Upwelling systems along the western margin of continents are the most productive marine ecosystems on the earth. In such systems the wind along the coast causes off-shore movement of surface water which results in upwelling of cold, highly CO 2 concentrated, nutrient-rich water to the illuminate zone where photosynthesis occurs (Fig. 1a ). Nutrients and CO 2 concentrations are essential for photosynthesis and their rise results in increased primary productivity in the water surface (growing

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... nt of organic matter). Dead organisms sink to the water/sediments zone where they are decomposed by microbes. In this process oxygen is consumed and strongly reducing conditions are established. The fate of magnetic phases in such an ecosystem is poorly resolved, despite the fact that upwelling systems are widespread and are active over millions of years. Considering such system magnetic phases can be formed, e.g., by biogenic processes or be destroyed by reductive dissolution.