The Old Black Sea (Lower-Middle Holocene) deep-sea sediments in the Black Sea basin contain carbonate laminae with a fixed position in the section - in the base of the typical sapropelic muds. The areal distribution of these laminae covers the whole continental slope and rise. They are usually lacking in the sediments of the abyssal plain. XRD, SEM and EDS studies show that the laminae comprise mainly authigenic carbonates - aragonite and magnesian calcite. Aragonite occurs as elongated rice-shaped monocrystals or as diverse aggregates of elongated crystal platelets. The magnesian calcite (6-14 mol % MgCO/sub 3/) forms aggregates of isometric grains with submicritic dimensions between the aragonite grains or individual laminae consisting of idiomorphic rhombohedral and/or skeleton crystals. Low-magnesian calcite is also found sometimes. Usually it is related to Holocene coccoliths without traces of recrystallization. The laminae do not show traces of lithification. A hemogenic-synsedimentary genesis of the carbonate laminae is suggested; their mineral composition witnesses marine chemical composition of the initial solutions with a high Mg/Ca ratio.