Natural radiocarbon (14C) abundances and stable carbon isotope (Δ13C) compositions were measured for sediment total organic carbon (TOC), and total lipid fractions of sediments, bottom water, and hydrate-water collected from two hydrocarbon seepage sites in Green Canyon, Northern Gulf of Mexico to determine the contribution of old carbon from seeps to sediment TOC and dissolved organic carbon (DOC) pools. Our results indicate that 40--60% of the organic carbon preserved in the sediments and 30% of the DOC in the deep water above the seeps were seep-derived 14C-depleted organic carbon. This new evidence along with our previous studies suggest that natural marine hydrocarbon seepage could be a significant source contributing old carbon to the marine environment. Our findings suggest that the global importance and the long-term impact of this contribution to biogeochemical carbon cycling in the ocean need to be more thoroughly investigated. ¿ 2001 American Geophysical Union