Critical Storytelling in Uncritical Times shares the stories of undergraduate students and educators in U.S. higher education. Storytellers in this volume grapple with issues of bullying, stigma ...surrounding mental health, cultural barriers, gender inequity, and other forms of struggle in educational settings.
Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, ...decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S‐to‐P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere‐asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.
Key Points
S‐to‐P receiver functions image lithospheric discontinuities beneath northern East African Rift
The Moho shallows from 28 km in the MER along the rift to 23 km in the northern Afar triple junction
The LAB is strong beneath the Ethiopian plateau but not imaged beneath the rift valley in Afar or MER
Geological storage of CO ₂ that has been captured at large, point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions. However, this technology ...will only be viable if it can be guaranteed that injected CO ₂ will remain trapped in the subsurface for thousands of years or more. A significant issue for storage security is the geomechanical response of the reservoir. Concerns have been raised that geomechanical deformation induced by CO ₂ injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO ₂ leakage. In this paper, we examine three large-scale sites where CO ₂ is injected at rates of ∼1 megatonne/y or more: Sleipner, Weyburn, and In Salah. We compare and contrast the observed geomechanical behavior of each site, with particular focus on the risks to storage security posed by geomechanical deformation. At Sleipner, the large, high-permeability storage aquifer has experienced little pore pressure increase over 15 y of injection, implying little possibility of geomechanical deformation. At Weyburn, 45 y of oil production has depleted pore pressures before increases associated with CO ₂ injection. The long history of the field has led to complicated, sometimes nonintuitive geomechanical deformation. At In Salah, injection into the water leg of a gas reservoir has increased pore pressures, leading to uplift and substantial microseismic activity. The differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site.
Based on observations in both the laboratory and the Earth, we develop the hypothesis that plate boundaries are lubricated by networks of melt‐rich shear zones. Such lubrication would serve to reduce ...effective strength and focus deformation at plates boundaries. This idea emerges from two sets of observations: (1) stress‐driven melt segregation and organization in experimentally deformed mantle rocks and (2) seismic anisotropy patterns as observed at three divergent plate boundaries (the Ethiopian Rift, the Reykjanes Ridge, and the East Pacific Rise). In all three tectonic settings, the magnitude of anisotropy is greatest at the probable locations of the lithosphere‐asthenosphere boundary within the plate boundary (“marginal LAB”). Seismic anisotropy in the upper mantle is controlled by the lattice preferred orientation (LPO) of predominant olivine and the alignment of melt structures. The observed patterns of anisotropy are controlled by the dip angle of the marginal LAB. When steeply dipping, shear wave splitting in vertically traveling waves (e.g., SKS phases) is most sensitive to the alignment of melt, and surface waves should reveal faster Rayleigh wave velocities than Love wave velocities (VSV > VSH). When shallowly dipping, shear wave splitting in vertically traveling body waves is controlled by olivine LPO, and surface waves show faster Love wave velocities than Rayleigh wave velocities (VSV < VSH). The formation of melt‐rich networks by stress‐driven segregation should be most effective where strain rates are highest. These melt‐lubricated shear zones will reduce effective viscosity relative to the direct extrapolation of viscosity values derived from laboratory creep experiments on homogenous samples. A composite model of anisotropic seismic properties is developed to test the hypothesis that melt segregates along the LAB, incorporating olivine fabrics with oriented and segregated melt over a range of length scales. This model is applied to observations from the three example plate boundaries, leaving the reader to speculate on the implications for interpretation of anisotropy patterns at other geodynamic settings.
The location and degree of material transfer between the upper and lower mantle are key to the Earth's thermal and chemical evolution. Sinking slabs and rising plumes are generally accepted as ...locations of transfer
, whereas mid-ocean ridges are not typically assumed to have a role
. However, tight constraints from in situ measurements at ridges have proved to be challenging. Here we use receiver functions that reveal the conversion of primary to secondary seismic waves to image the discontinuities that bound the mantle transition zone, using ocean bottom seismic data from the equatorial Mid-Atlantic Ridge. Our images show that the seismic discontinuity at depths of about 660 kilometres is broadly uplifted by 10 ± 4 kilometres over a swath about 600 kilometres wide and that the 410-kilometre discontinuity is depressed by 5 ± 4 kilometres. This thinning of the mantle transition zone is coincident with slow shear-wave velocities in the mantle, from global seismic tomography
. In addition, seismic velocities in the mantle transition zone beneath the Mid-Atlantic Ridge are on average slower than those beneath older Atlantic Ocean seafloor. The observations imply material transfer from the lower to the upper mantle-either continuous or punctuated-that is linked to the Mid-Atlantic Ridge. Given the length and longevity of the mid-ocean ridge system, this implies that whole-mantle convection may be more prevalent than previously thought, with ridge upwellings having a role in counterbalancing slab downwellings.
Background The UK Biobank is a unique resource for biomedical research, with extensive phenotypic and genetic data on half a million adultsfrom the general population. We aimed to examine the effect ...of neurodevelopmental copy number variants (CNVs) on the cognitive performance of participants. Methods We used Affymetrix Power Tools and PennCNV-Affy software to analyse Affymetrix microarraysof the first 152,728genotypedindividuals. We annotated a list of 93 CNVs and compared their frequencies with control datasets. Weanalysed the performance on seven cognitive tests of carriers of 12 CNVs associated with schizophrenia (N = 1,087) and of carriers of another 41neurodevelopmental CNVs (N = 484). Results The frequencies of the 93 CNVs in the Biobank subjects were remarkably similar to those among 26,628 controls from other datasets. Carriersof schizophrenia-associated CNVs and of the group of 41 other neurodevelopmental CNVs had impaired performance on the cognitive tests, with nine of 14 comparisons remaining statistically significant aftercorrection for multiple testing.They also had lower educational and occupational attainment (p-values between 10-7 and 10-18 ).Thedeficits in cognitive performance were modest (z score reductions between 0.01 and 0.51), compared to individuals with schizophrenia in the Biobank (z score reductions between 0.35 and 0.90). Conclusions This is the largest study on the cognitive phenotypes of CNVs to date. Adult carriers of neurodevelopmentalCNVs from the general population have significant cognitivedeficits. The UK Biobank will allow unprecedented opportunities for analysis of furtherphenotypic consequences of CNVs.
The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but ...it is unclear what causes this. We use the method of source‐side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe. We find significant amounts of splitting (≤2.4 s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice‐preferred orientation development in wadsleyite, ringwoodite, or MgSiO3‐perovskite. We invert for the orientation of several mechanisms—subject to uncertainties in mineralogy and deformation—and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth.
Key Points:
Shear wave splitting shows that the transition zone is anisotropic near slabs
Hydrous phases explain the data better than olivine alignment in deep slabs
The presence of hydrous phases implies water may be brought to the lower mantle
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