5632 research outputs found
Sort by
Carbon isotope fractionation by an ancestral rubisco suggests that biological proxies for CO₂ through geologic time should be reevaluated
No full textThe history of Earth’s carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO₂ is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. Therefore, we measured both biomass (ε_p) and enzymatic (ε_(Rubisco) carbon isotope fractionations of a cyanobacterial strain (Synechococcus elongatus PCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to ≫1 Ga. This strain, nicknamed ANC, grows in ambient pCO₂ and displays larger ε_p values than WT, despite having a much smaller ε_(Rubisco) (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Surprisingly, ANC ε_p exceeded ANC ε_(Rubisco) in all conditions tested, contradicting prevailing models of cyanobacterial carbon isotope fractionation. Such models can be rectified by introducing additional isotopic fractionation associated with powered inorganic carbon uptake mechanisms present in Cyanobacteria, but this amendment hinders the ability to accurately estimate historical pCO₂ from geological data. Understanding the evolution of rubisco and the CO₂ concentrating mechanism is therefore critical for interpreting the carbon isotope record, and fluctuations in the record may reflect the evolving efficiency of carbon fixing metabolisms in addition to changes in atmospheric CO₂
Partial Oxidation of Methane Enabled by Decatungstate Photocatalysis Coupled to Free Radical Chemistry
No full textThe decatungstate anion, [W₁₀O₃₂]⁴⁻ or DT, is a useful photocatalyst for organic transformations involving C–H functionalization. Herein, we leverage the unique photoredox properties of DT to generate a chlorine radical from chloride ion for the photochemical partial oxidation of methane. Under optimized conditions, the DT–chloride–iodine ensemble achieves methane to methyl trifluoroacetate conversion with >350 photocatalyst turnovers at ∼60% yield based on methane in trifluoroacetic acid solvent. Methyl trifluoroacetate exhibits excellent stability under reaction conditions with minimal amounts of degradation (<6%) detected after 41 h. Based on density functional theory calculations, we propose a mechanism that involves synergistic relationships among the DT, chloride, and iodine species with the following key features: (1) photoredox electron transfer reaction of DT with Cl⁻ to generate Cl•, (2) reaction of photoexcited DT with methane to generate methyl radicals via net hydrogen atom abstraction, (3) a Cl/I radical-based pathway in which methane is converted to MeTFA, and (4) reoxidation of reduced DT species by dioxygen. This mechanism takes advantage of the unique redox potential of DT and the ability of DT to mediate both electron transfer and hydrogen atom transfer reactions, ultimately generating an efficient pathway for aerobic methane partial oxidation
VISTA: Venus in Situ Transfer and Analysis Mission Concept
No full textAfter Magellan 30 years ago, US Venus exploration has relied on archived data, Earth-based and flyby observations of missions bound elsewhere, and international efforts such as Venus Express (ESA) and Akatsuki (JAXA) until the selection of NASA Discovery missions VERITAS and DAVINCI and participation in ESA's EnVision. These missions will address a significant number of major science questions about the past and present of Venus. Nevertheless, many additional and crucial questions about the history of Venus, including its similarities and differences from Earth, will remain unresolved even after the success of these new missions. Significant divergences in planetary evolution of Venus and Earth require knowledge that is not obtainable by the selected suite of upcoming missions, but can be attained by an innovative approach in the next 20 years if precursor science and technology paves the way.
In the same way that the Mars Exploration Rovers, Mars Science Lab, and Perseverance have provided measurements that unravel the mysteries of Mars, the Venus In Situ Transfer and Analysis mission concept (VISTA) provides an opportunity to obtain measurements that cannot be obtained by a simple, short-term mission to Venus. VISTA would provide detailed knowledge of the surface and atmosphere to better understand the origin and evolution of Venus, its geology and former habitability, and the interaction of its surface with the atmosphere. Since the surface environment of Venus is not conducive to long-term missions, we propose a long duration, sky-borne laboratory in the Venus upper atmosphere, maintained at temperatures and pressures favorable for extended laboratory measurements that shed light on the composition and history of minerals and rocks retrieved from the surface.
VISTA is a concept for a flagship mission to collect samples from multiple locations on the planet surface, and from the Venus atmosphere, and deliver them to a highly-capable, long-lived aerial laboratory for detailed analysis with modern instrumentation. Characterizing the composition, structure, and isotopic ratios of these samples will answer questions of surface composition across multiple geologic provinces. These measurements will help answer questions about the
fundamental branch points in the evolution of Venus. Studies of atmospheric aerosols will support models of cloud formation. The longevity of VISTA will provide further information on
atmospheric circulation, and provide a platform for detecting rare seismic and volcanic events.
Any in situ Venus mission faces significant technical and operational challenges. VISTA shares some challenges with past and current in situ concepts, and presents its own unique challenges (e.g. asset rendezvous, sample processing, and long-lived laboratory platform). This paper describes the architecture and trades of the VISTA mission concept for the aerial laboratory, (multiple) sampling landers, ascent vehicles, and sample retrievers
Energetic Constraints on the Pattern of Changes to the Hydrological Cycle under Global Warming
No full textThe response of zonal-mean precipitation minus evaporation (P − E) to global warming is investigated using a moist energy balance model (MEBM) with a simple Hadley cell parameterization. The MEBM accurately emulates zonal-mean P − E change simulated by a suite of global climate models (GCMs) under greenhouse gas forcing. The MEBM also accounts for most of the intermodel differences in GCM P − E change and better emulates GCM P − E change when compared to the “wet-gets-wetter, dry-gets-drier” thermodynamic mechanism. The intermodel spread in P − E change is attributed to intermodel differences in radiative feedbacks, which account for 60%–70% of the intermodel variance, with smaller contributions from radiative forcing and ocean heat uptake. Isolating the intermodel spread of feedbacks to specific regions shows that tropical feedbacks are the primary source of intermodel spread in zonal-mean P − E change. The ability of the MEBM to emulate GCM P − E change is further investigated using idealized feedback patterns. A less negative and narrowly peaked feedback pattern near the equator results in more atmospheric heating, which strengthens the Hadley cell circulation in the deep tropics through an enhanced poleward heat flux. This pattern also increases gross moist stability, which weakens the subtropical Hadley cell circulation. These two processes in unison increase P − E in the deep tropics, decrease P − E in the subtropics, and narrow the intertropical convergence zone. Additionally, a feedback pattern that produces polar-amplified warming partially reduces the poleward moisture flux by weakening the meridional temperature gradient. It is shown that changes to the Hadley cell circulation and the poleward moisture flux are crucial for understanding the pattern of GCM P − E change under warming
Geological and tectonic significance of rodingite in the Ess ophiolite, Arabian Shield, Saudi Arabia
No full textWe report the first study of rodingite in the Ess ophiolite, exposed at the northeast end of the Yanbu suture of the Arabian Shield. The rodingite forms thin cross-cutting dykes in serpentinite and irregular blocks in ophiolitic mélange. Both dykes and blocks of rodingite are bounded by greenish chloritite blackwall zones and may contain relics of ophiolitic metagabbro. The mineral assemblage in rodingite is (hydro)garnet+vesuvianite+diopside+chlorite+Mn-ilmenite±titanite. The bulk chemical change from gabbro to rodingite involves loss of SiO₂, K₂O, Na₂O, Al₂O₃, Fe₂O₃, and MgO compensated by a strong increase in CaO contents. Petrographic evidence indicates a two-stage process that formed diopside, hydrogarnet and chlorite first, followed by vesuvianite. We show that the major episode of rodingitization was contemporaneous with serpentinization, proceeding from temperatures near 400 °C down to about 250 °C, and predated the obduction of the Ess ophiolite section onto the Arabian Shield. This sequence of events is indicated by formation of chloritite blackwall, by shear planes cutting rodingite dikes, and by fragments of rodingite in fault breccias. We conclude that gabbroic protoliths were transformed to rodingite by Ca-metasomatism related to serpentinization of adjacent ultramafic rocks in an intraoceanic supra-subduction spreading environment. The calcium-rich hydrothermal solutions that drove rodingitization may have been derived from CO₂-bearing slab fluids released by subducted carbonate sediments, modified by reaction with and serpentinization of ultramafic mantle lithologies. The presence of rodingite and associated chloritite blackwall therefore indicates a distinctive feature of serpentinization in suprasubduction zone oceanic lithosphere, namely Ca-rich fluid fluxes during hot serpentinization on the seafloor. Because such features predate and are not associated with obduction, it is likely that subducted suprasubduction zone oceanic lithosphere would contain similar assemblages, able to modulate the transport of volatiles, Ca, and trace elements into the deep mantle
The Use of Narrative in Science and Health Communication: A Scoping Review
No full textBackground. Many people deny science and reject health recommendations despite widely distributed facts and statistics. Didactic science and health communication is often dry, and relies on the false assumption that people make purely evidence-based decisions. Stories can be a powerful teaching tool by capturing attention and evoking emotion.
Objective. We explore the impact and appeal of, and describe best practices for, using narrative (storytelling) versus didactic methods in science and health communication.
Patient involvement. No patients were involved in the review process.
Methods. We searched PubMed and Web of Science for articles either: assessing effectiveness of narrative science/health communication; assessing acceptability of (or preference for) narrative science/health communication; giving advice on how best to use narrative; and/or providing science-based explanations for how/why narrative succeeds.
Results. Narrative science/health communication is effective and appealing for audiences across a variety of topics and mediums, with supporting evidence across fields such as epidemiology, neuroscience, and psychology. Whether narrative or didactic messaging is most effective depends on the topic, audience, and objective, as well as message quality. However, combining narrative with didactic methods is likely to be more effective than using either strategy alone.
Discussion. Narrative science/health communication merits wider implementation and further research. Narrative communication creates openness to information by delaying the formulation of counterarguments.
Practical value. Science and health communicators should collaborate with cultural and storytelling experts, work directly with their target audiences throughout the message development and testing processes, and rely on popular story elements (e.g., first-person point of view, relatable protagonists) to improve the comprehension, engagement, and thoughtful consideration of their intended audience
UV and Lyα Halos of Lyα Emitters across Environments at z = 2.84
No full textWe present UV and Lyα radial surface brightness (SB) profiles of Lyα emitters (LAEs) at z = 2.84 detected with the Hyper Suprime-Cam on the Subaru Telescope. The depth of our data, together with the wide-field coverage including a protocluster, enable us to study the dependence of Lyα halos (LAHs) on various galaxy properties, including Mpc scale environments. UV and Lyα images of 3490 LAEs are extracted, and stacking the images yields SB sensitivity of ~ 1 x 10⁻²⁰ erg s⁻¹ cm⁻² arcsec⁻² in Lyα, reaching the expected level of optically thick gas illuminated by the UV background at z ∼ 3. Fitting of the two-component exponential function gives the scale-lengths of 1.56 ± 0.01 and 10.4 ± 0.3 pkpc. Dividing the sample according to their photometric properties, we find that, while the dependence of halo scale-length on environment outside of the protocluster core is not clear, LAEs in the central regions of protoclusters appear to have very large LAHs, which could be caused by combined effects of source overlapping and diffuse Lyα emission from cool intergalactic gas permeating the forming protocluster core irradiated by active members. For the first time, we identify UV halos around bright LAEs that are probably due to a few lower-mass satellite galaxies. Through comparison with recent numerical simulations, we conclude that, while scattered Lyα photons from the host galaxies are dominant, star formation in satellites evidently contributes to LAHs, and that fluorescent Lyα emission may be boosted within protocluster cores at cosmic noon and/or near bright QSOs
Ab Initio Derivation of Lattice-Gauge-Theory Dynamics for Cold Gases in Optical Lattices
No full textWe introduce a method for quantum simulation of U(1) lattice gauge theories coupled to matter, utilizing alkaline-earth(-like) atoms in state-dependent optical lattices. The proposal enables the study of both gauge and fermionic matter fields without integrating out one of them in one and two dimensions. We focus on a realistic and robust implementation that utilizes the long-lived metastable clock state available in alkaline-earth(-like) atomic species. Starting from an ab initio modeling of the experimental setting, we systematically carry out a derivation of the target U(1) gauge theory. This approach allows us to identify and address conceptual and practical challenges for the implementation of lattice gauge theories that—while pivotal for a successful implementation—have never been rigorously addressed in the literature: those include the specific engineering of lattice potentials to achieve the desired structure of Wannier functions and the subtleties involved in realizing the proper separation of energy scales to enable gauge-invariant dynamics. We discuss realistic experiments that can be carried out within such a platform using the fermionic isotope ¹⁷³Yb, addressing via simulations all key sources of imperfections, and provide concrete parameter estimates for relevant energy scales in both one- and two-dimensional settings
Near-Optimal Distributed Linear-Quadratic Regulator for Networked Systems
No full textThis paper studies the trade-off between the degree of decentralization and the performance of a distributed controller in a linear-quadratic control setting. We study a system of interconnected agents over a graph and a distributed controller, called κ-distributed control, which lets the agents make control decisions based on the state information within distance κ on the underlying graph. This controller can tune its degree of decentralization using the parameter κ and thus allows a characterization of the relationship between decentralization and performance. We show that under mild assumptions, including stabilizability, detectability, and a subexponentially growing graph condition, the performance difference between κ-distributed control and centralized optimal control becomes exponentially small in κ. This result reveals that distributed control can achieve near-optimal performance with a moderate degree of decentralization, and thus it is an effective controller architecture for large-scale networked systems
One-level density estimates for Dirichlet L-functions with extended support
No full textWe estimate the 1-level density of low-lying zeros of L(s,χ) with χ ranging over primitive Dirichlet characters of conductor in [Q/2,Q] and for test functions whose Fourier transform is supported in [−2−50/1093,2+50/1093]. Previously any extension of the support past the range [−2,2] was only known conditionally on deep conjectures about the distribution of primes in arithmetic progressions, beyond the reach of the Generalized Riemann Hypothesis (e.g Montgomery's conjecture). Our work provides the first example of a family of L-functions in which the support is unconditionally extended past the "trivial range" that follows from a simple application of the underlying trace formula (in this case orthogonality of characters). We also highlight consequences for non-vanishing of L(s,χ)