9N EPR Aromatic Carbon

OVERVIEW: Oceanic dissolved organic carbon (DOC) is one of the largest reduced carbon (C) pools on Earth. The majority of this DOC is housed in the deep ocean, where its net removal is extremely slow. The oldest, and presumably most refractory, molecular components of oceanic DOC are condensed aromatic compounds. The origin of these refractory aromatic molecules in the deep ocean is unknown, but recent discoveries of (nano)particulate graphite in venting fluids and marine-like isotopic signature of condensed aromatics in oceanic bottom waters suggests a hydrothermal source warrants further consideration. The proposed research builds upon this previous work by asking: Do hydrothermal vent systems control the formation and distribution of the refractory aromatic C that persists in the deep ocean? To answer this question, samples will be collected from hydrothermal vent fields along the well-studied East Pacific Rise (EPR) 9°N segment to target three main objectives: 1) quantify and characterize aromatic C and inorganic geochemistry along a hydrothermal continuum from a range of focused and diffuse fluid temperatures, 2) determine whether thermally altered marine organic matter is the main source of refractory aromatic C emitted by hydrothermal vents, 3) provide a preliminary model of the hydrothermal fluxes, dispersal, and fate of particulate and dissolved refractory aromatic C in the deep Eastern Pacific Ocean that can be validated and refined with future work. Results produced will significantly advance the understanding of slowly cycling aromatic C pools in the abyssal ocean and its sediments, which controls the sequestration of C on short and geologic timescales. INTELLECTUAL MERIT: The deep oceans are the final frontier in the Earth system and their role in global C cycling is still debated. The proposed research will examine the formation and fate of hydrothermal aromatic C within the global ocean C cycle framework. The significance of these findings will extend beyond the chemical oceanography community and be fruitful for marine geologists seeking to understand the impact of hydrothermal vent fields on the abyssal ocean. By measuring hydrothermal aromatic C fluxes, results of this work will also inform those studying the subsurface biosphere and assist in constraining fluxes between the deep and surface Earth C reservoirs. Using multiple analytical proxies to quantify and characterize graphite and soot-type molecules emitted across a broad geochemical range of venting fluids will allow for the determination of which hydrothermal conditions are favorable for the production of refractory aromatic C. By studying organic-inorganic interactions, novel datasets for elucidating linked biogeochemical processes will be produced. Complementary isotopic and molecular measurements will reveal whether hydrothermal aromatic C is sourced from the thermal alteration of preexisting marine organic matter, challenging the previously-held assumption of a mantle-derived CO 2 source. Assessment of off-axis water and sediment transects will confirm whether condensed aromatic C that persists in the deep ocean and its sediments is hydrothermal in origin.