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

海洋溶解有机碳（DOC）是地球上最大的还原碳储存库之一。大部分DOC都位于深海中，在那里它的循环非常缓慢。放射性碳测年表明，海洋DOC最古老的成分是缩合芳香族化合物。这些化合物也被认为是无反应性和持久性的。该项目将通过在已得到充分研究的东太平洋隆起9°N热液区进行实地考察，调查洋中脊热液喷口是否是这部分深海DOC的来源。这项工作的结果将促进对深海及其沉积物中缓慢循环的芳香碳库的理解，它在短时间和地质时间尺度上控制着碳的封存。该项目涉及三名早期职业研究员，一名博士后研究员和两名研究生。最近在排气流体中发现的（纳米）颗粒石墨和海底水体中凝聚芳烃的类海洋同位素特征表明，这些凝聚芳烃可能存在热液来源，值得进一步研究。拟议的研究建立在之前的工作基础上，提出了以下问题：热液喷口系统是否控制了深海中顽固的芳香族碳的形成和分布？为了回答这个问题，将从研究充分的东太平洋隆起9°N段的热液喷口区收集样本，以实现三个主要目标：1)通过集中和扩散的流体温度范围，量化和表征热液连续统中的芳香碳和无机地球化学特征；2)确定热蚀变的海洋有机质是否是热液喷口排放的难降解芳香碳的主要来源；3)提供热液通量，分散，以及东太平洋深处颗粒和溶解难熔芳族碳的命运，这些可以在未来的工作中得到验证和完善。使用多种分析代理来量化和表征在广泛的地球化学范围内的喷口流体中释放的石墨和煤烟型分子，将允许确定哪种热液条件有利于生产难熔芳香族碳。通过研究有机-无机相互作用，将产生新的数据集，用于阐明相互关联的生物地球化学过程。互补的同位素和分子测量将揭示热液芳香族碳是否来自先前存在的海洋有机质的热变化，挑战之前关于地幔来源的二氧化碳来源的假设。对离轴水和沉积物样带的评估将确认在深海及其沉积物中存在的凝聚芳香碳是否来源于热液。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.