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:
- Past hypoxic intervals in the Baltic Sea were characterized by
multidecadal oscillations in oxygen stress
- Regularly-paced internal oscillations caused by feedbacks in coupled
iron-phosphorus dynamics
- External loading of phosphorus and climate forcing dictate the
amplitude of internal oscillatory behavior
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.