High-temperature neutron diffraction study of deuterated brucite

To study the structural behavior of brucite at high temperature, we conducted in situ neutron diffraction experiments of a deuterated brucite powder sample, Mg(OD)₂, in the temperature range 313–583 K. The sample was stable up to 553 K, above which it started to decompose into periclase (MgO) and D₂O vapor. Rietveld analyses of the obtained data were performed using both single-site and three-site split-atom hydrogen models. Our results show that with increasing temperature, unit-cell parameter c increases at a rate ~7.7 times more rapidly than a. This large anisotropy of thermal expansion is primarily due to rapid increase in the interlayer thickness along the c-axis on heating. The amplitudes of thermal vibration for Mg, O, and D increase linearly with increasing temperature; however, the rate of the increase for the lighter D is much larger. In addition, D vibrates anisotropically with a higher magnitude within the (001) plane, as confirmed by our first-principles phonon calculations. On heating, the interatomic distances between a given D and its associated O and D from the adjacent [MgO₆] layer increase, whereas the O–D bond length decreases. This behavior suggests weakened D···O and D···D interlayer interactions but strengthened O–D bonding with increasing temperature.     

A study of candidate marine target impact craters in Arabia Terra,Mars

Abstract– Previous workers have proposed that a northern ocean existed early during Martian geologic history and the shorelines of that ocean would coincide roughly with the crustal dichotomy that divides the smooth, northern lowlands with the cratered, southern highlands. Arabia Terra is a region on Mars that straddles the crustal dichotomy, and several proposed shorelines are located in the area. Shallow marine impact craters on Mars likely would exhibit features like those on Earth, including characteristic morphological features that are distinctly different from that of craters formed on land. Common attributes of terrestrial marine impact craters include features of wet mass movement such as gravity slumps and debris flows; radial gullies leading into the crater depression; resurge deposits and blocks of dislocated materials; crater rim collapse or breaching of the crater wall; a central peak terrace or peak ring terrace; and subdued topography (an indicator of both age and possible flood inundation immediately following impact). In this article, these features have been used to evaluate craters on Mars as to a possible marine origin. This study used a simple quantification system to approximately judge and rank shallow marine impact crater candidates based on features observed in terrestrial analogs. Based on the quantification system, 77 potential shallow marine impact craters were found within an area bounded by 20°N and 40°N as well as 20°W and 20°E. Nine exemplary candidates were ranked with total scores of 70% or more. In a second, smaller study area, impact craters of approximately similar size and age were evaluated as a comparison and average total scores are 35%, indicating that there is some morphological difference between craters inside and outside the proposed shorelines. Results of this type of study are useful in helping to develop a general means of classification and characterization of potential marine craters.     

Modelling of anisotropic coal swelling and its impact on permeability behaviour for primary and enhanced coalbed methane recovery

Coal swelling/shrinkage during gas adsorption/desorption is a well-known phenomenon. For some coals the swelling/shrinkage shows strong anisotropy, with more swelling in the direction perpendicular to the bedding than that parallel to the bedding. Experimental measurements performed in this work on an Australian coal found strong anisotropic swelling behaviour in gases including nitrogen, methane and carbon dioxide, with swelling in the direction perpendicular to the bedding almost double that parallel to the bedding. It is proposed here that this anisotropy is caused by anisotropy in the coal's mechanical properties and matrix structure. The Pan and Connell coal swelling model, which applies an energy balance approach where the surface energy change caused by adsorption is equal to the elastic energy change of the coal solid, is further developed to describe the anisotropic swelling behaviour incorporating coal property and structure anisotropy. The developed anisotropic swelling model is able to accurately describe the experimental data mentioned above, with one set of parameters to describe the coal's properties and matrix structure and three gas adsorption isotherms. This developed model is also applied to describe anisotropic swelling measurements from the literature where the model was found to provide excellent agreement with the measurement. The anisotropic coal swelling model is also applied to an anisotropic permeability model to describe permeability behaviour for primary and enhanced coalbed methane recovery. It was found that the permeability calculation applying anisotropic coal swelling differs significantly to the permeability calculated using isotropic volumetric coal swelling strain. This demonstrates that for coals with strong anisotropic swelling, anisotropic swelling and permeability models should be applied to more accurately describe coal permeability behaviour for both primary and enhanced coalbed methane recovery processes.     

The global carbon market-mechanism landscape: pre and post 2020 perspectives

With market-mechanisms likely to achieve emission reductions at lower cost than alternative approaches, there is a presumption that they will be embraced by those who are serious about achieving ambitious reductions. Two broad messages exist; there is already considerable activity and some ambition in many parts of the world – a fragmented but embryonic ‘global’ trading landscape is emerging – and there are efforts at UN level to provide a unifying framework for these bottom-up developments. The topography of interest and response varies considerably across groups of countries, and there have been delays in making progress on a unifying framework. This article analyses the current carbon market landscape in terms of market dynamics and market-mechanism developments whilst undertaking an examination of how climate change negotiations under the United Nations Framework Convention on Climate Change (UNFCCC) is shaping the future carbon market landscape. This work shows that the combination of existing, emerging, and potential carbon market-mechanisms can be regarded as an emerging pre-2020 fragmented ‘global’ carbon market landscape based on differing bottom-up market based approaches. One outcome of a 2015 Climate Agreement could be a post-2020 global carbon market which would include new domestic and international market initiatives such as the Framework for Various Approaches and New Market Mechanism, together with reformed Kyoto mechanisms.

Policy relevance

With the 2015 Agreement under the United Nations Framework Convention on Climate Change (UNFCCC) expected to see Parties commit to ambitious mitigation commitments, post-2020 could see significant Party (& industry) investment in market-mechanisms and associated emissions units in an effort to achieve some of the abatement cost minimization offered by market approaches. This article is written for those who have an interest in understanding what is happening – and what is not happening – as regards the emergence of market-related approaches to GHG mitigation globally in the run up to the 21st Conference of the Parties (COP) of the UNFCCC which meets in Paris in December 2015, and what could be the shape of things to come post-2020.     

The role of ejecta in the small crater populations on the mid-sized saturnian satellites

We find evidence, by both observation and analysis, that primary crater ejecta play an important role in the small crater (less than a few km) populations on the saturnian satellites, and more broadly, on cratered surfaces throughout the Solar System. We measure crater populations in Cassini images of Enceladus, Rhea, and Mimas, focusing on image data with scales less than 500 m/pixel. We use recent updates to crater scaling laws and their constants (Housen, K.R., Holsapple, K.A. [2011]. Icarus 211, 856–875) to estimate the amount of mass ejected in three different velocity ranges: (i) greater than escape velocity, (ii) less than escape velocity and faster than the minimum velocity required to make a secondary crater (v_min), and (iii) velocities less than v_min. Although the vast majority of mass on each satellite is ejected at speeds less than v_min, our calculations demonstrate that the differences in mass available in the other two categories should lead to observable differences in the small crater populations; the predictions are borne out by the measurements we have made to date. In particular, Rhea, Tethys, and Dione have sufficient surface gravities to retain ejecta moving fast enough to make secondary crater populations. The smaller satellites, such as Enceladus but especially Mimas, are expected to have little or no traditional secondary populations because their escape velocities are near the threshold velocity necessary to make a secondary crater. Our work clarifies why the Galilean satellites have extensive secondary crater populations relative to the saturnian satellites. The presence, extent, and sizes of sesquinary craters (craters formed by ejecta that escape into temporary orbits around Saturn before re-impacting the surface, see Dobrovolskis, A.R., Lissauer, J.J. [2004]. Icarus 169, 462–473; Alvarellos, J.L., Zahnle, K.J., Dobrovolskis, A.R., Hamill, P. [2005]. Icarus 178, 104–123; Zahnle, K., Alvarellos, J.L., Dobrovolskis, A.R., Hamill, P. [2008]. Icarus 194, 660–674) is not yet well understood. Finally, our work provides further evidence for a “shallow” size–frequency distribution (slope index of ～2 for a differential power-law) for comets a few kilometers diameter and smaller.     

Three-Dimensional Modeling of Shallow Shear-Wave Velocities for Las Vegas, Nevada, Using Sediment Type

A three-dimensional model of near-surface shear-wave velocity in the deep alluvial basin underlying the metropolitan area of Las Vegas, Nevada (USA), is being developed for earthquake site response projections. The velocity dataset, which includes 230 measurements, is interpolated across the model using depth-dependent correlations of velocity with sediment type. The sediment-type database contains more than 1 400 well and borehole logs. Sediment sequences reported in logs are assigned to one of four units. A characteristic shear-wave velocity profile b developed for each unit by analyzing closely spaced pairs of velocity profiles and well or borehole logs. The resulting velocity model exhibits reasonable values and patterns, although it does not explicitly honor the measured shear-wave velocity profiles. Site response investigations that applied a preliminary version of the velocity model support a two-zone ground-shaking hazard model for the valley. Areas in which clay predominates in the upper 30 m are predicted to have stronger ground motions than the rest of the basin.     

Tsunami sediments and their foraminiferal assemblages

Tsunami hazard assessment begins with a compilation of past events that have affected a specific location. Given the inherent limitations of historical archives, the geological record has the potential to provide an independent dataset useful for establishing a richer, chronologically deeper time series of past events. Recent geological studies of tsunami are helping to improve our understanding of the nature and character of tsunami sediments. Wherever possible, geologists should be working to improve the research ‘tool kit’ available to identify past tsunami events. Marine foraminifera (single celled heterotrophic protists) have often been reported as present within tsunami-deposited sediments but in reality, little information about environmental conditions, and by analogy, the tsunami that deposited them, has been reported even though foraminifera have an enormous capacity to provide meaningful palaeo-environmental data. Here, we review what foraminifera are, describe their basic form and significance, summarise where they have been reported in tsunami sediments and identify what can be learnt from them. We review the gaps in our understanding and make recommendations to assist researchers who examine foraminiferal assemblages in order to enhance their use within tsunami geology.