GEOCHEMICAL RECORD ON EARLY DIAGENESIS OF RECENT BALTIC SEA SEDIMENTS

Crust-freezing technique was used to sample undisturbed short cores fr om two sample stations representing oxidized and reduced top sediments in the Baltic Sea. The structures and chemical properties of laminate d and unlaminated loose surface sediments are described. Laminations a re formed by accumulation of organic rich matter and associated format ion of sulfides during winters, and precipitation of authigenic mangan ese-carbonate from the watermass above during summers. We observed tha t the laminations are not any more visible in recent Gotland Deep (F-8 1) sediments possibly due to accelerated sulfate reduction and methano genesis close to the sediment surface. Early diagenetic processes, ass ociated with sulfur and carbon, were found to control the occurrence o f many elements. The distribution patterns of iron, manganese, molybde num, copper, lead, carbon and sulfur were found to be changed because they are dissolved to interstitial water. In anoxic sediments the temp oral framework is not always useful for evaluating metal trends, but s hifting chemical environments and associated diagenetic processes affe ct the shape of metal profiles. In oxidized conditions, the postdeposi tional changes due to mineralization of carbon and sulfur compounds oc cur at greater sediment depths, and they do not affect the chemical pr operties of surface sediments.