SEDIMENT ACCUMULATION IN SEPETIBA BAY (BRAZIL) DURING THE HOLOCENE: A REFLEX OF THE HUMAN INFLUENCE

The nature of sedimentation and sediment accumulation rates in Sepetiba Bay, Brazil were interpreted from grain-size patterns, natural radiochemical distributions and seismic stratigraphy. The grain-size analyses showed progressive upward fining of sediment in cores, and a higher percentage of clay in surficial deposits in 1996 than observed during a previous spatial survey in the 1970s. Based on 210Pb geochronology, accumulation rates range from 0.37 cm yr(-1) to 2.0 cm yr(-1) for the last hundred years. In contrast, seismic stratigraphy indicates a range from 0.01 to 0.17 cm yr(-1) over the last 7000 years. Particularly high accumulation rates are found in the northeast part of the bay, and, as a consequence of these high rates, the shoreline in the northern part of the bay prograded approximately 400 m in the last 100 years. An apparent increase in accumulation rates and a tendency for deposits to fine upward over the last similar to 100 years are attributed to human disturbance and soil erosion inland, which have been accelerated with economic development since the late 1970s.