Climate change during the medieval climate anomaly (MCA) and the little ice age (LIA) led to the expansion and reduction, respectively, of the hypoxic bottom cover in the Baltic Sea. Here we propose a modeling study to systematically analyze mechanisms by which climate change led to the observed trends and to validate model results to geochemical sediment core data. The interplay between physical and biogeochemical processes leads to complex dynamics controlling oxygen levels in the Baltic Sea. The sediments play an important role by acting both as a source and a sink of phosphate, which forms the main bio-limiting nutrient. However, it is poorly understood how climate change during the MCA led to the spreading of hypoxia. Already, various triggers have been proposed to explain the spreading of the hypoxia during the MCA, such as increased cyanobacterial production under warmer conditions, increased/decreased stratification due to changing rainfall patterns, and sedimentary release of phosphates. In the first part of the project (work package WP1), we will use a state-of-the-art ecosystem model to identify scenarios that can explain the link between climate change and hypoxia during the medieval period.

The model will be improved by implementing an early diagenetic module, which can vertically resolve chemical profiles in the sediment (WP2). Temperature-dependent rate expressions will be implemented for biogeochemical reactions. The sediment module will first be calibrated to the current state of sediments (WP3). Scenarios from WP1 that can successfully explain the oxygen trends will then be tested in model runs from the medieval period to the present day (WP4). The simulation of the medieval period can be validated by various sedimentary proxies, which can reconstruct trends in the redox conditions of the bottom water, the supply of metals from shelves to deeper basins, affecting phosphate sequestration, and the amount of phosphorus and organic matter preserved in sediments. The anticipated results of the project are the attribution of the spreading of hypoxia during the MCA to a mechanism and an improved understanding of the role of benthic dynamics affecting eutrophication in response to climate change.