Geophysical Constraints Research Articles

Variable dynamic styles of primordial heterogeneity preservation in the Earth's lower mantle

The evolution of the system Earth is critically influenced by the long-term dynamics, composition and structure of the mantle. While cosmochemical and geochemical constraints indicate that the lower mantle hosts an ancient primordial reservoir that may be enriched in SiO2 with respect to the upper mantle, geophysical observations and models point to efficient mass transfer and convective mixing across the entire mantle. Recent hypotheses of primordial-material preservation in a convecting mantle involve delayed mixing of intrinsically dense and/or intrinsically strong heterogeneity. Yet, the effects of composition-dependent rheology and density upon heterogeneity preservation and the dynamics of mantle mixing remain poorly understood. Here, we present two-dimensional numerical models in spherical geometry, investigating the preservation styles of primordial material as a function of its physical properties (i.e., viscosity and density contrasts). We establish multiple regimes of primordial-material preservation that can occur in terrestrial planets. These include (1) efficient mixing, (2) double-layered convection with or without topography, and (3) variable styles of partial heterogeneity preservation (e.g., as diffuse domains, piles or viscous blobs in the lower mantle). Some of these regimes are here characterised for the first time, and all regimes are put into context with each other as a function of model parameters. The viscous-blobs and diffuse-domains regimes can reconcile the preservation of primordial domains in a convecting mantle, potentially resolving the discrepancy between geochemical and geophysical constraints for planet Earth. Several, if not all, regimes characterised here may be relevant to understand the long-term evolution of terrestrial planets in general.

Thermal properties of harzburgite and dunite at 0.8–3 ​GPa and 300–823 ​K and implications for the thermal evolution of Tibet

Thermal diffusivity (D) and thermal conductivity (κ) of harzburgite and dunite from Luobusha ophiolite were simultaneously measured up to 3 ​GPa and 823 ​K using the transient plane-source method in a multi anvil apparatus. The results show that the values of D and κ of both samples systematically decrease with increasing temperature and increase with increasing pressure. By combination of the thermal physical data of rocks and minerals and geophysical constraints, we performed numerical simulation on the thermal evolution of Tibet vary over depth, distance and geologic ages. The present results provide new constraints on occurrence of partial melting and its geophysical significance beneath Tibetan crust.

Geophysical constraints on a modified crust below the Rayn Anticlines, Eastern Saudi Arabia

The Gravity anomaly signature for a regional Trans-Rayn Geophysical Traverse (TRGT) across Rayn Anticlines (RA) illustrates alternating Bouguer-Anomaly (B.A.) ‘highs’ and ‘lows’, highs over the anticline axes and lows in flanking sub-basins. We demonstrate for the first time that the bipolar gravity signatures actually camouflage a more extensive B.A. low of amplitude ~63 mgal and wavelength ~750 km, developed between the Qatar and Amar Arches. 2D-gravity modeling for TRGT infers a 3-layer crustal model for RA: (i) a regionally extensive upper crustal layer ~10–15 km thick of lighter density (by −0.02 g/cm3) compared to normal upper continental crust, (ii) a mid-crustal ~10 km thick transition layer of intermediate density and (iii) the lower mafic crust of higher than normal density. The Moho dips westward, from 37.3 below Ghawar-Anticline (GA) to 47 and 47.6 km below the KBS-Amar Arches, with a lateral density change in the lower crust following the same direction. Crustal thinning by as much as 22% occurred below GA; the altered crust directly correlating to the basement uplift on its top and mantle uplift at its base. An inverse correlation between crustal thicknesses and Vp/Vs is observed for the entire RA, displaying highest Vp/Vs (1.93) and Poisson’s ratio (0.443) for the GA crust.

Negative emissions and international climate goals\u2014learning from and about mitigation scenarios

For aiming to keep global warming well-below 2 °C and pursue efforts to limit it to 1.5 °C, as set out in the Paris Agreement, a full-fledged assessment of negative emission technologies (NETs) that remove carbon dioxide from the atmosphere is crucial to inform science-based policy making. With the Paris Agreement in mind, we re-analyse available scenario evidence to understand the roles of NETs in 1.5 °C and 2 °C scenarios and, for the first time, link this to a systematic review of findings in the underlying literature. In line with previous research, we find that keeping warming below 1.5 °C requires a rapid large-scale deployment of NETs, while for 2 °C, we can still limit NET deployment substantially by ratcheting up near-term mitigation ambition. Most recent evidence stresses the importance of future socio-economic conditions in determining the flexibility of NET deployment and suggests opportunities for hedging technology risks by adopting portfolios of NETs. Importantly, our thematic review highlights that there is a much richer set of findings on NETs than commonly reflected upon both in scientific assessments and available reviews. In particular, beyond the common findings on NETs underpinned by dozens of studies around early scale-up, the changing shape of net emission pathways or greater flexibility in the timing of climate policies, there is a suite of “niche and emerging findings”, e.g. around innovation needs and rapid technological change, termination of NETs at the end of the twenty-first century or the impacts of climate change on the effectiveness of NETs that have not been widely appreciated. Future research needs to explore the role of climate damages on NET uptake, better understand the geophysical constraints of NET deployment (e.g. water, geological storage, climate feedbacks), and provide a more systematic assessment of NET portfolios in the context of sustainable development goals.

Thermo-mechanical and stratigraphic numerical forward modelling: recent advances and their joint application in the Pannonian Basin

Basin analysis and subsidence history provide key insights into sedimentary basin forming mechanisms. Direct observations have long been the only source of information on their thermal and lithological architecture. State of the art modelling techniques today enable the prediction and computation of their formation and evolution constrained by geological field observations, geophysical and deep borehole data. Understanding the inherent connections between large-scale tectonic and local basin-scale surface processes requires the joint application of thermo-mechanical and stratigraphic modelling techniques. To this aim, we combined the thermo-mechanical lithospheric-scale numerical code Flamar and the high-resolution 3D deterministic stratigraphic software DionisosFlow. This joint modelling method quantifies forcing factors, such as crustal and lithospheric thinning, lithospheric flexure, sea-level and climatic variations associated with water and sediment influx and sediment compaction. The modelling shows the migration of extensional deformation in space and time creating deep half-grabens. After a rapid uplift event, the subsequent post-rift times are characterized by continuous kilometre-scale differential vertical movements. The modelled tectonic subsidence and uplift rates and half-graben geometries are imported into the 3D stratigraphic modelling code. Our modelling of a 120 km × 150 km area shows that such scenarios are associated with continental alluvial to shallow-water sedimentation and footwall erosion during the early stages of the syn-rift, followed by rapid deepening during the subsequent syn-rift evolution. Finally, the basins are filled by a large-scale prograding shelf-margin slope system during the post-rift times. We differentiate between unconformities caused by tectonics, sea-level variations or auto-cyclic processes. Our tectonic and stratigraphic results are compared with geological and geophysical constraints from the Pannonian Basin of Central Europe.

Crustal Poisson’s ratio tomography and velocity modeling across tectono-magmatic lake regions of Eastern Anatolia (Turkey): New geophysical constraints for crustal tectonics

The tomography of the Poisson ratio structures based on the Vp/Vs ratio provides much tighter constraints on the crust and uppermost mantle than Vp or Vs alone. In this study, high-resolution crustal tomography beneath tectono-magmatic lake regions (Lakes Van and Erçek) of Eastern Anatolia (Turkey) was used to estimate Poisson’s ratio from the Vp and Vs structures through the joint inversion analyses of the travel times of Vp and Vs using the station records of the permanent network of Kandilli Observatory and Earthquake Research Institute (KOERI, Turkey). The results of Poisson’s ratio anomaly reveal distinct and substantial variations in the different tectonic units and show a prominent, continuous and dense high Poisson’s ratio anomalies in the range of 0.27 to 0.30 for the lowermost crust/the uppermost mantle beneath the regions of Lakes Van and Erçek and comparably low average Poisson’s ratio (∼0.23) anomalies for the brittle crust in several local areas. High Poisson's ratios (over ∼0.29) in the basins of Lakes Van and Erçek and in the vicinity of their boundary faults appear to be due to partial melts. Low Poisson’s ratios (∼0.23) near seismogenetic faults at ∼14 km depth beneath the lake regions suggest weaker and more easily deformable crust. The high and low Poisson’s ratio structures suggest a strongly fault-controlled and highly heterogeneous crustal architecture beneath the lake regions. High Poisson’s ratios at variable depths suggest that the mafic content beneath the lake regions is higher than has ever been expected, thus indicating the existence of intrusive materials of possible mantle origin, interacting with faulted crustal rocks. The regional extent of the high Poisson’s ratios (0.27-0.30) of the lowermost crust/the uppermost mantle proposes strong interaction of partial melts with active tectonics in the basins of Lakes Van and Erçek. Key constraints from the estimated Poisson’s ratios and V-anomalies indicate the diverse and interactive nature of crustal deformations beneath the lake regions where the underplating of the mantle magma scenario is proposed to exert a key control on tectono-magmatic evolution of the regions of Lakes Van and Erçek and serves as a working hypothesis for further studies.

Semisupervised sequence modeling for elastic impedance inversion

Recent applications of machine learning algorithms in the seismic domain have shown great potential in different areas such as seismic inversion and interpretation. However, such algorithms rarely enforce geophysical constraints — the lack of which might lead to undesirable results. To overcome this issue, we have developed a semisupervised sequence modeling framework based on recurrent neural networks for elastic impedance inversion from multiangle seismic data. Specifically, seismic traces and elastic impedance (EI) traces are modeled as a time series. Then, a neural-network-based inversion model comprising convolutional and recurrent neural layers is used to invert seismic data for EI. The proposed workflow uses well-log data to guide the inversion. In addition, it uses seismic forward modeling to regularize the training and to serve as a geophysical constraint for the inversion. The proposed workflow achieves an average correlation of 98% between the estimated and target EI using 10 well logs for training on a synthetic data set.

High pressure thermoelasticity and sound velocities of Fe-Ni-Si alloys

The Earth's iron-dominant core is known to contain nickel from cosmochemical analysis and some amount of light elements from geophysical constraints on density and seismic wave velocities. Although there have been several studies to constrain thermoelastic properties of iron-alloys, there has been no systematic study on the effects of nickel and light elements on properties of iron using the same experimental methods and data analysis approach. We conducted nuclear resonant inelastic X-ray scattering and X-ray diffraction experiments on body-centered cubic and hexagonal close-packed (hcp) Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 up to 104 GPa and 86 GPa, respectively, and compare to similar measurements conducted on hcp-Fe up to 171 GPa. Specifically, we determine the Debye sound velocity from the low-energy transfer region of the (partial) phonon density of states (DOS) using the equation of state determined for each material and a new approach which utilizes information criteria and probability distributions. Nickel decreases the shear velocity of iron, while 10 at% Si has little to no effect on the shear velocity of Fe0.91Ni0.09. We observe that the shape of the phonon DOS of these alloys remains similar with increasing pressure. In the measured compression range, we therefore apply a generalized scaling law to describe the volume dependence of the phonon DOS and find that the vibrational Grüneisen parameters of hcp-Fe0.91Ni0.09 are nearly indistinguishable from those hcp-Fe and those for Fe0.8Ni0.1Si0.1 trend lower. From the vibrational free energy, we constrain the harmonic vibrational component of thermal pressure, which shows a significant positive deviation from theoretical calculations of hcp-Fe at pressures and temperatures of Earth's core. Collectively, our results demonstrate that the effects of nickel should be considered when modeling iron-rich planetary cores.

Fault pattern and seismotectonic potential at the south-western edge of the Ionian Subduction system (southern Italy): New field and geophysical constraints

The south-western edge of the Calabrian Arc in southern Italy has been investigated throughout a joint analysis of field, marine and geophysical data which provided constraints on the fault pattern and on the seismotectonic potential. The study was focused on a poorly known sector of a larger belt of seismically active faults slicing across the NE corner of Sicily, the so-called Tindari Fault System. Our data pointed out that the investigated area, including the mainland and the Ionian offshore, is deformed by oblique faulting with a general NW-SE tectonic trend. Earthquake distribution and seismic profiles pointed out active deformation in the offshore while the mainland is characterized by the occurrence of a NW-SE oriented, >20 km-long, structural belt. However, scarce seismicity has been recorded in the last 30 years alongside this tectonic structure, accounting for a possible silent segment of the larger fault system. Tomographic images revealed that the Moho discontinuity is deformed by a NE-dipping lithospheric tectonic structure which has been here retained the main mode of deformation and responsible for coseismic displacement in the area. As a whole, field and geophysical data agree with a general NW-SE trend segmented pattern of recent/active faults that have the potentiality of generating magnitude 6.5–7 earthquakes.

Geochemistry of the post-collisional Miocene mafic Tunceli Volcanics, Eastern Turkey: Implications for the nature of the mantle source and melting systematics

The East Anatolian Accretionary Complex (EAAC) comprises an ideal example of post-collisional volcanism within the Africa-Eurasia collision zone. The Miocene mafic Tunceli Volcanics, as a part of this post-collisional volcanic system, are located in the western termination of EAAC. The mafic Tunceli Volcanics are characterized by mildly alkaline and tholeiitic basalts, in which olivine, clinopyroxene and plagioclase characterize the main mineralogy. The role of fractional crystallization (FC) and assimilation combined with fractional crystallization (AFC) processes appear to be negligible in the petrogenesis of the primitive mafic Tunceli Volcanics. Relative enrichment in large ion lithophile elements (LILE), Th and La over high field strength elements (HFSE) and heavy rare earth elements (HREE) suggest contribution from a metasomatized mantle source. The wide range of ratios displayed by these elements also calls for some asthenospheric input for the genesis of these volcanics. The metasomatizing agents can be attributed to a past subduction event, probably during the closure of Neotethys. Considering also the geophysical constraints, which limits the lithospheric thickness to about 70–75 km around the region, a melt mixing between lithospheric and asthenospheric melts generated at different depths appear to be an important process in the petrogenesis of these lavas. The combined geochemical and geophysical data, therefore, necessitate a geodynamic model with some remnant lithospheric mantle underlying the Eastern Anatolian region.

Geophysical constraints on the structure and formation of Onepoto, Orakei, Pupuke and Tank Farm maar volcanoes, Auckland Volcanic Field

ABSTRACTWe report on a geophysical study of maar volcanoes in the Auckland Volcanic Field which was performed in 1975, whose results remain relevant. Modelled gravity and magnetic anomalies indicate that substantial funnel-shaped magnetised basaltic bodies underlie the mud-filled craters at Onepoto and Orakei Basin. These bodies may represent magma that solidified within the craters after initial explosive eruptive phases that involved direct interaction between magma and surface or near-surface water. The estimated volume of residual magnetised basalt beneath the crater is roughly equal to the ejected quantity of basalt at Onepoto and about 3 times larger at Orakei. Magnetic anomaly data show that the Lake Pupuke crater also partially filled with basaltic material after an explosive phase than in turn followed an initial effusive eruptive phase. In contrast, there is no magnetic indication of a substantial magnetised basaltic body beneath Tank Farm crater, so it appears that magma supply ceased at the end of the explosive phase.

Comment on “Channel flow, tectonic overpressure, and exhumation of high-pressure rocks in the Greater Himalayas” by Marques et al. (2018)

Abstract. The upward-tapering channel model proposed by Marques et al. (2018) for the Himalayas has a “base” that forms part of the subducting footwall and therefore does not close the channel. This configuration does not produce return flow, and no dynamic overpressure develops in the channel. The geometrical and kinematic configuration they actually use for their calculations differs from this and is both geologically and mechanically improbable. In addition, the fixed upper boundary condition in their models is mechanically unrealistic and inconsistent with geological and geophysical constraints from the Himalayan orogen. In reality, the dynamic pressures calculated from their model, which exceed lithostatic pressure by as much as 1.5 GPa, would cause elastic flexure or permanent deformation of the upper plate. I estimate that a flexural upwarp of 50 km of the upper plate would be required to balance forces, which would lead to geologically unrealistic topographic and gravity anomalies. The magnitude of the dynamic overpressure that could be confined is in fact limited by the shear strength of the upper plate in the Himalayas, which is likely to be < 120 MPa.

Diversity of food systems for securing future food availability

We depict the different models of likely future value-chains for agro food sector. The starting points of the reflection are the main coming geo-physical constraints acknowledged by most scientists (rise of sea level; climatic disaster higher frequency; scarcity of concentrated energy and other material resources). Thus the effects of the coming changes (and especially the effects of the global warming) upon agriculture are a regular study topic, while the effects of the other constraints, and the likely evolutions of the food systems as a whole, remain quite overlooked. When the scarcity of resources becomes general (as it is the case for oil and minerals in the coming decades), the present value-chains may no longer function. We draw from these evidences to design 6 models of food value-chains (including farming, processing and delivery systems). We therefore describe the models : “Today” (any food, in-store selling everywhere, at any time); “Amazon” (any food, at home in metropoles, at any time); “Chariot” (mainly local food, in streets of cities and villages, seasonal products); “Roman villa” (local food, at farm, seasonal products); “Survival” (energetic food, specific location, in response to disasters or to “hunger gap”); “Exportation food” (spices, salt etc., at any time). Only the last four together will be frugal enough to be compliant with the future geo-physical constraints. We also explore some consequences in terms of the future way of life, around the topics of agricultural work and cities.

Relating bed character and subglacial morphology using seismic data from Thwaites Glacier, West Antarctica

Seismic measurements on Thwaites Glacier show a spatially variable bed character, with implications for ice-sheet stability. The West Antarctic Ice Sheet is losing mass rapidly through outlet glaciers and ice streams in the Amundsen Sea Embayment, including Thwaites Glacier, where limited observations and modeling suggest that ice-flow rates depend on bed properties. Here we characterize bed properties of Thwaites Glacier based on amplitude analysis of reflection-seismic data from a ∼40-km-long profile collected in the approximate flow direction and two ∼10-km-long profiles transverse to flow. The upstream portion of the seismic profile reveals a ∼12-km long sedimentary basin with ice-flow-aligned bedforms capped by a continuous till layer that is likely soft and deforming (porosity ∼0.4–0.45), with several locations where water has pooled at the bed. Downstream of the sedimentary basin, the bed rises by ∼400 m over ∼25 km into subglacial highlands. Our seismic survey of these subglacial highlands reveals strong spatial variations in bed character across rugged topography (∼200 m amplitude at ∼2- to 5-km wavelength) resembling crag-and-tails. Till on the stoss sides (facing upglacier) of topographic highs is more consolidated (porosity ∼0.3–0.35 or lower), whereas the lee sides (facing downglacier) and flat regions exhibit porosity similar to the till of the upstream sedimentary basin. Modeling studies could use the observed correlation between bed character and bed aspect and slope to extend our observations to other parts of Thwaites Glacier, resulting in more-realistic models of future grounding-line retreat. Our findings highlight the need for more geophysical constraints on bed properties for important outlets in Antarctica and Greenland.

Geophysical Constraints on the Relationship Between Seamount Subduction, Slow Slip, and Tremor at the North Hikurangi Subduction Zone, New Zealand

AbstractWe use a prestack depth migration reflection image and magnetic anomaly data across the northern Hikurangi subduction zone, New Zealand, to constrain plate boundary structure and geometry of a subducting seamount in a region of shallow slow slip and recent International Ocean Discovery Program drilling. Our 3‐D model reveals the subducting seamount as a SW‐NE striking, lozenge‐shaped ridge approximately 40 km long and 15 km wide, with relief up to 2.5 km. This seamount broadly correlates with a 20‐km‐wide gap separating two patches of large (>10 cm) slow slip and the locus of tectonic tremor associated with the September–October 2014 Gisborne slow slip event. Largest slow slip magnitudes occurred where the décollement is underlain by a 3.0‐km‐thick zone of highly reflective subducting sediments. Wave speeds within this zone are 7% lower than adjacent and overlying strata, supporting the view that high fluid pressures within subducting sediments may facilitate shallow slow slip along the north Hikurangi margin.

Integrated Petrological and Geophysical Constraints on Magma System Architecture in the Western Galápagos Archipelago: Insights From Wolf Volcano

AbstractThe 2015 eruption of Wolf volcano was one of the largest eruptions in the Galápagos Islands since the onset of routine satellite‐based volcano monitoring. It therefore provides an excellent opportunity to combine geophysical and petrological data, to place detailed constraints on the architecture and dynamics of subvolcanic systems in the western archipelago. We present new geodetic models that show that pre‐eruptive inflation at Wolf was caused by magma accumulation in a shallow flat‐topped reservoir at ~1.1 km, whereas edifice‐scale deformation during the eruption was related to a deflationary source at 6.1–8.8 km. Petrological observations suggest that the erupted material was derived from both a subvolcanic mush and a liquid‐rich magma body. Using a combination of olivine‐plagioclase‐augite‐melt (OPAM) and clinopyroxene‐melt barometry, we show that the majority of magma equilibration, crystallization, and mush entrainment occurred at a depth equal to or greater than the deep geodetic source, with little petrological evidence of material sourced from shallower levels. Hence, our multidisciplinary study does not support a fully transcrustal magmatic system beneath Wolf volcano before the 2015 eruption but instead indicates two discrete storage regions, with a small magma lens at shallow levels and the major zone of magma storage in the lower crust, from which most of the erupted material was sourced. A predominance of lower crustal magma storage has previously been thought typical of subvolcanic systems in the eastern Galápagos Archipelago, but our new data suggest that this may also occur beneath the more active volcanoes of the western archipelago.

Joint interpretation of gravity and resistivity data from the Northern Kenya volcanic rift zone: Structural and geothermal significance

The Northern Kenya Rift has been the least studied sector of the Kenyan part of the East African Rift due to its remote location. Geothermal exploration conducted in the last ten years has greatly improved the geophysical constraints on the geology of this area. Recently, about 515 gravity stations have been surveyed around the Korosi, Paka and Silali volcanoes and analysed in conjuction with resistivity data from over 300 MT stations in order to jointly interpret the density and resistivity distribution of the Northern Kenya rift. Our models of the gravity data are in good agreement with previous MT and TEM resistivity studies of the individual volcanoes. The new Bouguer gravity map is characterized by a 10 to 15 km wide gravity high of 8 mGal amplitude striking NNE along the inner rift corresponding to resistivity > 50 Ωm below 2 km depth. Gravity lows due to structures shallower than 2 km depth at the Paka and Korosi volcanoes have been interpreted as low density bodies within their edifices, likely to consist of either unaltered near-surface pyroclastics or deeper tuffs altered at 60 to 180°C to hydrothermal smectite clay. Magnetotelluric (MT) resistivity models were used to further constrain the 2D gravity models. The high resistivity, low density near-surface rocks on the flanks of the volcanoes are interpreted to represent unaltered pyroclastics above the water table, whereas low resistivity, low density bodies underneath the Paka and Korosi volcanoes indicate low density tuffs, hydrothermally altered to hydrated smectite clay. Below 2 km depth the deeper high density zone beneath the volcanic inner rift is likely to be a combination of higher temperature, low porosity alteration associated with geothermal reservoirs and/or denser rocks related to intrusions. As expected, the greater proportion of dense lavas associated with fissure swarms mapped at the surface coincide with both relatively high gravity and MT resistivity south of Paka volcano. However, farther north between Paka and Silali volcanoes, the trends in higher gravity and resistivity lie west of the currently active fissure zone. This apparently inconsistent trend in the gravity and resistivity has been interpreted as lavas buried below recent tuffs and clastics associated with a former alignment of fissure eruptions in the rift about 7 km to the west of the current axis of eruption between Paka and Silali.

Geophysical constraints on microbial biomass in subseafloor sediments and coal seams down to 2.5\xa0km off Shimokita Peninsula, Japan

To understand the ability of microbial life to inhabit a deep subseafloor coalbed sedimentary basin, the correlation between fluid transport properties and the abundance of microbial cells was investigated based on core samples collected down to about 2.5 km below the seafloor during the Integrated Ocean Drilling Program Expedition 337 off the Shimokita Peninsula, Japan. The overall depth profiles for porosity and permeability exhibited a decreasing trend with increasing depth. However, at depths greater than 1.2 km beneath the seafloor, the transport characteristics of the sediments were highly variable, with the permeability ranging from 10−16 to 10−22 m2 and the pore size ranging from < 0.01 to 100 μm. This is mainly attributed to the diversity of the lithology, which exhibits a range of pore sizes and pore geometries. Fracture channels in coal seams had the highest permeability, while shale deposits had the smallest pore size and lowest permeability. A positive correlation between permeability and pore size was confirmed by the Kozeny-Carman equation. Cell abundance at shallower depths was positively correlated with porosity and permeability, and was less strongly correlated with pore size. These findings suggest that one of the factors affecting the decrease in microbial cell abundance with increasing depth was a reduction in nutrient and water supply to indigenous microbial communities as a result of a decrease in porosity and permeability due to sediment compaction. Anomalous regions with relatively high cell concentrations in coal-bearing units could be explained by the higher permeability and larger pore size for these units compared to the surrounding sediments. Nutrient transport through permeable cleats in coal layers might occur upwards toward the upper permeable sandstone layers, which are well suited for sustaining sizable microbial populations. Conversely, impermeable shale and siltstone with small pores (< 0.2 μm, which is smaller than microbial cell size) may act as barriers to water and energy-yielding substrates for deep microbial life. We propose that the pore size and permeability govern the threshold for microbial habitability in the deep subseafloor sedimentary biosphere.

A Boussinesq slurry model of the F-layer at the base of Earth’s outer core

SUMMARY Seismic observations suggest that a stably stratified layer, known as the F-layer, 150–300 km thick exists at the bottom of Earth’s liquid outer core. These observations contrast with the density inferred from the Preliminary Reference Earth Model (PREM), which assumes an outer core that is well-mixed and adiabatic throughout. The liquid core is composed primarily of iron alloyed with a light component. A thermal boundary layer produces the opposite effect on the density profile compared with the observations, and single phase, thermochemical models do not provide a sufficient dynamic description of how light element is transported across the F-layer into the overlying liquid outer core. We therefore propose that the layer can be explained by a slurry on the liquidus, whereby solid particles of iron crystallize from the liquid alloy throughout the layer. The slurry model provides a dynamic explanation of how light element can be transported across a stable layer. We make two key assumptions, the first of which is fast-melting where the timescale of freezing is considered short compared to other processes. The second assumption is that we consider a binary alloy where the light element is purely composed of oxygen, which is expelled entirely into the liquid during freezing. We present a steady state 1-D box model of a slurry formulated in a reference frame moving at the speed of inner core growth. We ascertain temperature, light element concentration and solid flux profiles by varying the layer thickness, inner core heat flux and thermal conductivity, since there is some uncertainty in these estimates. Our solutions demonstrate that the steady state slurry can satisfy the geophysical constraints on the density jump across the layer and the core–mantle boundary heat flux.

Geophysical constraints on the lithospheric structure in the northeastern South China Sea and its implications for the South China Sea geodynamics

An E-W oriented OBS2015-2 wide-angle refraction profile was shot in the northeastern South China Sea (SCS) between previous refraction profiles T1 and T2 in order to better understand the variability of the crustal composition and the role of specific tectonic features. P-wave velocity models established from forward and inversion modeling imaged a 12 to 15 km-thick thinned continental crust and a high velocity layer (HVL) in the lower crust which is interpreted as magmatic underplating. Profile OBS2015-2 cut across the Taiwan transfer zone (TTZ), which separates two consecutive rifted segments of different orientations in the northeastern SCS. The TTZ is a well-defined upper crustal feature characterized by a HVL with different thickness on both sides. We have defined the southward limit of the thinned continental domain with the typical oceanic domain as a continent-ocean boundary (COB). The COB between the drilling area of IODP legs 367–368 to the Manila trench is characterized by a sharp contrast between the low amplitude, irregular shape magnetic anomalies and the high amplitude, elongated shape magnetic anomalies, which are associated with the thinned continental domain and the oceanic domain, respectively. We further extend the COB into the 400 to 500 km wide unfolded Manila slab located east of the Manila trench by using mid-slab dVp tomographic velocities to define the boundary between subducted oceanic and thinned continental crust. The location of the reconstructed northeast SCS COB appears to step northwards toward Taiwan across a 400 km N-S segment, providing new constraints on SCS geodynamics at the end of spreading and Taiwan pre-collisional configuration.