Investigating the triggering mechanisms of palaeoceanographic disturbance across the Frasnian–Famennian, Late Permian and the Paleocene–Eocene Thermal Maximum: insights from osmium isotopes and geochemistry

This thesis utilizes osmium (Os) isotope, together with other geochemical proxies, to investigate the paleoclimate and palaeoceanography of three Earth history intervals: the Wuchiapingian–Changhsing boundary (WCB), the Frasnian–Famennian (F–F) boundary and the Paleocene–Eocene (P–E) boundary. High-resolution Os isotope chemostratigraphy of four globally correlated WCB sections show two separate Os isotope excursions to less radiogenic compositions that are coincident with the carbon isotope excursions (CIEs). The Os isotope shift is interpreted to reflect increased unradiogenic Os input from basaltic magmatism in South China, possibly related to the Emeishan large igneous province. Volcanism may have provided the isotopically light carbon that drove the negative carbon isotope excursions across the WCB. Organic petrology, Os isotope stratigraphy, major and trace element analyses, and programmed pyrolysis analysis from five F–F sections from western New York, USA show evidence of a wildfire event at the F–F boundary and yield an estimated pO2 level of ~25% for the Late Devonian. Furthermore, the Os isotope records does not support an extra-terrestrial impact or volcanic event as a trigger for the F–F mass extinction. The inferred high O2 level supports the hypothesis that pCO2 drawdown and climate cooling may have caused the F–F mass extinction. A multiproxy geochemical study (Os isotope, mercury, sulfur, platinum group elements) on two P–E boundary North Atlantic Ocean records suggests that both a comet impact and major volcanic activity likely contributed to the environmental perturbations during the P–E interval. Approximately 0.4 Gt of carbon is estimated to have been derived from the comet, thus the impact cannot have been responsible for the full manifestation of the P–E CIE. Other sources of carbon may have jointly driven the P–E thermal maximum. Climate simulations indicate that stratospheric sulfate aerosols from the impact may have caused transient cooling and reduced precipitation prior to the onset of substantial P–E warming

Learning Predictive Safety Filter via Decomposition of Robust Invariant Set

Ensuring safety of nonlinear systems under model uncertainty and external disturbances is crucial, especially for real-world control tasks. Predictive methods such as robust model predictive control (RMPC) require solving nonconvex optimization problems online, which leads to high computational burden and poor scalability. Reinforcement learning (RL) works well with complex systems, but pays the price of losing rigorous safety guarantee. This paper presents a theoretical framework that bridges the advantages of both RMPC and RL to synthesize safety filters for nonlinear systems with state- and action-dependent uncertainty. We decompose the robust invariant set (RIS) into two parts: a target set that aligns with terminal region design of RMPC, and a reach-avoid set that accounts for the rest of RIS. We propose a policy iteration approach for robust reach-avoid problems and establish its monotone convergence. This method sets the stage for an adversarial actor-critic deep RL algorithm, which simultaneously synthesizes a reach-avoid policy network, a disturbance policy network, and a reach-avoid value network. The learned reach-avoid policy network is utilized to generate nominal trajectories for online verification, which filters potentially unsafe actions that may drive the system into unsafe regions when worst-case disturbances are applied. We formulate a second-order cone programming (SOCP) approach for online verification using system level synthesis, which optimizes for the worst-case reach-avoid value of any possible trajectories. The proposed safety filter requires much lower computational complexity than RMPC and still enjoys persistent robust safety guarantee. The effectiveness of our method is illustrated through a numerical example

Observation of the out-of-plane orbital antidamping-like torque

The out-of-plane antidamping-like orbital torque fosters great hope for high-efficiency spintronic devices. Here we report experimentally the observation of out-of-plane antidamping-like torque that could be generated by z-polarized orbital current in ferromagnetic-metal/oxidized Cu bilayers, which is presented unambiguously by the magnetic field angle dependence of spin-torque ferromagnetic resonance signal. The oxidized Cu thickness dependence of orbital torque ratios highlights the interfacial effect would be responsible for the generation of orbital current. Besides that, the oxidized Cu thickness dependence of damping parameter further proves the observation of antidamping-like torque. This result contributes to enriching the orbital-related theory of the generation mechanism of the orbital torque

Topologically protected subradiant cavity polaritons through linewidth narrowing enabled by dissipationless edge states

Cavity polaritons derived from the strong light-matter interaction at the quantum level provide a basis for efficient manipulation of quantum states via cavity field. Polaritons with narrow linewidth and long lifetime are appealing in applications such as quantum sensing and storage. Here, we propose a prototypical arrangement to implement a whispering-gallery-mode resonator with topological mirror moulded by one-dimensional atom array, which allows to boost the lifetime of cavity polaritons over an order of magnitude. This considerable enhancement attributes to the coupling of polaritonic states to dissipationless edge states protected by the topological bandgap of atom array that suppresses the leakage of cavity modes. When exceeding the width of Rabi splitting, topological bandgap can further reduce the dissipation from polaritonic states to bulk states of atom array, giving arise to subradiant cavity polaritons with extremely sharp linewidth. The resultant Rabi oscillation decays with a rate even below the free-space decay of a single quantum emitter. Inheriting from the topologically protected properties of edge states, the subradiance of cavity polaritons can be preserved in the disordered atom mirror with moderate perturbations involving the atomic frequency, interaction strengths and location. Our work opens up a new paradigm of topology-engineered quantum states with robust quantum coherence for future applications in quantum computing and network.Comment: 19 pages,8 figure

The XMM-Newton Line Emission Analysis Program (X-LEAP) I: Emission Line Survey of O VII, O VIII, and Fe L-Shell Transitions

The XMM-Newton Line Emission Analysis Program (X-LEAP) is designed to study diffuse X-ray emissions from the Milky Way (MW) hot gas, as well as emissions from the foreground solar wind charge exchange (SWCX). This paper reports an all-sky survey of spectral feature intensities corresponding to the O VII, O VIII, and iron L-shell (Fe-L) emissions. These intensities are derived from 5418 selected XMM-Newton observations with long exposure times and minimal contamination from point or extended sources. For 90% of the measured intensities, the values are within $\approx$ 2-18 photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ (line unit; L.U.), $\approx$ 0-8 L.U., and $\approx$ 0-9 L.U., respectively. We report long-term variations in O VII and O VIII intensities over 22 years, closely correlating with the solar cycle and attributed to SWCX emissions. These variations contribute $\sim30\%$ and $\sim20\%$ to the observed intensities on average and peak at $\approx$ 4 L.U. and $\approx$ 1 L.U. during solar maxima. We also find evidence of short-term and spatial variations in SWCX, indicating the need for a more refined SWCX model in future studies. In addition, we present SWCX- and absorption-corrected all-sky maps for a better view of the MW hot gas emission. These maps show a gradual decrease in oxygen intensity moving away from the Galactic center and a concentration of Fe-L intensity in the Galactic bubbles and disk.Comment: 23 pages, 13 figures, Accepted by ApJ

The XMM-Newton Line Emission Analysis Program (X-LEAP) II: The Multi-scale Temperature Structures in the Milky Way Hot Gas

This paper presents the multi-scale temperature structures in the Milky Way (MW) hot gas, as part of the XMM-Newton Line Emission Analysis Program (X-LEAP), surveying the O VII, O VIII, and Fe-L band emission features in the XMM-Newton archive. In particular, we define two temperature tracers, $I_{\rm OVIII}/I_{\rm OVII}$ (O87) and $I_{\rm FeL}/(I_{\rm OVII}+I_{\rm OVIII})$ (FeO). These two ratios cannot be explained simultaneously using single-temperature collisional ionization models, which indicates the need for multi-temperature structures in hot gas. In addition, we show three large-scale features in the hot gas: the eROSITA bubbles around the Galactic center (GC); the disk; and the halo. In the eROSITA bubbles, the observed line ratios can be explained by a log-normal temperature distribution with a median of $\log T/{\rm K} \approx 6.4$ and a scatter of $\sigma_T \approx 0.2$ dex. Beyond the bubbles, the line ratio dependence on the Galactic latitude suggests higher temperatures around the midplane of the MW disk. The scale height of the temperature variation is estimated to be $\approx$2 kpc assuming an average distance of $5$ kpc for the hot gas. The halo component is characterized by the dependence on the distance to the GC, showing a temperature decline from $\log\,T/{\rm K}\,\approx\, 6.3$ to $5.8$. Furthermore, we extract the auto-correlation and cross-correlation functions to investigate the small-scale structures. O87 and FeO ratios show a consistent auto-correlation scale of $\approx$$5^\circ$ (i.e., $\approx$$400$ pc at 5 kpc), which is consistent with expected physical sizes of X-ray bubbles associated with star-forming regions or supernova remnants. Finally, we examine the cross-correlation between the hot and UV-detected warm gas, and show an intriguing anti-correlation.Comment: 12 pages, 7 figures. ApJ in pres

Greedy-based Value Representation for Optimal Coordination in Multi-agent Reinforcement Learning

Due to the representation limitation of the joint Q value function, multi-agent reinforcement learning methods with linear value decomposition (LVD) or monotonic value decomposition (MVD) suffer from relative overgeneralization. As a result, they can not ensure optimal consistency (i.e., the correspondence between individual greedy actions and the maximal true Q value). In this paper, we derive the expression of the joint Q value function of LVD and MVD. According to the expression, we draw a transition diagram, where each self-transition node (STN) is a possible convergence. To ensure optimal consistency, the optimal node is required to be the unique STN. Therefore, we propose the greedy-based value representation (GVR), which turns the optimal node into an STN via inferior target shaping and further eliminates the non-optimal STNs via superior experience replay. In addition, GVR achieves an adaptive trade-off between optimality and stability. Our method outperforms state-of-the-art baselines in experiments on various benchmarks. Theoretical proofs and empirical results on matrix games demonstrate that GVR ensures optimal consistency under sufficient exploration

Evidence of wildfires and elevated atmospheric oxygen at the Frasnian−Famennian boundary in New York (USA): Implications for the Late Devonian mass extinction

The Devonian Period experienced significant fluctuations of atmospheric oxygen (O2) levels (∼25−13%), for which the extent and timing are debated. Also characteristic of the Devonian Period, at the Frasnian−Famennian (F−F) boundary, is one of the “big five” mass extinction events of the Phanerozoic. Fossilized charcoal (inertinite) provides a record of wildfire events, which in turn can provide insight into the evolution of terrestrial ecosystems and the atmospheric composition. Here, we report organic petrology, programmed pyrolysis analysis, major and trace element analyses, and initial osmium isotope (Osi) stratigraphy from five sections of Upper Devonian (F−F interval) from western New York, USA. These data are discussed to infer evidence of a wildfire event at the F−F boundary. Based on the evidence for a wildfire at the F−F boundary we also provide an estimate of atmospheric O2 levels of ∼23−25% at this interval, which is in agreement with the models that predict elevated pO2 levels during the Late Devonian. This, coupled with our Os isotope records, support the currently published Osi data that lacks any evidence for an extra-terrestrial impact or volcanic event at the F−F interval, and therefore to act as a trigger for the F−F mass extinction. The elevated O2 level at the F−F interval inferred from this study supports the hypothesis that pCO2 drawdown and associated climate cooling may have acted as a driving mechanism of the F−F mass extinction