Iron-Phosphorus Feedbacks Drive Multidecadal Oscillations in Baltic Sea Hypoxia

Area of Science:

• Paleoceanography
• Geochemistry
• Climate Science

Background:

• The Baltic Sea has experienced intermittent hypoxia for approximately 8,000 years.
• Recurrent hypoxic events occurred during the Holocene Thermal Maximum and Medieval Climate Anomaly.
• Sedimentary phosphorus release is a known driver of hypoxia, but short-term iron-phosphorus cycling feedbacks were poorly understood.

Purpose of the Study:

• To investigate the short-term dynamics of iron-phosphorus cycling during past Baltic Sea hypoxic events.
• To understand the relationship between hypoxia intensity and geochemical cycling at high resolution.

Main Methods:

• Utilized Laser Ablation (LA)-ICP-MS scanning of sediment cores for ultra-high resolution geochemical analysis.
• Generated detailed records of past hypoxic events.
• Employed a box model to simulate iron-phosphorus cycling dynamics.

Main Results:

• Identified in-phase multidecadal oscillations in hypoxia intensity and iron-phosphorus cycling during past events.
• Demonstrated that these oscillations were likely driven by internal cycling instabilities under preindustrial phosphorus loads.
• Showed modulation of these oscillations by external climate forcing.

Conclusions:

• The study reveals inherent instabilities in Baltic Sea iron-phosphorus cycling that drive oscillatory hypoxia.
• These natural oscillations can complicate the recovery from hypoxia, particularly under future scenarios of reduced external loading.
• High-resolution geochemical analysis is crucial for understanding complex paleoenvironmental feedbacks.