Large cold anomalies in the deep mantle and mantle instability in the Cretaceous

Two large cold masses in the deep mantle have been delineated by using long-wavelength seismic tomographic models in conjunction with mineralogical experimental data at high pressure. These cold anomalies are found under the western Pacific and the Americas with temperatures more than 1000 degrees below the ambient mantle temperature. These strong cold anomalies existing in the lower mantle today would suggest that there might have existed not too long ago a substantial temperature jump across a thermal boundary layer between the upper and lower mantle. Numerical simulations in an axisymmetric spherical-shell model incorporating the two major phase transitions have shown that very large pools of cold material with temperatures of around 1500 K can be flushed down to the core–mantle boundary during this tumultuous gravitational instability. A correlation is found between the current locations of these very cold masses and regions of past subduction since the Cretaceous. Correlation analysis shows that the slab mass-flux into the lower mantle does not behave in a steady-state fashion. These findings may support the idea of a strong gravitational instability with origins in the transition zone, as suggested by numerical models of mantle convection.     

Making a difference on the ground: the challenge of demonstrating the effectiveness of decision support

Decision support has become a popular concept, especially in the context of climate change. Government agencies and researchers increasingly recognize that they should provide it, and resource managers and policy-makers increasingly need and demand it. This demand will only grow as climate change progresses. Those who will attempt to meet this growing demand will need to demonstrate “effectiveness”. This editorial raises a number of critical questions that need to be answered in the course of evaluating whether decision support is effective. The answers, if carefully considered early on, may help to design processes that may in fact produce more useable products, facilitate their use, and ultimately create intended and desirable outcomes.     

Towards a better understanding of magnesium-isotope ratios from marine skeletal carbonates

This study presents magnesium stable-isotope compositions of various biogenic carbonates of several marine calcifying organisms and an algae species, seawater samples collected from the western Dutch Wadden Sea, and reference materials. The aim of this study is to explore the influence of mineralogy, taxonomy and environmental factors (e.g., seawater isotopic composition, temperature, salinity) on magnesium-isotopic (δ²⁶Mg) ratios of skeletal carbonates. Using high-precision multi-collector inductively coupled plasma mass spectrometry, we observed that the magnesium-isotopic composition of seawater from the semi-enclosed Dutch Wadden Sea is identical to that of open marine seawater. We further found that a considerable component of the observed variability in δ²⁶Mg values of marine skeletal carbonates can be attributed to differences in mineralogy. Furthermore, magnesium-isotope fractionation is species-dependent, with all skeletal carbonates being isotopically lighter than seawater. While δ²⁶Mg values of skeletal aragonite and high-magnesium calcite of coralline red algae indicate the absence or negligibility of metabolic influences, the δ²⁶Mg values of echinoids, brachiopods and bivalves likely result from a taxon-specific level of control on Mg-isotope incorporation during biocalcification. Moreover, no resolvable salinity and temperature effect were observed for coralline red algae and echinoids. In contrast, Mg-isotope data of bivalves yield ambiguous results, which require further validation. The data presented here, point to a limited use of Mg isotopes as temperature proxy, but highlight the method’s potential as tracer of seawater chemistry through Earth’s history.     

Observations of magnetic flux compression in jet impact experiments

The astrophysical jet experiment at Caltech generates a T=2–5 eV, n=10²¹–10²² m⁻³ plasma jet using coplanar disk electrodes linked by a poloidal magnetic field. A 100 kA current generates a toroidal magnetic field; the toroidal field pressure inflates the poloidal flux surface, magnetically driving the jet. The jet travels at up to 50 km/s for ∼20–25 cm before colliding with a cloud of initially neutral gas. We study the interaction of the jet and the cloud in analogy to an astrophysical jet impacting a molecular cloud. Diagnostics include magnetic probe arrays, a 12-channel spectroscopic system and a fast camera with optical filters. When a hydrogen plasma jet collides with an argon target cloud, magnetic measurements show the magnetic flux compressing as the plasma jet deforms. As the plasma jet front slows and the plasma piles up, the density of the frozen-in magnetic flux increases.     

Updates to the MSFC Meteoroid Stream Model

The Marshall Space Flight Center (MSFC) Meteoroid Stream Model simulates particle ejection and subsequent evolution from comets in order to provide meteor shower forecasts to spacecraft operators for hazard mitigation and planning purposes. The model, previously detailed in Moser and Cooke (Earth Moon Planets 95, 141 (2004)), has recently been updated; the changes include the implementation of the RADAU integrator, an improved planetary treatment, and the inclusion of general relativistic effects in the force function. The results of these updates are investigated with respect to various meteoroid streams and the outcome presented.     

Stability of source mechanisms inverted from P‐wave amplitude microseismic monitoring data acquired at the surface

We study the stability of source mechanisms inverted from data acquired at surface and near‐surface monitoring arrays. The study is focused on P‐wave data acquired on vertical components, as this is the most common type of acquisition. We apply ray modelling on three models: a fully homogeneous isotropic model, a laterally homogeneous isotropic model and a laterally homogeneous anisotropic model to simulate three commonly used models in inversion. We use geometries of real arrays, one consisting in surface receivers and one consisting in ‘buried’ geophones at the near‐surface. Stability was tested for two of the frequently observed source mechanisms: strike‐slip and dip‐slip and was evaluated by comparing the parameters of correct and inverted mechanisms. We assume these double‐couple source mechanisms and use quantitatively the inversion allowing non‐double‐couple components to measure stability of the inversion. To test the robustness we inverted synthetic amplitudes computed for a laterally homogeneous isotropic model and contaminated with noise using a fully homogeneous model in the inversion. Analogously amplitudes computed in a laterally homogeneous anisotropic model were inverted in all three models. We show that a star‐like surface acquisition array provides very stable inversion up to a very high level of noise in data. Furthermore, we reveal that strike‐slip inversion is more stable than dip‐slip inversion for the receiver geometries considered here. We show that noise and an incorrect velocity model may result in narrow bands of source mechanisms in Hudson's plots.     

Review of ray-Born forward modeling for migration and diffraction analysis

The ray-Born approximation is a very useful tool for forward modeling of scattered waves. The fact that ray-Born modeling underlies most seismic migration techniques, and therefore shares their assumptions, is a justification in itself to consider it for forward modeling. The ray-Born approximation does not make an explicit distinction between specular reflections and nonspecular diffractions. It therefore allows the modeling of diffractions from structural discontinuities such as edges and tips, as well as caustic diffractions. In the simplest implementation ray-Born seismograms are multiple-free. Ray-Born modeling can be orders of magnitude faster than finite-difference modeling, both in two-and three dimensions.     

Emerging trends in urbanizing Palestine: neglected city-builders beyond the occupation

In this article, we draw attention to trends in land transformation in the West Bank since the Second Intifada, after which a surge of investment from Gulf countries entered Palestine, almost exclusively in the West Bank. The occupied Palestinian territories have attracted a great deal of attention from media and academics, yet the vast majority of scholarship focuses on the conflict and the variety of social, economic, and political repercussions of the on-going Israeli occupation. While the occupation has undeniably serious impacts on every aspect of life for Palestinians, the near-exclusive focus on the conflict means that significant new trends, such as urban mega-developments and emerging market-based urbanisation processes, have been largely overlooked. We outline three directions for future research: the assemblage of “neglected city-builders” shaping current urbanization processes in Palestine; the ideas, policies, and norms circulating in the West Bank; and the social, spatial, economic, and other impacts of these new urban developments.     

It starts at home? Climate policies targeting household consumption and behavioral decisions are key to low-carbon futures

Through their consumption behavior, households are responsible for 72% of global greenhouse gas emissions. Thus, they are key actors in reaching the 1.5 °C goal under the Paris Agreement. However, the possible contribution and position of households in climate policies is neither well understood, nor do households receive sufficiently high priority in current climate policy strategies. This paper investigates how behavioral change can achieve a substantial reduction in greenhouse gas emissions in European high-income countries. It uses theoretical thinking and some core results from the HOPE research project, which investigated household preferences for reducing emissions in four European cities in France, Germany, Norway and Sweden. The paper makes five major points: First, car and plane mobility, meat and dairy consumption, as well as heating are the most dominant components of household footprints. Second, household living situations (demographics, size of home) greatly influence the household potential to reduce their footprint, even more than country or city location. Third, household decisions can be sequential and temporally dynamic, shifting through different phases such as childhood, adulthood, and illness. Fourth, short term voluntary efforts will not be sufficient by themselves to reach the drastic reductions needed to achieve the 1.5 °C goal; instead, households need a regulatory framework supporting their behavioral changes. Fifth, there is a mismatch between the roles and responsibilities conveyed by current climate policies and household perceptions of responsibility. We then conclude with further recommendations for research and policy.