Elevated organic matter (OM) export flux promotes marine anoxia, thus increasing carbon sequestration efficiency and decreasing atmospheric carbon dioxide levels. However, the mechanisms that trigger and sustain anoxic events—particularly those associated with nutrient-poor, oligotrophic surface waters—remain poorly constrained. Mediterranean Sea sapropels are well-preserved sediments deposited during episodic anoxic events throughout the Plio-Pleistocene; as such, they may provide unique insight into the biogeochemical and ecological drivers of—and responses to—marine anoxia. Using biomarker distributions, we demonstrate that anaerobic ammonium oxidizing (anammox) bacteria and diazotrophic endosymbionts of mat- and/or raft-forming diatoms were both abundant during sapropel events, particularly in the Ionian and Libyan seas. In these sapropels, the carbon isotope compositions of anammox biomarkers directly capture progressive ¹³C-depletion in deep-water dissolved inorganic carbon, indicating sustained carbon sequestration. To explain these observations, we propose a reinforcing feedback whereby initial nutrient and/or circulation perturbations promote fixed nitrogen loss via intensified anammox and heterotrophic denitrification, which in turn favors proliferation of rapidly sinking diatom-diazotroph symbiotic consortia, increases OM burial flux, and sustains anoxia. This mechanism resolves the long-standing conundrum that small and buoyant diazotrophs are apparently associated with high OM export during periods of marine anoxia and oligotrophy.

升高的有机物 (OM) 输出通量会促进海洋缺氧，从而提高碳封存效率并降低大气二氧化碳水平。然而，触发和维持缺氧事件的机制——特别是那些与营养贫乏、贫营养的地表水相关的机制——仍然缺乏约束。地中海腐泥是保存完好的沉积物，在整个上新世 - 更新世的间歇性缺氧事件中沉积；因此，它们可以为海洋缺氧的生物地球化学和生态驱动因素以及对海洋缺氧的反应提供独特的见解。使用生物标志物分布，我们证明厌氧氨氧化 (anammox) 细菌和形成垫子和/或形成筏的硅藻的固氮内共生体在腐烂事件期间都很丰富，特别是在爱奥尼亚海和利比亚海。在这些腐殖质中，深水溶解无机碳中的¹³ C-消耗，表明持续的碳封存。为了解释这些观察结果，我们提出了一种强化反馈，即初始养分和/或循环扰动通过加剧的厌氧氨氧化和异养反硝化促进固定氮损失，这反过来又促进快速下沉的硅藻 - 固氮共生体的增殖，增加 OM 埋藏通量，并维持缺氧。这种机制解决了长期存在的难题，即在海洋缺氧和贫营养期间，小型和漂浮的固氮菌显然与高 OM 输出相关。