The carbon isotope composition of total dissolved inorganic carbon (DIC) was measured on 111 samples of interstitial water from Sites U1316-U1318 drilled during Integrated Ocean Drilling Program Expedition 307 to the Porcupine Seabight, southwest of Ireland. The analyses were performed to aid in understanding processes involved in the origin and development of Challenger Mound, a deepwater coral mound that was the focus of the expedition. Site U1317 was drilled into the northwest shoulder of Challenger Mound, whereas Sites U1316 and U1318 penetrated siliciclastic slope facies that accumulated downslope and upslope of the mound, respectively. The delta (super 13) C (sub DIC) values in interstitial water range from -18.3 ppm to +3.9 ppm Vienna Peedee belemnite. Maximum and minimum values occur in juxtaposition in the two deepest samples recovered from Site U1317 and correspond to a maximum in dissolved methane. At all three sites, downhole profiles of delta (super 13) C (sub DIC) values show a marked change in slope across a major unconformity between the middle Miocene and upper Pliocene series. At Site U1317, delta (super 13) C (sub DIC) values decrease from -2.5 ppm to -8.5 ppm from the top of the core to the unconformity at approximately 122 meters below seafloor (mbsf). Below this depth, values increase overall to a maximum value of +3.9 ppm. At Sites U1316 and U1318, the delta (super 13) C (sub DIC) values decrease markedly over the uppermost 10 m to values lower than -15 ppm before increasing with depth to approximately -6 ppm at the middle Miocene/upper Pliocene unconformity. Below the unconformity, delta (super 13) C (sub DIC) values increase with depth toward maxima ranging between 0 ppm and +1.5 ppm.