An accumulation rate in a well-developed mangrove forest has been associated with relative sea-level rise on an island off the coast of Rio de Janeiro State. This rate was calculated by ²¹⁰Pb dating models from a single sediment core. Results indicate an accumulation rate of approximately 1.7 mm/y for the past approximately 100 years. This rate is almost identical to the ongoing eustatic mean rise in global sea level, indicating a tectonically stable mangrove habitat. Organic C (OC), total N, δ¹³C_(OC), and δ¹⁵N values from selected core intervals suggest a constant source of accumulating vegetal debris, dominated by C3-type vegetation with insignificant input of marine-derived organic matter, and a stable subaerial mangal habitat.