Tom Jilbert*1,2,3, Bo G. Gustafsson2,4, Simon Veldhuijzen3, Daniel C. Reed3,5 Niels A. G. M. van Helmond3, Martijn Hermans1,3 and Caroline P. Slomp3
1Aquatic Biogeochemistry Research Unit (ABRU), Ecosystems and Environment Research Program, Faculty of Biological and Environmental Sciences, P.O. Box 65, 00014 University of Helsinki, Finland, 2Tvärminne Zoological Station, University of Helsinki, J.A. Palménintie 260, 10900 Hanko, Finland,3Department of Earth Sciences (Geochemistry), Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands, 4Baltic Nest Institute, Baltic Sea Centre, Stockholm University, S-106 91 Stockholm, Sweden,5Fisheries & Oceans Canada, Bedford Institute of Oceanography, Canada
Corresponding author: Tom Jilbert (tom.jilbert@helsinki.fi)
Key Points:
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.