The results of mineralogical and geochemical studies of deep-sea carbonate buildups dredged from areas of modem volcanism in the rift valley of the northern Atlantic Ocean were obtained for the first time. Morphology, anatomy, chemical and microelement compositions of buildups are characterized. Mineralized biofilms with bacteriomorphic structures were revealed. Using the methods of X-ray diffraction and RSMZ analysis, the main carbonate minerals of the buildups were diagnosed and studied, and the phenomenon of the anomalous nature of the isotopic composition of carbon and oxygen in them was revealed. It has been suggested that the cause of the abnormally heavy oxygen isotopic composition in carbonate buildups is the borrowing of oxygen by carbonate-forming bacteria directly from sulfates, often distinguished by isotopeheavy oxygen.