Showing 451 - 460 of 733 Items
An improved comparison of atmospheric Ar/N2 time series and paired ocean-atmosphere model predictions
Date: 2008-11-16
Creator: Nicolas Cassar, Galen A. McKinley, Michael L. Bender, Robert Mika, Mark, Battle
Access: Open access
- Ar/N2 variations in the atmosphere reflect ocean heat fluxes, air-sea gas exchange, and atmospheric dynamics. Here atmospheric Ar/N2 time series are compared to paired ocean-atmosphere model predictions. Agreement between Ar/N2 observations and simulations has improved in comparison to a previous study because of longer time series and the introduction of automated samplers at several of the atmospheric stations, as well as the refinement of the paired ocean-atmosphere models by inclusion of Ar and N2 as active tracers in the ocean component. Although analytical uncertainties and collection artifacts are likely to be mainly responsible for observed Ar/N2 outliers, air parcel back-trajectory analysis suggests that some of the variability in Ar/N2 measurements could be due to the low-altitude history of the air mass collected and, by extension, the local oceanic Ar/N2 signal. Although the simulated climatological seasonal cycle can currently be evaluated with Ar/N2 observations, longer time series and additional improvements in the signal-to-noise ratio will be required to test other model predictions such as interannual variability, latitudinal gradients, and the secular increase in atmospheric Ar/N2 expected to result from ocean warming. Copyright 2008 by the American Geophysical Union.
Effects of stochasticity on the length and behaviour of ecological transients
Date: 2021-07-01
Creator: Alan Hastings, Karen C. Abbott, Kim Cuddington, Tessa B. Francis, Ying Cheng, Lai, Andrew Morozov
Access: Open access
- There is a growing recognition that ecological systems can spend extended periods of time far away from an asymptotic state, and that ecological understanding will therefore require a deeper appreciation for how long ecological transients arise. Recent work has defined classes of deterministic mechanisms that can lead to long transients. Given the ubiquity of stochasticity in ecological systems, a similar systematic treatment of transients that includes the influence of stochasticity is important. Stochasticity can of course promote the appearance of transient dynamics by preventing systems from settling permanently near their asymptotic state, but stochasticity also interacts with deterministic features to create qualitatively new dynamics. As such, stochasticity may shorten, extend or fundamentally change a system's transient dynamics. Here, we describe a general framework that is developing for understanding the range of possible outcomes when random processes impact the dynamics of ecological systems over realistic time scales. We emphasize that we can understand the ways in which stochasticity can either extend or reduce the lifetime of transients by studying the interactions between the stochastic and deterministic processes present, and we summarize both the current state of knowledge and avenues for future advances.
Species identification based on a semi-diagnostic marker: Evaluation of a simple conchological test for distinguishing blue mussels Mytilus edulis L. And M. trossulus Gould
Date: 2021-07-01
Creator: Vadim Khaitov, Julia Marchenko, Marina Katolikova, Risto VƤinƶlƤ, Sarah E., Kingston, David B. Carlon, Michael Gantsevich, Petr Strelkov
Access: Open access
- Cryptic and hybridizing species may lack diagnostic taxonomic characters leaving researchers with semi-diagnostic ones. Identification based on such characters is probabilistic, the probability of correct identification depending on the species composition in a mixed population. Here we test the possibilities of applying a semi-diagnostic conchological character for distinguishing two cryptic species of blue mussels, Mytilus edulis and M. trossulus. These ecologically, stratigraphically and economically important molluscs co-occur and hybridize in many areas of the North Atlantic and the neighboring Arctic. Any cues for distinguishing them in sympatry without genotyping would save much research effort. Recently these species have been shown to statistically differ in the White Sea, where a simple character of the shell was used to distinguish two mussel morphotypes. In this paper, we analyzed the associations between morphotypes and species-specific genotypes based on an abundant material from the waters of the Kola Peninsula (White Sea, Barents Sea) and a more limited material from Norway, the Baltic Sea, Scotland and the Gulf of Maine. The performance of the āmorphotype testā for species identification was formally evaluated using approaches from evidence-based medicine. Interspecific differences in the morphotype frequencies were ubiquitous and unidirectional, but their scale varied geographically (from 75% in the White Sea to 15% in the Baltic Sea). In addition, salinity-related variation of this character within M. edulis was revealed in the Arctic Barents Sea. For every studied region, we established relationships between the proportions of the morphotypes in the populations as well as between the proportions of the morphotypes in samples and the probabilities of mussels of different morphotypes being M. trossulus and M. edulis. We provide recommendations for the application of the morphotype test to mussels from unstudied contact zones and note that they may apply equally well to other taxa identified by semi-diagnostic traits.
Atmospheric measurements of the terrestrial O2 : CO2 exchange ratio of a midlatitude forest
Date: 2019-07-10
Creator: Mark O. Battle, J. William Munger, Margaret Conley, Eric Sofen, Rebecca, Perry, Ryan Hart, Zane Davis, Jacob Scheckman, Jayme Woogerd, Karina Graeter, Samuel Seekins, Sasha David, John Carpenter
Access: Open access
- Measurements of atmospheric O2 have been used to quantify large-scale fluxes of carbon between the oceans, atmosphere and land since 1992 (Keeling and Shertz, 1992). With time, datasets have grown and estimates of fluxes have become more precise, but a key uncertainty in these calculations is the exchange ratio of O2 and CO2 associated with the net land carbon sink (B). We present measurements of atmospheric O2 and CO2 collected over a 6-year period from a mixed deciduous forest in central Massachusetts, USA (42.537 N, 72.171 W). Using a differential fuel-cellbased instrument for O2 and a nondispersive infrared analyzer for CO2, we analyzed airstreams collected within and 5m above the forest canopy. Averaged over the entire period of record, we find these two species covary with a slope of -1:081Ā±0:007 mol of O2 per mole of CO2 (the mean and standard error of 6 h periods). If we limit the data to values collected on summer days within the canopy, the slope is -1:03Ā±0:01. These are the conditions in which biotic influences are most likely to dominate. This result is significantly different from the value of -1.1 widely used in O2-based calculations of the global carbon budget, suggesting the need for a deeper understanding of the exchange ratios of the various fluxes and pools comprising the net sink.
Inclusive decay BāĪ·X
Date: 1996-01-01
Creator: Y. Kubota, M. Lattery, M. Momayezi, J. K. Nelson, S., Patton, R. Poling, V. Savinov, S. Schrenk, R. Wang, M. S. Alam, I. J. Kim, Z. Ling, A. H. Mahmood, J. J. OāNeill, H. Severini, C. R. Sun, F. Wappler, G. Crawford, C. M. Daubenmier, R. Fulton, D. Fujino, K. K. Gan, K. Honscheid, H. Kagan, R. Kass, J. Lee, M. Sung, C. White, A. Wolf, M. M. Zoeller, F. Butler
Access: Open access
- Using data samples taken at the Ī„(4S) resonance and nearby continuum e+e- annihilation with the CLEO-II detector at CESR, we have measured the inclusive branching fraction B(BāĪ·X)=(17.6Ā±1.1Ā±1.2)%, and the momentum distribution of the Ī· mesons from B meson decay. The Ī· yield cannot be explained as arising solely from the decay of intermediate charmed mesons. Ā© 1996 The American Physical Society.
Exclusive hadronic B decays to charm and charmonium final states
Date: 1994-01-01
Creator: M. S. Alam, I. J. Kim, B. Nemati, J. J. ONeill, H., Severini, C. R. Sun, M. M. Zoeller, G. Crawford, C. M. Daubenmier, R. Fulton, D. Fujino, K. K. Gan, K. Honscheid, H. Kagan, R. Kass, J. Lee, R. Malchow, F. Morrow, Y. Skovpen, M. Sung, C. White, F. Butler, X. Fu, G. Kalbfleisch, W. R. Ross, P. Skubic, J. Snow, P. L. Wang, M. Wood, D. N. Brown, J. Fast
Access: Open access
- We have fully reconstructed decays of both BĀ»0 and B- mesons into final states containing either D, D*, D**, , or c1 mesons. This allows us to obtain new results on many physics topics including branching ratios, tests of the factorization hypothesis, color suppression, resonant substructure, and the B - BĀ»0 mass difference. Ā© 1994 The American Physical Society.
Thermal fractionation of air in polar firn by seasonal temperature gradients
Date: 2001-07-01
Creator: Jeffrey P. Severinghaus, Alexi Grachev, Mark Battle
Access: Open access
- Air withdrawn from the top 5-15 m of the polar snowpack (firn) shows anomalous enrichment of heavy gases during summer, including inert gases. Following earlier work, we ascribe this to thermal diffusion, the tendency of a gas mixture to separate in a temperature gradient, with heavier molecules migrating toward colder regions. Summer warmth creates a temperature gradient in the top few meters of the firn due to the thermal inertia of the underlying firn and causes gas fractionation by thermal diffusion. Here we explore and quantify this process further in order to (1) correct for bias caused by thermal diffusion in firn air and ice core air isotope records, (2) help calibrate a new technique for measuring temperature change in ice core gas records based on thermal diffusion [Severinghaus et al., 1998], and (3) address whether air in polar snow convects during winter and, if so, whether it creates a rectification of seasonality that could bias the ice core record. We sampled air at 2-m-depth intervals from the top 15 m of the firn at two Antarctic sites, Siple Dome and South Pole, including a winter sampling at the pole. We analyzed 15N/14N, 40Ar/36Ar, 40Ar/38Ar, 18O/16O of O2, O2/N2, 84Kr/36Ar, and 132Xe/36Ar. The results show the expected pattern of fractionation and match a gas diffusion model based on first principles to within 30%. Although absolute values of thermal diffusion sensitivities cannot be determined from the data with precision, relative values of different gas pairs may. At Siple Dome, Ī“40Ar/4 is 66 Ā± 2% as sensitive to thermal diffusion as Ī“15N, in agreement with laboratory calibration; Ī“18O/2 is 83 Ā± 3%, and Ī“84Kr/48 is 33 Ā± 3% as sensitive as Ī“15N. The corresponding figures for summer South Pole are 64 Ā± 2%, 81 Ā± 3%, and 34 Ā± 3%. Accounting for atmospheric change, the figure for Ī“O2/N2/4 is 90 Ā± 3% at Siple Dome. Winter South Pole shows a strong depletion of heavy gases as expected. However, the data do not fit the model well in the deeper part of the profile and yield a systematic drift with depth in relative thermal diffusion sensitivities (except for Kr, constant at 34 Ā± 4%), suggesting the action of some other process that is not currently understood. No evidence for wintertime convection or a rectifier effect is seen.
Quantum lifetime of two-dimensional holes
Date: 2007-08-01
Creator: J. P. Eisenstein, D. Syphers, L. N. Pfeiffer, K. W. West
Access: Open access
- The quantum lifetime of two-dimensional holes in a GaAs/AlGaAs double quantum well is determined via tunneling spectroscopy. At low temperatures the lifetime is limited by impurity scattering but at higher temperatures hole-hole Coulomb scattering dominates. Our results are consistent with the Fermi liquid theory, at least up to rs = 11. At the highest temperatures the measured width of the hole spectral function becomes comparable to the Fermi energy. A new, tunneling-spectroscopic method for determining the in-plane effective mass of the holes is also demonstrated. Ā© 2007 Elsevier Ltd. All rights reserved.
Superconformal coset equivalence from level-rank duality
Date: 1997-11-17
Creator: Stephen G. Naculich, Howard J. Schnitzer
Access: Open access
- We construct a one-to-one map between the primary fields of the N = 2 superconformal Kazama-Suzuki models G(m, n, k) and G(k, n, m) based on complex Grassmannian cosets, using level-rank duality of Wess-Zumino-Witten models. We then show that conformal weights, superconformal U(1) charges, modular transformation matrices, and fusion rules are preserved under this map, providing strong evidence for the equivalence of these coset models. Ā© 1997 Elsevier Science B.V.
The guanine nucleotide exchange factor trio mediates axonal development in the Drosophila embryo
Date: 2000-01-01
Creator: Jack Bateman, Huidy Shu, David Van Vactor
Access: Open access
- Recent analysis of Rho subfamily GTPases in Drosophila revealed roles for Rac and Cdc42 during axonogenesis. Here, we describe the identification and characterization of the Drosophila counterpart of Trio, a guanine nucleotide exchange factor (GEF) that associates with the receptor phosphatase LAR and regulates GTPase activation in vertebrate cells. Mutants deficient in trio activity display defects in both central and peripheral axon pathways reminiscent of pheno-types observed in embryos deficient in small GTPase function. Double mutant analysis shows that trio interacts with Rac in a dose-sensitive manner but not with Rho. Moreover, reduction of trio activity potentiates the phenotype of mutations in the LAR homolog Dlar, suggesting that these proteins collaborate in orchestrating the cytoskeletal events that underlie normal axonogenesis.