Global mass wasting during the Middle Ordovician: Meteoritic trigger or plate-tectonic environment?

Mass wasting at continental margins on a global scale during the Middle Ordovician has recently been related to high meteorite influx. Although a high meteorite influx during the Ordovician should not be neglected, we challenge the idea that mass wasting was mainly produced by meteorite impacts over a period of almost 10 Ma. Having strong arguments against the impact-related hypothesis, we propose an alternative explanation, which is based on a re-evaluation of the mass wasting sites, considering their plate-tectonic distribution and the global sea level curve. A striking and important feature is the distribution of most of the mass wasting sites along continental margins characterised by periods of magmatism, terrane accretion and continental or back-arc rifting, respectively, related to subduction of oceanic lithosphere. Such processes are commonly connected with seismic activity causing earthquakes, which can cause downslope movement of sediment and rock. Considering all that, it seems more likely that most of this mass wasting was triggered by earthquakes related to plate-tectonic processes, which caused destabilisation of continental margins resulting in megabreccias and debris flows. Moreover, the period of mass wasting coincides with sea level drops during global sea level lowstand. In some cases, sea level drops can release pore-water overpressure reducing sediment strength and hence promoting instability of sediment at continental margins. Reduced pore-water overpressure can also destabilise gas hydrate-bearing sediment, causing slope failure, and thus resulting in submarine mass wasting. Overall, the global mass wasting during the Middle Ordovician does not need meteoritic trigger.     

Towards New Swedish Recommendations for Cautious Perimeter Blasting

A Swedish table of blast damage depths has, in one form or other, been in use since the late-1970s. Its history and the underlying theory are described. New experimental and theoretical findings that point out a number of shortcomings in the table and the underlying theory are presented and discussed. A revised version of the blast damage table was recently introduced in conjunction with new but incomplete recommendations for cautious perimeter blasting. The new table leaves the difficult task of taking into account factory like decoupling, water in the borehole, the rock properties, type of initiation, charge length and the actual bit diameter to the user. This paper discusses different ways of doing this, based on the experimental findings and a recent formula for the prediction of the lengths of radial cracks behind the half-casts. The material presented in this paper is meant to supplement and extend the new Swedish recommendations for cautious perimeter blasting of tunnels, shafts, pits and road cuts.     

Testing the fidelity of thermometers at ultrahigh temperatures

A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H₂O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO₂ that is lower than the assumed value of unity.     

Leucoxene fish as a micro-kinematic indicator

Asymmetric aggregates of fine-grained leucoxene and quartz are reported from siliceous L-S tectonites deformed by progressive simple shear. The leucoxene fish, morphologically similar to mica fish, consistently yield shear senses in agreement with other kinematic criteria. We interpret the leucoxene as a pre- or early syntectonic alteration product of detrital heavy-minerals in a siliciclastic protolith. The leucoxene fish behave as passive markers, and they record kinematics by modification of pre-existing aggregates rather than syndeformational mineral growth. For siliceous metasedimentary rocks otherwise lacking in micro-kinematic indicators, leucoxene fish may provide an alternative to quartz c-axis analysis.     

Generalized LSG models with spatially varying Coriolis parameter

In this paper, we derive and study approximate balance models for nearly geostrophic shallow water flow where the Coriolis parameter is permitted to vary across the domain as long as it remains nondegenerate. This situation includes, for example, the β-plane approximation to the shallow water equations at mid-latitudes. Our approach is based on changing configuration space coordinates in the underlying variational principle in such a way that consistent asymptotics in the transformed Lagrangian leads to a degenerate Lagrangian structure. In this paper, we restrict our attention to first-order models. We show that the resulting models can be formulated in terms of an advected potential vorticity with a nonlinear vorticity inversion relation. We study the associated solvability conditions and identify a subfamily of models for which these conditions are satisfied without additional restrictions on the data. Finally, we provide the link between our framework and the theory of constrained Hamiltonian systems.     

Trends in the frequency of synoptic types in central-southern Chile in the period 1961–2012 using the Jenkinson and Collison synoptic classification

Atmospheric circulation patterns in southern Chile (42° 30′ S) were studied in order to determine and analyse the most characteristic synoptic types and their recent trends, as well as to gain an understanding of how they are associated with low-frequency variability patterns. According to the Jenkinson and Collison (J&C) classification method, a 16-point grid of sea-level pressure data was employed. The findings reveal that some synoptic types show statistically significant trends with a 95% confidence level, positively for anticyclonic westerly hybrids (AW) and advective types for third and fourth quadrant wind flows (W, NW, and N) and negatively for SW and cyclonic hybrids (CS and CSW). A model has been constructed of the linear regression of some weather types with teleconnections that most affect Chile: the undetermined types (U), AW were associated with El Niño or the warm phase of the Pacific Decadal Oscillation (PDO), whereas the cyclonic northerly and cyclonic northeasterly types (CN and CNE) were associated with La Niña or cool phase of the PDO. The weather types associated with Antarctic Oscillation (AAO) in its positive phase are anticyclonic northerly and northeasterly and northerly advection types, while in its negative phase are cyclonic southwesterly and advection types.     

Planform of convection with strongly temperature-dependent viscosity

Abstract

This paper experimentally investigates the convective planform near critical in a fluid layer whose temperature-dependent viscosity varies from top to bottom by up to a factor of 1500. Convection occurs in three different planforms: rolls, hexagons and squares. The square planform, which appears only for fluids with viscosity variation greater than about 50, replaces the hexagonal convection pattern as the Rayleigh number increases much above critical. The large amplitude of hexagonal convection with strong viscosity variation precludes studying the hexagon-square transition with perturbation methods of the type used to study the hexagon-roll transitions at smaller viscosity variations.     

Targeting carbon dioxide removal in the European Union

In principle, many climate policymakers have accepted that large-scale carbon dioxide removal (CDR) is necessary to meet the Paris Agreement’s mitigation targets, but they have avoided proposing by whom CDR might be delivered. Given its role in international climate policy, the European Union (EU) might be expected to lead the way. But among EU climate policymakers so far there is little talk on CDR, let alone action. Here we assess how best to ‘target’ CDR to motivate EU policymakers exploring which CDR target strategy may work best to start dealing with CDR on a meaningful scale. A comprehensive CDR approach would focus on delivering the CDR volumes required from the EU by 2100, approximately at least 50 Gigatonnes (Gt) CO₂, according to global model simulations aiming to keep warming below 2°C. A limited CDR approach would focus on an intermediate target to deliver the CDR needed to reach ‘net zero emissions’ (i.e. the gross negative emissions needed to offset residual positive emissions that are too expensive or even impossible to mitigate). We argue that a comprehensive CDR approach may be too intimidating for EU policymakers. A limited CDR approach that only addresses the necessary steps to reach the (intermediate) target of ‘net zero emissions’ is arguably more achievable, since it is a better match to the existing policy paradigm and would allow for a pragmatic phase-in of CDR while avoiding outright resistance by environmental NGOs and the broader public.

Key policy insights

• Making CDR an integral part of EU climate policy has the potential to significantly reshape the policy landscape.

• Burden sharing considerations would probably play a major role, with comprehensive CDR prolonging the disparity and tensions between progressives and laggards.

• Introducing limited CDR in the context of ‘net zero’ pathways would retain a visible primary focus on decarbonization but acknowledge the need for a significant enhancement of removals via ‘natural’ and/or ‘engineered’ sinks.

• A decarbonization approach that intends to lead to a low level of ‘residual emissions’ (to be tackled by a pragmatic phase-in of CDR) should be the priority of EU climate policy.

     

Intraseasonal variability of sea level and circulation in the Gulf of Thailand: the role of the Madden–Julian Oscillation

Intraseasonal variability of the tropical Indo-Pacific ocean is strongly related to the Madden–Julian Oscillation (MJO). Shallow seas in this region, such as the Gulf of Thailand, act as amplifiers of the direct ocean response to surface wind forcing by efficient setup of sea level. Intraseasonal ocean variability in the Gulf of Thailand region is examined using statistical analysis of local tide gauge observations and surface winds. The tide gauges detect variability on intraseasonal time scales that is related to the MJO through its effect on local wind. The relationship between the MJO and the surface wind is strongly seasonal, being most vigorous during the monsoon, and direction-dependent. The observations are then supplemented with simulations of sea level and circulation from a fully nonlinear barotropic numerical ocean model (Princeton Ocean Model). The numerical model reproduces well the intraseasonal sea level variability in the Gulf of Thailand and its seasonal modulations. The model is then used to map the wind-driven response of sea level and circulation in the entire Gulf of Thailand. Finally, the predictability of the setup and setdown signal is discussed by relating it to the, potentially predictable, MJO index.     

Two-dimensional modelling of some CFC replacement compounds

The Cambridge 2-D Eulerian model has been used to study the potential atmospheric distributions and lifetimes of a number of CFC replacement compounds and their degradation products. The study has focused on HFC 134a and HCFCs 123, 141b and 142b and the major products formed by their atmospheric degradation. The loss of these compounds and their products by hydroxyl radical attack, photolysis and in-cloud hydrolysis have been investigated. The study has shown that HCFCs 141b and 142b have sufficiently long lifetimes to enter the stratosphere in significant quantities, where degradation leads to an increase in the total stratospheric chlorine concentration. The study has also highlighted areas where further experimental work would be valuable, in particular characterisation of the product channels for the degradation reactions and determination of the removal rates of the products in the aqueous phase.