Mariana Borehole

Volatiles elements such as helium (He), carbon (C) and nitrogen (N) 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 in turn, the global distribution of 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 offer 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., CO 2 , CH 4 , NH 4 + ) 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. 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 temperature; as such the dominant biological and geochemical processes at each borehole are expected to vary. This proposal piggybacks on a 25-day cruise scheduled for 2022 (PIs Wheat, Lang and Seewald; Funded Proposal #1922671) and will leverage rich datasets previously gathered at these sites, while adding important estimates of volatile sources and sinks with whole community assessments of metagenomic DNA (relative abundance of different microbial groups and microbial potential functions) and metatranscriptomic RNA (activity of these microbes and their functions).