Growth rates of marine botryoidal aragonite cements from steep (35-45 degrees ) marginal slope deposits in the Bahamas and Belize have been determined by accelerator mass spectrometer radiocarbon dating of samples taken at the base and top of individual botryoids. The pore-filling cements, which range from approximately 11,000-13,000 years old, grew at average rates of 8-10 mm/100 yr with maximum rates > 25 mm/100 yr. Radiocarbon dating of coexisting skeletal components indicates that cementation was syndepositional. Microsampling transects across individual botryoids for stable-isotope analyses show little variation in delta 13 C and delta 18 O, supporting the conclusion that cementation was extremely rapid. Although the cements show a progressive depletion in isotopic composition of approximately 1 per thousand (delta 13 C) and 2 per thousand (delta 18 O) from 13 ka to 11 ka, the average variation (sigma 1 ) within individual pore-filling cements, ranging in size from 2 mm to 32 mm (bottom to top), was 0.11 per thousand (delta 13 C) and 0.14 per thousand (delta 18 O). Results of this study provide the first quantitative data on growth rates of marine carbonate cements in a marginal slope environment. The data indicate that marginal slope deposits may lithify within several tens of years and suggest that "geologically instantaneous" cementation may be critical in stabilizing steep carbonate slope deposits at or above angles of repose.