598 research outputs found
Tracing Earth's O2 evolution using Zn/Fe ratios in marine carbonates
Through Earth history, atmospheric oxygen has increased from initial values near zero to its present day level of about 21 % by volume; concomitantly, changes in ocean redox conditions have fundamentally altered global biogeochemical cycles. While there is a reasonable understanding of where oxygen history begins and ends, the quantitative timetable of oxygenation that links the endpoints has proven contentious. Equilibrium between marine surface environments and the overlying atmosphere suggests that carbonate-based redox proxies could refine palaeoredox records in time and space. Here we explore the use of Zn/Fe ratios to infer the evolution of atmospheric O2 through time, based on marine carbonate rocks that are well characterised in terms of depositional age, environmental setting, and diagenetic history. While Fe and Zn in the shallow ocean are mainly sourced from hydrothermal inputs, their redox sensitivities differ significantly, so that geological intervals with higher O2 would be characterised by stepped increases in Zn/Fe as preserved in shallow marine carbonates. Therefore, Zn/Fe analyses of ancient carbonates allow us to constrain past atmospheric pO2 levels, providing a secular record of atmospheric O2 over the past 3.5 billion years. In particular, we corroborate an earlier proposal that for much of the Proterozoic Eon, O2 levels were as low as 0.1-1 % of present atmospheric level. We conclude that Zn/Fe in shallow marine carbonate rocks has potential to provide a quantitative tracer for the long-term redox evolution of the oceans and the rise of atmospheric O
Integrated chronological control on an archaeologically significant Pleistocene river terrace sequence: the Thames-Medway, eastern Essex, England
Late Middle Pleistocene Thames-Medway deposits in eastern Essex comprise both large expanses of Palaeolithic artefact-bearing river sands/gravels and deep channels infilled with thick sequences of fossiliferous fine-grained estuarine sediments that yield valuable palaeoenvironmental information. Until recently, chronological control on these deposits was limited to terrace stratigraphy and limited amino-acid racemisation (AAR) determinations. Recent developments in both this and optically stimulated luminescence (OSL) dating make them potentially powerful tools for improving the chronological control on such sequences. This paper reports new AAR analyses and initial OSL dating from the deposits in this region. These results will help with ongoing investigation of patterns of early human settlement.
Using AAR, the attribution by previous workers of the interglacial channel deposits to both MIS 11 (Tillingham Clay) and MIS 9 (Rochford and Shoeburyness Clays) is reinforced. Where there are direct stratigraphic relationships between AAR and OSL as with the Cudmore Grove and Rochford Clays and associated gravels, they agree well. Where OSL dating is the only technique available, it seems to replicate well, but must be treated with caution since there are relatively few aliquots. It is suggested on the basis of this initial OSL dating that the gravel deposits date from MIS 8 (Rochford and Cudmore Grove Gravels) and potentially also MIS 6 (Dammer Wick and Barling Gravels). However, the archaeological evidence from the Barling Gravel and the suggested correlations between this sequence and upstream Thames terraces conflict with this latter age estimate and suggest that it may need more investigation
Towards a population of HMXB/NS microquasars as counterparts of low-latitude unidentified EGRET sources
The discovery of the microquasar LS 5039 well within the 95% conficence
contour of the Unidentified EGRET Source (UES) 3EG J1824-1514 was a major step
towards the possible association between microquasars (MQs) and UESs. The
recent discovery of precessing relativistic radio jets in LS I +61 303, a
source associated for long time with 2CG 135+01 and with the UES 3EG
J0241+6103, has given further support to this idea. Finally, the very recently
proposed association between the microquasar candidate AX J1639.0-4642 and the
UES 3EG J1639-4702 points towards a population of High Mass X-ray Binary
(HMXB)/Neutron Star (NS) microquasars as counterparts of low-latitude
unidentified EGRET sources.Comment: 12 pages, 7 figures. Proceedings of the Conference "The
Multiwavelength Approach to Unidentified Gamma-ray Sources", to appear in the
journal Astrophysics and Space Scienc
Theoretical overview on high-energy emission in microquasars
Microquasar (MQ) jets are sites of particle acceleration and synchrotron
emission. Such synchrotron radiation has been detected coming from jet regions
of different spatial scales, which for the instruments at work nowadays appear
as compact radio cores, slightly resolved radio jets, or (very) extended
structures. Because of the presence of relativistic particles and dense photon,
magnetic and matter fields, these outflows are also the best candidates to
generate the very high-energy (VHE) gamma-rays detected coming from two of
these objects, LS 5039 and LS I +61 303, and may be contributing significantly
to the X-rays emitted from the MQ core. In addition, beside electromagnetic
radiation, jets at different scales are producing some amount of leptonic and
hadronic cosmic rays (CR), and evidences of neutrino production in these
objects may be eventually found. In this work, we review on the different
physical processes that may be at work in or related to MQ jets. The jet
regions capable to produce significant amounts of emission at different
wavelengths have been reduced to the jet base, the jet at scales of the order
of the size of the system orbital semi-major axis, the jet middle scales (the
resolved radio jets), and the jet termination point. The surroundings of the
jet could be sites of multiwavelegnth emission as well, deserving also an
insight. We focus on those scenarios, either hadronic or leptonic, in which it
seems more plausible to generate both photons from radio to VHE and high-energy
neutrinos. We briefly comment as well on the relevance of MQ as possible
contributors to the galactic CR in the GeV-PeV range.Comment: Astrophysics & Space Science, in press (invited talk in the
conference: The multimessenger approach to the high-energy gamma-ray
sources", Barcelona/Catalonia, in July 4-7); 10 pages, 6 figures, 2 tables
(one reference corrected
Unidentified gamma-ray sources off the Galactic plane as low-mass microquasars?
A subset of the unidentified EGRET gamma-ray sources with no active galactic
nucleus or other conspicuous counterpart appears to be concentrated at medium
latitudes. Their long-term variability and their spatial distribution indicate
that they are distinct from the more persistent sources associated with the
nearby Gould Belt. They exhibit a large scale height of 1.3 +/- 0.6 kpc above
the Galactic plane. Potential counterparts for these sources include
microquasars accreting from a low-mass star and spewing a continuous jet.
Detailed calculations have been performed of the jet inverse Compton emission
in the radiation fields from the star, the accretion disc, and a hot corona.
Different jet Lorentz factors, powers, and aspect angles have been explored.
The up-scattered emission from the corona predominates below 100 MeV whereas
the disc and stellar contributions are preponderant at higher energies for
moderate (~15 deg) and small (~1 deg) aspect angles, respectively. Yet, unlike
in the high-mass, brighter versions of these systems, the external Compton
emission largely fails to produce the luminosities required for 5 to 10 kpc
distant EGRET sources. Synchrotron-self-Compton emission appears as a promising
alternative.Comment: 11 pages, 5 figures. Contributed paper to the "Multiwavelength
Approach to Unidentified Gamma-Ray Sources", Eds. K.S. Cheng & G.E. Romero,
to appear in Astrophysics and Space Science journa
A thermodynamic and kinetic basis for understanding chromium depletion in Ni-Cr-Fe alloys
Thermodynamic and kinetic models are constructed to describe the development of the chromium depleted zone in Ni-Cr-Fe alloys heated in the range 773-1173 K. The models are interactive and constitute a computer program called DEPLETE. The thermodynamic model is constructed using the Kohler method for the description of the free energy of a multi-component system. It provides the chromium concentration at the carbide-matrix interface as a function of alloy composition and temperature. The kinetic model tracks the shape of the chromium profile as a function of time at temperature and grain size. Model results show that the interfacial chromium concentration decreases for increasing carbon concentration and decreasing heat treatment temperature. Experimental verification of the model is made using high resolution energy despersive X-ray analysis via STEM. Measured results agree well with model results for the dependence of chromium depletion on various input parameters as well as the magnitude and shape of the chromium depleted zone. Experimental measurements also show that the grain boundary carbides are of the form M7C3 where M is about 96% chromium. Results confirm that carbide precipitation at the grain boundary is controlled by volume diffusion of chromium in the matrix and that in the temperature range 873 to 1073 K the chromium concentration in the grain boundary accurately approximates the carbide-matrix interfacial concentration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25692/1/0000246.pd
Efficacy and safety of strontium ranelate in the treatment of osteoporosis in men
CONTEXT: Strontium ranelate reduces vertebral and nonvertebral fracture risk in postmenopausal osteoporosis.
OBJECTIVE: The objective of this study was to determine the efficacy and safety of strontium ranelate in osteoporosis in men over 2 years (main analysis after 1 year).
DESIGN: This was an international, unbalanced (2:1), double-blind, randomized placebo-controlled trial (MALEO [MALE Osteoporosis]).
SETTING: This international study included 54 centers in 14 countries.
PARTICIPANTS: PARTICIPANTS were 261 white men with primary osteoporosis.
INTERVENTION: Strontium ranelate at 2 g/d (n = 174) or placebo (n = 87) was administered.
MAIN OUTCOME MEASURES: Lumbar spine (L2-L4), femoral neck, and total hip bone mineral density (BMD), biochemical bone markers, and safety were measured.
RESULTS: Baseline characteristics were similar in both groups in the whole population (age, 72.9 Âą 6.0 years; lumbar spine BMD T-score, -2.7 Âą 1.0; femoral neck BMD T-score, -2.3 Âą 0.7). Men who received strontium ranelate over 2 years had greater increases in lumbar spine BMD than those who received placebo (relative change from baseline to end, 9.7% Âą 7.5% vs 2.0% Âą 5.5%; between-group difference estimate (SE), 7.7% (0.9%); 95% confidence interval, 5.9%-9.5%; P < .001). There were also significant between-group differences in relative changes in femoral neck BMD (P < .001) and total hip BMD (P < .001). At the end of treatment, mean levels of serum cross-linked telopeptides of type I collagen, a marker of bone resorption, were increased in both the strontium ranelate group (10.7% Âą 58.0%; P = .022) and the placebo group (34.9% Âą 65.8%; P < .001). The corresponding mean changes of bone alkaline phosphatase, a marker of bone formation, were 6.4% Âą 28.5% (P = .005) and 1.9% Âą 25.4% (P = .505), respectively. After 2 years, the blood strontium level (129 Âą 66 Îźmol/L) was similar to that in trials of postmenopausal osteoporosis. Strontium ranelate was generally well tolerated.
CONCLUSIONS: The effects of strontium ranelate on BMD in osteoporotic men were similar to those in postmenopausal osteoporotic women, supporting its use in the treatment of osteoporosis in men
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV
The Lambda(b) differential production cross section and the cross section
ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum
pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7
TeV using data collected by the CMS experiment at the LHC. The measurements are
based on Lambda(b) decays reconstructed in the exclusive final state J/Psi
Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and
Lambda to proton pion, using a data sample corresponding to an integrated
luminosity of 1.9 inverse femtobarns. The product of the cross section times
the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls
faster than that of b mesons. The measured value of the cross section times the
branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06
+/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for
anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are
statistical and systematic, respectively.Comment: Submitted to Physics Letters
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