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Sample description and geochemistry collected from Mariana forearc boreholes with the HOG sampler Project Name: Collaborative Research: Collaborative Research: Characterization of Subduction Channel Processes, Borehole Sampling at Active Serpentinite Mud Volcanoes on the Mariana Funding: NSF-OCE-1921654 Susan Lang, Woods Hole Oceanographic InstitutionForearc NSF-OCE-1922671 Geoff Wheat, University of Alaska Fairbanks NSF-OCE-1921361 Jeff Seewald, Woods Hole Oceanographic Institution   Permits: Mariana Trench National Wildlife Refuge, U.S. Fish & Wildlife Service, Department of the Interior Permit Number: 12541-22009   Project goals: The overarching goal of the project was to understand the reaction processes that occur at depth during subduction. Boreholes drilled during the recent International Ocean Discovery Program Expedition 366 were targeted to collect pristine fluids.    The specific goals of the project were to (1) determine the water-rock reactions that control the chemical composition of deep-sourced fluids ascending from atop a subducting slap (2) identify the processes that regulate the nonbiological formation of methane, and assess its potential metabolic impact (3) characterize the origin and abundance of organic carbon molecules to constrain redox and pH conditions within the subduction channel and shallow subsurface environment and (4) relate the permeability of fluid pathways to seamount structure   Samples were collected during a 25 day expedition with a deep-sea submersible. A CORK-Lite was be deployed at three newly cased boreholes: U1492D (Yinazao), U1497D (Fantangisña), and U1496C (Asùt Tesoru). Fluids discharging from these three new boreholes, and a fourth already cased borehole (U1200C, South Chamorro), were collected for inorganic and organic geochemical analysis. Dataset Description: Table 1. Sample Logs. Summary of samples collected by the Hydrothermal Organic Geochemistry (HOG) sampler on R/V Kilo Moana (KM22-14) with ROV Jason, on dives J2_1468 through J2_1478, November 20, 2022 to December 17, 2022.  Fluid samples were collected into either 2 L or 11 L kynar bags that were acid washed (10% HCl soak overnight, followed by through rinsing, overnight Milli-Q soak, and further rinsing). Additional samples were collected by filtering fluids in situ through sterivex filters, flat filters loaded with RNA later, or copper tubes used for helium gas sampling. Partway through the expedition, a long tube was added to the intake of the sampler that reached deeper into the borehole. Samples collected after this addition recovered higher purity fluids. N.D. is no data. Table 2. Geochemical Data including pH, alkalinity, hydrolizable amino acids, free amino acids, total acetate, total formate, stable isotopes of dissolved inorganic carbon (δ13C), and stable isotopes of water (δ2H and δ18O). 

Leading deep-sea research expeditions requires a breadth of training and experience, and the opportunities for Early Career Researchers (ECRs) to obtain focused mentorship on expedition leadership are scarce. To address the need for leadership training in deep-sea expeditionary science, the Crustal Ocean Biosphere Research Accelerator (COBRA) launched a 14-week virtual Master Class with both synchronous and asynchronous components to empower students with the skills and tools to successfully design, propose, and execute deep-sea oceanographic field research. The Master Class offered customized and distributed training approaches and created an open-access syllabus with resources, including reading material, lectures, and on-line resources freely-available on the Master Class website (cobra.pubpub.org). All students were Early Career Researchers (ECRs, defined here as advanced graduate students, postdoctoral scientists, early career faculty, or individuals with substantial industry, government, or NGO experience) and designated throughout as COBRA Fellows. Fellows engaged in topics related to choosing the appropriate deep-sea research asset for their Capstone “dream cruise” project, learning about funding sources and how to tailor proposals to meet those source requirements, and working through an essential checklist of pre-expedition planning and operations. The Master Class covered leading an expedition at sea, at-sea operations, and ship-board etiquette, and the strengths and challenges of telepresence. It also included post-expedition training on data management strategies and report preparation and outputs. Throughout the Master Class, Fellows also discussed education and outreach, international ocean law and policy, and the importance and challenges of team science. Fellows further learned about how to develop concepts respectfully with regard to geographic and cultural considerations of their intended study sites. An assessment of initial outcomes from the first iteration of the COBRA Master Class reinforces the need for such training and shows great promise with one-quarter of the Fellows having submitted a research proposal to national funding agencies within six months of the end of the class. As deep-sea research continues to accelerate in scope and speed, providing equitable access to expedition training is a top priority to enable the next generation of deep-sea science leadership.