Deep‐water anoxia over the Frasnian–Famennian boundary (La Serre,France): a tectonically induced oceanic anoxic event?

The Frasnian–Famennian (F‐F) transition is well exposed in the basinal setting of La Serre (Montagne Noire, France), where oxygen‐deficient conditions prevailed in bottom environments. In this paper, we study the F‐F transition using inorganic geochemistry (notably, major elements, redox‐sensitive and/or sulphide‐forming trace metals, and productivity proxies). The results are compared with published data concerning the La Serre section, other F‐F sections and other geological formations also deposited in basinal settings. The very high enrichment in redox and productivity tracers indicates that anoxic conditions prevailed across the F‐F boundary, and that surface productivity was intense. Such deep‐water restricted conditions coupled with high productivity occurred in various places in Late Devonian oceans. Recycling alone cannot account for the intense nutrient supply and therefore the most probable source was via land runoff. We discuss the Eovariscan episode of uplift that stimulated rock weathering, and hence long‐term nutrient supply to the seas.     

Analysis and modeling of spatial correlation structure in small-scale rainfall in Central Oklahoma

Spatial correlation structure in small-scale rainfall is analyzed based on a dense cluster of raingauges in Central Oklahoma. This cluster, called the EVAC PicoNet, consists of 53 gauges installed in 25 measurement stations covering an area of about 3 km by 3 km. Two raingauges are placed in 24 stations and five in the central station. Three aspects of the estimated spatial correlation functions are discussed: dependence on time-scale ranging from 1 min to 24 h, inter-storm variability, and dependence on rainfall intensity. The results show a regular dependence of the correlogram parameters on the averaging time-scale, large differences of the correlograms in the individual storms, and the dominance of storms with high spatial variability on the average large sample characteristics. The authors also demonstrate and discuss the ambiguities in correlation estimates conditioned on rainfall intensities. The findings of this study have implications for raingauge network design, rainfall modeling, and conclusive evaluation of radar and satellite estimates of rainfall.     

Effects of non-thermal tails in Maxwellian electron distributions on synchrotron and Compton processes

We investigate how the presence of a non-thermal tail beyond a Maxwellian electron distribution affects the synchrotron process as well as Comptonization in plasmas with parameters typical for accretion flows on to black holes. We find that the presence of the tail can significantly increase the net (after accounting for self-absorption) cyclo-synchrotron emission of the plasma, which then provides seed photons for Compton upscattering. Thus, the luminosity in the thermally Comptonized spectrum is enhanced as well. The importance of these effects increases with both increasing Eddington ratio and black hole mass. The enhancement of the Comptonized synchrotron luminosity can be as large as ∼10³ and ∼10⁵ for stellar and supermassive black holes, respectively, when the energy content in the non-thermal tail is 1 per cent.
The presence of the tail only weakly hardens the thermal Comptonization spectrum but it leads to the formation of a high-energy tail beyond the thermal cut-off, which two effects are independent of the nature of the seed photons. Since observations of high-energy tails in Comptonization spectra can constrain the non-thermal tails in the electron distribution and thus the Comptonized synchrotron luminosity, they provide upper limits on the strength of magnetic fields in accretion flows. In particular, the measurement of an MeV tail in the hard state of Cyg X-1 by McConnell et al. implies the magnetic field strength in this source to be at most an order of magnitude below equipartition.     

The Frasnian–Famennian biotic crisis: How many (if any) bolide impacts?

The prime causation of the mid-Late Devonian mass extinction near the Frasnian–Famennian (F–F) boundary remains uncertain. Nevertheless, geochemical evidence has been presented recently as decisive evidence of a giant bolide impact occurring precisely at the F–F boundary, which promoted the global mortality episode. Palaeobiological data, however, imply a gradual global change, which is otherwise seen as a record of either multiple extraterrestrial catastrophes or of impact-triggered Earth-bound mechanisms. Sedimentological (mega-tsunami), physical (craters, microtektites), and geochemical records remain either elusive in many aspects, or incompatible with the predicted impact crisis pattern. Biotic succession across the F–F horizon is still poorly known, especially in continental domains, to evidence a synchronous ("bedding-plane") killing event at the close of the crisis. Instead, the commonly documented stepwise loss of biomass and an unproved distinctive "dead zone" are hard to explain simply as sampling artifacts. The assumed mass mortality precisely at the F–F boundary may be limited mainly to the pelagic realm. The underestimated role of early Variscan tectonism and associated volcanic-hydrothermal processes, resulting in thermal and nutrient pulses, as possible prime controls of the F–F crisis is suggested, as well as resemblances to the superplume-conditioned eventful mid-Cretaceous interval, exemplified in the Cenomanian-Turonian mass extinction. Additional shocks, generated by minor cometary strikes, are not excluded but may have affected some F–F biotas or areas. Received: 9 June 1998 / Accepted: 2 November 1998     

Validation of Galileo orbits using SLR with a focus on satellites launched into incorrect orbital planes

The space segment of the European Global Navigation Satellite System (GNSS) Galileo consists of In-Orbit Validation (IOV) and Full Operational Capability (FOC) spacecraft. The first pair of FOC satellites was launched into an incorrect, highly eccentric orbital plane with a lower than nominal inclination angle. All Galileo satellites are equipped with satellite laser ranging (SLR) retroreflectors which allow, for example, for the assessment of the orbit quality or for the SLR–GNSS co-location in space. The number of SLR observations to Galileo satellites has been continuously increasing thanks to a series of intensive campaigns devoted to SLR tracking of GNSS satellites initiated by the International Laser Ranging Service. This paper assesses systematic effects and quality of Galileo orbits using SLR data with a main focus on Galileo satellites launched into incorrect orbits. We compare the SLR observations with respect to microwave-based Galileo orbits generated by the Center for Orbit Determination in Europe (CODE) in the framework of the International GNSS Service Multi-GNSS Experiment for the period 2014.0–2016.5. We analyze the SLR signature effect, which is characterized by the dependency of SLR residuals with respect to various incidence angles of laser beams for stations equipped with single-photon and multi-photon detectors. Surprisingly, the CODE orbit quality of satellites in the incorrect orbital planes is not worse than that of nominal FOC and IOV orbits. The RMS of SLR residuals is even lower by 5.0 and 1.5 mm for satellites in the incorrect orbital planes than for FOC and IOV satellites, respectively. The mean SLR offsets equal \(-44.9, -35.0\), and \(-22.4\) mm for IOV, FOC, and satellites in the incorrect orbital plane. Finally, we found that the empirical orbit models, which were originally designed for precise orbit determination of GNSS satellites in circular orbits, provide fully appropriate results also for highly eccentric orbits with variable linear and angular velocities.     

Estimation of radar-rainfall error spatial correlation

The study presents a theoretical framework for estimating the radar-rainfall error spatial correlation (ESC) using data from relatively dense rain gauge networks. The error is defined as the difference between the radar estimate and the corresponding true areal rainfall. The method is analogous to the error variance separation that corrects the error variance of a radar-rainfall product for gauge representativeness errors. The study demonstrates the necessity to consider the area–point uncertainties while estimating the spatial correlation structure in the radar-rainfall errors. To validate the method, the authors conduct a Monte Carlo simulation experiment with synthetic fields with known error spatial correlation structure. These tests reveal that the proposed method, which accounts for the area–point distortions in the estimation of radar-rainfall ESC, performs very effectively. The authors then apply the method to estimate the ESC of the National Weather Service’s standard hourly radar-rainfall products, known as digital precipitation arrays (DPA). Data from the Oklahoma Micronet rain gauge network (with the grid step of about 5 km) are used as the ground reference for the DPAs. This application shows that the radar-rainfall errors are spatially correlated with a correlation distance of about 20 km. The results also demonstrate that the spatial correlations of radar–gauge differences are considerably underestimated, especially at small distances, as the area–point uncertainties are ignored.