The effects of degassing on magmatic gas waves and long period eruptive precursors at silicic volcanoes

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125 (10), (2020): e2020JB019755, https://doi.org/10.1029/2020JB019755Cyclical ground deformation, associated seismicity, and elevated degassing are important precursors to explosive eruptions at silicic volcanoes. Regular intervals for elevated activity (6–30 hr) have been observed at volcanoes such as Mount Pinatubo in the Philippines and Soufrière Hills in Montserrat. Here, we explore a hypothesis originally proposed by Michaut et al. (2013, https://doi.org/10.1038/ngeo1928) where porosity waves containing magmatic gas are responsible for the observed periodic behavior. We use two‐phase theory to construct a model where volatile‐rich, bubbly, viscous magma rises and decompresses. We conduct numerical experiments where magma gas waves with various frequencies are imposed at the base of the model volcanic conduit. We numerically verify the results of Michaut et al. (2013, https://doi.org/10.1038/ngeo1928) and then expand on the model by allowing magma viscosity to vary as a function of dissolved water and crystal content. Numerical experiments show that gas exsolution tends to damp the growth of porosity waves during decompression. The instability and resultant growth or decay of gas wave amplitude depends strongly on the gas density gradient and the ratio of the characteristic magma extraction rate to the characteristic magma degassing rate (Damköhler number, Da). We find that slow degassing can lead to a previously unrecognized filtering effect, where low‐frequency gas waves may grow in amplitude. These waves may set the periodicity of the eruptive precursors, such as those observed at Soufrière Hills Volcano. We demonstrate that degassed, crystal‐rich magma is susceptible to the growth of gas waves which may result in the periodic behavior.J. S. J. and D. B. were supported by NSF grant EAR‐1645057. C. M. has received financial support of the IDEXLyon Project of the University of Lyon in the frame of the Programme Investissements dAvenir (ANR‐16‐IDEX‐0005)

Dilation theory and systems of simultaneous equations in the predual of an operator algebra. II

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46272/1/209_2005_Article_BF01163170.pd

Velocity Amplitudes in Global Convection Simulations: The Role of the Prandtl Number and Near-Surface Driving

Several lines of evidence suggest that the velocity amplitude in global simulations of solar convection, U, may be systematically over-estimated. Motivated by these recent results, we explore the factors that determine U and we consider how these might scale to solar parameter regimes. To this end, we decrease the thermal diffusivity $\kappa$ along two paths in parameter space. If the kinematic viscosity $\nu$ is decreased proportionally with $\kappa$ (fixing the Prandtl number $P_r = \nu/\kappa$), we find that U increases but asymptotes toward a constant value, as found by Featherstone & Hindman (2016). However, if $\nu$ is held fixed while decreasing $\kappa$ (increasing $P_r$), we find that U systematically decreases. We attribute this to an enhancement of the thermal content of downflow plumes, which allows them to carry the solar luminosity with slower flow speeds. We contrast this with the case of Rayleigh-Benard convection which is not subject to this luminosity constraint. This dramatic difference in behavior for the two paths in parameter space (fixed $P_r$ or fixed $\nu$) persists whether the heat transport by unresolved, near-surface convection is modeled as a thermal conduction or as a fixed flux. The results suggest that if solar convection can operate in a high-$P_r$ regime, then this might effectively limit the velocity amplitude. Small-scale magnetism is a possible source of enhanced viscosity that may serve to achieve this high-$P_r$ regime.Comment: 34 Pages, 8 Figures, submitted to a special issue of "Advances in Space Research" on "Solar Dynamo Frontiers

The Psyche Gravity Investigation

The objective of the NASA Psyche mission gravity science investigation is to map the mass distribution within asteroid (16) Psyche to elucidate interior structure and to resolve the question of whether this metal-rich asteroid represents a remnant metal core or whether it is a primordial body that never melted. Measurements of gravity will be obtained via the X-band telecommunication system on the Psyche spacecraft, collected from progressively lower mapping altitudes. Orbital gravity will allow an estimate of GM to better than 0.001 km3 s−2. A spherical harmonic model of gravity to degree and order 10 will be achievable and, in concert with spherical harmonic data sets from topography and magnetometry, as well as surface composition data, will provide information regarding the spatial and radial distribution of mass that will be used to constrain the origin and evolution of (16) Psyche

Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction

The fossil record and recent molecular phylogenies support an extraordinary early-Cenozoic radiation of crown birds (Neornithes) after the Cretaceous-Paleogene (K-Pg) mass extinction [1–3 ]. However, questions remain regarding the mechanisms underlying the survival of the deepest lineages within crown birds across the K-Pg boundary, particularly since this global catastrophe eliminated even the closest stem-group relatives of Neornithes [4 ]. Here, ancestral state reconstructions of neornithine ecology reveal a strong bias toward taxa exhibiting predominantly non-arboreal lifestyles across the K-Pg, with multiple convergent transitions toward predominantly arboreal ecologies later in the Paleocene and Eocene. By contrast, ecomorphological inferences indicate predominantly arboreal lifestyles among enantiornithines, the most diverse and widespread Mesozoic avialans [5–7 ]. Global paleobotanical and palynological data show that the K-Pg Chicxulub impact triggered widespread destruction of forests [8, 9 ]. We suggest that ecological filtering due to the temporary loss of significant plant cover across the K-Pg boundary selected against any flying dinosaurs (Avialae [10 ]) committed to arboreal ecologies, resulting in a predominantly non-arboreal postextinction neornithine avifauna composed of totalclade Palaeognathae, Galloanserae, and terrestrial total-clade Neoaves that rapidly diversified into the broad range of avian ecologies familiar today. The explanation proposed here provides a unifying hypothesis for the K-Pg-associated mass extinction of arboreal stem birds, as well as for the post-K-Pg radiation of arboreal crown birds. It also provides a baseline hypothesis to be further refined pending the discovery of additional neornithine fossils from the Latest Cretaceous and earliest Paleogene.Also supported by:- a 50th Anniversary Prize Fellowship at the University of Bath.- a Smithsonian NMNH Deep Time Peter Buck Postdoctoral Fellowship.- NSF grants DGE-1650441 and DEB-1700786.</p

Multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes

The hypothesis that a single mushroom-like mantle plume head can generate a large igneous province within a few million years has been widely accepted(1). The Siberian Traps at the Permian Triassic boundary(2) and the Deccan Traps at the Cretaceous Tertiary boundary(3) were probably erupted within one million years. These large eruptions have been linked to mass extinctions. But recent geochronological data(4-11) reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years. This observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal, single-stage plume model suggests. Here we present numerical experiments to demonstrate that the entrainment of a dense eclogite-derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations. Such a process may contribute to multiple episodes of large igneous provinces.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62705/1/nature03697.pd

A framework for evaluating the influence of climate, dispersal limitation, and biotic interactions using fossil pollen associations across the late Quaternary

Environmental conditions, dispersal lags, and interactions among species are major factors structuring communities through time and across space. Ecologists have emphasized the importance of biotic interactions in determining local patterns of species association. In contrast, abiotic limits, dispersal limitation, and historical factors have commonly been invoked to explain community structure patterns at larger spatiotemporal scales, such as the appearance of late Pleistocene no-analog communities or latitudinal gradients of species richness in both modern and fossil assemblages. Quantifying the relative influence of these processes on species co-occurrence patterns is not straightforward. We provide a framework for assessing causes of species associations by combining a null-model analysis of co-occurrence with additional analyses of climatic differences and spatial pattern for pairs of pollen taxa that are significantly associated across geographic space. We tested this framework with data on associations among 106 fossil pollen taxa and paleoclimate simulations from eastern North America across the late Quaternary. The number and proportion of significantly associated taxon pairs increased over time, but only 449 of 56 194 taxon pairs were significantly different from random. Within this significant subset of pollen taxa, biotic interactions were rarely the exclusive cause of associations. Instead, climatic or spatial differences among sites were most frequently associated with significant patterns of taxon association. Most taxon pairs that exhibited co-occurrence patterns indicative of biotic interactions at one time did not exhibit significant associations at other times. Evidence for environmental filtering and dispersal limitation was weakest for aggregated pairs between 16 and 11 kyr BP, suggesting enhanced importance of positive species interactions during this interval. The framework can thus be used to identify species associations that may reflect biotic interactions because these associations are not tied to environmental or spatial differences. Furthermore, temporally repeated analyses of spatial associations can reveal whether such associations persist through time