Collaborative Research: Volatile Sources and Sinks across the Mariana Forearc

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

Volatile elements such as helium (He), carbon (C), and nitrogen (N) are exchanged between Earth’s surface (atmosphere and oceans) and interior (crust and mantle) at volcanic arcs. When an oceanic tectonic plate sinks, or subducts, underneath a continental plate, forming a volcanic arc, these volatiles delivered by the oceanic plate are released in volcanos and seeps across the entire arc system. The efficiency of this transfer controls the composition of Earth’s mantle, atmospheric evolution, and possibly, the global distribution of microscopic life. Despite the importance of global-scale deep volatile cycles, volatile fluxes have not been extensively studied in submarine volcanic environments, as these regions are notoriously difficult to access. Subseafloor boreholes were recently installed in four active serpentinite mud volcanoes in the Southern Mariana forearc. These boreholes offer a unique opportunity to sample large volumes of deeply sourced volatile-laden fluids. This project will sample the boreholes for He, C, and N isotopes, as well as microbiology, to determine how deep volatile cycling is controlled by complex interactions between geochemical and biological processes in the Mariana forearc. These data will enable calculation of forearc volatile fluxes, quantifying biological and geochemical reactions by comparing fluxes of reactive volatiles (C, N) with inert indicators of mantle contributions (e.g., He) and the relative activity levels of known microbial metabolic pathways across the Mariana forearc. Results will be shared through peer-reviewed publications. In addition, a series of four classroom lectures aimed at a high-school level audience will be developed that leverages data and experiences from this project. Volatile elements are exchanged between Earth’s exterior (atmosphere and oceans) and interior (crust and mantle) at convergent margins. Volatiles are input via subduction and released in volcanos and seeps across the entire arc system. The efficiency of this transfer controls mantle heterogeneities, Earth’s redox conditions, atmospheric evolution, and possibly, the global distribution of microscopic life. Despite the importance of global-scale deep volatile cycles, volatile fluxes have not been extensively studied in submarine volcanic forearcs, as these regions are notoriously difficult to access. Recently emplaced boreholes provide a unique opportunity to sample large volumes of deeply-sourced pristine volatile-laden fluids across a wide expanse of the submarine Mariana forearc. However, volatile outfluxes may be complicated by the fact that shallow forearc temperatures are amenable to life, and thus some of these deeply derived (i.e., from the subducting slab and/or mantle) volatiles (e.g., CO2, CH4, NH4+) may be altered by subsurface microbes. This project will determine forearc volatile fluxes, quantifying biological and geochemical reactions by comparing fluxes of reactive volatiles (C, N) with inert indicators of mantle contributions (e.g., He) and the relative activity levels of known microbial metabolic pathways across a transect of the Mariana forearc. This project will leverage previously emplaced boreholes that have never been sampled for He, C, and N isotopes, or microbiology, to determine how volatile recycling efficiency is controlled by complex interactions between geochemical and biological processes in the Mariana forearc. Cased boreholes were recently installed in four active serpentinite mud volcanoes in the Southern Mariana forearc. Together, these four boreholes form a trench-parallel transect, enabling access to deep fluids from active serpentinite mud volcanoes across the Mariana forearc. This project will gather geochemical and biological data from these boreholes, enabling a systematic assessment of forearc volatile fluxes and the key processes (e.g., calcite precipitation, heterotrophy, autotrophy, respiration) mediating those fluxes. This will determine volatile (He, C and N) fluxes between Earth reservoirs and quantify their sources (slab, mantle wedge, crust) and sinks (geochemical and biological). Boreholes are located at different distances from the trench and the emanating fluids have varying pH and temperatures, so the dominant biological and geochemical processes at each borehole are expected to vary. In addition to training two PhD students and undergraduate students, the PIs have a broad outreach plan that incorporates media outlets and collaborations with science museums, local high schools, and undergraduate researchers. Specifically, the PIs will develop a series of four classroom lectures and activities aimed at a high-school level audience that leverages data and experiences from this project. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.