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A Double-Edged Sword: The Role of Sulfate in Anoxic Marine Phosphorus Cycling Through Earth History
No full textModern anoxic marine sediments release phosphorus (P) to seawater, driving feedbacks at multiple timescales. On sub-Myr timescales, anoxic P regeneration amplifies ocean deoxygenation; on multi-Myr timescales, it stabilizes atmospheric O₂. Some authors have extended this thinking to the Precambrian: by analogy, widespread ocean anoxia would imply extensive P regeneration from sediments. However, this neglects the role of sulfate in P regeneration. While abundant in seawater today, sulfate was scarce in the Precambrian. Here a simple model is used to isolate the role of sulfate in anoxic P cycling. The model reproduces known feedbacks driven by anoxic P regeneration in a high-sulfate world, but shows muted P regeneration in low-sulfate, anoxic sediments. The transition between “limited” and “enhanced” P regeneration is thus controlled by sulfate. As a result, sulfate is a double-edged sword wielded by the anoxic P cycle: it can amplify oxygenation or deoxygenation depending on environmental context
Neuromorphic Engineering: In Memory of Misha Mahowald
No full textWe review the coevolution of hardware and software dedicated to neuromorphic systems. From modest beginnings, these disciplines have become central to the larger field of computation. In the process, their biological foundations become more relevant, and their realizations increasingly overlap. We identify opportunities for significant steps forward in both the near and more distant future
The case for the angrite parent body as the archetypal first-generation planetesimal: Large, reduced and Mg-enriched
No full textAngrites are silica-undersaturated achondrites formed very early in the history of the Solar System, and the most volatile-depleted known meteorites. As such, the study of angrites can provide critical insights into the early stages of planetary formation, melting and differentiation. Yet, understanding the origins of angrites and the nature of their parent body has long been hindered by the initially small number of specimens available. Here, we leverage (i) the rapidly growing number of known angrites, and (ii) equilibrium crystallization experiments at various pressure, temperature and oxygen fugacity conditions (P-T-fO₂), to revisit the petrogenesis of angrites and constrain key features of the angrite parent body (APB), such as its composition and size.
We observe that quenched (i.e., volcanic) angrites define two compositional groups, which we show are readily related by fractional crystallization. This crystallization trend converges on an olivine-clinopyroxene-plagioclase (Ol + Cpx + Plag) multiple saturation boundary, whose composition is sampled by D’Orbigny, Sahara 99555 and NWA 1296. Using the observation that some quenched specimens represent primitive angritic melts, we derive a self-consistent bulk composition for the APB. We find that this composition matches the proposed Mg/Si ratio of 1.3 derived from the angrite δ³⁰Si values, and yields a core size (18 ± 6 wt%) in agreement with the siderophile elements depletion in the APB mantle. Our results support a primary control of nebular fractionation (i.e., partial condensation) on the composition of the APB. To establish the liquid phase equilibria of angrites, a series of 1 atmosphere and high-pressure crystallization experiments (piston cylinder and internally heated pressure vessel) were performed on a synthetic powder of D’Orbigny. The results suggest that the APB was a large (possibly Moon-sized) body, formed from materials condensed at relatively high-temperature (∼1300–1400 K), and whose fO₂ changed from mildly reducing (∼IW-1.5) to relatively oxidizing (∼IW+1 ± 1) in the ∼ 3 Myr between its core formation and the crystallization of D’Orbigny-like (Group 2) angrites. Based on its timing of accretion and differentiation, its composition, redox, and size, we argue that the APB represents the archetype of the first-generation of refractory-enriched planetesimals and embryos formed in the innermost part of the inner Solar System (<1 AU), and which accreted in the telluric planets
Demographic predictors of treatment and complications for adult spinal deformity: An analysis of the national inpatient sample
No full textPurpose: To examine the role of demographics on surgical management and inpatient complications in patients with spinal deformity between 2010 and 2014 via retrospective analysis.
Methods: Data were obtained from the National Inpatient Sample (NIS). International Classification of Diseases 9th revision codes were used to identify patients with a primary diagnosis of adult spinal deformity (ASD). Multivariable Poisson regression analyses were used to determine whether any individual demographic variables were predictive of surgical management, surgical complexity, postoperative complications and revision operations.
Results: 17,433 patients were identified for analysis. Surgical intervention was performed for 94.5% of patients with a primary diagnosis of ASD. Patients at urban teaching hospitals were the most likely to receive surgery (OR= 2.13; 95% CI 1.51-2.95; p < 0.001) relative to rural patients. Female patients were the majority undergoing surgery and were more likely to receive a complication or require a revision when controlling for surgical complexity. Medicare patients were the least likely to undergo surgery and the most likely to receive complex fusion when undergoing an operation. Medicare patients were the least likely to experience complications (OR=0.89; 95% CI 0.80-0.98; p = 0.022) after adjusting for surgical complexity. With regards to race and ethnicity, Hispanics had a decreased likelihood of receiving a revision surgery.
Conclusion: There were substantial differences in rates of surgical management, postoperative complications, and revisions among individuals of different demographics including sex, insurance status, ethnicity and hospital teaching status. Further research evaluating the effect of demographics in spine surgery is warranted to fully understand their influence on patient outcomes
The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156
No full textThe distribution of white dwarf rotation periods provides a means for constraining angular momentum evolution during the late stages of stellar evolution, as well as insight into the physics and remnants of double degenerate mergers. Although the rotational distribution of low-mass white dwarfs is relatively well constrained via asteroseismology, that of high-mass white dwarfs, which can arise from either intermediate-mass stellar evolution or white dwarf mergers, is not. Photometric variability in white dwarfs due to rotation of a spotted star is rapidly increasing the sample size of high-mass white dwarfs with measured rotation periods. We present the discovery of 22.4 minute photometric variability in the light curve of EGGR 156, a strongly magnetic, ultramassive white dwarf. We interpret this variability as rapid rotation, and our data suggest that EGGR 156 is the remnant of a double degenerate merger. Finally, we calculate the rate of period change in rapidly-rotating, massive, magnetic WDs due to magnetic dipole radiation. In many cases, including EGGR 156, the period change is not currently detectable over reasonable timescales, indicating that these WDs could be very precise clocks. For the most highly-magnetic, rapidly-rotating massive WDs, such as ZTF J1901+1450 and RE J0317−853, the period change should be detectable and may help constrain the structure and evolution of these exotic white dwarfs
Tandem electrocatalytic N₂ fixation via proton-coupled electron transfer
No full textNew electrochemical ammonia (NH₃) synthesis technologies are of interest as a complementary route to the Haber–Bosch process for distributed fertilizer generation, and towards exploiting ammonia as a zero-carbon fuel produced via renewably sourced electricity. Apropos of these goals is a surge of fundamental research targeting heterogeneous materials as electrocatalysts for the nitrogen reduction reaction (N₂RR). These systems generally suffer from poor stability and NH₃ selectivity; the hydrogen evolution reaction (HER) outcompetes N₂RR. Molecular catalyst systems can be exquisitely tuned and offer an alternative strategy, but progress has been thwarted by the same selectivity issue; HER dominates. Here we describe a tandem catalysis strategy that offers a solution to this puzzle. A molecular complex that can mediate an N₂ reduction cycle is partnered with a co-catalyst that interfaces the electrode and an acid to mediate proton-coupled electron transfer steps, facilitating N−H bond formation at a favourable applied potential (−1.2 V versus Fc⁺/⁰) and overall thermodynamic efficiency. Certain intermediates of the N₂RR cycle would be otherwise unreactive via uncoupled electron transfer or proton transfer steps. Structurally diverse complexes of several metals (W, Mo, Os, Fe) also mediate N₂RR electrocatalysis at the same potential in the presence of the mediator, pointing to the generality of this tandem approach
Additive manufacturing of micro-architected metals via hydrogel infusion
No full textMetal additive manufacturing (AM) enables the production of high value and high performance components with applications from the aerospace to biomedical fields. Layer-by-layer fabrication circumvents the geometric limitations of traditional metalworking techniques, allowing topologically optimized parts to be made rapidly and efficiently. Existing AM techniques rely on thermally initiated melting or sintering for part shaping, a costly and material-limited process. We report an AM technique that produces metals and alloys with microscale resolution via vat photopolymerization (VP). 3D-architected hydrogels are infused with metal precursors, then calcined and reduced to convert the hydrogel scaffolds into miniaturized metal replicas. This approach represents a paradigm shift in VP; the material is selected only after the structure is fabricated. Unlike existing VP strategies, which incorporate target materials or precursors into the photoresin during printing, our method does not require re-optimization of resins and curing parameters for different materials, enabling quick iteration, compositional tuning, and the ability to fabricate multimaterials. We demonstrate AM of metals with critical dimensions of ~40 µm that are challenging to fabricate using conventional processes. Such hydrogel-derived metals have highly twinned microstructures and unusually high hardness, providing a pathway to create advanced metallic micromaterials
Properties of Redlen cadmium zinc telluride with respect to x-ray spectroscopy
No full textWe present a study of a sample pixelated cadmium zinc telluride (CZT) detector using CZT purchased from Redlen Technologies. We demonstrate that the material shows good uniformity across the 2 cm × 2 cm × 3 mm crystal in terms of leakage current, gain, and spectral resolution. We find that the detector produces very good spectral resolution for energies up to at least 105 keV, achieving a full-width at half-maxima of 450 eV at 14 keV up to 880 eV at 105 keV using only single-pixel events. Though our analysis of spectra including multiple-pixel events is somewhat limited, we also produce a spectrum including events in which photon energy is deposited across two adjacent pixels. We find that this degrades the energy resolution by up to 30%, but this result can likely be improved using more rigorous calibrations. Additionally, we investigate depth-of-interaction effects, showing that spectral resolution can be improved by 3% to 7% for energies between 86 and 105 keV by removing events beyond a certain depth. Performing this cut reduces efficiency, removing 13% to 21% of photons from the resulting spectral lines
Stability Constrained Reinforcement Learning for Real-Time Voltage Control
No full textDeep reinforcement learning (RL) has been recognized as a promising tool to address the challenges in real-time control of power systems. However, its deployment in real-world power systems has been hindered by a lack of formal stability and safety guarantees. In this paper, we propose a stability constrained reinforcement learning method for real-time voltage control in distribution grids and we prove that the proposed approach provides a formal voltage stability guarantee. The key idea underlying our approach is an explicitly constructed Lyapunov function that certifies stability. We demonstrate the effectiveness of the approach in case studies, where the proposed method can reduce the transient control cost by more than 30% and shorten the response time by a third compared to a widely used linear policy, while always achieving voltage stability. In contrast, standard RL methods often fail to achieve voltage stability
Not All Labels Are Equal: Rationalizing The Labeling Costs for Training Object Detection
No full textDeep neural networks have reached high accuracy on object detection but their success hinges on large amounts of labeled data. To reduce the labels dependency, various active learning strategies have been proposed, based on the confidence of the detector. However, these methods are biased towards high-performing classes and lead to acquired datasets that are not good representatives of the testing set data. In this work, we propose a unified frame-work for active learning, that considers both the uncertainty and the robustness of the detector, ensuring that the network performs well in all classes. Furthermore, our method leverages auto-labeling to suppress a potential distribution drift while boosting the performance of the model. Experiments on PASCAL VOC07+12 and MS-COCO show that our method consistently outperforms a wide range of active learning methods, yielding up to a 7.7% improvement in mAP, or up to 82% reduction in labeling cost. Code is available at https://github.com/NVlabs/AL-SSL