Precisely-dated, high-resolution stable isotope and trace element data from a stalagmite from La Garma Cave, northern Spain, reveal several stages of distinct climatic variability along the northern ...Iberian Atlantic margin, and provide new constraints on the latitude of North Atlantic westerlies during the Younger Dryas Event (YD). Westerly wind position (reconstructed using sub-annually resolved Mg data as a proxy for sea spray contributions associated with wind strength at this coastal cave site) during the early YD (12.85-12.15 kyr) oscillated meridionally, resembling the decadal-scale component of the modern North Atlantic Oscillation (NAO). Northward repositioning of westerly storm tracks over northern Iberia began at 12.15 kyr, consistent with other high-resolution wind proxy reconstructions from central and northern Europe, but occurred more gradually nearer the Atlantic margin. From 12.15 kyr to the YD termination, atmospheric circulation resembled a persistently positive NAO, with westerlies reaching their maximum northward extent at 11.8 kyr (reflected by a Mg concentration minimum at this time). Air temperature (reflected by our delta 18O data) and Iberian wind strength were predominantly coupled throughout the YD suggesting that temperature modulated sea-ice extent, and consequently controlled westerly wind latitude. However our data suggest that abrupt warming at 12.1 kyr was followed by much more gradual northward shifts in westerly position, and that a lag existed between the warming and sea-ice retreat. This gradual return of the westerlies to the north beginning at 12.1 kyr is consistent with inferred changes in wind strength at other European sites. Additionally, atmospheric circulation inferred from our northern Iberian wind strength proxy record generally tracked low-frequency meridional shifts in Intertropical Convergence Zone position, corroborating past research that suggested closely coupled low- and high-latitude atmospheric teleconnections over this period.
The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip ...water δ
O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ
O is most similar to the amount-weighted precipitation δ
O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ
O records the recharge-weighted δ
O. This implies that the δ
O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ
O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.
Superconducting magnet training is one of the accelerator related issues attracting attention due to significant operational costs and time budget associated to it. It is especially worrisome that ...magnets based on the "next-generation" Nb 3 Sn technology are affected by long training. While various efforts are underway to better understand and resolve the problem a parallel path could also be investigated, a path bypassing the issue. Following the concept of fast induced over-current during magnet powering, FNAL has developed an upgradable capacitor-based device to discharge through a superconducting magnet at quench detection or operator chosen time. The 0.4 F/1 kV device has been tested on a 1-m-long dipole-coil in a "mirror" magnet configuration and conclusive results on magnet training elimination have been observed. In this paper we discuss the main characteristics of the device, compare simulated response and actual performance, elaborate on test drivers and outcomes. Next steps and perspectives for future use are debated.
Thin films of β-Ga2O3 doped with Si are grown on Al2O3 (0001) and β-Ga2O3 (100) substrates by metal organic vapor phase epitaxy. Homoepitaxial growth of single-phase, smooth β-Ga2O3 layers doped with ...Si, with a dislocation density not exceeding those of the melt grown substrate, was demonstrated. The interplay between growth conditions, structural and electrical properties of the Ga-oxide layers is studied. XRD and HRTEM show that Si-doping in the concentration range 1017–1018cm−3 does not deteriorate the quality of the Ga-oxide layers compared to undoped ones. The different nature of defects in undoped and doped material is investigated by HRTEM. It is found out that the density of twins and stacking faults on a-planes are an order of magnitude lower in β-Ga2O3:Si than in undoped material, while defects on inclined planes have rarely been observed in β-Ga2O3.
•The present paper reports on MOVPE growth and characterization study of Si-doped β-Ga2O3 layers.•The Si doping potential in the MOVPE process is studied - such an investigation does not exist in the literature.•Homoepitaxial growth of Si-doped β-Ga2O3 layers on native melt growth substrates is demonstrated.•The different nature of defects in undoped and doped material is investigated by HRTEM.•The density of twins and stacking faults on a-planes are an order of magnitude lower in β-Ga2O3:Si than in undoped material.
Classic Maya populations living in peri-urban states were highly dependent on seasonally distributed rainfall for reliable surplus crop yields. Despite intense study of the potential impact of ...decadal to centennial-scale climatic changes on the demise of Classic Maya sociopolitical institutions (750-950 CE), its direct importance remains debated. We provide a detailed analysis of a precisely dated speleothem record from Yok Balum cave, Belize, that reflects local hydroclimatic changes at seasonal scale over the past 1600 years. We find that the initial disintegration of Maya sociopolitical institutions and population decline occurred in the context of a pronounced decrease in the predictability of seasonal rainfall and severe drought between 700 and 800 CE. The failure of Classic Maya societies to successfully adapt to volatile seasonal rainfall dynamics likely contributed to gradual but widespread processes of sociopolitical disintegration. We propose that the complex abandonment of Classic Maya population centres was not solely driven by protracted drought but also aggravated by year-to-year decreases in rainfall predictability, potentially caused by a regional reduction in coherent Intertropical Convergence Zone-driven rainfall.
The cylindrical drift chamber is the most innovative part of the MEG II detector, the upgraded version of the MEG experiment. The MEG II chamber differs from the MEG one because it is a single volume ...cylindrical structure, instead of a segmented one, chosen to improve its resolutions and efficiency in detecting low energy positrons from muon decays at rest. In this paper, we show the characteristics and performances of this fundamental part of the MEG II apparatus and we discuss the impact of its higher resolution and efficiency on the sensitivity of the MEG II experiment. Because of its innovative structure and high quality resolution and efficiency the MEG II cylindrical drift chamber will be a cornerstone in the development of an ideal tracking detector for future positron-electron collider machines.
PDF
