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Abstract Thaw-induced permafrost landslides, or thaw slumps, exemplify the alarming potential of rapid Arctic climate change to couple with unstable geomorphic feedbacks and escalate landscape hazards, sediment and solute fluxes, and carbon emissions. An emergent hot spot of thaw slump activity in the western Canadian Arctic provides an opportunity to understand the geomorphic context that predisposes some regions to extreme thaw sensitivity. Previous research suggests that slump concentration in this region may result from postglacial incision of ice-cored moraine along the historical margin of the Laurentide Ice Sheet. We assess this hypothesis in the Aklavik Range of the Northwest Territories, where we mapped 217 thaw slumps and reconstructed more than 3 m/k.y. of bedrock canyon incision since the Last Glacial Maximum. We find that thaw slumps tend to form between fluvially incised bedrock and relict till hillslopes. More than 75% of thaw slumping occurs within 500 m of incised bedrock, and hillslope normalized steepness explains over 33% of slump size variability. We interpret intense regional thaw slumping as a climate-sensitive landscape response to postglacial base-level change propagating into ice-cored hillslopes. This finding implies that the end-member permafrost disturbances observed in the western Canadian Arctic may be limited outside regions that share analogous geomorphic context. 

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Prostaglandins (PGs), locally acting lipid signals, regulate female reproduction, including oocyte development. However, the cellular mechanisms of PG action remain largely unknown. One cellular target of PG signaling is the nucleolus. Indeed, across organisms, loss of PGs results in misshapen nucleoli, and changes in nucleolar morphology are indicative of altered nucleolar function. A key role of the nucleolus is to transcribe ribosomal RNA (rRNA) to drive ribosomal biogenesis. Here we take advantage of the robust, in vivo system of Drosophila oogenesis to define the roles and downstream mechanisms whereby PGs regulate the nucleolus. We find that the altered nucleolar morphology due to PG loss is not due to reduced rRNA transcription. Instead, loss of PGs results in increased rRNA transcription and overall protein translation. PGs modulate these nucleolar functions by tightly regulating nuclear actin, which is enriched in the nucleolus. Specifically, we find that loss of PGs results in both increased nucleolar actin and changes in its form. Increasing nuclear actin, by either genetic loss of PG signaling or overexpression of nuclear targeted actin (NLS-actin), results in a round nucleolar morphology. Further, loss of PGs, overexpression of NLS-actin or loss of Exportin 6, all manipulations that increase nuclear actin levels, results in increased RNAPI-dependent transcription. Together these data reveal PGs carefully balance the level and forms of nuclear actin to control the level of nucleolar activity required for producing fertilization competent oocytes. 

Tibiofemoral compression forces present during locomotion can result in high stress and risk damage to the knee. Powered assistance using a knee exoskeleton may reduce the knee load by reducing the work required by the muscles. However, the exact effect of assistance on the tibiofemoral force is unknown. The goal of this study was to investigate the effect of knee extension assistance during the early stance phase on the tibiofemoral force. Nine able-bodied adults walked on an inclined treadmill with a bilateral knee exoskeleton with assistance and with no assistance. Using an EMG-informed neuromusculoskeletal model, muscle forces were estimated, then utilized to estimate the tibiofemoral contact force. Results showed a 28% reduction in the knee moment, which resulted in approximately a 15% decrease in knee extensor muscle activation and a 20% reduction in subsequent muscle force, leading to a significant 10% reduction in peak and 9% reduction in average tibiofemoral contact force during the early stance phase (p < 0.05). The results indicate the tibiofemoral force is highly dependent on the knee kinetics and quadricep muscle activation due to their influence on knee extensor muscle forces, the primary contributor to the knee load. 

High-quality science education is essential for students to become scientifically literate. Model-Evidence Link (MEL) diagrams and build-a-MEL (baMEL) diagrams are instructional scaffolds that create an opportunity for students to build scientific understanding through the evaluation of the connections between evidence and alternative explanations of a scientific phenomenon. The MELs and baMELs allow for a natural incorporation of three-dimensional learning that has been recommended by the Next Generation Science Standards to enhance students’ comprehension. Through this science teaching methodology, students are able to see that by diagramming and then writing about one’s thoughts about the connections between evidence and explanations, one can deepen their understanding of scientific concepts. 

Abstract Accurate helium White Dwarf (DB) masses are critical for understanding the star’s evolution. DB masses derived from the spectroscopic and photometric methods are inconsistent. Photometric masses agree better with currently accepted DB evolutionary theories and are mostly consistent across a large range of surface temperatures. Spectroscopic masses rely on untested HeiStark line-shape and Van der Waals broadening predictions, show unexpected surface temperature trends, and are thus viewed as less reliable. To test this conclusion, we present in this paper detailed HeiStark line-shape measurements at conditions relevant to DB atmospheres (T_electron≈12,000–17,000 K,n_electron≈ 10¹⁷cm⁻³). We use X-rays from Sandia National Laboratories’Z-machine to create a uniform ≈120 mm long hydrogen–helium mixture plasma. Van der Waals broadening is negligible at our experimental conditions, allowing us to measure HeiStark profiles only. Hβ, which has been well-studied in our platform and elsewhere, serves as then_ediagnostic. We find that HeiStark broadening models used in DB analyses are accurate within errors at tested conditions. It therefore seems unlikely that line-shape models are solely responsible for the observed spectroscopic mass trends. Our results should motivate the WD community to further scrutinize the validity of other spectroscopic and photometric input parameters, like atmospheric structure assumptions and convection corrections. These parameters can significantly change the derived DB mass. Identifying potential weaknesses in any input parameters could further our understanding of DBs, help elucidate their evolutionary origins, and strengthen confidence in both spectroscopic and photometric masses. 

It is a pleasure to present the second special issue of The Earth Scientist sponsored by the MEL Project team (https://serc.carleton.edu/mel/index.html)! The Model-Evidence Link (MEL) and MEL2 projects have been sponsored by the National Science Foundation (Grant Nos. 1316057, 1721041, and 2027376) to Temple University and the University of Maryland, in partnership with the University of North Georgia, TERC, and the Planetary Science Institute. In 2016 we shared with you the four MEL diagram activities, covering the topics of climate change, the formation of the Moon, fracking and earthquakes, and wetlands use, as well as a rubric for assessment. This issue brings to you our four new build-a-MEL activities on the origins of the Universe, fossils and Earth’s past, freshwater resources, and extreme weather. Additionally, there are articles about a new NGSS-aligned rubric and transfer task to help students apply their new skills in other situations and about teaching with MEL and build-a-MEL activities. Our goals with all of these activities are to help students learn Earth science content by engaging in scientific practices, notably the evaluation of alternative explanatory models (by looking at the connections between lines of evidence and the competing models) and argumentation. The team has tested these activities in multiple middle and high school classrooms. Our research has shown the activities to be effective in learning both content and skills, and our partner teachers report that students enjoy the activities. These activities are freely available for teachers to use. We hope that you and your students will also find them to be effective and enjoyable approaches to learning about complex and sometimes controversial socioscientific issues within Earth Science. 

Our Earth’s climate is dynamic and ever changing. The fossil record provides evidence of early organisms and information about climate changes of the past. By exploring evidence of variations in the fossil record, students can better understand the issues related to climate change today. In the build-a-MEL activity described here, students are asked to evaluate different lines of evidence and make a judgement about how they connect to alternative explanatory models. Critical thinking skills are enhanced while students engage in a process of negotiation about the evidence, and students are hopefully better prepared to address the complexity of issues related to our current climate change situation from studying fossil evidence.