Tight coupling between enrichment of iron and manganese in North Sea suspended matter and sedimentary redox processes: evidence for seasonal variability

Suspended matter and sediments from the North Sea were analysed for Al, Fe and Mn. A seasonal variation of Mn and Fe over Al ratios is observed. For most of the year suspended matter is enriched in Mn and Fe relative to average soil composition. No enrichment, and even depletion, occurs during a short period of the year (April-May) coinciding with the occurrence of the spring phytoplankton bloom. Surface sediments from the Southern Bight show the inverse image, with Mn and Fe enrichment occurring only in April-May and no enrichment during the rest of the year. These observations suggest a tight coupling between: (1) the change of redox potential in surface sediments, as triggered by the advection of organic matter from the spring bloom; (2) the outflow of reduced iron and manganese from sediments to the watercolumn; and (3) the enrichment of suspended matter in iron and manganese due to the oxidation-precipitation of reduced iron and manganese. The role of the coastal sediments investigated here as an important site of organic matter mineralization is stressed by the dissolved oxygen profiles indicating a transition from 100