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Atmospheric carbon dioxide and oxygen concentrations are partially linked via the geological cycle of organic carbon (Fig. 1A-C; e.g., CO2 + H2O ↔ CH2O + O2). The history of these two biologically active components, controls on their concentrations, and implications for the complexity of the biosphere and habitability of Earth have been hotly debated, but generally considered independently. Ribulose bisphosphate carboxylase/oxygenase, RuBisCo, is the enzyme responsible for all oxygenic photosynthesis, carbon fixation, and is the gatekeeper of energy flow to the animal kingdom. Since RuBisCo also fixes O2 as part of photorespiration, O2 and CO2 compete for the active site of RuBisCo. Episodes of enhanced organic carbon burial contributed to removing carbon and releasing oxygen to the environment, particularly after the advent of land biota, so dramatically increased the O2/CO2 ratio (Fig. 1B). This increase in O2/CO2 should have influenced the efficiency of RuBisCo, shifting the balance towards the energy-sapping photorespiration and limiting the carbon fixation ability of plants and algae, thereby reducing new productivity and the energy cascade to the higher trophic levels within the ecosystem. However, the complexity of the modern ecosystem has emerged and thrived amidst this backdrop of increasing O2/CO2 throughout the Phanerozoic, which raises key research questions regarding evolution and habitability. To what extent can the biosphere adapt to variations caused by geological cycles? Are there Gaia-like feedbacks between life and their physical environment that assist in maintaining Earth’s habitability? Does the biosphere itself limit the range of environmental possibilities? Here we link the history of Phanerozoic O2 and CO2 concentrations and draw together the evolution of marine algal primary producers and the diversity history of marine animals to explore feedbacks between life and the environment. We emphasise that spatially resolved coupled redox and fossil evidence may be key to understanding feedbacks between the biosphere and the geosphere, as well as the drivers and limits on habitability. 

Cosmogenic nuclide techniques have advanced the geosciences by providing tools for exposure age dating, burial dating, quantification of denudation rates and more. Advances in geochemistry, accelerator mass spectrometry and atom trap trace analyses are ushering in a new cosmogenic nuclide era, by improving the sensitivity of measurements to ultra- trace levels that now allow new applications of these techniques to numerous Earth surface processes. The advances in cosmogenic nuclide techniques have equipped the next generation of geoscientists with invaluable tools for understanding the planet, but addressing pressing needs requires rising to an even greater challenge: imbuing within the cosmogenic community, and the geosciences as a whole, a commitment to justice, equity, diversity and inclusion that matches our dedication to scientific research. In this Primer, we review the state of the art and recent exciting breakthroughs in the use of cosmogenic nuclide techniques, focusing on erosion factories over space and time, and new perspectives on ice sheet stability. We also highlight promising ways forward in enhancing inclusion in the field, as well as obstacles that remain to be overcome. 

It might be highly effective if students could transition dynamically between individual and collaborative learning activities, but how could teachers manage such complex classroom scenarios? Although recent work in AIED has focused on teacher tools, little is known about how to orchestrate dynamic transitions between individual and collaborative learning. We created a novel technology ecosystem that supports these dynamic transitions. The ecosystem integrates a novel teacher orchestration tool that provides monitoring support and pairing suggestions with two AI-based tutoring systems that support individual and collaborative learning, respectively. We tested the feasibility of this ecosystem in a classroom study with 5 teachers and 199 students over 22 class sessions. We found that the teachers were able to manage the dynamic transitions and valued them. The study contributes a new technology ecosystem for dynamically transitioning between individual and collaborative learning, plus insight into the orchestration functionality that makes these transitions feasible. 

We show that non-obtuse trapezoids are uniquely determined by their Dirichlet Laplace spectrum. This extends our previous result [Hezari et al., Ann. Henri Poincare 18(12), 3759–3792 (2017)], which was only concerned with the Neumann Laplace spectrum. 

Ionic reactions are the most common reactions used in chemical synthesis. In relatively low dielectric constant solvents (e.g., dichloromethane, toluene), ions usually exist as ion pairs. Despite the importance of counterions, a quantitative description of how the paired ’counterion’ affects the reaction kinetic is still elusive. We introduce a general and quantitative model, namely transition-state expansion (TSE), that describes how the size of a counterion affects the transition- state structure and the kinetics of an ionic reaction. This model could rationalize the counterion effects in nucleophilic substitutions and gold-catalyzed enyne cycloisomerizations. 

How oxygen levels in Earth’s atmosphere and oceans evolved has always been a central question in Earth System Science. Researchers have developed numerous tracers to tackle this question, utilizing geochemical characteristics of different elements. Iodine incorporated in calcium carbonate (including biogenic) minerals, reported as I/Ca, is a proxy for dissolved oxygen in seawater. Here we review the rationale behind this proxy, its recent applications, and some potential future research directions.