. Meanwhile, in the upper ocean, the high-frequency air-sea coupling helped maintain the shallower mixed and isothermal layers by diurnal heating and cooling at the sea surface, which led to a higher mean SST. In contrast, the barely coupled experiments underestimated SST and therefore convective activities. Overall, our results demonstrated that high-frequency air-sea coupling (1 hour or shorter) could improve the reproducibility of the intensity and temporal variation in both diurnal convection and upper ocean processes.

The uncertainty in present-day anthropogenic forcing is dominated by uncertainty in the strength of the contribution from aerosol. Much of the uncertainty in the direct aerosol forcing can be attributed to uncertainty in the anthropogenic fraction of aerosol in the present-day atmosphere, due to a lack of historical observations. Here we present a robust relationship between total present-day aerosol optical depth and the anthropogenic contribution across two multi-model ensembles and a large single-model perturbed parameter ensemble. Using observations of aerosol optical depth, we determine a reduced likely range of the anthropogenic component and hence a reduced uncertainty in the direct forcing of aerosol.

The parameters of sporadic $D_s$ layer of electric conductivity caused by ultraenergetic relativistic electron (URE) precipitations are determined due to indirect electromagnetic method. Previously we determined the southern boundaries of these precipitations in the frames of the following supposition: the effect of a normal wave reflection and its conversion into other normal waves on the boundary between disturbed and undisturbed parts of a radio path might have been ignored. Now we show by accurate simulation that it was true for strong and moderate disturbances. For the powerful URE disturbances the effect is significant. In order to obtain this result we had to change a traditional problem statement about a normal wave conversion in the terrestrial waveguide and to solve numerically this new problem with a ${}^{\prime \prime}$volume${}^{\prime \prime}$ inhomogeneity.

An altitude-triggered lightning flash with 8 leader/return stroke sequences containing 15 attempted leaders and 8 stroked leaders was observed with a high-speed camera and a mirrorless camera. The path and velocity characteristics of these leaders are investigated in detail. These leaders propagated along three different paths and had different development processes. Attempted leaders are found to die out in three ways: slow down and then disappear in somewhere of the path, give up propagating along the path and switch to propagate along the channel of a branch, be caught up and merged by other leaders propagating along the same path. Propagations of attempted leaders are not progressive, with some of them not always reaching as far as previous one did. The terminal height of attempted leaders ranges from over 1617m to 875m above the ground. A branching node is found to be the critical point determining a leader to attach the ground or not. Average 2-D speed of attempted leaders range from 2.7×105m/s to 21.0×105m/s. Some of attempted leaders even propagated in a higher speed than stroked leaders before they died out. There is no inevitable relation between the initial speed and their final fate. A critical value of propagation speed between attempted leaders and stroked leaders reported here is found to be 4×106m/s. Attempted leaders are found to slow down before propagating to the two branching nodes along the path.