[ 1] Natural radiocarbon measurements on methane ((14)C-CH(4)) extracted from the Cariaco Basin water column show that 98% of the methane in Cariaco Basin waters is derived from fossil ( radiocarbon-free) sources. Previous work on Cariaco Basin methane ( CH(4)) considered only a diagenetic sediment source. Similar measurements of sediment (14)C-CH(4) indicate that sediment CH(4) is produced from modern particulate material; thus the sediment and water column CH(4) have distinct sources. Using time-dependent CH(4) geochemical box models which include a fossil seep CH(4) source term, we estimate 1) 0.024-0.028 Tg y(-1) of seep CH(4) are added to the Cariaco Basin water column, 2) the water column CH(4) will reach a steady-state concentration by the year 2065, and 3) the seep CH(4) inputs possibly began in 1967, following the July 30 Caracas earthquake. Oxidizing this CH(4) to dissolved inorganic carbon does not appear to affect Cariaco Basin (14)C chronologies.