ESC Publications - Cambridge Univesity
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Insights into magma chamber processes from the relationship between fabric and grain shape in troctolitic cumulates
Full text linkThe strength of foliations defined by shape preferred orientation of plagioclase in troctolitic cumulates from the Layered Series of the Skaergaard intrusion, and the Rum Eastern Layered Intrusion, increases as the grains become more tabular, due either to the greater propensity of highly non-equant grains to be re-arranged by magmatic currents or tectonic disruption of poorly consolidated mush, or by the effects of a pre-existing shape preferred orientation on final grain shape in fully solidified rocks. The stratigraphic evolution of grain shape, microstructures and fabrics in the lowest 320m of the Skaergaard Layered Series records the progressive inflation of the chamber to its final size. During the earliest stages of solidification, the extent of in situ nucleation and growth on the chamber floor decreased upwards through the stratigraphy, due to the development of a thermally insulating blanket of mush on the floor. An upwards increase in foliation strength as the chamber inflated to its final size was a result of the increasing strength of convection of the bulk magma and an increasing contribution to the floor mush of crystals derived from the walls of the enlarging magma chamber. Plagioclase in the troctolites in the open-system magma chamber of the Rum Eastern Layered Intrusion is generally more equant than that in the Skaergaard intrusion, perhaps related to the slower crystal growth on the margins of the continuously replenished Rum chamber. Significant sub-solidus modification of original igneous microstructures is observed in Rum troctolites from parts of the stratigraphy recording frequent replenishment events
Quantifying the relationship between short‐wavelength dynamic topography and thermomechanical structure of the upper mantle using calibrated parameterization of anelasticity
Full text linkOceanic residual depth varies on ≤5000 km wavelengths with amplitudes of ±1 km. A component of this short‐wavelength signal is dynamic topography caused by convective flow in the upper ~300 km of the mantle. It exerts a significant influence on landscape evolution and sea‐level change, but its contribution is often excluded in geodynamic models of whole‐mantle flow. Using seismic tomography to resolve buoyancy anomalies in the oceanic upper mantle is complicated by the dominant influence of lithospheric cooling on velocity structure. Here, we remove this cooling signal from global surface wave tomographic models, revealing a correlation between positive residual depth and slow residual velocity anomalies at depths <300 km. To investigate whether these anomalies are of sufficient amplitude to account for short‐wavelength residual depth variations, we calibrate an experimentally derived parameterization of anelastic deformation at seismic frequencies to convert shear wave velocity into temperature, density, and diffusion creep viscosity. Asthenospheric temperature anomalies reach +150°C in the vicinity of major magmatic hotspots and correlate with geochemical and geophysical proxies for potential temperature along mid‐ocean ridges. Locally, we find evidence for a 150 km‐thick, low‐viscosity asthenospheric channel. Incorporating our revised density structure into models of whole‐mantle flow yields reasonable agreement with residual depth observations and suggests that ±30 km deviations in local lithospheric thickness account for a quarter of total amplitudes. These predictions remain compatible with geoid constraints and substantially improve the fit between power spectra of observed and predicted dynamic topography. This improvement should enable more accurate reconstruction of the spatio‐temporal evolution of Cenozoic dynamic topography
Deep Earth explorers
Full text linkThe Cambridge Deep Earth Seismology group has an exhibition at the Sedgwick Museum, Cambridge, aimed at increasing understanding of our planet and changing perceptions of geophysics – and geophysicists. Group members Jennifer Jenkins, Jess Bartlet and Sanne Cottaar tell us more
Timing the Extant Avian Radiation: The Rise of Modern Birds, and the Importance of Modeling Molecular Rate Variation
Full text linkUnravelling the phylogenetic relationships among the major groups of living birds has been described as the greatest outstanding problem in dinosaur systematics. Recent work has identified portions of the avian tree of life that are particularly challenging to reconstruct, perhaps as a result of rapid cladogenesis early in crown bird evolutionary history (specifically, the interval immediately following the end-Cretaceous mass extinction). At face value this hypothesis enjoys support from the crown bird fossil record, which documents the first appearances of most major crown bird lineages in the early Cenozoic—in line with a model of rapid post-extinction niche filling among surviving avian lineages. However, molecular-clock analyses have yielded strikingly variable estimates for the age of crown birds, and conflicting inferences on the impact of the end-Cretaceous mass extinction on the extant bird radiation. This uncertainty has often been ascribed to a patchy avian fossil record, but the possibility of model misspecification in molecular divergence time analyses represents an important and relatively underexplored alternative hypothesis. Here, we highlight the necessity of further developing and using models that account for co-ordinated variation in rates of molecular evolution across a phylogeny (e.g. molecular early burst) as a means of assessing support for a rapid post-Cretaceous radiation of crown birds. We discuss how relationships between life-history and substitution rates can mislead divergence time studies that do not account for directional changes in substitution rates over time, and suggest that these effects might have caused some of the variation in existing molecular date estimates for birds. We suggest multiple paths forward that could help resolve this and similar conflicts within other major eukaryotic clades
The Biogeography of Coelurosaurian Theropods and Its Impact on Their Evolutionary History
No full textThe Coelurosauria are a group of mostly feathered theropods that gave rise to birds, the only dinosaurians that survived the Cretaceous-Paleogene extinction event and are still found today. Between their first appearance in the Middle Jurassic up to the end Cretaceous, coelurosaurians were party to dramatic geographic changes on the Earth’s surface, including the breakup of the supercontinent Pangaea, and the formation of the Atlantic Ocean. These plate tectonic events are thought to have caused vicariance or dispersal of coelurosaurian faunas, influencing their evolution. Unfortunately, few coelurosaurian biogeographic hypotheses have been supported by quantitative evidence. Here, we report the first, broadly sampled quantitative analysis of coelurosaurian biogeography using the likelihood-based package BioGeoBEARS. Mesozoic geographic configurations and changes are reconstructed and employed as constraints in this analysis, including their associated uncertainties. We use a comprehensive time-calibrated coelurosaurian evolutionary tree produced from the Theropod Working Group phylogenetic data matrix. Six biogeographic models in the BioGeoBEARS package with different assumptions about the evolution of spatial distributions are tested against geographic constraints. Our results statistically favor the DIVALIKE+J and DEC+J models, which allow vicariance and founder events, supporting continental vicariance as an important factor in coelurosaurian evolution. Ancestral range estimation indicates frequent dispersal events via the Apulian route (connecting Europe and Africa during the Early Cretaceous) and the Bering land bridge (connecting North America and Asia during the Late Cretaceous). These quantitative results are consistent with commonly inferred Mesozoic dinosaurian dispersals and continental-fragmentation-induced vicariance events. In addition, we recognize the importance of Europe as a dispersal center and gateway in the Early Cretaceous, as well as other vicariance events such as those triggered by the disappearance of land bridges
Pennaraptoran systematics
No full textNew and important pennaraptoran specimens continue to be discovered on a regular basis. Yet, with these discoveries the number of viable phylogenetic hypotheses has increased, including ones that challenge the traditional exclusive grouping of dromaeosaurids and troodontids within a monophyletic Deinonychosauria. This chapter section will cover recent efforts to address prevailing phylogenetic uncertainties and controversies, both between and within key clades, including deinonychosaurian monophyly, the phylogenetic position of anchiornithines and scansoriopterygids and the interrelationships of enantiornithines. Whilst recent discoveries mainly from Asia have created much of the latest uncertainties and controversies, brand new material, particularly from Asia, promises to rather fittingly address them. Further curatorship of long-standing phylogenetic datasets and more prevalent use of extended analytical protocols will be essential to meeting this challenge, especially for groups whose boundaries have been blurred. As it becomes increasingly difficult to study all fossil materials owing to their growing numbers and ever disparate locations, broader use of digital fossils and online character databases for character coding is acutely needed to ensure that errors arising from remote rather than first-hand scoring are reduced as far as possible, particularly at this time of rapid data accumulation. Recent taxonomic revisions and newly described taxa also present opportunities to update and revisit clade definitions, e.g., designating neotypes for reference taxa like Troodon formosus
Phylogenetic definitions for Caprimulgimorphae (Aves) and major constituent clades under the International Code of Phylogenetic Nomenclature
Full text linkPhylogenetic nomenclature, a system of taxonomic nomenclature in which taxon names are defined based on phylogenetic relationships, has been widely adopted in recent decades, particularly by vertebrate palaeontologists. However, formal regulation of this taxonomic system had been non-existent until the recent implementation of the International Code of Phylogenetic Nomenclature (PhyloCode). To fulfil the requirements of the PhyloCode, we explicitly establish phylogenetic definitions that we recommended in a recent phylogenetic study on the avian taxon Caprimulgimorphae (which includes nightjars, potoos, frogmouths, swifts, hummingbirds, and others) and many of its major constituent subclades. Two new names are coined: Sedentaves (for the smallest crown clade uniting Steatornis and Nyctibiidae) and Letornithes (for the smallest crown clade uniting Podargidae and Daedalornithes). We also briefly review the fossil record and diagnostic morphological apomorphies of caprimulgimorph clades for which relevant information is available
Icequake source mechanisms for studying glacial sliding
Full text linkImproving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strengt
Estimating the carbon content of the deep mantle with Icelandic melt inclusions
Full text linkEarth's carbon budget is central to our understanding of the long-term co-evolution of life and the planet. Direct observations of surface reservoirs allow for the detailed quantification of their carbon content. However, the carbon content of Earth's deep interior remains poorly constrained. Here we study olivine-hosted melt inclusions from two Icelandic eruptions, with those from the Miðfell eruption allowing us to investigate the carbon content of the deep mantle. Comparison with the previously studied Borgarhraun eruption highlights the presence of deep, plume-sourced mantle material within the Miðfell source region. Miðfell contains trace element-depleted melt inclusions undersaturated in CO2, which have high CO2/Ba () and CO2/Nb (), though some inclusions preserve even greater relative carbon enrichment. These observations allow us to reconstruct the CO2 content of the bulk Miðfell source as being . By identifying that Miðfell is a mixture of depleted and deep mantle components, we can estimate a CO2 content for the deep mantle component of ; a concentration that is over ten times higher than depleted MORB mantle estimates. Assuming that the deep mantle component identified in Miðfell is representative of a global reservoir, then with our new CO2 estimate and by considering a range of representative mantle fractions for this reservoir, we calculate that it contains up to 14 times more carbon than that of the atmosphere, oceans, and crust combined. Our result of elevated CO2/Ba and CO2/Nb ratios, and carbon enrichment support geochemical bulk Earth carbon models that call for the presence of carbon-rich deep mantle domains to balance Earth's relatively carbon-poor upper mantle and surface environment
Morphological Analysis of Archetypal Calcite Cement
No full textArchetypal cements from their deposition to the time they are sampled preserve a record of subsurface fluid evolution that can cover many millions of years. Their morphological development after nucleating on fissure walls is simulated by graphical models showing maturation from isolated to competitive to parallel growth. Graphical models are not available for the cementation of pores that are more complex and diverse. The reconstruction of cement growth in pores is accomplished here using multiple cathodoluminescent (CL) growth zones.
Sediment pores are filled in two stages separated by a cementation threshold associated with a rapid drop in permeability. Pre-threshold cement is centripetal ordered, regularly distributed, and nurtured by a megapore network. Post-threshold cement is disordered, sporadically distributed, and nurtured by labile micropore connections.
The crystallographic form of euhedral crystals at cement's growing front has environmental significance while the anhedral shape of aggregate crystals it leaves behind is controlled by geometrical selection.
Growth of seeded cement is divided into epitaxial and mantle stages; syntaxial when these two are not identified. The epitaxial stage begins with a seed at multiple points then coalesces, simplifies, and morphs towards the Wulff or equilibrium surface. The extent of epitaxial growth is correlated with the size of crystals forming pore walls. Increments of mantle growth can be initiated over the whole surface of a pre-existing crystal, or can be restricted to its edges and/or corners. Restricted seeding is often linked with changes of crystal habit resulting in the reorientation of the crystal's fastest growth direction and the direction of maturation.
Some impingement intercrystalline boundaries have offset growth zones explained by dissolution; such a self-cannibalization process is new. The existing explanation that some enfacial junctions are due to pauses in growth is confirmed by the arrangement of growth zones around triple points, but a new type is recorded formed from continuously growing crystals whose intercrystalline boundaries become modified.
The presence of the cement's internal growth surfaces allow its development to be reconstructed, providing the foundation for further cement studies; however, one example is presented where the disproportionate filling of pores in adjacent millimeter-thick layers is obscure because its crystals lack internal growth surfaces