866 research outputs found
Theoretical feedbacks between neoproterozoic glaciations and eukaryotic evolution
Physiological consequences of transitions in the level of selection explain the existence of Neoproterozoic lichen symbioses. Between-individual differentiation within groups causes physiological buffering, giving altruists higher fitness than cheaters in variable environments. Altruists increase from rarity. Reciprocal feedback between a genotype frequency and an abiotic state may be equivalent to kin selection.
At contemporary degassing rate, biotic silicate weathering enhancement 10 – 20 times greater than present would have been required for a biologically-triggered snowball Earth. However, biotic enhancement could have compensated by declining abiotic silicate weathering rate at low temperatures – sufficiently to accentuate a tectonically-driven glaciation.
Marine carbonate speciation causes a slushball ocean to become a net CO2 source at large reservoir sizes, making glacial duration shorter than a hard snowball by a factor of the atmospheric CO2Â degassing fraction. Equilibrated oceans require a carbonate weathering:burial ratio Wcarb : Bcarb > 1 for deglaciation by CO2(g) increase.
If adequate biotic silicate weathering enhancement rapidly occurs after deglaciation, subsequent reglaciation will occur within 3 – 5Myrs. The contrast with the ~50Ma separating Neoproterozoic events invokes the elapse of a coevolutionary “succession time”, before which biotically-triggered reglaciation is improbable. Low post-glacial silicate weathering is consistent with Si-depleted cap carbonates. Phanerozoic reduction in glacial susceptibility resulted from physiological evolution of biotic weathering and/or calcification.
Emergence of terminal cellular differentiation in Ediacara/Metazoa required an environmental context E, provided by Neoproterozoic glaciations, in which the high fitness cost was adaptive. Restriction of the biosphere to refugia experiencing extreme, repeated founder effects raised kin selection for altruism. Between-group isolation limited cheat migration, accentuating group viability selection for altruism. Both processes occurred globally, over multi-million year timescales; explaining subsequent proliferation of Ediacaran macrobiota
Improved Semileptonic Form Factor Calculations in Lattice QCD
We investigate the computational efficiency of two stochastic based
alternatives to the Sequential Propagator Method used in Lattice QCD
calculations of heavy-light semileptonic form factors. In the first method, we
replace the sequential propagator, which couples the calculation of two of the
three propagators required for the calculation, with a stochastic propagator so
that the calculations of all three propagators are independent. This method is
more flexible than the Sequential Propagator Method but introduces stochastic
noise. We study the noise to determine when this method becomes competitive
with the Sequential Propagator Method, and find that for any practical
calculation it is competitive with or superior to the Sequential Propagator
Method. We also examine a second stochastic method, the so-called ``one-end
trick", concluding it is relatively inefficient in this context. The
investigation is carried out on two gauge field ensembles, using the
non-perturbatively improved Wilson-Sheikholeslami-Wohlert action with N_f=2
mass-degenerate sea quarks. The two ensembles have similar lattice spacings but
different sea quark masses. We use the first stochastic method to extract
-improved, matched lattice results for the semileptonic form
factors on the ensemble with lighter sea quarks, extracting f_+(0)
Evaluation of Tropospheric Water Vapor Simulations from the Atmospheric Model Intercomparison Project
Probing the Evolution of the Galaxy Interaction/Merger Rate Using Collisional Ring Galaxies
We present the results from our program to determine the evolution of the
galaxy interaction/merger rate with redshift using the unique star-forming
characteristics of collisional ring galaxies. We have identified 25 distant
collisional ring galaxy candidates (CRGCs) in a total of 162 deep Hubble Space
Telescope Wide Field/Planetary Camera-2 images obtained from the HST Archives.
Based on measured and estimated redshifts, these 25 CRGCs all lie in the
redshift interval of 0.1 < z < 1. Using the local collisional ring galaxy
volume density and the new ``standard'' cosmology, we find that in order to
account for the number of identified CRGCs in our surveyed fields, the galaxy
interaction/merger rate, parameterized as (1 + z)^m, must increase steeply with
redshift.We determine a minimum value of m = 5.2 0.7, though m could be
as high as 7 or 8. We can rule out a non-evolving (m = 0) and weakly evolving
(m = 1-2) galaxy interaction/merger rate at greater than the 4 sigma level of
confidence.Comment: Accepted in the Astrophysical Journal (11 pages, 4 figures). Higher
resolution version of the figures is available at
http://www.astro.cornell.edu/~vassilis/papers
Modeling the source of GW150914 with targeted numerical-relativity simulations
In fall of 2015, the two LIGO detectors measured the gravitational wave
signal GW150914, which originated from a pair of merging black holes. In the
final 0.2 seconds (about 8 gravitational-wave cycles) before the amplitude
reached its maximum, the observed signal swept up in amplitude and frequency,
from 35 Hz to 150 Hz. The theoretical gravitational-wave signal for merging
black holes, as predicted by general relativity, can be computed only by full
numerical relativity, because analytic approximations fail near the time of
merger. Moreover, the nearly-equal masses, moderate spins, and small number of
orbits of GW150914 are especially straightforward and efficient to simulate
with modern numerical-relativity codes. In this paper, we report the modeling
of GW150914 with numerical-relativity simulations, using black-hole masses and
spins consistent with those inferred from LIGO's measurement. In particular, we
employ two independent numerical-relativity codes that use completely different
analytical and numerical methods to model the same merging black holes and to
compute the emitted gravitational waveform; we find excellent agreement between
the waveforms produced by the two independent codes. These results demonstrate
the validity, impact, and potential of current and future studies using
rapid-response, targeted numerical-relativity simulations for better
understanding gravitational-wave observations.Comment: 11 pages, 3 figures, submitted to Classical and Quantum Gravit
Pincer-plus-one ligands in self-assembly with palladium(ii): a molecular square and a molecular tetrahedron
The combination of a palladium(ii) precursor with a diimine-phenol ligand and an oxidant (H2O2 or O2) under different conditions has, serendipitously, given both a molecular square and a molecular tetrahedron by self-assembly of building blocks comprising palladium(ii) centres coordinated to the oxidised forms of the ligand
Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry
A simple and accurate low-blank method has been developed for the analysis of total dissolved copper, cadmium, lead, and iron in a small volume (1.3–1.5 mL per element) of seawater. Pre-concentration and salt-separation of a stable isotope spiked sample are achieved by single batch extraction onto nitrilotriacetate (NTA)-type Superflow® chelating resin beads (100–2400 beads depending on the element). Metals are released into 0.1–0.5 M HNO[subscript 3], and trace metal isotope ratios are determined by ICPMS. The benefit of this method compared to our previous Mg(OH)2 coprecipitation method is that the final matrix is very dilute so cone-clogging and matrix sensitivity suppression are minimal, while still retaining the high accuracy of the isotope dilution technique. Recovery efficiencies are sensitive to sample pH, number of resin beads added, and the length of time allowed for sample–resin binding and elution; these factors are optimized for each element to yield the highest recovery. The method has a low procedural blank and high sensitivity sufficient for the analysis of pM–nM open-ocean trace metal concentrations. Application of this method to samples from the Bermuda Atlantic Time-Series Study station provides oceanographically consistent Cu, Cd, Pb, and Fe profiles that are in good agreement with other reliable data for this site. In addition, the method can potentially be modified for the simultaneous analysis of multiple elements, which will be beneficial for the analysis of large number of samples.National Science Foundation (U.S.) (NSF frant OCE-0751409)Kuwait Foundation for the Advancement of SciencesNational Science Foundation (U.S.) (NSF award OCE-0751409)National Science Foundation (U.S.) (NSF award OCE-0926197
A selected ion flow tube study of the reactions of gas-phase cations with PSCl3
A selected ion flow tube was used to investigate the positive ion chemistry of thiophosphoryl chloride, PSCl. Rate coefficients and ion product branching ratios have been determined at room temperature for reactions with nineteen cations ; HO, CF, CF, NO, NO, SF, SF, CF, O, HO, NO, O, CO, CO, N, N, Ar, F and Ne (in order of increasing recombination energy). Complementary data described in the previous paper have been obtained for this molecule via the observation of threshold photoelectron photoion coincidences. For ions whose recombination energies are in the range 10-22 eV, comparisons are made between the product ion branching rations of PSCl from photoionisation and from ion-molecule reactions. In most instances, the data from the two experiments are well correlated, suggesting that long-range charge transfer is the dominant mechanism for these ion-molecule reactions ; the agreement is particularly good for the atomic ions Ar, F and Ne. Some reactions (e.g. O + PSCl), however, exhibit significant differences; short-range charge transfer must then be occurring following the formation of an ion-molecule complex. For ions whose recombination energies are less than 10 eV (i.e. HO, CF, CF and NO), reactions can only occur via a chemical process in which bonds are broken and formed, because the recombination energy of the cation is less than the ionisation energy of PSCl
The regulation of oxygen to low concentrations in marine oxygen-minimum zones
The Bay of Bengal hosts persistent, measurable, but sub-micromolar, concentrations of oxygen in its oxygen-minimum zone (OMZ). Such low-oxygen conditions are not necessarily rare in the global ocean and seem also to characterize the OMZ of the Pescadero Basin in the Gulf of California, as well as the outer edges of otherwise anoxic OMZs, such as can be found, for example, in the Eastern Tropical North Pacific. We show here that biological controls on oxygen consumption are required to allow the semistable persistence of low-oxygen conditions in OMZ settings; otherwise, only small changes in physical mixing or rates of primary production would drive the OMZ between anoxic and oxic states with potentially large swings in oxygen concentration. We propose that two controls are active: an oxygen-dependent control on oxygen respiration and an oxygen inhibition of denitrification. These controls, working alone and together, can generate low-oxygen concentrations over a wide variability in ocean mixing parameters. More broadly, we discuss the oxygen regulation of organic matter cycling and N2 production in OMZ settings. Modern biogeochemical models of nitrogen and oxygen cycling in OMZ settings do contain some of the parameterizations that we explore here. However, these models have not been applied to understanding the persistence of low, but measurable, concentrations of oxygen in settings like the Bay of Bengal, nor have they been applied to understanding what biological/physical processes control the transition from a weakly oxygenated state to a “functionally” anoxic state with implications for nitrogen cycling. Therefore, we believe that the approach here illuminates the relationship between oxygen and the biogeochemical cycling of carbon and nitrogen in settings like the Bay of Bengal. Furthermore, we believe that our results could further inform large-scale ocean models seeking to explore how global warming might influence the spread of low-oxygen waters, influencing the cycles of oxygen, carbon, and nitrogen in OMZ settings
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