Interstitial waters and diagenetic changes of marine sediments from Oslofjord and Skagerrak

The recent cla y sediments in the middle Oslofjord and northern Skagerrak areas generally show the following succession of geochemical en vironments downward s from the se dimen t-seawater interface : a thin (0- 5 cm) top oxic layer, a post-oxic zone (anoxic, nonsulfidic) with nitrate and manganese oxide
reduction, and then the lowermost sulfidic zone with frequent pyrite framboids. The upper part of the anoxic environments seems to be the locus of authigenic calcite growth on foraminifera test surfaces. However, these calcite rhombs are corroded together with the foraminiferous substratum with further burial, so they are not preserved towards depth. The authigenic calcite rhombs are found in all the Skagerrak cores, but are missing or in small quantities in the Oslofjord cores. The associated pore waters are close to calcite saturation, but the Mg to Ca ratio is so high that one would rather expect dolomite to form. Most probably, however, the dolomite formation is inhibited by surface poisoning species such as sulfate ions . The pore waters seem close to saturation with all the clay minerals: chlorite, kaolinite, smectite and illite .