Changes since the onset of acid deposition among calcium-sensitive cladoceran taxa within softwater lakes of Ontario, Canada     

Adam Jeziorski  Andrew M. Paterson  John P. Smol 《Journal of Paleolimnology》2012,48(2):323-337

Aqueous calcium (Ca) concentrations are currently decreasing in many softwater lakes on the Boreal Shield. As the onset of these declines often pre-date direct monitoring programs, indirect techniques are required to examine the impacts of reduced Ca availability on aquatic communities with relatively high Ca demands such as the Cladocera (Class: Branchiopoda). Among the Cladocera, the family Daphniidae has been identified as a taxonomic group potentially useful for inferring past Ca concentrations due to their high Ca demands and preservation in lake sediments. Here, we use a ??top/bottom?? paleolimnological analysis to compare present-day cladoceran communities preserved in the surface sediments of 36 softwater lakes in south-central Ontario, Canada, which are potentially vulnerable to Ca decline (i.e. small headwater systems with present-day lakewater [Ca]?<?3?mg?L^?1), with the communities present in lake sediments deposited prior to the onset of regional acid deposition. To distinguish the potential impacts of lake acidification from those of Ca availability (as Ca and pH trends are strongly correlated in this region), the study lakes were chosen to be evenly distributed about a present-day lakewater pH of 6 and Ca concentration of 1.5?mg?L^?1 (threshold values). Despite the importance of pH as an explanatory variable for the present-day assemblages, a comparison of the sedimentary remains from the two time periods indicate there have been large declines since pre-industrial times in the relative abundances of Ca-rich Daphnia spp. (particularly of the Daphnia longispina species complex), regardless of present-day pH, accompanied by increases in the Ca-poor species Holopedium glacialis. These observations suggest that recent declines in Ca concentration may have already fallen below baseline conditions, with marked implications for ecosystem function due to the differential responses among cladoceran taxa.  相似文献   

Stellar and galactic environment survey (SAGE)     

M. A. Barstow  M. R. Burleigh  N. J. Bannister  J. S. Lapington  M. P. Kowalski  R. G. Cruddace  K. S. Wood  F. Auchere  M. F. Bode  G. E. Bromage  B. Gibson  A. C. Cameron  A. Cassatella  F. Delmotte  M.-F. Ravet  J. G. Doyle  C. S. Jeffery  B. Gaensicke  C. Jordan  N. Kappelmann  K. Werner  R. Lallement  D. de Martino  S. A. Matthews  K. J. H. Phillips  G. Del Zanna  M. Orio  E. Pace  I. Pagano  J. H. M. M. Schmitt  B. Y. Welsh 《Astrophysics and Space Science》2009,320(1-3):231-238

This paper describes a proposed high resolution soft X-ray and Extreme Ultraviolet (EUV) spectroscopy mission to carry out a survey of Stellar and Galactic Environments (SAGE). The payload is based on novel diffraction grating technology which has already been proven in a sub-orbital space mission and which is ready to fly on a satellite platform with minimal development. Much of the technical detail of the instrumentation has been reported elsewhere and we concentrate our discussion here on the scientific goals of a SAGE base-line mission, demonstrating the scientific importance of high resolution spectroscopy in the Extreme Ultraviolet for the study of stars and the local interstellar medium.  相似文献   

The orbital-thermal evolution and global expansion of Ganymede     

Michael T. Bland  Adam P. Showman 《Icarus》2009,200(1):207-221

The tectonically and cryovolcanically resurfaced terrains of Ganymede attest to the satellite's turbulent geologic history. Yet, the ultimate cause of its geologic violence remains unknown. One plausible scenario suggests that the Galilean satellites passed through one or more Laplace-like resonances before evolving into the current Laplace resonance. Passage through such a resonance can excite Ganymede's eccentricity, leading to tidal dissipation within the ice shell. To evaluate the effects of resonance passage on Ganymede's thermal history we model the coupled orbital-thermal evolution of Ganymede both with and without passage through a Laplace-like resonance. In the absence of tidal dissipation, radiogenic heating alone is capable of creating large internal oceans within Ganymede if the ice grain size is 1 mm or greater. For larger grain sizes, oceans will exist into the present epoch. The inclusion of tidal dissipation significantly alters Ganymede's thermal history, and for some parameters (e.g. ice grain size, tidal Q of Jupiter) a thin ice shell (5 to 20 km) can be maintained throughout the period of resonance passage. The pulse of tidal heating that accompanies Laplace-like resonance capture can cause up to 2.5% volumetric expansion of the satellite and contemporaneous formation of near surface partial melt. The presence of a thin ice shell and high satellite orbital eccentricity would generate moderate diurnal tidal stresses in Ganymede's ice shell. Larger stresses result if the ice shell rotates non-synchronously. The combined effects of satellite expansion, its associated tensile stress, rapid formation of near surface partial melt, and tidal stress due to an eccentric orbit may be responsible for creating Ganymede's unique surface features.  相似文献   

The XMM Cluster Survey: forecasting cosmological and cluster scaling-relation parameter constraints     

Martin Sahlén  Pedro T. P. Viana  rew R. Liddle  A. Kathy Romer  Michael Davidson  Mark Hosmer  Ed Lloyd-Davies  Kivanc Sabirli  Chris A. Collins  Peter E. Freeman  Matt Hilton  Ben Hoyle  Scott T. Kay  Robert G. Mann  Nicola Mehrtens  Christopher J. Miller  Robert C. Nichol  S. Adam Stanford  Michael J. West  † 《Monthly notices of the Royal Astronomical Society》2009,397(2):577-607

We forecast the constraints on the values of  σ₈, Ω_m  and cluster scaling-relation parameters which we expect to obtain from the XMM Cluster Survey (XCS). We assume a flat Λ cold dark matter Universe and perform a Monte Carlo Markov Chain analysis of the evolution of the number density of galaxy clusters that takes into account a detailed simulated selection function. Comparing our current observed number of clusters shows good agreement with predictions. We determine the expected degradation of the constraints as a result of self-calibrating the luminosity–temperature relation (with scatter), including temperature measurement errors, and relying on photometric methods for the estimation of galaxy cluster redshifts. We examine the effects of systematic errors in scaling relation and measurement error assumptions. Using only  ( T , z )  self-calibration, we expect to measure Ω_m to ±0.03 (and  Ω_Λ  to the same accuracy assuming flatness), and σ₈ to ±0.05, also constraining the normalization and slope of the luminosity–temperature relation to ±6 and ±13 per cent (at 1σ), respectively, in the process. Self-calibration fails to jointly constrain the scatter and redshift evolution of the luminosity–temperature relation significantly. Additional archival and/or follow-up data will improve on this. We do not expect measurement errors or imperfect knowledge of their distribution to degrade constraints significantly. Scaling-relation systematics can easily lead to cosmological constraints 2σ or more away from the fiducial model. Our treatment is the first exact treatment to this level of detail, and introduces a new 'smoothed ML' (Maximum Likelihood) estimate of expected constraints.  相似文献   

Matter temperature during cosmological recombination     

Douglas Scott  Adam Moss 《Monthly notices of the Royal Astronomical Society》2009,397(1):445-446

The temperature of the atomic matter in the Universe is held to that of the cosmic background radiation until decoupling at   z ∼ 100  . After this, it cools faster than the radiation [  ∝ (1 + z )²  rather than  (1 + z )  ] and would have fallen to about 20 mK today if astrophysical feedback processes had not heated up the interglactic medium. We show how the derivative of the Compton coupling equation helps numerically to follow the decoupling process.  相似文献   

Recent arrival of faint cluster galaxies on the red sequence: luminosity functions from 119 deg² of CFHTLS     

Ting Lu  David G. Gilbank  Michael L. Balogh  Adam Bognat 《Monthly notices of the Royal Astronomical Society》2009,399(4):1858-1876

The global star formation rate has decreased significantly since   z ∼ 1  , for reasons that are not well understood. Red-sequence galaxies, dominating in galaxy clusters, represent the population that have had their star formation shut off, and may therefore be the key to this problem. In this work, we select 127 rich galaxy clusters at  0.17 ≤ z ≤ 0.36  , from 119 deg² of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS) optical imaging data, and construct the r '-band red-sequence luminosity functions (LFs). We show that the faint end of the LF is very sensitive to how red-sequence galaxies are selected, and an optimal way to minimize the contamination from the blue cloud is to mirror galaxies on the redder side of the colour–magnitude relation. The LFs of our sample have a significant inflexion centred at     , suggesting a mixture of two populations. Combining our survey with low-redshift samples constructed from the Sloan Digital Sky Survey, we show that there is no strong evolution of the faint end of the LF (or the red-sequence dwarf-to-giant ratio) over the redshift range  0.2 ≲ z ≲ 0.4  , but from   z ∼ 0.2  to ∼0 the relative number of red-sequence dwarf galaxies has increased by a factor of ∼3, implying a significant build-up of the faint end of the cluster red sequence over the last 2.5 Gyr.  相似文献   

Are ¹²CO lines good indicators of the star formation rate in galaxies?     

E. Bayet  M. Gerin  T. G. Phillips  A. Contursi 《Monthly notices of the Royal Astronomical Society》2009,399(1):264-272

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Probing the microarcsecond structure of the Circinus megamasers through diffractive interstellar scintillation     

J. N. McCallum  S. P. Ellingsen  J. E. J. Lovell  C. J. Phillips  J. E. Reynolds 《Monthly notices of the Royal Astronomical Society》2009,392(4):1339-1362

We have used the Long Baseline Array to observe the Circinus Galaxy H₂O megamasers on two epochs separated by 49 d. The masers were observed to be highly variable, consistent with previous studies. We observe rapid variations in the linewidth and central velocity of the maser's spectral profile. The characteristics of these spectral variations suggest that the masers have an anisotropic structure, with axial ratios ≲5, and display velocity gradients in their microarcsecond structure. We also find a strong anticorrelation between the linewidth and the modulation index of the maser features and a relationship between the modulation index and characteristic time-scale of the variations. These various phenomena can all be simply explained through a model where the rapid variations in the masers are produced by quenched diffractive scintillation in the presence of modest velocity gradients in the maser emission.
The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers.
In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s⁻¹ for the second epoch. We conclude that any future attempts to measure a time delay in this source should utilize intercontinental baselines to increase the chances of success.  相似文献   

Stellar And Galactic Environment survey (SAGE)     

M. A. Barstow  M. P. Kowalski  R. G. Cruddace  K. S. Wood  F. Auchere  N. J. Bannister  M. F. Bode  G. E. Bromage  M. R. Burleigh  A. C. Cameron  A. Cassatella  F. Delmotte  J. G. Doyle  B. Gaensicke  B. Gibson  C. S. Jeffery  C. Jordan  N. Kappelmann  R. Lallement  J. S. Lapington  D. de Martino  S. A. Matthews  M. Orio  E. Pace  I. Pagano  K. J. H. Phillips  M.-F. Ravet  J. H. M. M. Schmitt  B. Y. Welsh  K. Werner  G. Del Zanna 《Experimental Astronomy》2009,23(1):169-191

This paper describes a proposed high resolution soft X-ray and Extreme Ultraviolet spectroscopy mission to carry out a survey of Stellar and Galactic Environments (SAGE). The payload is based on novel diffraction grating technology which has already been proven in a sub-orbital space mission and which is ready to fly on a satellite platform with minimal development. We discuss the goals of a SAGE base-line mission and demonstrate the scientific importance of high resolution spectroscopy in the Extreme Ultraviolet for the study of stars and the local interstellar medium.  相似文献   

Magnesium isotopic composition of igneous rock standards measured by MC-ICP-MS   总被引：2，自引：0，他引：2  

Fang Huang  Justin Glessner  Adam Ianno  Craig Lundstrom  Zhaofeng Zhang 《Chemical Geology》2009,268(1-2):15-23

To provide inter-lab comparison for high-precision Mg isotope analysis, Mg isotope compositions (expressed as δ²⁶Mg relative to DSM-3) for commercially accessible peridotite, basalt, andesite, and granite geo-standards have been measured by multi-collector inductively coupled mass-spectrometry (Nu-Plasma) using sample-standard bracketing method. There is a large tolerance of matrix cations during the measurement of Mg isotopes, as intensity ratios of ²³Na/²⁴Mg and ²⁷Al/²⁴Mg of about 20% only change the δ²⁶Mg by less than 0.1‰, and low ⁵⁵Mn/²⁴Mg (<0.1) and ⁵⁸Ni/²⁴Mg (<0.01) do not cause significant mass bias either. Concentration match between samples and standards within 90% is adequate to obtain accurate isotope analysis, which also mitigates the isobaric interference of ¹²C¹⁴N⁺ on ²⁶Mg. Organic matrix from chemical purification can cause significant analytical errors when the mass of Mg processed is small. The long-term reproducibility of δ²⁶Mg_DSM-3 for samples with relatively higher MgO content is about 0.11‰ (2SD), and granites with lower MgO content is about 0.2‰ (2SD). Although the standards in this study have wide ranges of major element compositions with SiO₂ from 40 to 70 wt.% and MgO from 0.75 to 49.6 wt.%, they exhibit a variation of Mg isotopic compositions with δ²⁶Mg from −0.07 to −0.40‰. δ²⁶Mg do not correlate with SiO₂ or MgO contents, suggesting homogenous Mg isotope compositions in igneous rocks at the level of current precision, relative to low temperature samples including sediments and riverine and sea waters. Our data do not support a non-chondritic Mg isotope composition of the Earth.  相似文献