Old carbon in living organisms and young CaC0₃ cements from abyssal brine seeps

ABYSSAL brine springs at the base of the Florida Escarpment in the Gulf of Mexico (∼3,280 m, 26^° O2' N 84^° 55' W) are surrounded by communities of abundant heterotrophic organisms^1-4 and carbonate-cemented crusts⁵. These organisms are apparently supported by local chemosynthetic primary production because the ¹³C content of their tissues is lower than that found in typical photosynthetically fixed organic carbon⁶, and the enzymes associated with chemosynthetic metabolism are present in their tissues^7,8. Here we report that fossil methane is the dominant source of carbon found in living tissues and recent authigenic carbonate minerals associated with abyssal brine seeps at the base of the Florida Escarpment. Most organic carbon and authigenic carbonates adjacent to the seeps contain progressively less modern carbon as their ¹³C contents approach that of methane carried in the brine. Incorporation of fossil methane (≤ = 1.3% modern carbon) into living tissues and carbonate cements results in recently formed materials which are depleted in ¹⁴C. Therefore, ¹⁴C cannot be used to indicate the age of authigenic materials produced at the pore-water-seepage environments that speckle the continental margins of the world.