Hypoxia has occurred intermittently in the Baltic Sea since the establishment of brackish-water conditions at ∼8,000 years B.P., principally as recurrent hypoxic events during the Holocene Thermal Maximum (HTM) and the Medieval Climate Anomaly (MCA). Sedimentary phosphorus release has been implicated as a key driver of these events, but previous paleoenvironmental reconstructions have lacked the sampling resolution to investigate feedbacks in past iron-phosphorus cycling on short timescales. Here we employ Laser Ablation (LA)-ICP-MS scanning of sediment cores to generate ultra-high resolution geochemical records of past hypoxic events. We show that in-phase multidecadal oscillations in hypoxia intensity and iron-phosphorus cycling occurred throughout these events. Using a box model, we demonstrate that such oscillations were likely driven by instabilities in the dynamics of iron-phosphorus cycling under preindustrial phosphorus loads, and modulated by external climate forcing. Oscillatory behavior could complicate the recovery from hypoxia during future trajectories of external loading reductions.

中文翻译：

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铁磷反馈驱动波罗的海缺氧的多年代际振荡

自 BP 约 8,000 年建立咸水条件以来，波罗的海间歇性地发生了缺氧，主要是在全新世最高温度 (HTM) 和中世纪气候异常 (MCA) 期间反复发生的缺氧事件。沉积磷释放被认为是这些事件的关键驱动因素，但之前的古环境重建缺乏采样分辨率来调查过去铁磷循环在短时间内的反馈。在这里，我们采用激光烧蚀 (LA)-ICP-MS 扫描沉积物岩心来生成过去缺氧事件的超高分辨率地球化学记录。我们表明，在这些事件中发生了缺氧强度和铁磷循环的同相数十年振荡。使用盒子模型，我们证明，这种振荡可能是由工业化前磷负荷下铁磷循环动力学的不稳定性驱动的，并受到外部气候强迫的调节。在外部负荷减少的未来轨迹中，振荡行为可能会使缺氧恢复复杂化。