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Fossil methane source dominates Cariaco Basin water column methane geochemistry

Natural radiocarbon measurements on methane (14C‐CH4) 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 (CH4) considered only a diagenetic sediment source. Similar measurements of sediment 14C‐CH4 indicate that sediment CH4 is produced from modern particulate material; thus the sediment and water column CH4 have distinct sources. Using time‐dependent CH4 geochemical box models which include a fossil seep CH4 source term, we estimate 1) 0.024–0.028 Tg y−1 of seep CH4 are added to the Cariaco Basin water column, 2) the water column CH4 will reach a steady‐state concentration by the year 2065, and 3) the seep CH4 inputs possibly began in 1967, following the July 30 Caracas earthquake. Oxidizing this CH4 to dissolved inorganic carbon does not appear to affect Cariaco Basin 14C chronologies.