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Most of the geologic CO₂entering Earth’s atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO₂and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite–talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul’s transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO_2(aq)concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a δ¹³C of −3.40 ± 0.04‰, and the enrichment of CO₂in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO₂. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul’s Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO₂. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO₂-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO₂. 

Early engagement in undergraduate research opportunities promotes improved critical thinking and scientific reasoning, increased academic performance, enhanced retention both within STEM majors and in college overall, and improved satisfaction with college. It is therefore critical to create pathways for early-stage college students to engage in undergraduate research. Transdisciplinary Grand Challenges programs at large public universities provide an opportunity to engage undergraduates in research that is directly tied to their community’s needs. The objective of this paper is to present the development and implementation of a science communication fellowship to engage early-stage undergraduate students in research. We created the Grand Challenge Water Science Communication Fellowship, in which students work with mentors (faculty, research scientists, graduate students) to create a communication project to educate the public on a water resources related issue that is currently being researched. The research used to produce the communication project can either be the student’s own or the research of their mentor. Students select their own communications venues (e.g., paintings, podcasts, videos, infographics) and work individually with their mentor and together as a cohort to develop and refine their individual projects. The projects are presented at the end of the semester-long fellowship program at the University’s Undergraduate Research Conference, which is open to the public. Participating students and mentors represent a wide variety of backgrounds, including biology, physics, environmental engineering, mechanical engineering, economics, environmental science, and geography. Several tangible benefits were seen for both students and mentors in the program’s first year. Students formed an active multidisciplinary cohort that created a sense of belonging to the university; most of the students are now working in the research lab of their mentor; and students from the prior year’s cohort and organizing and mentoring the next year’s cohort. Research mentors have obtained broader visibility of their research by using the produced communications pieces in grant proposals, research papers, presentations, websites, and other public avenues for knowledge sharing. In the second year of the program, we now aim to use qualitative and quantitative surveys to understand if participation in the program increases students’ self-efficacy and research identity. Survey questions ask students to evaluate aspects such as, how active their role was in planning the project, sense of responsibility for project progress, sense of belonging to a community of researchers, and intention to persist in a research experience. Results will be used to scale this opportunity and create similar communication fellowships for other Grand Challenges and disciplinary programs at the university.