Iron‐Phosphorus Feedbacks Drive Multidecadal Oscillations in Baltic Sea Hypoxia

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.