Active fluid venting sites characterized by living chemosynthetic communities and extensive carbonate precipitates are widespread features along the Costa Rica margin and in other subduction systems. During the first phase of SFB 574 about 200 such vent sites were discovered and >500 authigenic carbonates sampled. Several types of carbonate lithologies were identified: Chemoherm and seepage-associated crusts, gas hydrate-associated carbonates, calcareous concretions and dolostones. Stable C- and O-isotopes reveal that all types are depleted in d13C and enriched in d18O. The C-source in all cases is either thermogenic or biogenic methane or a mixture thereof. All samples have equilibrium d18O-signatures heavier than todays ocean ranging from +1 permil SMOW (e.g., cement of chemoherm and seepage-associated carbonates) to +4 permil SMOW (e.g., filling of a bioturbation tube; buried fluid channels).