Linked variations in sediment accumulation rates and sea-level in Guanabara Bay, Brazil, over the last 6000years

Variable sediment accumulation rates in cores collected in Guanabara Bay, Rio de Janeiro State, indicate that filling of the bay started before 6000years ago and that it was not uniform through time. Comparison of these variable sediment accumulation rates and sea-level fluctuations over the last 5500years suggests a linear relation between them. The sea-level drop between 5500 and 5000years ago, for example, corresponds to higher accumulation rates between 5130 and 4350calendaryearsBP. At this time, the sea-level change from 5 to 2m above present sea-level caused a drop of wave base level and consequently an increase of sediment supply to deeper parts of Guanabara Bay. Between approximately 4350 and 2000–1800years ago sea level became stable with consequent reduction in the accumulation rate. From this time until approximately 500years ago accumulation rates increased again because of another sea-level drop from 2m to the present sea-level. The rate of the second sea-level drop was not as high as the first one, and neither was the accumulation rate increase. Although radiocarbon dates suggest a dramatic reduction of accumulation rates between ~4000–2000years ago, estimates for the last 150years based on 210Pb indicate a progressive increase in the accumulation rates despite the presumed stability and perhaps even a small elevation of modern sea-level in the last 150years as deduced from tide gauge records. Accumulation rates increase from 0.14cm·yr−1 to 0.49cm·yr−1 for the period of 1922 to 1951 and reach 0.60cm·yr−1 from 1963 to 1993. In the last five years accumulation rates double, increasing from 0.60cm·yr−1 to 1.25cm·yr−1. These higher accumulation rates during stable or rising sea-level are likely due to local increases in deforestation, expansion of farming land, dredging of channels, and increased paving of roadways.