Earth's Core Research Articles

Antipodal waveform observations of seismic waves diffracting and refracting at the base of Earth's outer core

We approach this analysis of the inner-outer core boundary (IOCB) with antipodal waveform data in the distance range 179.0°–180° to test the hypothesis whether propagation at the base of the outer core is commensurate with diffraction and/or refraction. The propagation paths observed cover about two-thirds of the IOCB surface. Seismic data from seven diameters are examined—Tonga to Algeria (station code TAM), Sulawesi to Amazon (PTGA), northern Chile to Hainan Island (QIZ), two between central Chile and the mainland China (XAN, ENH), and two diameters from New Zealand to both Portugal (PTO) and Spain (ECAL). The adequacy of global Earth models—both 1-D and 1-D core +3D mantle—in fitting the antipodal observations is found to be deficient. Lateral heterogeneity among the propagation paths are mapped and projected to the Earth's surface for context. We stack data to increase signal-to-noise, and model waveform data via the 3D spectral element method on the EarthSimulator4. With the exception of one path to ENH China, none of the global Earth models match amplitudes of diffracted waves or the stacked amplitudes relative to PKIKP, which travels along the antipodal diameter. Energetic arrivals observed in the waveform data set are modeled as a combination of refraction within a low velocity zone (LVZ) at the base of the outer core, and diffraction around this structure. The observations at the base of the outer core may be further subdivided between paths with a thin low velocity zone (QIZ, XAN, PTGA), and those with a thicker, slower LVZ (TAM). The former paths are characterized by a thin LVZ (20–50 km thick, layer–gradient, respectively) with a velocity of approximately 10.0 km/s. The second group of paths shows large amplitude, complicated waveforms for TAM, corroborated by data from PTO and ECAL. TAM waveforms are modeled with a negative gradient from 100 to 50–75 km thickness above the IOCB, to a basal velocity of 8.8 km/s at IOCB. We also modeled the prospect of non-zero rigidity at the bottom of the outer core, but found no evidence of sensitivity in the core data. The structures fitting the data correlate across the inner–outer core boundary, with a velocity anomaly in PKPbc data juxtaposed immediately above the TAM path beneath SE Asia. More generally, the TAM IOCB path when projected up to the core-mantle boundary (CMB) apparently aligns with regions between the large low shear velocity provinces (LLSVP) at the CMB. Other paths with thinner or negligible LVZ at the IOCB partially underlie the LLSVP when projected to the CMB.

Onset of oscillatory magnetoconvection under rapid rotation and spatially varying magnetic field

The onset of periodic time-varying magnetoconvection in a regime relevant to the Earth's outer core is investigated in this study. A rapidly rotating plane fluid layer subject to an axially varying horizontal magnetic field is considered under the dynamical regimes of stronger magnetic diffusion compared to thermal and viscous diffusion rates. Dynamically specific convective instabilities, both inside and outside the tangent cylinder regions of the Earth's core, have been investigated by using appropriate patterns of the imposed mean magnetic field. The hallmark of convection onset, with such axially varying mean field, is a viscous oscillatory mode weakly modified by the magnetic field. This modified viscous oscillatory (mVO) mode is observed to exist over a wide range of the strength of the imposed field, making it a dynamically appropriate flow structure with characteristics of outer core convection. An optimal Prandtl number Pr* is found through numerical experiments where the mVO mode can be the most unstable to infinitesimal perturbations. It is further shown that this optimal state admits oscillations for earthlike regimes where thermal diffusion is much less than magnetic diffusion. Also, the formation of columnar convection rolls from isolated vortices is demonstrated as a result of combinations of the classical viscous oscillatory and mVO modes in the rapidly rotating limit (Ekman number E→0). Overall, the qualitative characteristics of magnetoconvection modes for the various imposed patterns are found to be similar despite representing distinct regions in the Earth's outer core.

Origin of 182W Anomalies in Ocean Island Basalts

AbstractOcean island basalts (OIB) show variable 182W deficits that have been attributed to either early differentiation of the mantle or core‐mantle interaction. However, 182W variations may also reflect nucleosynthetic isotope heterogeneity inherited from Earth's building material, which would be evident from correlated 182W and 183W anomalies. Some datasets for OIB indeed show hints for such correlated variations, meaning that a nucleosynthetic origin of W isotope anomalies in OIB cannot be excluded. We report high‐precision W isotope data for OIB from Samoa and Hawaii, which confirm previously reported 182W deficits for these samples, but also demonstrate that none of these samples have resolvable 183W anomalies. These data therefore rule out a nucleosynthetic origin of the 182W deficits in OIB, which most likely reflect the entrainment of either core material or an overabundance of late‐accreted materials within OIB mantle sources. If these processes occurred over Earth's history, they may have also been responsible for shifting the 182W composition of the bulk mantle to its modern‐day value. We also report Mo isotope data for some Hawaiian OIB, which reveal no resolved nucleosynthetic Mo isotopic anomalies. This is consistent with inheritance of 182W deficits in OIB from the addition of either core or late‐accreted material, but only if these materials have a non‐carbonaceous (NC) meteorite‐like heritage. As such, these data rule out significant contributions of carbonaceous chondrite (CC)‐like materials to either Earth's core or late accretion.

Early Water Delivery to Terrestrial Planet Regions during the Stages of Jupiter’s Formation and Migration in the Grand Tack Model

The formation and subsequent migration of gas giants could significantly affect the material mixing in the solar system. In this study, we use N-body simulations to investigate how much water is transported into the region of the terrestrial planet formation during the growth and migration phases of Jupiter in the Grand Tack model. We found that Jupiter’s growth was accompanied by significant mass transport and that a substantial amount of water (about 10 times Earth’s ocean mass for the initial planetesimal distribution based on the minimum-mass solar nebula) was transported into the terrestrial planet region. The total amount delivered increased further during Jupiter’s migration phase (totaling about 10–40 times Earth’s ocean mass), which was less dependent on simulation parameters. In addition, at these stages, terrestrial planets were not fully grown. Therefore, water supplied during these early stages could interact with metallic iron during the core formation of protoplanets and/or growing Earth. Since hydrogen in water molecules can dissolve into their cores, this could explain the density deficit observed in the current Earth core. Notably, Jupiter could play an important role as a “barrier” in explaining the dichotomy of the isotopic compositions between noncarbonaceous (NC) and carbonaceous (CC) meteorites. This study’s results show that Jupiter’s growth necessitates some mixing of NC and CC materials.

The influence of uncertain mantle density and viscosity structures on the calculations of deep mantle flow and lateral motion of plumes

SUMMARY Mantle plumes form from thermal boundary layers, such as Earth's core–mantle boundary. As plumes rise towards the surface, they are laterally deflected by the surrounding mantle flow that is governed by deep mantle density and viscosity structures. The lateral motions of mantle plumes carry information of deep mantle structure and dynamics and are used to setup reference frames by which absolute plate motions are reconstructed. In this study, we compare two methods to compute deep mantle flow and lateral motion of plumes. In mantle convection (MC) models, the mantle flow field and lateral motions of plumes are determined by solving conservation equations forward-in-time from given initial conditions. In plume advection (PA) models, approximate viscosity and present-day density structures are used to calculate present-day mantle flow which is then propagated backward-in-time assuming zero thermal diffusion, and plume conduits are represented by continuous lines and are passively advected within the background mantle flow. The question is how assumptions in PA models influence the predictions of deep mantle flow and plume lateral motions. Here, we perform purely thermal MC models and thermochemical MC models with intrinsically dense materials in the lowermost mantle. The deep mantle flow and plume lateral motions are determined accurately in each MC model. We also perform PA models using the approximated present-day viscosity and temperature structures in these MC models. We find that PA models without considering temperature-dependence of viscosity and/or only using long wavelength present-day temperature structure (up to degree 20) often lead to an average of ∼50–60 per cent and ∼60–200 per cent differences of present-day mantle flow velocities than purely thermal MC models and thermochemical MC models, respectively. By propagating inaccurate flow fields backward-in-time in PA models often cause even larger errors of mantle flow velocities in the past. Even using the same parameters and starting from the same present-day mantle flow fields as in MC models, the PA models still show an average of ∼10–30 per cent misfit of mantle flow velocities after ∼40 Ma. In addition, we show that errors of mantle flow fields in PA models can cause ∼100–600 per cent differences of plume lateral motions than that constrained in MC models in the past 60 Ma. Even we use the mantle flow in MC models to advected virtual plumes in PA models, the virtual plumes could still show ∼50–300 per cent difference of lateral motions than dynamic plumes in MC models if the virtual plumes do not start with the same locations and/or shapes as plumes in MC models. We also find virtual plumes in PA models initiated at different locations and/or with different shapes can be later advected to similar locations, suggesting that the lateral motions of plumes in PA models can be non-unique. Therefore, it is important to consider the build-in assumptions of PA models when interpreting their predictions on deep mantle flow field and plume lateral motions. The accuracy of PA models would improve as we gain better understanding on Earth's deep mantle structure and dynamics.

First-principles study of thermoelasticity and structural phase diagram of CaO

We present pressure $(P)$ and temperature $(T)$ variations of elastic and anisotropic properties, solid-solid (rocksalt, B1 to cesium chloride, B2) and solid-liquid structural phase transitions of CaO. We employed first-principles density functional theory supplemented with an anharmonic contribution to phonon dynamics. Good agreement is obtained for all properties up to the pressure and temperature range relevant to the Earth's mantle and outer core. Born elastic criteria are generalized for arbitrary stress to elaborate the structural phase diagram from a thermoelastic viewpoint. We propose a self-consistent computational scheme to incorporate the effect of thermal hysteresis into Lindemann's melting law. This improvised melting law exhibits quantitative agreement with reported findings for the high-$P$ melting curve. We find the triple point (i.e., the coexistence of B1, B2, and liquid phases) at 23 GPa and 4600 K. By examining the pressure-term included elastic properties, we can show that the solid-solid transition is mechanical in a pressure range of 0--200 GPa and temperature up to 3000 K. The softening of shear elastic constant ${C}_{44}$ drives the B1-B2 phase transition. This assertion is corroborated by examining the phonon dispersion curve and mode Gr\"uneisen parameter at different pressures for both B1 and B2 phases. The solid-liquid phase boundary can be treated accurately through the temperature-dependent thermodynamic Gr\"uneisen parameter. Furthermore, in this paper, we predict a negative melting slope $>140\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ with a peak temperature of 7800 K, suggesting a smaller molar volume on the liquid side than that of the solid phase. This finding is supported by electronic band structure calculation. It is proposed that the hybridization of the empty $3d$ band of the cation with $s$ orbitals lowers the conduction band to cross the Fermi energy at the Brillouin zone center and leads the insulator-to-metal/semimetal phase transition $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Different electronic states on solid and liquid sides make the liquid phase more compressible than the solid phase, eventually reducing the melting temperature with pressure.

Figure data for "Unveiling the effect of Ni on the formation and structure of Earth's inner core"

Figure data for "Unveiling the effect of Ni on the formation and structure of Earth's inner core"

Self-diffusion coefficient and sound velocity of Fe-Ni-O fluid: Implications for the stratification of Earth's outer core

It is experimentally reported that the stratified layer atop Earth's outer core is hundreds of kilometers thick with a maximum sound velocity reduction of 0.3% relative to the preliminary reference Earth model. However, why the sound velocity atop the outer core is reduced remains theoretically unclear. In this paper, the Ni and vital light O in the outer core were both considered to have implications for the stratification of Earth's core, including the stratification thickness and the sound velocity profile. Ab initio molecular dynamics simulations were performed on the Fe-Ni-O fluid under the conditions of Earth's outer core, and the self-diffusion coefficients and ion-ion dynamic structure factors were calculated. The self-diffusion coefficient of O was calculated to be (19.56 ± 0.83) × 10−9 m2s−1 at the core-mantle boundary. Combining the diffusion equation with the time evolution of the O self-diffusion coefficient, the calculated stratification thickness at present is 194.7 km. The calculated ion-ion dynamic structural factors indicate that the sound velocity in the outmost outer core near the stratified layer is 7.86 km/s, which is lower than the preliminary reference Earth model values (∼8.05 km/s). These results show that Fe-Ni-O liquid atop the outer core featuring an appropriate thickness and a reduced sound velocity, thereby shedding light on the dynamic behavior of Earth's core.

Comparisons of the core and mantle compositions of earth analogs from different terrestrial planet formation scenarios

The chemical compositions of Earth's core and mantle provide insight into the processes that led to their formation. N-body simulations, on the other hand, generally do not contain chemical information, and seek to only reproduce the masses and orbits of the terrestrial planets. These simulations can be grouped into four potentially viable scenarios of Solar System formation (Classical, Annulus, Grand Tack, and Early Instability) for which we compile a total of 433 N-body simulations. We relate the outputs of these simulations to the chemistry of Earth's core and mantle using a melt-scaling law combined with a multi-stage model of core formation. We find the compositions of Earth analogs to be largely governed by the fraction of equilibrating embryo cores (kcore_emb) and the initial embryo masses in N-body simulations, rather than the simulation type, where higher values of kcore_emb and larger initial embryo masses correspond to higher concentrations of Ni, Co, Mo, and W in Earth analog mantles and higher concentrations of Si and O in Earth analog cores. As a result, larger initial embryo masses require smaller values of kcore_emb to match Earth's mantle composition. On the other hand, compositions of Earth analog cores are sensitive to the temperatures of equilibration and fO2 of accreting material. Simulation type may be important when considering magma ocean lifetimes, where Grand Tack simulations have the largest amounts of material accreted after the last giant impact. However, we cannot rule out any accretion scenarios or initial embryo masses due to the sensitivity of Earth's mantle composition to different parameters and the stochastic nature of N-body simulations. We use our compiled simulations to explore the relationship between initial embryo masses and the melting history of Earth analogs, where the complex interplay between the timing between impacts, magma ocean lifetimes, and volatile delivery could affect the compositions of Earth analogs formed from different simulation types. Comparing the last embryo impacts experienced by Earth analogs to specific Moon-forming scenarios, we find the characteristics of the Moon-forming impact are dependent on the initial conditions in N-body simulations where larger initial embryo masses promote larger and slower Moon-forming impactors. Mars-sized initial embryos are most consistent with the canonical hit-and-run scenario onto a solid mantle. Our results suggest that constraining the fraction of equilibrating impactor core (kcore) and the initial embryo masses in N-body simulations could be significant for understanding both Earth's accretion history and characteristics of the Moon-forming impact.

中性子回折実験で探る高温高圧下でのSiを含んだhcp鉄の水素化挙動

The identification of light elements in the Earth's core has been an open question for more than 50 years. From seismic observation, the Earth’s core is believed to contain some light elements because it is about 10% less dense than pure Fe at the corresponding pressure and temperature conditions. Among the candidates of light elements in the core, hydrogen is one of the promising light elements. Neutron diffraction is very useful in systems when detecting light atoms surrounded by heavy atoms because its scattering intensity is independent of atomic number. Hence, many neutron diffraction studies have been performed on the iron-hydrogen binary system. On the other hand, it is likely that there is more than one type of light elements dissolved in the core, and interactions between light elements are also involved. Here, we focused on an Fe-Si-H system, and we investigated the sites occupied by deuterium and its site occupancy of hcp Fe0.95Si0.05 Hx at 14.7 GPa and 800 K by in-situ neutron diffraction experiments. So far, this pressure condition is the highest for neutron diffraction experiments at high pressure and high temperature, where crystal structure analysis has been conducted.

Dynamic mode decomposition of the geomagnetic field over the last two decades

Earth's magnetic field, which is generated in the liquid outer core through the dynamo action, undergoes changes on timescales of a few years to several million years, yet the underlying mechanisms responsible for the field variations remain to be elucidated. In this study, we apply a novel data analysis technique developed in fluid dynamics, namely the dynamic mode decomposition, to analyze the geomagnetic variations over the last two decades when continuous satellite observations are available. The dominant dynamic modes are extracted by solving an eigen-value problem, so one can identify modes with periods longer than the time span of data. Our analysis show that similar dynamic modes are extracted from the geomagnetic secular variation and secular acceleration, justifying the validity of applying the dynamic mode decomposition method to geomagnetic field. We reveal that the geomagnetic field variations are characterized by a global mode with period of 58 years, a localized mode with period of 16 years and an equatorially trapped mode with period of 8.5 years. These modes are possibly related to magnetohydrodynamic waves in the Earth's outer core.

On the Model of Equatorial Magnetic‐Archimedes‐Coriolis Waves Propagating at Earth's Core Surface and the Potential Implications

AbstractThe purposes of this work are (a) clarifying the specific latitude range in which the currently physical model of the equatorial trapped Magnetic‐Archimedes‐Coriolis (namely eMAC) waves propagating atop the Earth's core can own the enough accuracy to describe the hydromagnetic waves; (b) presenting the systematically analytical expressions to represent the physical properties (e.g., the equatorial confinement and latitudinal distribution, damping rate , eigen‐period T) of the eMAC waves. Here, the new results indicate that: (a) the eMAC wave model can own the high accuracy (i.e., the relative errors are less than 5%) to describe the core waves in the regions with latitude below 25°; (b) the equatorial confinement and latitudinal distribution law is essentially governed by a specific solution form with the typical Hermite polynomial term of degree ; (c) the damping rate can be estimated by (μ being the vacuum permeability, being the core electrical conductivity, H being the stratified layer thickness of the core), showing that the magnetic diffusivity () can cause the ohmic dissipation of the waves; besides, the H value is predicted to be larger than 20 km, when matches the observed 8.5 years period. This work also presents the analytical models for the perturbated magnetic fields due to the eMAC waves, presenting that the azimuthal perturbed magnetic field (with degree n = 1) is mainly confined to the equatorial regions with latitude below ∼15°, the profile of which coincides with the azimuthal core surface flows inferred from the geomagnetic observations.

Cadmium isotope fractionation during metal-silicate partitioning – Results and implications for Earth's volatile accretion

Metal-silicate partitioning experiments were carried out at 1.5 GPa and 1508 to 1843 K to constrain Cd partitioning and isotope fractionation during core formation. At the studied conditions, there was no significant stable isotope fractionation during Cd partitioning between metal and silicate phases with a mean ∆114Cdmet-sil = −0.02 ± 0.09‰ (2SD, n = 7). Two experiments that investigated sulphide-silicate partitioning of Cd yielded fractionation factors of −0.04 ± 0.06‰ and − 0.23 ± 0.07‰ (2SE), whereby the latter result was obtained for a short run that may not represent full equilibrium. In summary, the findings suggest that Cd isotope fractionation during segregation of Earth's core was either absent or very minor. The Cd partitioning data of this and previous investigations were combined in multiple linear regression analyses to better constrain Cd metal-silicate partitioning during core formation. In accord with earlier work, the analyses reveal that Cd metal-silicate partitioning is not significantly impacted by temperature and pressure but affected by the S content of the metal phase. In addition, it is shown that the presence of C and Si in the metal reduce the siderophile character of Cd. Based on estimates for the composition of Earth's core, the data suggest a metal-silicate partition coefficient DCd of about 0.4 for a single-stage core formation event. However, given uncertainties about the light element composition of Earth's core, DCd values larger than 1 cannot be ruled out at present for core formation. The results of this study and data on the composition of the bulk silicate Earth and chondritic meteorites were applied in mass balance calculations to constrain the Cd signature of Earth's main stage accretion material prior to delivery of the late veneer. The modelling indicates Earth's main stage of accretion involved material with an average Cd isotope composition that was lighter than that of known carbonaceous and enstatite chondrites. Most likely, this reflects either poor characterisation of these meteorites by the few precise data currently available or that a significant fraction of the terrestrial volatile inventory was acquired from material not directly related to carbonaceous and enstatite chondrites. Furthermore, terrestrial accretion most likely did not encompass the addition of a volatile-rich late veneer exceeding 2% of Earth's mass, in accord with accretion models, which invoke that volatile delivery occurred primarily during main stage accretion, alongside core formation.

Elasticity and Viscosity of hcp Iron at Earth's Inner Core Conditions From Machine Learning‐Based Large‐Scale Atomistic Simulations

AbstractAlthough considerable efforts have been made in the last years to examine the physical properties of hexagonal close‐packed (hcp) iron at extreme conditions, it remains challenging to explain many geophysical observations in Earth's inner core. Here we examine the elastic and plastic behavior of hcp iron and the effects of structural defects at inner core conditions using large‐scale atomistic simulations coupled with machine learning‐based interatomic potential. Our results suggest that the seismic anisotropy pattern in the inner core can be ascribed to the elastic anisotropy (6%) of hcp iron. The observed low shear wave velocity is largely produced by viscous grain boundaries in iron polycrystal. We also found highly mobile and abundant vacancies in hcp iron yield a viscous strength (1015±1) that is consistent with the geophysical observations. Therefore, our findings highlight the role played by structural defects and lessen the demand for light elements to explain the observed seismic data.

Earth's Paleomagnetic Field and its Changes Through Time

Paleomagnetic data is the only way to access the behavior of the geomagnetic field of internal origin through geological time. Therefore, the complete description of the paleosecular variation (PSV) is fundamental for unraveling the evolution of the Earth's core. Modeling of PSV relies on observing the fluctuations in the direction and intensity of the field on the surface, the evaluation of the paleomagnetic data dispersion in different times and situations of low and high reversal rates, the kinematics of the virtual poles during reversals, and other aspects. Magneto-cyclostratigraphy is a powerful tool to constrain the age of sedimentary formations, and the recognition of patterns of magnetic variation represents a breakthrough in the development of cyclostratigraphy. This paper reviews some of the contributions to the subject of the Laboratory of Paleomagnetism at Instituto de Astronomia, Geofísica e Ciências Atmosféricas the University of São Paulo.

Ab initio determination on diffusion coefficient and viscosity of FeNi fluid under Earth's core condition.

The Earth's outer core is mainly composed of Fe and Ni. The geodynamo of the Earth's core are closely correlated with the transport properties of the fluid in the Earth's core. We selected the typical FeNi fluid, and systemically calculated its diffusion coefficient and viscosity under Earth's core condition by quantum molecular dynamics simulation. The diffusion coefficients are almost constant along the core adiabatic curve. The self-diffusion coefficients of Ni along the core adiabatic curve range from 2.47 × 10-9 to 3.37 × 10-9 m2s-1. The diffusion coefficient increases with temperature increase, while viscosity decrease with temperature increase. The calculations on the transport properties suggest that the Ni impurities have a negligible effect on the diffusion coefficient and viscosity of Earth's core.

Iron Hydride in the Earth's Inner Core and Its Geophysical Implications

AbstractHydrogen is potentially a key light element in the Earth's core. Determining the stability of iron hydride is essential for Earth's core mineralogy applications. We investigated the thermal stabilities of a range of Fe‐H binaries at core P‐T conditions. It is concluded that face‐centered cubic phase FeH is stable in the Earth's inner core. The high mobility of hydrogen in the Fe lattice suggests that hydrogen is transferred to a superionic state under the inner core conditions, where the superionic state transfer temperature of H in Fe fcc lattice is ∼500 K higher than that in hcp Fe system. The H concentration in the inner core is estimated to be ∼0.92 wt% to explain its density deficit, this value was further constrained to ∼0.21 wt% by matching the density jump at the inner‐core boundary. H alongside other light elements are required to account for the geophysical observations of the Earth's inner core.

On convective instabilities in a rotating fluid with stably stratified layer and thermally heterogeneous boundary

The onset of convection in a rotating plane layer due to a vertical temperature gradient is studied in this paper. The background stratification is modulated by lateral temperature variations and stable stratification aimed at understating the Earth's outer core convection subject to thermal core–mantle interaction. At the top boundary, sinusoidal and Gaussian temperature variations are imposed apart from the reference case of isothermal condition used in the classical Rayleigh–Benard convection. The additional modulating conditions break the top–bottom flow symmetry leading to flow localization and asymmetry that exhibit modified temporal dynamics unlike that of the classical Rayleigh–Benard cells. The threshold for convection is lowered with flows occurring in surplus heat flux regions caused by the imposed conditions. Despite flow suppression in the stable layer, rapid rotation favors the penetration of convection rolls with smaller wavelengths. The lateral variations in temperature imposed at the top boundary enhance such axial penetration with a laterally varying penetrative extent resulting in a modified clustered flow structure unlike the reference case. With both modulating conditions imposed, the onset of overstable modes is favored for low Prandtl numbers, a regime which is relevant to the Earth's core conditions. With rapid rotation, a novel mode of traveling wave instability occurs at the onset of convection, the propagation direction of which is controlled by the lateral temperature gradients at the top boundary. The onset of oscillatory modes is suppressed by the imposition of the modulating conditions indicated by the significant lowering of the transition Prandtl number.

‘Another wall of turf’

Excavation of a well-preserved stretch of the rampart of the Antonine Wall east of Watling Lodge, at Laurieston, allowed for the first micromorphological study of its earthen building materials. This revealed that the rampart core, as well as the cheeks, were constructed in well layered courses of turf blocks – but sourced from different grasslands. The evidence differs from macroscopic observations made at other sections in this area where the rampart material had been interpreted as representing a loose earth core with clay cheeks. Our results show that even when the characteristic striped sections indicative of turf are not visible in the field, thin-section analysis can confirm the use of intact soil blocks with the remains of grassed surfaces. It now seems possible that the visible variation between materials in the eastern and western sectors of the Antonine Wall may simply be due to different types of turf used, varying in subsoil composition and topsoil formation, and representing differences in landscape management and survival of vegetation. Combined with macroscopic field recording to identify Roman building practices, our analysis of this section at Laurieston also shows the care that was taken to construct a level, well-draining base for the rampart to avoid slumping and moisture build-up. Further excavations and thin-section analysis elsewhere along the Wall are now needed to confirm whether turf was more extensively used than so far anticipated for the eastern sector and whether the Antonine Wall could possibly have been built completely of turf. Such conclusions would suggest a much more standardised construction process and more extensive grassland exploitation than considered up until now. While our results demonstrate the importance of micromorphological analysis for understanding this earthen UNESCO World Heritage site, our interdisciplinary approach may also have wider relevance for research on linear earthworks in different geographical and chronological settings.

Designing polyacrylic acid capped luminescent rare earth core–shell nanoparticles for simultaneous Cu(II) and temperature sensing

Cu2+ is an essential trace element for humans and animals, and the precise detection of Cu2+ can help diagnose many diseases. However, the current Cu2+ detection technology still needs to be improved in terms of the biocompatibility of the material, the penetration of the excitation source, and the detection limit. In this paper, the polyacrylic acid (PAA) capped NaYF4:Yb3+/Er3+@NaYF4 core–shell nanoparticles (NPs) with a particle size of about 22.5 nm is proposed. Under the excitation of 980 nm with strong penetration, the Cu2+ concentration can be detected by the emission intensity of NPs at 520 nm, and the detection limit is 0.018 μM. In addition, this material can also perform temperature detection within 296–373 K with a relative sensitivity of 1.23 % K−1. Therefore, this NPs is a promising multifunctional material that can detect temperature and Cu2+ concentration.