P>Authigenic carbonates at methane seeps provide valuable information on the evolution of seepage dynamics. In this study, we report on an unusual type of 13C-depleted [delta 13C: -44.8 parts per thousand to -8.3 parts per thousand V-PDB (Vienna PeeDee Belemnite)] calcite-cemented grapestone concretion composed of small spheroidal nodules, < 0.1-15 cm across, associated with an ancient seep setting in Eocene marine sediments (Pobiti Kamani area, NE Bulgaria). Field observations suggest that the nodules started to grow close to the ancient sea floor and apparently were agitated by fluid flow, promoting their spheroidal shape. The precipitation of calcite between sand grains started at multiple closely spaced nucleation sites, induced by locally elevated calcite supersaturation in response to anaerobic microbial consumption of ascending methane. Consecutive nodule growth continued deeper in the sediment, with the formation of dark and light concentric bands surrounding one or more nodule cores. The occurrence of similar grapestone concretions at seeps might indicate changing redox conditions at the seep site.