3,693 research outputs found
Unexpected Effect of Internal Degrees of Freedom on Transverse Phonons in Supercooled Liquids
We show experimentally that in a supercooled liquid composed of molecules
with internal degrees of freedom the internal modes contribute to the frequency
dependent shear viscosity and damping of transverse phonons, which results in
an additional broadening of the transverse Brillouin lines. Earlier, only the
effect of internal modes on the frequency dependent bulk viscosity and damping
of longitudinal phonons was observed and explained theoretically in the limit
of weak coupling of internal degrees of freedom to translational motion. A new
theory is needed to describe this new effect. We also demonstrate, that the
contributions of structural relaxation and internal processes to the width of
the Brillouin lines can be separated by measurements under high pressure
Calcium-mediated stabilisation of soil organic carbon
Soils play an essential role in the global cycling of carbon and understanding the stabilisation mechanisms behind the preservation of soil organic carbon (SOC) pools is of globally recognised significance. Until recently, research into SOC stabilisation has predominantly focused on acidic soil environments and the interactions between SOC and aluminium (Al) or iron (Fe). The interactions between SOC and calcium (Ca) have typically received less attention, with fewer studies conducted in alkaline soils. Although it has widely been established that exchangeable Ca (CaExch) positively correlates with SOC concentration and its resistance to oxidation, the exact mechanisms behind this relationship remain largely unidentified. This synthesis paper critically assesses available evidence on the potential role of Ca in the stabilisation of SOC and identifies research topics that warrant further investigation. Contrary to the common view of the chemistry of base cations in soils, chemical modelling indicates that Ca2+ can readily exchange its hydration shell and create inner sphere complexes with organic functional groups. This review therefore argues that both inner- and outer-sphere bridging by Ca2+ can play an active role in the stabilisation of SOC. Calcium carbonate (CaCO3) can influence occluded SOC stability through its role in the stabilisation of aggregates; however, it could also play an unaccounted role in the direct sorption and inclusion of SOC. Finally, this review highlights the importance of pH as a potential predictor of SOC stabilisation mechanisms mediated by Al- or Fe- to Ca, and their respective effects on SOC dynamics
Potential accumulation of toxic trace elements in soils during enhanced rock weathering
Terrestrial enhanced rock weathering (ERW) is a carbon dioxide removal technology that aims at accelerating one of the most powerful negative feedbacks on Earth's climate, the chemical weathering of silicates. To achieve this, ERW proposes to spread ground silicate rock on agricultural soils. According to many models, global application rates of 40 tonnes of ground basaltic rock per hectare and per year would be necessary to sequester a significant amount of CO2, representing up to 24% of the current net annual increase in atmospheric CO2. When assessing the viability of ERW as a global geo-engineering strategy, a pivotal but overlooked question to address is whether ERW may lead to toxic trace element accumulation in soils at unauthorized and potentially harmful levels. This study evaluates the legal sustainability of ERW with regard to trace element contents in soils. We compare different trace element accumulation scenarios considering a range of rock sources, application rates and national regulatory limits. The results indicate that, at the suggested annual application rate of 40 tonnes per hectare, the first regulatory limits would be exceeded after 6 and 10 years for copper and nickel, respectively. This study argues in favour of close tailoring of ERW deployment to local conditions in order to tap into its climate mitigation potential while preserving long-term soil uses
Free energy of an SU(2) monopole-antimonopole pair
We induce an external monopole-antimonopole pair in an SU(2) lattice
gauge system and measure its free energy as a way to probe the vacuum
structure. We discuss the motivation and computational methodology of the
investigation and illustrate our preliminary results.Comment: LATTICE98(confine
Potential between external monopole and antimonopole in SU(2) lattice glu odynamics
We present the results of a study of the free energy of a monopole pair in
pure
SU(2) theory at finite temperature, both below and above the deconfinement
tran sition. We find a Yukawa potential between monopoles in both phases. At
low temp erature, the screening mass is compatible with the lightest glueball
mass. At hi gh temperature, we observe an increased screening mass with no
apparent disconti nuity at the phase transition.Comment: LATTICE 99 (Topology and Confinement
A cascading influence of calcium carbonate on the biogeochemistry and pedogenic trajectories of subalpine soils, Switzerland
The effects of the spatial predictability of food on the defence behaviour and distribution of juvenile convict cichlids (Cichlasoma nigrofasciatum) /cTamara C. Grand. --
Examines changes in the competitive behaviour and distribution of individuals in response to varying levels of spatial predictability for food. Groups of 6 juvenile convict cichlids competed for Daphnia magna prey under varying conditions
Evidence linking calcium to increased organo-mineral association in soils
Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO-bearing) or without carbonates (CaCO-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δC values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δC values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δC values. Bulk SOC content was twice as high in the CaCO-bearing profiles, which also had lower bulk δC values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δC values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δC values
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