The thin layer of soil at the earth's surface supports life, storing water and nutrients for plant uptake. These processes occur in the soil pore space, often half the soil volume, but our understanding of how this volume responds to environmental change is poor. Convention, has been to predict soil porosity, or its reciprocal bulk density (BD), from soil texture using pedotransfer functions (PTFs). A texture based approach, invariant to environmental change, prevents feedback from land use or climate change to soil porosity. Moreover, PTFs are often limited to mineral soils with < 20% soil organic matter (SOM) content. Here, we develop an analytical model to predict soil porosity, or BD, as a function of SOM. We test it on two comprehensive, methodologically consistent, temperate national-scale topsoil data sets (0-15 cm) (Wales, n = 1385; Great Britain, n = 2570). The purpose of the approach is to generate an analytical function suitable for predicting soil porosity change with SOM content, while providing insight into the main grain-scale factors determining the porosity emergence. The newly developed function covering the entire SOM gradient allows for impacts of land use, management or climate change to feedback on soil porosity or bulk density through decadal dynamic changes in SOM. 

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Analytical modelling of soil porosity and bulk density across the soil organic matter and land-use continuum

Fabric evolution and liquefaction resistance in multiple-liquefaction process: a micromechanical study using DEM-clump

The orientation of, and contacts between, grains of sand reflect the processes that deposit the sands. Grain orientation and contact geometry also influence mechanical properties. Quantifying and understanding sand microstructure thus provide an opportunity to understand depositional processes better and connect microstructure and macroscopic properties. Using x-ray computed microtomography, we compare the microstructure of naturally-deposited beach sands and laboratory sands created by air pluviation in which samples are formed by raining sand grains into a container. We find that naturally-deposited sands have a narrower distribution of coordination number (i.e., the number of grains in contact) and a broader distribution of grain orientations than pluviated sands. The naturally-deposited sand grains orient inclined to the horizontal, and the pluviated sand grains orient horizontally. We explain the microstructural differences between the two different depositional methods by flowing water at beaches that re-positions and reorients grains initially deposited in unstable grain configurations.

https://eartharxiv.org/repository/object/2254/download/4649/

Microstructural differences between naturally-deposited and laboratory beach sands.

Lowermost Mantle Structure Beneath the Central Pacific Ocean: Ultralow Velocity Zones and Seismic Anisotropy

SUMMARY Observational data of asteroids can be explained by considering them as an agglomerate of granular material. Understanding the mechanical properties of these objects is relevant for many scientiﬁc reasons: space missions design, evaluation of impact threats to our planet, and understanding the nature of asteroids and their implication in the origin of the solar system. In-situ measurements of mechanical properties require complex and costly space missions. Here a laboratory-scale characterization of wave propagation in granular media is presented using a novel experimental setup as well as numerical simulations. The pressure inside an asteroid is still a matter of debate, but it deﬁnitely presents a pressure gradient towards the interior. This is why impact characterization needs to be performed as a function of the conﬁning pressure. Our experimental setup allows for the simultaneous measurement of the external conﬁning pressure, internal pressure, total strain, and acceleration in a 50 cm side squared box ﬁlled up with a billion grains. We study the propagation of impact-generated and shaker-born seismic body waves in the 500 Hz range. Through subsequent compression-relaxation cycles, it was observed that the granular media behaves on average like a solid with a constant elastic modulus during each compression. Effective medium theory (EMT) for granular media explains the data at low pressure. After each compression-relaxation cycle, the elastic modulus increases, and a high hysteresis is observed: relaxation shows a more complex behavior than compression. We show that seismic waves generated by both impact and vibration travels at the pressure wave speed. Thanks to a numerical model, we measure a strong wave attenuation α ∼ 3 . 4 Np/m. We found that the wave speed increases with the conﬁning pressure with a p 1 / 2 dependency, in disagreement with theoretical models that predicts a shallower dependency. The dependency of the elasticity with the conﬁning pressure can be explained by a modiﬁed EMT model with a coordination number proportional to the pressure, or equivalently by a mesoscopic nonlinear model based on third-order nonlinear elastic energy. The interpretation of these models is a deep reorganization in the particle contact network.

Elastic waves generated by impact and vibration in confined granular media

Coordination number controls elastic moduli, seismic velocity, and force transmission in sands and is thus a critical factor controlling the resistance of sands to deformation. Previous studies quantified relationships between coordination number, porosity, grain size, sphericity, and effective stress in pluviated or modeled sands. Here, we determine if these relationships hold in naturally-deposited beach sands. We collect samples while preserving their microstructures and use x-ray computed microtomography images to characterize grain properties. Similar to pluviated and modeled sand studies, we find that average coordination numbers and porosities for freshly deposited natural sands are 8.1 ± 2.8 and 0.37 ± 0.01, respectively. The range and standard deviation in coordination numbers of the natural beach sands are, however, significantly higher than observed in pluviated and modeled sand studies. At the same effective stress and porosities, coordination number is linearly proportional to grain surface area except for the smallest and largest grains. Coordination number depends non-linearly on sphericity. We attribute the higher ranges and standard deviations of coordination numbers in the natural sands to its broader grain size distribution, and we propose that the largest grains limit grain rearrangement, which influences spatial distributions of coordination numbers in natural sands.

https://eartharxiv.org/repository/object/1857/download/3922/

Coordination Numbers in Natural Beach Sand

Defining Earth's elusive thermal budget in the presence of a hidden reservoir

Significance Ocean island basalts, thought to originate from deep mantle material, exhibit a correlation between tungsten and helium isotopic signatures. The source of this correlation remains elusive: While mantle helium isotope heterogeneities are often attributed to a primitive, undegassed lower mantle reservoir, additional processes must be invoked to further explain the correlation with tungsten isotope signatures. We show that direct interaction between the core and the deep mantle can naturally explain the tungsten and helium isotopic composition of ocean island basalts. This possibility undermines the long-standing view that the processing of the Earth’s mantle must be inefficient to preserve primordial signals.

Long-term core–mantle interaction explains W-He isotope heterogeneities

Convection in planetary mantles is in the so‐called mixed heating mode; it is driven by heating from below, due to a hotter core, as well as heating from within, due to radiogenic heating and secular cooling. Thus, in order to model the thermal evolution of terrestrial planets, we require the parameterization of heat flux for mixed heated convection in particular. However, deriving such a parameterization from basic principles is an elusive task. While scaling laws for purely internal heating and purely basal heating have been successfully determined using the idea that thermal boundary layers are marginally stable, recent theoretical analyses have questioned the applicability of this idea to convection in the mixed heating mode. Here, we present a scaling approach that is rooted in the physics of convection, including the boundary layer stability criterion. We show that, as long as interactions between thermal boundary layers are properly accounted for, this criterion succeeds in describing relationships between thermal boundary layer (TBL) properties for mixed heated convection. The surface heat flux of a convecting fluid is locally determined by the properties of the upper TBL, as opposed to globally determined. Our foundational scaling approach can be readily extended to nearly any complexity of convection within planetary mantles.

Amy Ferrick

Papers

Coordination Numbers in Natural Beach Sand

Defining Earth's elusive thermal budget in the presence of a hidden reservoir

Long-term core–mantle interaction explains W-He isotope heterogeneities

Microstructural differences between naturally-deposited and laboratory beach sands.

Generalizing Scaling Laws for Mantle Convection With Mixed Heating