We found methane in North Atlantic surface waters to be in equilibrium relative to the present atmospheric CH4 partial pressure, in agreement with earlier measurements in the Pacific. In the atmosphere, CH4 has more than doubled over the last 150 years, based on historical records and ice-core analysis. The CH4 concentration of surface waters tends to have followed this atmospheric increase through time, which implies that the surface methane in regions of North Atlantic deep-water formation, as in the Greenland and Labrador seas, must have also been steadily rising. Usually deep ocean waters are low in methane, which has been explained exclusively by microbial oxidation. However, the observed undersaturation of methane in deep waters relative to the present atmospheric partial pressure must be related to lower methane concentrations in surface waters in the past. The distributions of methane and CCl3F (CFC11) in the subpolar North Atlantic show that the degree of saturations of both trace gases is highly correlated with each other at almost a 1:1 ratio. Taking into account that methane has been increasing in the atmosphere over a longer period of time than CFC11 and moreover, that CFC11 behaves in the ocean like a noble gas, we derive a 50-year turnover time for CH4 consumption in the newly formed deep waters of the northern Atlantic.
G. Rehder*, R.S. Keir*, M. Rhein1, and E.
Last updated: December 19, 2000