Stable isotope records of carbonates from up to 20 m long sediment cores from the forearc basin of the Sunda arc display significant 13C depletion, deviating from expected normal marine levels by 5 to 40? (Peedee belemnite (PDB)). This depletion is interpreted to be caused by methane seepage and associated authigenic carbonate precipitation in sediments near the seafloor. About 2-¿m-sized globular carbonate particles found in isotopically depleted samples support this interpretation. Their shape suggests microbial mediation of the precipitation process. Investigation of benthic foraminiferal assemblages in cores which record seep influence indicate times of high abundance with an up to twentyfold increase of the population and a dominance of opportunistic infaunal taxa. We relate these "events" to the seepage of methane, thus providing an understanding of the duration and age of this postdepositional process: postulated seep activity is most intense between 3--7 kyr B.P. and 27--33 kyr B.P. As cores with 13C isotope depletion were taken atop active fault zones in the forearc, we suggest a tectonic control of the seepage. Records of seep activity thus may have the potential to archive imprints of tectonic activity.