199 research outputs found
Aquatic and terrestrial proxy evidence for Middle Pleistocene palaeolake and lakeâshore development at two Lower Palaeolithic sites of Schöningen, Germany
The archaeological sites in the openâcast mine of Schöningen, Germany, represent outstanding archives for understanding Middle Pleistocene interglacialâglacial transitions and human adaption. Aquatic microfossil and pollen assemblages from the âReinsdorf sequenceâ, likely correlated to Marine Isotope Stage 9, document environmental changes from a thermal maximum to succeeding glacial conditions recorded in two sequences of excavation sites 12 II and 13 II. Multiâproxy analyses enable detailed reconstruction of lakeâshore and landscape developments despite variable microfossil preservation in changing carbonateâ and organicârich deposits. Rich aquatic vegetation with abundant charophytes suggests repeated phases with water depths of 0.5â2 m at site 13 II, while even greater temporary depths are deduced for 12 II DB. Mesorheophilic and mesotitanophilic ostracod species indicate stream inflows with mediumâlow calcium contents of >18 mg Ca L â1 originating from nearby springs. Diatoms point to mesoâeutrophic conditions and an alkaline pH of the lake water. Interglacial conditions with thermophile forests but no aquatic microfossils preserved, suggesting a dry or only temporarily flooded site, mark the beginning of the sequence. Continuous presence of aquatic organisms and overall dominance of small tychoplanktonic diatoms during a subsequent cool steppe phase provide evidence for increased water depths and unstable habitats characterized by erosion and probably prolonged periods of lake ice cover. During the succeeding boreal forestâsteppe phase, surface runoff into the productive, shallow lake decreased due to a more extensive vegetation cover. Concurrently, intensified groundwater input in contact with the nearby salt wall caused elevated salinities. Following a lake level drop, stream inflows and lake levels increased again towards the end of the Reinsdorf sequence and promoted development of a diverse fauna and flora at the lake shore; thereby maintaining an attractive living and hunting environment for early humans during a phase of generally cooler temperatures and landscape instability at the transition into a glacial period
The unexpectedly short Holocene Humid Period in Northern Arabia
The early to middle Holocene Humid Period led to a greening of todayâs arid Saharo-Arabian desert belt. While this phase is well defined in North Africa and the Southern Arabian Peninsula, robust evidence from Northern Arabia is lacking. Here we fill this gap with unprecedented annually to sub-decadally resolved proxy data from Tayma, the only known varved lake sediments in Northern Arabia. Based on stable isotopes, micro-facies analyses and varve and radiocarbon dating, we distinguish five phases of lake development and show that the wet phase in Northern Arabia from 8800â7900 years BP is considerably shorter than the commonly defined Holocene Humid Period (~11,000â5500 years BP). Moreover, we find a two century-long peak humidity at times when a centennial-scale dry anomaly around 8200 years BP interrupted the Holocene Humid Period in adjacent regions. The short humid phase possibly favoured Neolithic migrations into Northern Arabia representing a strong human response to environmental changes
Imprints of the Little Ice Age and the severe earthquake of AD 2001 on the aquatic ecosystem of a tropical maar lake in El Salvador
Using a 530-year sediment record from the maar Lake Apastepeque, El Salvador, and based on diverse geochemical and biological (cladocerans, chironomids, diatoms, ostracods, testate amoebae) indicators, we estimated climatic and environmental alterations during the Little Ice Age (LIA) and reconstructed the recent history of the lake. Results demonstrate relatively humid conditions in the mid-elevations (500âm a.s.l.) of El Salvador during most parts of the LIA, resulting in high lake levels. Contrarily, the first part of the LIA was characterized by drier climates comparable to studies from Mexico and Belize, which correlated this phase with the Spörer minimum. Regional comparison with palaeorecords from the northern Neotropics reveals a high heterogeneity in local expressions of the LIA in Central America, likely connected to the high topographic heterogeneity of the region. Since the beginning of the 20th century, Lake Apastepeque has experienced enhanced human impact expressed as increased nutrient supply. The most recent period was characterized by significant environmental disturbance, which we relate to an upper-crustal earthquake, one of the strongest over the last 500âyears, that affected the region on 13th February 2001 (Mwâ=â6.6, epicentre at 10âkm depth, 30âkm from the lake). The release of toxic bottom components such as hydrogen sulphide and high turbidity and turbulence of water caused major species turnover in the lake ecosystem, resulting in a massive fish kill and colonization by large cladocerans. Modern sediments still show slightly altered biota communities compared to pre-earthquake assemblages, indicating that the ecosystem has still not fully recovered
Neutrino and Antineutrino Inclusive Charged-current Cross Section Measurements with the MINOS Near Detector
The energy dependence of the neutrino-iron and antineutrino-iron inclusive
charged-current cross sections and their ratio have been measured using a
high-statistics sample with the MINOS Near Detector exposed to the NuMI beam
from the Main Injector at Fermilab. Neutrino and antineutrino fluxes were
determined using a low hadronic energy subsample of charged-current events. We
report measurements of neutrino-Fe (antineutrinoFe) cross section in the energy
range 3-50 GeV (5-50 GeV) with precision of 2-8% (3-9%) and their ratio which
is measured with precision 2-8%. The data set spans the region from low energy,
where accurate measurements are sparse, up to the high-energy scaling region
where the cross section is well understood.Comment: accepted by PR
Light Vector Mesons in the Nuclear Medium
The light vector mesons (, , and ) were produced in
deuterium, carbon, titanium, and iron targets in a search for possible
in-medium modifications to the properties of the meson at normal nuclear
densities and zero temperature. The vector mesons were detected with the CEBAF
Large Acceptance Spectrometer (CLAS) via their decays to . The rare
leptonic decay was chosen to reduce final-state interactions. A combinatorial
background was subtracted from the invariant mass spectra using a
well-established event-mixing technique. The meson mass spectrum was
extracted after the and signals were removed in a nearly
model-independent way. Comparisons were made between the mass spectra
from the heavy targets () with the mass spectrum extracted from the
deuterium target. With respect to the -meson mass, we obtain a small
shift compatible with zero. Also, we measure widths consistent with standard
nuclear many-body effects such as collisional broadening and Fermi motion.Comment: 15 pages, 18 figures, 3 table
photoproduction on the proton for photon energies from 0.675 to 2.875 GeV
Differential cross sections for the reaction have been
measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged
photon beam with energies from 0.675 to 2.875 GeV. The results reported here
possess greater accuracy in the absolute normalization than previous
measurements. They disagree with recent CB-ELSA measurements for the process at
forward scattering angles. Agreement with the SAID and MAID fits is found below
1 GeV. The present set of cross sections has been incorporated into the SAID
database, and exploratory fits have been extended to 3 GeV. Resonance couplings
have been extracted and compared to previous determinations.Comment: 18 pages, 48 figure
First measurement of coherent -meson photoproduction on deuteron at low energies
The cross section and decay angular distributions for the coherent \phi meson
photoproduction on the deuteron have been measured for the first time up to a
squared four-momentum transfer t =(p_{\gamma}-p_{\phi})^2 =-2 GeV^2/c^2, using
the CLAS detector at the Thomas Jefferson National Accelerator Facility. The
cross sections are compared with predictions from a re-scattering model. In a
framework of vector meson dominance, the data are consistent with the total
\phi-N cross section \sigma_{\phi N} at about 10 mb. If vector meson dominance
is violated, a larger \sigma_{\phi N} is possible by introducing larger t-slope
for the \phi N \to \phi N process than that for the \gamma N \to \phi N
process. The decay angular distributions of the \phi are consistent with
helicity conservation.Comment: 6 page
Search for medium modification of the meson
The photoproduction of vector mesons on various nuclei has been studied using
the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory. The
vector mesons, , , and , are observed via their decay to
, in order to reduce the effects of final state interactions in the
nucleus. Of particular interest are possible in-medium effects on the
properties of the meson. The spectral function is extracted from
the data on various nuclei, carbon, iron, and titanium, and compared to the
spectrum from liquid deuterium, which is relatively free of nuclear effects. We
observe no significant mass shift for the meson; however, there is some
widening of the resonance in titanium and iron, which is consistent with
expected collisional broadening.Comment: 8 pages, 4 figure
Electroproduction of mesons at GeV measured with the CLAS spectrometer
Electroproduction of exclusive vector mesons has been studied with the
CLAS detector in the kinematical range GeV,
GeV, and GeV. The
scaling exponent for the total cross section as was
determined to be . The slope of the four-momentum transfer
distribution is GeV. Under the assumption of
s-channel helicity conservation (SCHC), we determine the ratio of longitudinal
to transverse cross sections to be . A 2-gluon exchange model
is able to reproduce the main features of the data.Comment: Phys Rev C, 15 pages, 18 figure
Photodisintegration of He into p+t
The two-body photodisintegration of He into a proton and a triton has
been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at Jefferson
Laboratory. Real photons produced with the Hall-B bremsstrahlung-tagging system
in the energy range from 0.35 to 1.55 GeV were incident on a liquid He
target. This is the first measurement of the photodisintegration of He
above 0.4 GeV. The differential cross sections for the He
reaction have been measured as a function of photon-beam energy and
proton-scattering angle, and are compared with the latest model calculations by
J.-M. Laget. At 0.6-1.2 GeV, our data are in good agreement only with the
calculations that include three-body mechanisms, thus confirming their
importance. These results reinforce the conclusion of our previous study of the
three-body breakup of He that demonstrated the great importance of
three-body mechanisms in the energy region 0.5-0.8 GeV .Comment: 13 pages submitted in one tgz file containing 2 tex file and 22
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