An anomalous Upper Jurassic limestone bed, 3 m thick, is present within the volcaniclastic infill of a forearc basin, exposed on Alexander Island, Antarctica. The limestone, formally designated the Gateway Pass Limestone Bed, contains laminated crusts and blocks, including hardgrounds, showing radial-fibrous and botryoidal calcite cements. Carbon isotope values are very negative (delta 13 C of -33.5 per thousand to -44.6 per thousand PDB) indicating a methane-oxidation origin for the carbonate. The limestone bed formed around a submarine cold seep in the Antarctic Peninsula forearc basin adjacent to the accretionary prism. In contrast to adjacent nearly barren sea floor the limestone also yields a "benthic island" assemblage containing abundant gastropods and bivalves. The seep is directly comparable to certain Jurassic seeps at the convergent margin of the northeast Pacific, and both are related to subduction at the eastern proto-Pacific margin.