EARLY DIAGENESIS AND CHEMICAL CHARACTERISTICS OF INTERSTITIAL WATER AND SEDIMENTS IN THE DEEP DEPOSITION BOTTOMS OF THE BALTIC PROPER

The nutrient load into the brackish Baltic Sea has increased over the last century. This increase, together with varying deep-water supply, has resulted in a large-scale oxygen deficiency in the deeper basins o f the Baltic proper. Moreover, since 1976 hydrogen sulphide has been p resent in the bottom water of the eastern Gotland basin. In January 19 90 it was found up to a depth of 130 m. We collected sediment cores at six locations in the deep basins of the Baltic proper. After extracti ng the interstitial waters several carbon and nutrient fractions were analyzed, and the interstitial water itself was analyzed with respect to Eh, pH, alkalinity, iron, manganese, ammonium, phosphorus, hydrogen sulphide, chlorinity, sulphate and major alkali and earth metals (Mg, Ca, Na, K, Sr). Thermodynamic equilibrium calculations in combination with data on carbon and nutrient concentration distributions were use d as an indirect tool for determining early diagenetic processes that could have been responsible for the variation in the measured paramete rs observed in the interstitial water and sediments. High concentratio ns of nutrients and sulphide were found in the interstitial water, esp ecially in the southern and central Baltic proper. Sulphate reduction and subsequent metal sulphide precipitation have induced high carbonat e alkalinity and increasing pH values in the euxinic interstitial wate r with increasing depth in the sediment, and hence it is possible that calcium carbonates and mixed manganese carbonates were precipitated. The dissolution and precipitation of calcium carbonates probably contr ol the upper limit of both the calcium concentration and carbonate alk alinity. Most of the examined interstitial waters were saturated or su persaturated with both pure and mixed manganese carbonates as well as with apatite and anapaite. This indicates that these minerals exhibit slow reaction kinetics or that they are paragenetically formed accordi ng to qualitative observation rules, e.g., the Ostwald step rule or Os twald ripening. The iron concentration was mainly determined by the re sult of precipitation and dissolution of amorphous oxyhydroxide and ac id volatile monosulphide in the oxidized and reduced zones, respective ly. Thermodynamic solubility calculations also suggest that there is a n ongoing precipitation of manganese sulphide in the Gotland deep. Dep letion of sodium, magnesium and strontium in the interstitial water, a s compared to prediagenetic conditions, was probably primarily due to an exchange removal of Fe(III) oxyhydroxides (anoxic condition) from c ertain clay minerals. A close correlation was noted between magnesium and strontium, which implies that the diagenesis of these two elements is similar. The carbon and nitrogen in the sediment samples were pres ent mainly in organic forms; the inorganic fractions normally constitu ted less than or equal to 10% of the total amount of these elements. H owever, the inorganic carbon content in the central part of the Gotlan d deep was relatively high (2% of d.w. or about 30% of the total carbo n content). This enrichment of inorganic carbon was probably mainly du e to enhanced authigenic precipitation of mixed manganese carbonates. Phosphorus, on the other hand, was very abundant in inorganically boun d fractions. In terms of early diagenesis, the amounts of inorganic ph osphorus were governed by the precipitation and dissolution of Fe(III) oxyhydroxides.