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Carbon stable isotopic composition (δ13C) of dissolved inorganic carbon (DIC) in interstitial waters, as well as concentrations of other pore fluid chemical species, have been widely used to infer rates of subseafloor metabolic activities and carbonate diagenesis (Claypool et al., 2006; D’Hondt et al., 2004). Shipboard analyses of interstitial water chemistry and headspace gas at the Eirik Drift sites exhibit downhole sulfate and methane concentration profiles that indicate the presence of the sulfate/​methane interface (see the “Expedition 303 summary” chapter). To further explore chemical signatures of subseafloor metabolisms and early diagenesis, we measured δ13C of DIC in interstitial waters from the Eirik Drift Sites U1305, U1306, and U1307 (Fig. F1). This report provides results of our shore-based analysis of interstitial waters from the three sites.