373 research outputs found
Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (<i>Porites</i>)
The "δ<sup>11</sup>B-pH" technique was applied to modern and ancient corals <i>Porites</i> from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector – Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25‰, allowing a precision of about ±0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient K<sub><i>D</i></sub> for different aragonite species. Modern coral δ<sup>11</sup>B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50±0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous <i>p</i>CO<sub>2</sub> to be 440±25 ppmV at around 11.5 kyr BP for corals at Marquesas and ~500 ppmV near Tahiti where it was assumed that <i>p</i>CO<sub>2</sub> in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in <i>p</i>CO<sub>2</sub> between the ocean and the atmosphere at Marquesas (Δ<i>p</i>CO<sub>2</sub>) indicates that the surface waters behave as a moderate CO<sub>2</sub> sink or source (−53 to 20 ppmV) during El Niño-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO<sub>2</sub> (21 to 92 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO<sub>2</sub> released with Δ<i>p</i>CO<sub>2</sub> of +185±25 ppmV. This last finding provides further evidence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO<sub>2</sub> content
Planetesimal Formation In Self-Gravitating Discs
We study particle dynamics in local two-dimensional simulations of
self-gravitating accretion discs with a simple cooling law. It is well known
that the structure which arises in the gaseous component of the disc due to a
gravitational instability can have a significant effect on the evolution of
dust particles. Previous results using global simulations indicate that spiral
density waves are highly efficient at collecting dust particles, creating
significant local over-densities which may be able to undergo gravitational
collapse. We expand on these findings, using a range of cooling times to mimic
the conditions at a large range of radii within the disc. Here we use the
Pencil Code to solve the 2D local shearing sheet equations for gas on a fixed
grid together with the equations of motion for solids coupled to the gas solely
through aerodynamic drag force. We find that spiral density waves can create
significant enhancements in the surface density of solids, equivalent to 1-10cm
sized particles in a disc following the profiles of Clarke (2009) around a
solar mass star, causing it to reach concentrations several orders of magnitude
larger than the particles mean surface density. We also study the velocity
dispersion of the particles, finding that the spiral structure can result in
the particle velocities becoming highly ordered, having a narrow velocity
dispersion. This implies low relative velocities between particles, which in
turn suggests that collisions are typically low energy, lessening the
likelihood of grain destruction. Both these findings suggest that the density
waves that arise due to gravitational instabilities in the early stages of star
formation provide excellent sites for the formation of large,
planetesimal-sized objects.Comment: 11 pages, 8 figures, accepted for publication in MNRA
Tidal Truncation of Circumplanetary Discs
We analyse some properties of circumplanetary discs. Flow through such discs
may provide most of the mass to gas giant planets, and such discs are likely
sites for the formation of regular satellites. We model these discs as
accretion discs subject to the tidal forces of the central star. The tidal
torques from the star remove the disc angular momentum near the disc outer edge
and permit the accreting disc gas to lose angular momentum at the rate
appropriate for steady accretion. Circumplanetary discs are truncated near the
radius where periodic ballistic orbits cross, where tidal forces on the disc
are strong. This radius occurs at approximately 0.4 r_H for the planet Hill
radius r_H. During the T Tauri stage of disc accretion, the disc is fairly
thick with aspect ratio H/r > 0.2 and the disc edge tapering occurs over a
radial scale ~ H ~ 0.1 r_H. For a circular or slightly eccentric orbit planet,
no significant resonances lie within the main body of the disc. Tidally driven
waves involving resonances nonetheless play an important role in truncating the
disc, especially when it is fairly thick. We model the disc structure using one
dimensional time-dependent and steady-state models and also two dimensional SPH
simulations. The circumplanetary disc structure depends on the variation of the
disc turbulent viscosity with radius and is insensitive to the angular
distribution of the accreting gas. Dead zones may occur within the
circumplanetary disc and result in density structures. If the disc is turbulent
throughout, the predicted disc structure near the location of the regular
Jovian and Saturnian satellites is smooth with no obvious feature that would
favor formation at their current locations.Comment: Accepted for publication in MNRA
Lifting the lid: a clinical audit on commode cleaning
Many healthcare-associated infections (HCAIs) are preventable by infection control procedures designed to interrupt the transmission of organisms from a source. Commodes are in use constantly throughout healthcare facilities. Therefore commode surfaces are constantly handled, and any pathogens present have the potential to be transferred to not only other surfaces but also, more importantly, to patients, thus compromising patient safety. In order to examine the effectiveness and thoroughness of cleaning commodes an audit was undertaken to assess compliance with evidence-based practice. This audit demonstrates a cycle which includes defining best practice, implementing best practice, monitoring best practice and taking action to improve practice. The audit results confirmed an issue that the authors had long suspected. That is, that commodes allocated to individual patients are not always cleaned after every use. Using adenosine triphosphate (ATP) bioluminescence as an indicator of organic soiling also demonstrated that commodes that were considered clean were not always cleaned to a high standard. Implementing the audit recommendations improves staff knowledge through education, standardises cleaning procedures and ultimately improves patient safety
Increasing the frequency of hand washing by healthcare workers does not lead to commensurate reductions in staphylococcal infection in a hospital ward
Hand hygiene is generally considered to be the most important measure that can be applied to prevent the spread of healthcare-associated infection (HAI). Continuous emphasis on this intervention has lead to the widespread opinion that HAI rates can be greatly reduced by increased hand hygiene compliance alone. However, this assumes that the effectiveness of hand hygiene is not constrained by other factors and that improved compliance in excess of a given level, in itself, will result in a commensurate reduction in the incidence of HAI. However, several researchers have found the law of diminishing returns to apply to hand hygiene, with the greatest benefits occurring in the first 20% or so of compliance, and others have demonstrated that poor cohorting of nursing staff profoundly influences the effectiveness of hand hygiene measures. Collectively, these findings raise intriguing questions about the extent to which increasing compliance alone can further reduce rates of HAI.
In order to investigate these issues further, we constructed a deterministic Ross-Macdonald model and applied it to a hypothetical general medical ward. In this model the transmission of staphylococcal infection was assumed to occur after contact with the transiently colonized hands of HCWs, who, in turn, acquire contamination only by touching colonized patients. The aim of the study was to evaluate the impact of imperfect hand cleansing on the transmission of staphylococcal infection and to identify, whether there is a limit, above which further hand hygiene compliance is unlikely to be of benefit.
The model demonstrated that if transmission is solely via the hands of HCWs, it should, under most circumstances, be possible to prevent outbreaks of staphylococcal infection from occurring at a hand cleansing frequencies <50%, even with imperfect hand hygiene. The analysis also indicated that the relationship between hand cleansing efficacy and frequency is not linear - as efficacy decreases, so the hand cleansing frequency required to ensure R0<1 increases disproportionately.
Although our study confirmed hand hygiene to be an effective control measure, it demonstrated that the law of diminishing returns applies, with the greatest benefit derived from the first 20% or so of compliance. Indeed, our analysis suggests that there is little benefit to be accrued from very high levels of hand cleansing and that in most situations compliance >40% should be enough to prevent outbreaks of staphylococcal infection occurring, if transmission is solely via the hands of HCWs. Furthermore we identified a non-linear relationship between hand cleansing efficacy and frequency, suggesting that it is important to maximise the efficacy of the hand cleansing process
Continental aridification and the vanishing of Australia\u27s megalakes
The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50-47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The final disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridification that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water filling these lakes at different times during the last full glacial cycle. © 2011 Geological Society of America
Recent developments in planet migration theory
Planetary migration is the process by which a forming planet undergoes a
drift of its semi-major axis caused by the tidal interaction with its parent
protoplanetary disc. One of the key quantities to assess the migration of
embedded planets is the tidal torque between the disc and planet, which has two
components: the Lindblad torque and the corotation torque. We review the latest
results on both torque components for planets on circular orbits, with a
special emphasis on the various processes that give rise to additional, large
components of the corotation torque, and those contributing to the saturation
of this torque. These additional components of the corotation torque could help
address the shortcomings that have recently been exposed by models of planet
population syntheses. We also review recent results concerning the migration of
giant planets that carve gaps in the disc (type II migration) and the migration
of sub-giant planets that open partial gaps in massive discs (type III
migration).Comment: 52 pages, 18 figures. Review article to be published in "Tidal
effects in Astronomy and Astrophysics", Lecture Notes in Physic
Accreting Protoplanets in the LkCa 15 Transition Disk
Exoplanet detections have revolutionized astronomy, offering new insights
into solar system architecture and planet demographics. While nearly 1900
exoplanets have now been discovered and confirmed, none are still in the
process of formation. Transition discs, protoplanetary disks with inner
clearings best explained by the influence of accreting planets, are natural
laboratories for the study of planet formation. Some transition discs show
evidence for the presence of young planets in the form of disc asymmetries or
infrared sources detected within their clearings, as in the case of LkCa 15.
Attempts to observe directly signatures of accretion onto protoplanets have
hitherto proven unsuccessful. Here we report adaptive optics observations of
LkCa 15 that probe within the disc clearing. With accurate source positions
over multiple epochs spanning 2009 - 2015, we infer the presence of multiple
companions on Keplerian orbits. We directly detect H{\alpha} emission from the
innermost companion, LkCa 15 b, evincing hot (~10,000 K) gas falling deep into
the potential well of an accreting protoplanet.Comment: 35 pages, 3 figures, 1 table, 9 extended data item
Genomics accelerated isolation of a new stem rust avirulence gene - wheat resistance gene pair
Stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt) is a devastating disease of the global staple crop wheat. Although this disease was largely controlled by genetic resistance in the latter half of the 20th century, new strains of Pgt with increased virulence, such as Ug99, have evolved by somatic hybridisation and mutation. These newly emerged strains have caused significant losses in Africa and other regions and their continued spread threatens global wheat production. Breeding for disease resistance provides the most cost-effective control of wheat rust diseases. A number of race-specific rust resistance genes have been characterised in wheat and most encode immune receptors of the nucleotide-binding leucine-rich repeat (NLR) class. These receptors recognize pathogen effector proteins often known as avirulence (Avr) proteins. However, only two Avr genes have been identified in Pgt to date, AvrSr35 and AvrSr50 and none in other cereal rusts, which hinders efforts to understand the evolution of virulence in rust populations. The Sr27 resistance gene was first identified in a wheat line carrying an introgression of the 3R chromosome from Imperial rye. Although not deployed widely in wheat, Sr27 is widespread in the artificial crop species Triticosecale (triticale) which is a wheat-rye hybrid and is a host for Pgt. Sr27 is effective against Ug99 and other recently emerged Pgt strains. Here we identify both the Sr27 gene in wheat and the corresponding AvrSr27 gene in Pgt and show that virulence to Sr27 can arise experimentally and in the field through deletion mutations, copy number variation and expression level polymorphisms at the AvrSr27 locus
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