Methane concentrations in various marine environments

Methane concentrations have been measured in the surface waters and at depth for several different marine environments. Measurements have also been made in rain water collected in Washington, D.C., over the Pacific Ocean, and in Hawaii. In tropical open ocean areas, surface water concentrations average 4.5 × 10−5 ml/l and decrease to values approaching 0.6 × 10−5 ml/l at a depth of 5000 meters. In natural anoxic conditions (i.e., Cariaco Trench, Black Sea, Lake Nitinat) concentrations can increase as much as four orders of magnitude. In the case of Lake Nitinat, 1.6 ml/l is found at a depth of 200 meters. Nearshore and bay concentrations can be 2–3 orders of magnitude higher than open ocean surface values. Open ocean surface waters are slightly supersaturated with methane, whereas estuary systems are highly supersaturated. Open ocean surface values remain fairly constant, whereas bay and river systems vary considerably, depending on the time of year, addition of pollutants, and tidal mixing. Rain water analyses yielded an average concentration of 5.0 × 10−5 ml/l for the Hawaiian and Pacific samples, and an average concentration of 8.8 × 10−5 ml/l was found for the Washington, D.C., area. Atmospheric methane concentrations over the open ocean remain fairly constant at 1.4 ppm. Atmospheric values in Washington, D.C., at NRL averaged 1.7 ppm.