237 research outputs found
Depositional Environment of CoralâRudist Associations in the Upper Cretaceous Cardenas Formation (Central Mexico)
In the Cardenas Formation (central Mexico), a 175 m thick sedimentary sequence of Maastrichtian age was analyzed with respect to its palaeontology and sedimentology. A wide variety of lithological and palaeontological features characterize this sequence comprising unfossiliferous and fossil-bearing sand- and siltstones, and diverse rudist and coralârudist associations in carbonate or mixed carbonate/clastic lithologies. A total of 24 rudist and coralârudist associations are exposed in the investigated section, which are grouped into 5 limestone units. Radiolitid assemblages, coralârudist reefs, coral-domiÂnaÂted reefs, and hippuritid-dominated reefs are present. The stacking pattern of these reef intervals indicates a general transgressive trend through the entire section. Smaller-scale facies trends could be distinguished within each limestone unit, comprising deepening-upward sequences, defined by a shorefaceâcalcareous algaeâradiolitidâmarl facies transition, and shallowing-upward sequences defiÂned by a hippuritidâactaeonellidâcoral/rudist facies transition. This cyclic sedimentation pattern is obscured by an episodic input of clastic sediments derived from the uplifting Sierra Madre Oriental, which in turn triggered either the development or decline of reefs
A new rhizangiid genus from the Miocene of North America (Sclerangia n. gen.; Florida, USA)
The colonial rhizangiid genus Sclerangia n. gen. is described from the Miocene of the USA (Chipola Formation, Florida). The new genus is characterized by plocoid to subcerioid polyp integration, cylindrical to subcylindrical or tympanoid corallites, and an endotheca that is generally absent or made of a very small number of vesicular dissepiments. Compared to other rhizangiid genera, the new taxon is characterized by rather weakly dentate septa. In addition, the new genus differs from all the rhizangiid genera by its consistent encrustation of dead gastropod shells that are inhabited by sipunculid worms
Study of temperature dependent atomic correlations in MgB
We have studied the evolution with temperature of the local as well as the
average crystal structure of MgB using the real-space atomic pair
distribution function (PDF) measured by high resolution neutron powder
diffraction. We have investigated the correlations of the B-B and B-Mg nearest
neighbor pair motion by comparing, in the wide temperature range from T=10 K up
to T=600 K, the mean-square displacements (MSD) of single atoms with the
mean-square relative displacements (MSRD) obtained from the PDF peak
linewidths. The results show that the single atom B and Mg vibrations are
mostly decoupled from each other, with a small predominance of positive (in
phase) correlation factor for both the B-B and B-Mg pairs. The small positive
correlation is almost temperature independent, in contrast with our theoretical
calculations; this can be a direct consequence of the strong decay processes of
the anharmonic phonons
DNA barcoding reveals the coral âlaboratory-ratâ, Stylophora pistillata encompasses multiple identities
Stylophora pistillata is a widely used coral âlab-ratâ species with highly variable morphology and a broad biogeographic range (Red Sea to western central Pacific). Here we show, by analysing Cytochorme Oxidase I sequences, from 241 samples across this range, that this taxon in fact comprises four deeply divergent clades corresponding to the Pacific-Western Australia, Chagos-Madagascar-South Africa, Gulf of Aden-Zanzibar-Madagascar, and Red Sea-Persian/Arabian Gulf-Kenya. On the basis of the fossil record of Stylophora, these four clades diverged from one another 51.5-29.6â
Mya, i.e., long before the closure of the Tethyan connection between the tropical Indo-West Pacific and Atlantic in the early Miocene (16â24â
Mya) and should be recognised as four distinct species. These findings have implications for comparative ecological and/or physiological studies carried out using Stylophora pistillata as a model species, and highlight the fact that phenotypic plasticity, thought to be common in scleractinian corals, can mask significant genetic variation
Spin in Density-Functional Theory
The accurate description of open-shell molecules, in particular of transition
metal complexes and clusters, is still an important challenge for quantum
chemistry. While density-functional theory (DFT) is widely applied in this
area, the sometimes severe limitations of its currently available approximate
realizations often preclude its application as a predictive theory. Here, we
review the foundations of DFT applied to open-shell systems, both within the
nonrelativistic and the relativistic framework. In particular, we provide an
in-depth discussion of the exact theory, with a focus on the role of the spin
density and possibilities for targeting specific spin states. It turns out that
different options exist for setting up Kohn-Sham DFT schemes for open-shell
systems, which imply different definitions of the exchange-correlation energy
functional and lead to different exact conditions on this functional. Finally,
we suggest some possible directions for future developments
Pharmacodynamics of bisphosphonates in arthritis.
Inflammatory arthritis is a group of autoimmune diseases characterized by chronic inflammation of the joints. Rheumatoid arthritis, the most common form of arthritis, is associated with local joint destruction and systemic bone loss. Osteoclasts, the only cells of the body able to resorbe bone, are key players in these two types of bone loss. Bisphosphonates are analogs of pyrophosphate that inhibit osteoclast action and bone resorption. They are indicated in pathology associated with excess resorption. Besides their effect on bone they also exhibit extra-osseous properties, acting on tumor cells, inflammation and angiogenesis. As a result, they have been trialed in the context of arthritis. It is now clear that they do not have any significant direct effect on disease activity or pain. If their indication in the prevention of glucocorticoid-induced osteoporosis is clear, any beneficial effects on bone erosions are still controversial but interesting preliminary results warrant further investigations
Low Luminosity Type II Supernovae: Spectroscopic and Photometric Evolution
In this paper we present spectroscopic and photometric observations for four
core collapse supernovae (SNe), namely SNe 1994N, 1999br, 1999eu and 2001dc.
Together with SN 1997D, we show that they form a group of exceptionally
low-luminosity events. These SNe have narrow spectral lines (indicating low
expansion velocities) and low luminosities at every phase (significantly lower
than those of typical core-collapse supernovae). The very low luminosity during
the ^{56}Co radioactive decay tail indicates that the mass of ^{56}Ni ejected
during the explosion is much smaller (M(Ni) ~ 2-8 x 10^{-3} Mo) than the
average (M(Ni) ~ 6-10 x 10^{-2} Mo). Two supernovae of this group (SN 1999br
and SN 2001dc) were discovered very close to the explosion epoch, allowing us
to determine the lengths of their plateaux (~ 100 days) as well as
establishiing the explosion epochs of the other, less-completely observed SNe.
It is likely that this group of SNe represent the extreme low-luminosity tail
of a single continuous distribution of SN II-P events. Their kinetic energy is
also exceptionally low. Although an origin from low mass progenitors has also
been proposed for low-luminosity core-collapse SNe, recent work provides
evidence in favour of the high mass progenitor scenario. The incidence of these
low--luminosity SNe could be as high as 4-5 of all type II SNe.Comment: 23 pages, 26 figures, accepted for publication in MNRA
- âŠ