Water distribution in the lower mantle: Implications for hydrolytic weakening

The presence of water in lower mantle minerals is thought to have substantial effects on the rheological properties of the Earth's lower mantle in what is generally known as “hydrolytic weakening”. This weakening will have profound effects on global convection, but hydrolytic weakening in lower mantle minerals has not been observed experimentally and thus the effect of water on global dynamics remains speculative. In order to constrain the likelihood of hydrolytic weakening being important in the lower mantle, we use first principles methods to calculate the partitioning of water (strictly protons) between mineral phases of the lower mantle under lower mantle conditions. We show that throughout the lower mantle water is primarily found either in the minor Ca-perovskite phase or in bridgmanite as an Al3+–H+ pair. Ferropericlase remains dry. However, neither of these methods of water absorption creates additional vacancies in bridgmanite and thus the effect of hydrolytic weakening is likely to be small. We find that water creates significant number of vacancies in bridgmanite only at the deepest part of the lower mantle and only for very high water contents (>1000 ppm). We conclude that water is thus likely to have only a limited effect on the rheological properties of the lower mantle

Elastic properties of ferropericlase at lower mantle conditions and its relevance to ULVZs

The elasticity of FexMg1 − xO was examined under lowermost mantle temperature and pressure conditions using density functional theory (DFT). The addition of iron decreases the shear modulus of MgO but has varying effects on the bulk modulus depending on the spin state of the iron. The spin state of iron in FexMg1 − xO is dependent on pressure and temperature but also on the concentration of iron. At 136 GPa, Fe in low concentrations (75%) it is nearly entirely in the high spin state. There is, as expected, a large decrease in seismic velocities with iron substitution. However, the effect of Fe is greater at high-temperatures than at low-temperatures, meaning it is difficult to extrapolate low-temperature experimental results. We cannot simultaneously match the density and seismic velocities of ULVZs with Fe-enriched ferropericlase. This is reflected in (dln Vs/dln Vp)T,P, which in ULVZs is generally observed to be about 3, but does not exceed about 1.5 for Fe-enriched periclase. A mixture of ferropericlase and ferrous perovskite can cause Vs decreases of up to 45%, which allows the range of ULVZ Vp, Vs and densities to be matched. We also find that (dln Vs/dln Vp)T,P increases up to as much as 3 but this value is strongly dependent on the bounds of the mixing geometry. We conclude, therefore, that the properties of ULVZs can be readily explained by a lower mantle with a single phase that is heavily enriched in Fe

Ferrous iron partitioning in the lower mantle

We used density functional theory (DFT) to examine the partitioning of ferrous iron between periclase and bridgmanite under lower mantle conditions. To study the effects of the three major variables — pressure, temperature and concentration — these have been varied from 0 to 150 GPa, from 1000 to 4000 K and from 0 to 100% total iron content. We find that increasing temperature increases KD, increasing iron concentration decreases KD, while pressure can both increase and decrease KD. We find that KD decreases slowly from about 0.32 to 0.06 with depth under lower mantle conditions. We also find that KD increases sharply to 0.15 in the very lowermost mantle due to the strong temperature increases near the CMB. Spin transitions have a large effect on the activity of ferropericlase which causes KD to vary with pressure in a peak-like fashion. Despite the apparently large changes in KD through the mantle, this actually results in relatively small changes in total iron content in the two phases, with View the MathML sourceXFefp ranging from about 0.20 to 0.35, before decreasing again to about 0.28 at the CMB, and View the MathML sourceXFebd has a pretty constant value of about 0.04–0.07 throughout the lower mantle. For the very high Fe concentrations suggested for ULVZs, Fe partitions very strongly into ferropericlase

The effect of water on the post-spinel transition and evidence for extreme water contents at the bottom of the transition zone

The transition of ringwoodite to bridgmanite and periclase (the post-spinel transition) is a strong control on the 660 phase discontinuity and the boundary between the transition zone and the lower mantle. The transition zone may contain significant amounts of water and thus the effect of water on the post-spinel transition must be known to correctly determine its properties. In this paper we examine the transition of ringwoodite to bridgmanite and periclase in both dry and wet conditions using density functional theory (DFT). In the dry case we calculate a high negative Clapeyron slope ( MPa/K at 1873 K). We also find that the Clapeyron slope is significantly nonlinear with temperature and much lower at 1000 K (−1.31 MPa/K) or if determined by linear interpolation from 1000 K (−2.37 MPa/K). The addition of water causes a large broadening of the transition through the development of a phase loop. Seismic studies suggest that the 660 km discontinuity is narrower than 2 km. For this to be the case our results suggest that the water content at the bottom of the transition zone needs to be either less than ∼700 ppm or, alternatively, above ∼8000 ppm (assuming an effective transition width near the maximum transition width). In the latter case this is above the saturation limit for bridgmanite and so will be accompanied by the production of a free water phase/hydrous melt. The hydration of ringwoodite also causes the onset of the transition to deepen with 1 wt% water increasing the depth of the transition by about 8 km. This is relatively small compared to seismically observed variations in the 660 km discontinuity of around 35 km and so water alone cannot account for the observed 660 km discontinuity topography. Water causes no substantial changes to the Clapeyron slope of the transition, so the 660 km topography could be explained by thermal variations of ∼500 K

Hydrous silicate melts and the deep mantle H2O cycle

We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO2-H2O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H2O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where ≳ 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H2O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with ∼0.02-0.04 wt% H2O may be indicative of the H2O content of the transition zone and lower mantle, resulting in a bulk mantle H2O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing

The innate immune sensor Toll-like receptor 2 controls the senescence-associated secretory phenotype

Cellular senescence is a stress response program characterized by a robust cell cycle arrest and the induction of a proinflammatory senescence-associated secretory phenotype (SASP) that is triggered through an unknown mechanism. Here, we show that, during oncogene-induced senescence (OIS), the Toll-like receptor 2 (TLR2) and its partner TLR10 are key mediators of senescence in vitro and in murine models. TLR2 promotes cell cycle arrest by regulating the tumor suppressors p53-p21 , p16 , and p15 and regulates the SASP through the induction of the acute-phase serum amyloids A1 and A2 (A-SAAs) that, in turn, function as the damage-associated molecular patterns (DAMPs) signaling through TLR2 in OIS. Last, we found evidence that the cGAS-STING cytosolic DNA sensing pathway primes TLR2 and A-SAAs expression in OIS. In summary, we report that innate immune sensing of senescence-associated DAMPs by TLR2 controls the SASP and reinforces the cell cycle arrest program in OIS

Development and internal validation of a clinical rule to improve antibiotic use in children presenting to primary care with acute respiratory tract infection and cough: a prognostic cohort study

BACKGROUND: Antimicrobial resistance is a serious threat to public health, with most antibiotics prescribed in primary care. General practitioners (GPs) report defensive antibiotic prescribing to mitigate perceived risk of future hospital admission in children with respiratory tract infections. We developed a clinical rule aimed to reduce clinical uncertainty by stratifying risk of future hospital admission. METHODS: 8394 children aged between 3 months and 16 years presenting with acute cough (for ≤28 days) and respiratory tract infection were recruited to a prognostic cohort study from 247 general practitioner practices in England. Exposure variables included demographic characteristics, parent-reported symptoms, and physical examination signs. The outcome was hospital admission for respiratory tract infection within 30 days, collected using a structured, blinded review of medical records. FINDINGS: 8394 (100%) children were included in the analysis, with 78 (0·9%, 95% CI 0·7%-1·2%) admitted to hospital: 15 (19%) were admitted on the day of recruitment (day 1), 33 (42%) on days 2-7; and 30 (39%) on days 8-30. Seven characteristics were independently associated (p<0·01) with hospital admission: age <2 years, current asthma, illness duration of 3 days or less, parent-reported moderate or severe vomiting in the previous 24 h, parent-reported severe fever in the previous 24 h or a body temperature of 37·8°C or more at presentation, clinician-reported intercostal or subcostal recession, and clinician-reported wheeze on auscultation. The area under the receiver operating characteristic (AUROC) curve for the coefficient-based clinical rule was 0·82 (95% CI 0·77-0·87, bootstrap validated 0·81). Assigning one point per characteristic, a points-based clinical rule consisting of short illness, temperature, age, recession, wheeze, asthma, and vomiting (mnemonic STARWAVe; AUROC 0·81, 0·76-0·85) distinguished three hospital admission risk strata: very low (0·3%, 0·2-0·4%) with 1 point or less, normal (1·5%, 1·0-1·9%) with 2 or 3 points, and high (11·8%, 7·3-16·2%) with 4 points or more. INTERPRETATION: Clinical characteristics can distinguish children at very low, normal, and high risk of future hospital admission for respiratory tract infection and could be used to reduce antibiotic prescriptions in primary care for children at very low risk. FUNDING: National Institute for Health Research (NIHR)

Pre-hospital transdermal glyceryl trinitrate for transient ischaemic attack: Data from the RIGHT-2 trial

\ua9 2024 The Author(s). European Journal of Neurology published by John Wiley &amp; Sons Ltd on behalf of European Academy of Neurology.Background and purpose: Ambulance trials assessing interventions in suspected stroke patients will recruit patients with currently active symptoms that will resolve into transient ischaemic attack (TIA). The safety and efficacy of glyceryl trinitrate (GTN) in the pre-specified subgroup of patients with TIA in the Rapid Intervention with Glyceryl Trinitrate in Hypertensive Stroke Trial 2 (RIGHT-2) was assessed. Methods: RIGHT-2 was a pre-hospital-initiated multicentre randomized sham-controlled blinded-endpoint trial that randomized patients with presumed ultra-acute stroke within 4 h of symptom onset to transdermal GTN or sham. Final diagnosis was determined by site investigators. The primary outcome was a shift in modified Rankin Scale (mRS) scores at 90 days analysed using ordinal logistic regression reported as adjusted common odds ratio with 95% confidence intervals (CIs). Secondary outcomes included death or dependence (mRS &gt;2). Results: In all, 109 of 1149 (9.5%) patients had a final diagnosis of TIA (GTN 57, sham 52) with mean age 73 (SD 13) years, 19 (17.4%) had pre-morbid mRS &gt;2, and onset to randomization was 80 min (interquartile range 49, 105). GTN lowered blood pressure by 7.4/5.2 mmHg compared with sham by hospital arrival. At day 90, GTN had no effect on shift in mRS scores (common odds ratio for increased dependence 1.47, 95% CI 0.70–3.11) but was associated with increased death or dependence (mRS &gt;2): GTN 29 (51.8%) versus sham 23 (46.9%), odds ratio 3.86 (95% CI 1.09–13.59). Conclusions: Pre-hospital ultra-acute transdermal GTN did not improve overall functional outcome in patients with investigator-diagnosed TIA compared with sham treatment