Calcite cement in Recent submarine and littoral sediments from Denmark is characterized by a high and variable magnesium content. A survey of the magnesium content and distribution in individual crystals has been made by use of energy dispersive X-ray analyses and was found to vary from 1.5 to 11.7 wt%. The maximum content corresponds to an elemental composition of approximately (Ca (sub .55) , Mg (sub .45) ) Co 3 . The distribution of the magnesium reveals a characteristic zonation in the crystals. The magnesium content increases rapidly from the center towards the crystal margin, but shows a significant drop in concentration in the outermost 1-2 mu m of the crystal periphery. The precipitation of the cement is considered to have originated from oxidized methane. The incorporation and distribution of magnesium in the calcite crystal lattice is believed to be governed by an increasing Mg/Ca ration in the brine under the influence of selective removal of Ca ions during crystallization. The abrupt decrease in the surface layers of the crystals is most likely due to interruption of the methane supply, which involves a subsequent leaching of magnesium ions at the time the interstitial solution returns to normal marine composition, in which magnesian calcite is a metastable phase.