Early diagenesis and chemical characteristics of interstitial water and sediments in the deep deposition bottoms of the Baltic proper

Abstract

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 of the Baltic proper. Moreover, since 1976 hydrogen sulphide has been present in the bottom water of the eastern Gotland basin. In January 1990 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 extracting 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 used as an indirect tool for determining early diagenetic processes that could have been responsible for the variation in the measured parameters observed in the interstitial water and sediments. High concentrations of nutrients and sulphide were found in the interstitial water, especially in the southern and central Baltic proper. Sulphate reduction and subsequent metal sulphide precipitation have induced high carbonate alkalinity and increasing pH values in the euxinic interstitial water 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 control the upper limit of both the calcium concentration and carbonate alkalinity. Most of the examined interstitial waters were saturated or supersaturated 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 according to qualitative observation rules, e.g., the Ostwald step rule or Ostwald ripening. The iron concentration was mainly determined by the result of precipitation and dissolution of amorphous oxyhydroxide and acid volatile monosulphide in the oxidized and reduced zones, respectively. Thermodynamic solubility calculations also suggest that there is an ongoing precipitation of manganese sulphide in the Gotland deep. Depletion of sodium, magnesium and strontium in the interstitial water, as compared to prediagenetic conditions, was probably primarily due to an exchange removal of Fe(III) oxyhydroxides (anoxic condition) from certain 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 present mainly in organic forms; the inorganic fractions normally constituted ≤10% of the total amount of these elements. However, the inorganic carbon content in the central part of the Gotland deep was relatively high (2% of d.w. or about 30% of the total carbon content). This enrichment of inorganic carbon was probably mainly due to enhanced authigenic precipitation of mixed manganese carbonates. Phosphorus, on the other hand, was very abundant in inorganically bound fractions. In terms of early diagenesis, the amounts of inorganic phosphorus were governed by the precipitation and dissolution of Fe(III) oxyhydroxides.