Tom Jilbert*^1,2,3, Bo G. Gustafsson^2,4, Simon Veldhuijzen³, Daniel C. Reed^3,5 Niels A. G. M. van Helmond³, Martijn Hermans^1,3 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.