Collaborative Research: Hydrothermal vent systems mediate the formation and fate of refractory aromatic carbon in the deep ocean

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Oceanic dissolved organic carbon (DOC) is one of the largest reservoirs of reduced carbon on Earth. Most of this DOC is housed in the deep ocean, where it cycles extremely slowly. Radiocarbon dating shows that the oldest components of oceanic DOC are condensed aromatic compounds. These compounds are also presumed to be unreactive and persistent. This project will investigate whether mid-ocean ridge hydrothermal vents are a source for this fraction of deep ocean DOC, through a field campaign at the well-studied East Pacific Rise 9°N hydrothermal site. Results of this work will advance the understanding of slowly cycling aromatic carbon pools in the abyssal ocean and its sediments, which controls the sequestration of carbon on short and geologic timescales. The project involves three early career researcher investigators, a postdoctoral investigator, and two graduate students.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 for those condensed aromatics and 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 carbon 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 9°N segment to target three main objectives: 1) quantify and characterize aromatic carbon 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 carbon emitted by hydrothermal vents, 3) provide a preliminary model of the hydrothermal fluxes, dispersal, and fate of particulate and dissolved refractory aromatic carbon in the deep Eastern Pacific Ocean that can be validated and refined with future work. 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 carbon. 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 carbon is sourced from the thermal alteration of preexisting marine organic matter, challenging the previously-held assumption of a mantle-derived CO2 source. Assessment of off-axis water and sediment transects will confirm whether condensed aromatic carbon that persists in the deep ocean and its sediments is hydrothermal in origin.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.