Abstract
Hypoxia has occurred intermittently in the Baltic Sea since the
establishment of brackish-water conditions at ~8000
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 simple
box model, we demonstrate that such oscillations were likely driven by
instabilities in the dynamics of iron-phosphorus cycling under
pre-industrial phosphorus loads, and modulated by external climate
forcing. Oscillatory behavior could complicate the recovery from hypoxia
during future trajectories of external loading reductions.