2,042 research outputs found
On the stratigraphic integrity of leaf-wax biomarkers in loess paleosols
Paleoenvironmental and paleoclimate reconstructions based on molecular
proxies, such as those derived from leaf-wax biomarkers, in loess-paleosol
sequences represent a promising line of investigation in Quaternary research.
The main premise of such reconstructions is the synsedimentary deposition of
biomarkers and dust, which has become a debated subject in recent years. This
study uses two independent approaches to test the stratigraphic integrity of
leaf-wax biomarkers: (i) long-chain n-alkanes and fatty acids are
quantified in two sediment-depth profiles in glacial till on the Swiss
Plateau, consisting of a Holocene topsoil and the underlying B and C
horizons. Since glacial sediments are initially very poor in organic matter,
significant amounts of leaf-wax biomarkers in the B and C horizons of those
profiles would reflect postsedimentary root-derived or microbial
contributions. (ii) Compound-specific radiocarbon measurements are conducted
on n-alkanes and n-alkanoic (fatty) acids from several depth intervals in
the loess section "Crvenka", Serbia, and the results are compared to
independent estimates of sediment age.
<br><br>
We find extremely low concentrations
of plant-wax n-alkanes and fatty acids in the B and C horizons below the
topsoils in the sediment profiles. Moreover, compound-specific radiocarbon
analysis yields plant-wax <sup>14</sup>C ages that agree well with published
luminescence ages and stratigraphy of the Serbian loess deposit. Both
approaches confirm that postsedimentary, root-derived or microbial
contributions are negligible in the two investigated systems. The good
agreement between the ages of odd and even homologues also indicates that
reworking and incorporation of fossil leaf waxes is not particularly relevant
either
Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN
The transverse stratification of active galactic nuclei (AGN) jets is
suggested by observations and theoretical arguments, as a consequence of
intrinsic properties of the central engine (accretion disc + black hole) and
external medium. On the other hand, the one-component jet approaches are
heavily challenged by the various observed properties of plasmoids in radio
jets (knots), often associated with internal shocks. Given that such a
transverse stratification plays an important role on the jets acceleration,
stability, and interaction with the external medium, it should also induce
internal shocks with various strengths and configurations, able to describe the
observed knots behaviours. By establishing a relation between the transverse
stratification of the jets, the internal shock properties, and the multiple
observed AGN jet morphologies and behaviours, our aim is to provide a
consistent global scheme of the various AGN jet structures. Working on a large
sample of AGN radio jets monitored in very long baseline interferometry (VLBI)
by the MOJAVE collaboration, we determined the consistency of a systematic
association of the multiple knots with successive re-collimation shocks. We
then investigated the re-collimation shock formation and the influence of
different transverse stratified structures by parametrically exploring the two
relativistic outflow components with the specific relativistic hydrodynamic
(SRHD) code AMRVAC. We were able to link the different spectral classes of AGN
with specific stratified jet characteristics, in good accordance with their
VLBI radio properties and their accretion regimes.Comment: 16 pages, 12 figures, accepted for publication in A&
Hyperbolicity preserving HLL solver for two-layer shallow-water equations applied to dam-break flows
River morphodynamics and sediment transportDebris and hyperconcentrated flow
SST-GATE: A dual mirror telescope for the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) will be the world's first open
observatory for very high energy gamma-rays. Around a hundred telescopes of
different sizes will be used to detect the Cherenkov light that results from
gamma-ray induced air showers in the atmosphere. Amongst them, a large number
of Small Size Telescopes (SST), with a diameter of about 4 m, will assure an
unprecedented coverage of the high energy end of the electromagnetic spectrum
(above ~1TeV to beyond 100 TeV) and will open up a new window on the
non-thermal sky. Several concepts for the SST design are currently being
investigated with the aim of combining a large field of view (~9 degrees) with
a good resolution of the shower images, as well as minimizing costs. These
include a Davies-Cotton configuration with a Geiger-mode avalanche photodiode
(GAPD) based camera, as pioneered by FACT, and a novel and as yet untested
design based on the Schwarzschild-Couder configuration, which uses a secondary
mirror to reduce the plate-scale and to allow for a wide field of view with a
light-weight camera, e.g. using GAPDs or multi-anode photomultipliers. One
objective of the GATE (Gamma-ray Telescope Elements) programme is to build one
of the first Schwarzschild-Couder prototypes and to evaluate its performance.
The construction of the SST-GATE prototype on the campus of the Paris
Observatory in Meudon is under way. We report on the current status of the
project and provide details of the opto-mechanical design of the prototype, the
development of its control software, and simulations of its expected
performance.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
Non-parametric comparison of histogrammed two-dimensional data distributions using the Energy Test
When monitoring complex experiments, comparison is often made between regularly acquired histograms of data and reference histograms which represent the ideal state of the equipment. With the larger HEP experiments now ramping up, there is a need for automation of this task since the volume of comparisons could overwhelm human operators. However, the two-dimensional histogram comparison tools available in ROOT have been noted in the past to exhibit shortcomings. We discuss a newer comparison test for two-dimensional histograms, based on the Energy Test of Aslan and Zech, which provides more conclusive
discrimination between histograms of data coming from different distributions than methods provided in a recent ROOT release.The Science and Technology Facilities Council, U
GPS water level measurements for Indonesia's Tsunami Early Warning System
On Boxing Day 2004, a severe tsunami was generated by a strong earthquake in Northern Sumatra causing a large number of casualties. At this time, neither an offshore buoy network was in place to measure tsunami waves, nor a system to disseminate tsunami warnings to local governmental entities. Since then, buoys have been developed by Indonesia and Germany, complemented by NOAA's Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys, and have been moored offshore Sumatra and Java. The suite of sensors for offshore tsunami detection in Indonesia has been advanced by adding GPS technology for water level measurements. <br><br> The usage of GPS buoys in tsunami warning systems is a relatively new approach. The concept of the German Indonesian Tsunami Early Warning System (GITEWS) (Rudloff et al., 2009) combines GPS technology and ocean bottom pressure (OBP) measurements. Especially for near-field installations where the seismic noise may deteriorate the OBP data, GPS-derived sea level heights provide additional information. <br><br> The GPS buoy technology is precise enough to detect medium to large tsunamis of amplitudes larger than 10 cm. The analysis presented here suggests that for about 68% of the time, tsunamis larger than 5 cm may be detectable
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