Pii: S0025-3227(00)00015-3

Three major depositional basins (the Gdansk, Bornholm and Eastern Gotland basins) of the Baltic Proper, which together account for .50% of the depositional areas in the entire Baltic Sea, have accumulated increased amounts of sediment and organic carbon during the last 50 years, as is shown in Pb-dated sediment cores. The shallow Arkona Basin has had constant accumulation rates and rate increases of bulk sediment, organic carbon and aluminium in the Bornholm Basin are parallel and are interpreted to reflect increased material input from land or from erosion of shallow-water areas. In the Gdansk and Eastern Gotland basins, the accumulation rates of mineral matter have risen at lower rates than those of organic carbon and point to preferential enrichment of organic matter. At increasing rates of burial, the material in the Gdansk and Eastern Gotland basins appears to have been depleted in phosphorus relative to nitrogen and organic carbon. Using C:N:P ratios from a sediment trap mooring in the Gotland Basin and from fluffy layer material in the Pomeranian Bight as characteristic ratios for sedimenting material, we find evidence for significant increases in the ratios of C to P in sediments younger than 15–25 years in cores from the Eastern Gotland basin, coincident with H2S build-up in deep waters. Paired oxygen and phosphorus concentration data from the sub-halocline water column of the Gotland Basin covering the time since 1970 suggest that the phosphate reflux is fed from two sources: At the transition from oxic to anoxic conditions, iron-bound phosphate is suddenly liberated at the sediment–water interface and results in a concentration jump of approximately 2 mmol dm phosphate in deep water. During anoxic periods with H2S in bottom waters, phosphate diffuses out of the sediment and adds to the dissolved phosphate pool. Our data imply that the sediment contributed approximately 14,000 t a of phosphorus to the water column of the deep Gotland Basin since the early 1970s. Compared to total P input to the Baltic Sea (1993: 39,000 t), the reflux of P from anoxic sediments in the Gotland Basin thus is a major source of P for the Baltic Sea. On time scales of years and decades, the reflux of phosphorus from the sediments may be the reason for discrepancies between the ratios of nitrogen and phosphorus in the Baltic Sea and for a lack of effects of declining phosphate discharge into this large ecosystem. q 2000 Elsevier Science B.V. All rights reserved.