Carbon and nutrient geochemistry of the surface sediments of the Gulf of Riga, Baltic Sea

The distribution of different forms of carbon, nitrogen and phosphorus and standard physical sediment parameters of the surface sediments of the Gulf of Riga, a shallow semi-enclosed brackish bay of the Baltic Sea, has been examined. Several measured parameters were highly correlated. This is valid almost irrespectively of differences in depth below seafloor and sediment characteristics as well as location and extent of burial/diagenesis. Preferential release of nitrogen (N) relative to carbon during organic matter decomposition seems to occur during halmyrolysis and/or almost instantaneously upon settling at the sediment surface because the C-org/N-org ratio is uniform (10.2) within all types of sediment. Preferential release of phosphorus (P) relative to both C and N also occurs and organic matter decomposition appears independent of the amount of C and N. Inorganic C and N in sediments normally constitute only a small proportion of the total C and N concentrations. The amount of inorganically bound nitrogen depends to a great deal on the surface area of the adsorbing species and the clay mineral composition. Inorganic P usually constitutes more than 50% of the total amounts of P, irrespective of physical and chemical differences between samples. Concerning early diagenesis the amount of inorganic P is controlled mainly by the release/fixation on species prone to oxidation/reduction reactions. An estimate of the total amount of annually sequestered C, N and P in the accumulation sediments suggests 44,000, 5800 and 1100 ton/year, respectively. For N and P this sequestering constitutes around 6% and 40% of the total annual load.