Acceleration Period Research Articles

Competition between rate strengthening and gravitational acceleration induces stick-slips of inclined granular flow

This study conducted experiments on dry granular mass released on an inclined flume. The periodic acceleration and deceleration of the flow, that is, the stick-slip phenomenon, was observed during the experiments. To investigate the stick-slip motion mechanism, numerical simulations were conducted with the inclusion of the shear-strengthening μ(I)-rheology into the elasto-plastic models of granular flow. The stick-slip phenomenon was captured naturally without the modification of the empirical friction law. The results revealed that the competition between the rate strengthening implemented by the μ(I)-rheology and the gravitational acceleration along the inclined plane induces stick-slips. By considering the experimental results in combination with the simulation results, the effects of the particle size, gate size opening, surface roughness, and frictional parameters of μ(I)-rheology on the stick-slip phenomenon were elucidated.

Effect of interface properties between functionalized cellulose nanocrystals and tricalcium silicate on the early hydration mechanism of cement

The hydration process is a critical stage in the development of the properties of cement-based materials，however, the influence of cellulose nanocrystals (CNC) on cement during the early hydration is controversial. This study uses pure tricalcium silicate (C3S) as the object to analyze the influence of carboxyl CNC (CCNC) and sulfonic CNC (SCNC) on hydration parameters such as hydration rate, types, quality, and pore structure of hydration products during the early hydration period. The dissolution of C3S and the interaction between fluids and minerals at the C3S/CNC interface is the core mechanism of the early hydration. Ultraviolet and energy spectrum analyzed adsorption between CNC and C3S from a physical perspective, and molecular dynamics simulations deeply discussed the adsorption behavior between C3S/Water/CNC at nano-scale. The results show that CNC prolongs the hydration induction and acceleration period, delays the early hydration of C3S; CCNC exhibits a more significant effect than SCNC. Under the action of Ca-O and hydrogen bonds, CNC will adsorb on the surface of C3S and cover its surface, reducing the number of waters on the surface of C3S. Hydrophilic CNC molecules attract water molecules close to the CNC chain, and solidifying water molecules prevents it from entering the hydration surface. CNC also restricts the diffusion of Ca2+ on the surface of C3S to outside, improving the stability of Ca-O coordination, promoting the accumulation of hydration products at the C3S surface, and inhibiting the dissolution of C3S. Understanding the effect of functionalized CNCs on the diffusion and sedimentation mechanism of water molecules and Ca2+ during hydration can provide insights for guiding the design of nanomaterials with strong compatibility with cement systems and enlarging the application of functionalized nanomaterials in cement-based materials engineering fields.

Hydration mechanism and kinetic response of a C4A3$-C2S binary system

The physical properties of C4A3$-C2S (ye´elimite-belite) clinker system will change with the proportion of C4A3$ or C2S, but the effect of mineral proportion change on hydration mechanism is not systematic and unclear. The C4A3$-C2S binary system was established by experiments, and its hydration process was characterized. The kinetic response of C4A3$-C2S binary hydration system was studied by Kondo model. The results showed that the change of design mineral ratio directly affects the hydration process and the hydration mechanism. The experiment and analysis show that when the design mineral ratio of C2S was lower than 65 wt%, the dominant factor of hydration was mainly controlled by nucleation mechanism (The hydration acceleration period is particularly significant), and when the ratio of C2S was higher, it would be controlled by diffusion mechanism. The precise value of the boundary reaction control is between 65 wt% (design mineral ratio of C2S) to 75 wt%. Only when the hydration kinetics of the binary system is controlled by the diffusion mechanism, the CH product can be stably crystallized. The classical Kondo model is suitable for the C4A3$-C2S hydration system.

Centrifuge modeling of loess slope failure induced by rising water level utilizing intact sample

Understanding the failure process of bottom-saturated loess slopes is of great significance in loess areas where flood irrigation is commonly utilized to alleviate the scarcity of precipitation. In this study, the failure process of a loess slope with an increased groundwater level was recreated and the multiple failure modes of loess landslides were revealed. A centrifugal model test was conducted using a groundwater-recharge device. An intact loess sample retrieved from the Heifangtai Loess Terrace was employed in the test. The model test was monitored using a video recorder, high-speed camera, and soil/pore water pressure sensors, and the results were validated by utilizing an intermittently investigated field landslide. Based on the monitored data and acceleration, the test was divided into three periods: the initial acceleration period with no water inlet (0–40 g), bottom saturation period (40–60 g), and failure occurrence period (60–80 g). The soil/pore water pressure and degree of deformation were relatively low, with a steadily increasing trend during the first two periods. With the enrichment of the soil water content, retrogressive sliding, deep subsidence, and surface sinkhole failures occurred successively up to areas with relatively high pore water pressure during the last period. The results of the field landslide investigation showed multiple failure modes, as observed in the model test. The results suggest that the coexistence of multiple failure modes could gradually evolve into croplands and promote water infiltration into the deep loess, increasing the groundwater level and accelerating the failure process of the slope. Despite the overall effects of multiple failure patterns on the evolution of the slope, each failure pattern had a relatively independent evolutionary process within a certain area, which could be further analyzed for the early recognition of loess landslides. This study also indicates that the challenges of centrifuge modeling for water-related materials with intact soil samples are the boundary conditions and data monitoring within the model.

How small differences grow over time – the snowball effect of grit on practice in sport

ABSTRACT Domain-specific practice is crucial for expertise. While much is known about practice, less is understood about what motivates individuals to start and continue with the intensive domain-specific practice needed for expert performance. We demonstrate in a large sample (N = 388) of Australian elite youth soccer players that they retrospectively report different amounts of practice at various stages of development and that acquisition is driven by the personality trait of grit. Players reported consistently engaging in domain-specific activities every year from age 8 until 13 years. The estimated logged hours experienced a marked acceleration at the age of 13 years, resulting in an enhanced and curvilinear pattern of practice accumulation. Grittier players, however, start accumulating more practice at the beginning of the talent development process and continue to consistently log more hours throughout the years than less gritty players. Consequently, initially small differences in practice engagement between those with high and low grit scores, snowball into sizable ones at the age of 15 years. The impact of grit on practice accumulation is, however, more subtle, as grit’s two components, consistency of interest and perseverance of effort, exert a different pattern of influence. The driving factor of the initial differences and their consistent snowball effect until age 13 years is a consistency of interest, whereas the perseverance of effort aspect impacts the practice acceleration period from age 13 to age 15 years. We elaborate on these findings to explain the complex relationship between grit and practice on the path to performance excellence.

Sensors Layout Optimization Design of Rocket Sled Test System.

The rocket sled, as a ground dynamic test system, combines the characteristics of the wind tunnel test and the flight test. However, some practical factors, such as shock wave interference, ground effect, and high-intensity aerodynamic noise will cause serious interference and even failure of the uniformly distributed sensors during horizontal sliding in a wide speed range. The AGARD HB-2 standard model is employed as the payload to simulate the aerodynamic and aeroacoustic characteristics during the variable acceleration period, aiming to optimize the test sensors layout. It is observed that in the high Mach number flow fields, strong coupling behaviors among complex waves will occur. The peak of wake vortex strength will appear at 1.5 s and gradually diminish over time. In addition, when the vortex between the load and the booster is monitored, its position shifts forward in the subsonic stage, then gradually moves backward and expands in the supersonic stage. Acoustic directivity is pronounced at subsonic and transonic speeds, pointing towards 75° and 135° relative to the sliding speed, respectively. These results can provide technical support for sensor layout and high-precision testing in rocket sled tests.

Effect of waste oyster shell powder on the micro- and macroproperties and sustainable performance of cement-based materials

A large amount of oyster shell waste is generated globally every year. Proper disposal of discarded oyster shells is crucial for reducing environmental pollution and achieving sustainable development. The aim of this paper was to study the feasibility of using waste oyster shells as a cementitious material to replace cement. The effects of waste oyster shell powder (OSP) with different water–binder ratios (w/b = 0.5, w/b = 0.2) and different substitution rates (10 %, 20 %) on the properties of cement-based materials were studied. The experimental results are as follows: The addition of oyster shell powder reduces the compressive strength, ultrasonic pulse velocity (UPV), surface electrical resistivity and cumulative heat of hydration of the mortar, but the strength loss is smaller for a water-to-binder ratio of 0.2 than for a water-to-binder ratio of 0.5. A good linear relationship between the compressive strength and UPV was also found. A good exponential relationship exists between the surface electrical resistivity and compressive strength. The heat of hydration results show that when the water–binder ratio is 0.5, replacing cement with waste OSP promotes aluminate phase hydration and advances the acceleration period. Finally, the carbon emissions of the mortar were normalized according to the compressive strength, and using OSP instead of cement at a low water–binder ratio was more conducive to reducing CO2 emissions.

Interface friction between neighbor diamondene nanoribbons: A molecular dynamics study

Due to its unique carbon-carbon bond topology, diamondene exhibits different frictional properties compared to pristine graphene. This study evaluates the friction properties of diamondene ribbon (DR) for potential applications in the design of dynamic nanodevices. For a DR sample on a DR substrate at a specific temperature, it has a maximum static friction force (MSFF) that is about 38 times greater than that of a graphene counterpart with the same size. The rate of MSFF to the contact surface demonstrates the shear strength of a material. As an intrinsic property, the shear strength between DRs measures approximately 3.58 MPa at 10 K, which is much greater than that between graphene ribbons. When the pulling force on the DR sample exceeds MSFF, the sample undergoes a period of acceleration before reaching a stable sliding velocity. The viscous damping coefficient between DRs increases with increasing pulling force on the DR sample while decreasing with increasing temperature. The dynamic friction force behaves in a temperature-dependent manner due to the unstable bond topology of DRs at high temperatures. These studies address previous research lack regarding friction between carbon materials.

Mean flow generated by asymmetric periodic excitation in an annular channel

Surface waves of a shallow liquid layer confined in an annular channel of length L~\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ilde{L}}}$$\\end{document} are generated by periodic acceleration of the channel along its azimuthal direction. We show that if the oscillations are anharmonic, e.g., having the form of a saw tooth, a mean mass flow is generated and even the mean speed over one period is zero. The flow is studied both experimentally and numerically. In the experiment, the velocity field and surface deflection are derived from the frames of videos recorded with a Go-Pro Hero 4 camera with custom rectilinear lens. We show resonance curves of the mean square of the water surface deflection and of the averaged mass flow. Both curves show almost coinciding maxima for multiples of the fundamental resonance frequency f~r=c~0/(2L~),\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ilde{f}}}_r=\ ilde{c}_0/(2{{\ ilde{L}}}),$$\\end{document} where c~0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ilde{c}}}_0$$\\end{document} is the shallow water wave velocity. The measurements are confirmed by numerical solutions of an integrated boundary layer model including inertia and dissipation.

Effect of ball milling-Ca(OH)2 coupling activation on the cementitious properties of ferrous extraction tailing of nickel slag: Basic properties, hydration mechanism and heavy metal safety

In order to improve the reactivity of ferrous extraction tailing of nickel slag (FETNS), This paper delves into the influence of ball milling-activator Ca(OH)2 on the cementitious activity of FETNS. Furthermore, it assesses the combined activation's impact on the working performance, mechanical properties, shrinkage properties, early hydration process, hydration products, and heavy metal element leaching of the FETNS-OPC composite cementitious system. The results show that the coupling activation significantly enhanced the reactivity of FETNS and prolonged the acceleration period of the composite cementitious system during the hydration process, thereby promoting the hydration reaction. This not only shortened the setting time of the composite cementitious system, but also significantly improved its early compressive strength. The microscopic test results showed that the coupling activation improved the reaction degree of the composite cementitious system, promoted the formation of more reaction products, and increased the compactness of the internal structure. In addition, the coupling activation also enhanced the degree of polymerization of the composite cementitious system, and these factors together led to an increase in compressive strength. However, it is worth noting that with the increase of FETNS reactivity under the coupling activation, the autogenous shrinkage change rate and drying shrinkage change rate of the composite cementitious system also increased accordingly. Finally, the results of the heavy metal leaching test showed that the coupling activation reduced the leaching value of heavy metal elements in FETNS, ensuring the safety of practical application.

Multicomponent entropic cosmology model with generalized entropy

Entropic cosmology provides a concrete physical understanding of the late accelerated expansion of the universe. The acceleration appears to be a consequence of entropy associated with information storage in the universe. Therefore, the assumption of an ad-hoc dark energy is not necessary. In this paper, we investigate the implications of a multicomponent model (radiation and non-relativistic matter) that includes a subdominant power-law term within a thermodynamically admissible model. We use a generic power-law entropy and the temperature of the universe horizon results from the requirement that the Legendre structure of thermodynamics is preserved. We analyse the behaviour for different combinations of the parameters and compare them with other cosmological models, the observed redshift dependencies of the Hubble parameter H and the luminosity distance data obtained from supernovae. This is an early attempt to analyse a multicomponent entropic cosmological model. Furthermore, the analysis is based on a entropy scaling with an arbitrary power of the Hubble radius instead of a specific entropy. This allows us to simultaneously infer different models, compare them and conserve the scaling exponent as a parameter that can be fitted with the observational data, thus providing information about the form of the actual cosmological entropy and temperature. We show that the introduced correction term is able to explain different acceleration and deceleration periods in the late-time universe by solving the model numerically. We discuss the advantages and disadvantages of entropic models compared to mainstream cosmology.

Influences of alumina type and sulfate content on hydration and physicochemical changes in Portland–limestone cement

This study explored the influence of alumina type and gypsum content on the hydration, mechanical properties, and chemical changes of Portland–limestone cement (PLC) using isothermal calorimetry, inductively coupled plasma-optical emission spectroscopy (ICP-OES), ionic chromatography (IC), compressive strength testing, X-ray diffraction (XRD), 27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR) spectroscopy, and mercury intrusion porosimetry (MIP) testing. The addition of alumina shortened the induction period, accelerated the acceleration period, and increased the mechanical strength of the PLC pastes with 3 and 7 wt% gypsum. The acceleration effect and strength enhancement were not distinct in the PLC pastes with less gypsum (0.1 wt%). The addition of γ-alumina led to the greatest accelerating effect and strength improvement in the PLC pastes with 3 and 7 wt% gypsum, and the compressive strength achieved was comparable to those of pure Ordinary Portland cement (OPC) pastes without limestone. The addition of γ-alumina induced the rapid dissolution of gypsum and consumption of sulfate ions. The chemical change affected by the addition of alumina varied greatly depending on the gypsum content, and trends in AFt and AFm formation in the PLC pastes with different gypsum contents were confirmed. Finally, pore size refinement was observed through the addition of alumina to PLC pastes with 3 and 7 wt% gypsum.

The impact of school climate on academic burnout of Chinese students: the mediating effect of psychological capital

BackgroundStudents are in their teenage years during middle school, which is a period of acceleration and transition for individual physical and mental development. Therefore, junior high school students are easily influenced by their internal and external environment, leading to academic burnout. The main activity venue for middle school students in school homework is the external environment that affects their academic performance. Psychological capital is the student’s own resource and the external environment that affects their academic performance.AimsThis study aims to explore the interrelationship among school atmosphere, psychological capital, and academic burnout, and construct a theoretical model among them. This study provides practical and feasible methods and guidance suggestions for educational institutions and teachers, helps students improve their mental health level, enhances the positive perception level of the school atmosphere, and alleviates their academic burnout.MethodsBuilding upon previous research on school climate, psychological capital, and academic burnout, this study employed the following research tools: The Perceived School Climate Questionnaire (PSC), the Positive Psychological Questionnaire (PPQ), and the High School Students’ Academic burnout Scale. A questionnaire survey was conducted on 1,267 students from three different regions of Chongqing, involving various types of junior high schools.ResultsThe findings of this study demonstrate that both the external school climate and internal psychological capital exert an impact on academic burnout. A positive school climate can foster the development of psychological capital, subsequently enhancing academic performance. Additionally, psychological capital directly influences academic burnout. The higher the level of positivity is, the more pronounced the alleviating effect becomes. The school climate can directly affect academic burnout, and it can also indirectly affect academic burnout through the mediation of psychological capital, with psychological capital playing a partial mediating role.ConclusionTo avoid academic burnout, schools should actively create a good school atmosphere and mobilize students’ learning enthusiasm; teachers should provide targeted psychological counseling to reduce students’ negative emotional perception; students should correct their learning attitude and recognize their own learning status.

Localising pulsations in the hard X-ray and microwave emission of an X-class flare

Aims. The aim of this work is to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA, we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from ∼7 s in the impulsive phase to ∼35 s later in the flare. Methods. We analysed the temporal, spatial, and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare. We reconstructed images for individual peaks in the impulsive phase and performed spectral fitting at high cadence throughout the first phase of pulsations. Results. Our imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission change in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux, showing a “soft-hard-soft” spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase. Conclusions. Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave emission. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration over time related to the associated filament eruption.

Turbulent–turbulent transient concept in pulsating flows

The turbulence behaviour of current-dominated pulsating flows has been investigated. Direct numerical simulations have been carried out for Stokes lengths over a range of $l_s^+=5\unicode{x2013}26$ , and amplitudes spanning 90 % of the current-dominated regime, about a mean flow of $\overline {Re}=6275$ . The results show that the turbulence response in intermediate and low-frequency pulsations is governed by a multistage turbulent–turbulent transition process, which bears a strong similarity to the multistage response of non-periodic acceleration. During the early acceleration period, the flow enters a pretransition stage, in which a new laminar perturbation boundary layer forms at the wall, and the streamwise velocity streaks are stretched. If the low-speed streaks destabilise prior to the deceleration period, then the flow enters a transition stage in which the perturbation boundary layer undergoes a bypass-like transition process. A unique feature of pulsating flows is the ongoing mechanism of turbulence decay, which initiates during the deceleration period and constitutes the main transient turbulence mechanism for much of the cycle. For high-frequency pulsations, the perturbation boundary layer fails to reach the pretransition stage prior to the deceleration period. Instead, the flow alternates between two inertial stages which are characterised by two layers of amplified viscous force; one growing at the wall, and one detached and moving towards the core.

Enhancing seismic assessment and risk management of buildings: A neural network-based rapid visual screening method development

Some of the existing buildings are designed based on lower design standards or even without considering seismic design standards. Recent earthquakes have further highlighted the vulnerability of these buildings when subjected to severe seismic activity. Consequently, it has become imperative to conduct seismic vulnerability assessments of the existing building stock. Therefore, the assessment of the existing building stock is required through the utilization of Rapid Visual Screening (RVS) methods. However, the existing conventional RVS methods used in seismic building assessments have shown limited accuracy. Furthermore, because these methods were developed based on expert opinions and/or due to access limitations to detailed assessment-based generated data used for their development, further enhancing them is challenging. To address these limitations, a new RVS method, which leverages Neural Networks (NN) and building-specific parameters, for reinforced concrete, adobe mud, bamboo, brick, stone, and timber buildings has been proposed in this study. Unlike conventional methods that rely on site seismicity class, the developed data-driven approach incorporates building-specific parameters such as the fundamental structural period and building spectral acceleration. The developed RVS method is specifically tailored to analyze diverse types of buildings in regions with varying seismicity risks, all in preparation for an impending earthquake. In this study, the developed RVS method demonstrated a promising 68% test accuracy, effectively representing the building performance against earthquakes. These findings illustrate the potential of the developed NN based RVS method in assessing existing buildings, thereby mitigating potential loss of life and property during imminent earthquake and alleviating the associated economic burden. Furthermore, this study introduces a new RVS method that can pave the way for future advancements in the field of seismic vulnerability assessment of existing buildings.

Creep deformation monitoring of landslides in a reservoir area

Effective utilization of reservoirs facilitates the distribution of water resources in both time and space, providing strong support for the sustainable growth of the economy and society. However, the periodic water level fluctuations of a reservoir during its operation may lead to geological hazards such as landslides. Here, we conducted a comparative analysis of the deformation processes of reservoir and non-reservoir landslides in the Jilintai area between 2017 and 2022 using Interferometric Synthetic Aperture Radar (InSAR) technology and wavelet analysis. Results show that more unstable locations were found on shaded slopes with a soil moisture of 6–10% than on sunny ones with a soil moisture of 15–19%. Both reservoir and non-reservoir landslides showed continuous and prolonged creep displacement over time. The deformation curve of the reservoir landslides displayed a step-like trend, whereas it was a linear trend without the discernible acceleration period for the non-reservoir landslides. Furthermore, the wavelet analysis revealed that the deformation of reservoir landslides follows a one-year period, almost synchrony with reservoir level changes. The results from this work deepen our understanding of kinematic processes of reservoir and non-reservoir landslides, which is crucial for reasonable and effective landslide monitoring and prevention.

Classical Larmor formula through the Unruh effect for uniformly accelerated electrons

We investigate the connection between the classical Larmor formula and the quantum Unruh effect by computing the emitted power by a uniformly accelerated charged particle and its angular distribution in the coaccelerated frame. We consider a classical particle accelerated with nonzero charge only for a finite period and then take the infinite-time limit after removing the effects due to the initial charging and final discharging processes. We show that the result found for the interaction rates agrees with previous studies in which the period of acceleration with nonzero charge was taken to be infinite from the beginning. We also show that the power and angular distribution of emission, which is attributed either to the emission or absorption of a Rindler photon in the coaccelerated frame, is given by the Larmor formula, confirming that, at tree level, it is necessary to take into account the Unruh effect in order to reproduce the classical Larmor radiation formula in the coaccelerated frame. Published by the American Physical Society 2024

One-loop effective action of the IKKT model for cosmological backgrounds

We study cosmological solutions of the IKKT model with k = –1 FLWR geometry, taking into account one-loop corrections. A previously discussed covariant quantum spacetime is found to be stabilized through one-loop effects at early times, without adding a mass term to the model. At late times, this background is modified and goes through a period of acceleration, before settling down to a coasting FLRW geometry with scale parameter a(t) ~ t. This is qualitatively close to observation without any fine-tuning, irrespective of the detailed matter content of the universe.

Increased black carbon (soot) accumulation during the Anthropocene in a less-developed region of Xinjiang, northwestern China

Records of environmental changes at regional or continental scales in less-developed regions may aid clarification of the onset of the Anthropocene. A sediment core was collected from Bosten Lake in Xinjiang Province, northwestern China, to investigate historical variations of combustion indicators of black carbon (BC), char and soot and to reflect human influences over the past 150 years. Results show that a rapid increase in soot fluxes (over 1.5 times on average) happened after 1950, consistent with the Great Acceleration period as well as establishment of the People's Republic of China, following a rapidly developing economy. Soot flux peaked around the 1960, coincident with wasteland reclamation in Xinjiang Province, decreasing after the year 2000 owing to environmental protection requirements such as desulfurization. Average char/soot ratios before and after 1950 were 0.56 and 0.37, respectively, such low ratios suggest a predominant contribution of soot from long-range transport, while the industrialization in China since the 1950 s may have caused further reduction in the ratios. Sedimentary soot deposition flux in less-developed areas thus recorded key anthropogenic activities occurring in China and supports the onset of the Anthropocene in the mid-20th century, as proposed by the Anthropocene Working Group.