The diagenesis in fine-grained sediments from a 300 to 400-years-old Dunkirk deposit, exposed on the intertidal flat, was studied at a site in the Eastern Scheldt. A new in situ pore water sampling technique that allowed repeated sampling at exactly the same place was used to monitor the seasonal fluctuations in interstitial water composition. The pore-water sampler proved to be useful for monitoring seasonal variations in the pore-water composition, because spatial variability as a disturbing factor could be ruled out. The use of this sampler is advised for unconsolidated sediments in salt marshes, intertidal flats and other shallow (< 0.5 m) water bodies. Subrecent Dunkirk deposits, exposed on intertidal flats in the Eastern Scheldt, contain predominantly non-metabolizable organic carbon. Consequently, rates of organic carbon mineralization and nutrient regeneration in these sediments are low underneath the superficial 0 to 2 cm of sediment in which metabolizable organic matter may temporarily accumulate. The contribution of these sediments to the carbon and nutrient dynamics in the Eastern Scheldt is negligible, because of its restricted surface area and its refractory organic matter. The organic carbon from these types of sediments, after erosion, will behave as inert stuffing in the recent fine-grained deposits, accumulating elsewhere in the Eastern Scheldt.