Methane concentrations in the Black Sea average 0.1 ml/l at depths below 500 m and about 7 10^-4 ml/l in surface waters. The concentration gradient is causing an upward flux of about 47 ml of methane/m²/year across the interface between the deeper hydrogen sulfide waters and shallow oxygenated surface waters. The unsaturated hydrocarbons (ethylene and propylene) are present in the surface waters at twice the concentration of the saturated hydrocarbons (ethane and propane). In the deeper waters, the saturated hydrocarbons exceed the unsaturated hydrocarbons by several orders of magnitude. The methane appears to be originating from microbiologic degradation of organic matter in the sedime ts. Sediment gases taken from as deep as 800 cm in some cores were found to contain 92 percent methane with a KC¹³ of about -68. Small amounts of ethane, propane, and butane also are being formed with the methane.

A layer of sediment containing about 20 percent organic matter is present at about 50 cm in both the east and west basins of the Black Sea. This organic matter was found to be very high in aromatic hydrocarbons and asphaltic compounds and very low in paraffin hydrocarbons. The paraffins showed a strong predominance of odd-chain lengths in the carbon range from C₂₃ to C₃₃. This organic matter is believed to be partly land derived.